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• 151. Aad, G.
KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik. KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik. KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik. KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik.
Search for light scalar top-quark pair production in final states with two leptons with the ATLAS detector in root s=7 TeV proton-proton collisions2012Inngår i: European Physical Journal C, ISSN 1434-6044, E-ISSN 1434-6052, Vol. 72, nr 11, s. 2237-Artikkel i tidsskrift (Fagfellevurdert)

A search is presented for the pair production of light scalar top quarks in root s = 7 TeV proton-proton collisions recorded with the ATLAS detector at the Large Hadron Collider. This analysis uses the full data sample collected during 2011 running that corresponds to a total integrated luminosity of 4.7 fb(-1). Light scalar top quarks are searched for in events with two opposite-sign leptons (e, mu), large missing transverse momentum and at least one jet in the final state. No excess over Standard Model expectations is found, and the results are interpreted under the assumption that the light scalar top decays to a b-quark in addition to an on-shell chargino whose decay occurs through a virtual W boson. If the chargino mass is 106 GeV, light scalar top-quark masses up to 130 GeV are excluded for neutralino masses below 70 GeV.

• 152. Aad, G.
KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik. KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik. KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik. KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik.
Search for pair-produced massive coloured scalars in four-jet final states with the ATLAS detector in proton-proton collisions at root s=7 TeV2013Inngår i: European Physical Journal C, ISSN 1434-6044, E-ISSN 1434-6052, Vol. 73, nr 1, s. 2263-Artikkel i tidsskrift (Fagfellevurdert)

A search for pair-produced massive coloured scalar particles decaying to a four-jet final state is performed by the ATLAS experiment at the LHC in proton-proton collisions at root s = 7 TeV. The analysed data sample corresponds to an integrated luminosity of 4.6 fb(-1). No deviation from the Standard Model is observed in the invariant mass spectrum of the two-jet pairs. A limit on the scalar gluon pair production cross section of 70 pb (10 pb) is obtained at the 95 % confidence level for a scalar gluon mass of 150 GeV (350 GeV). Interpreting these results as mass limits on scalar gluons, masses ranging from 150 GeV to 287 GeV are excluded at the 95 % confidence level.

• 153. Aad, G.
KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik. KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik. KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik. KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik.
Search for second generation scalar leptoquarks in pp collisions at root s=7 TeV with the ATLAS detector2012Inngår i: European Physical Journal C, ISSN 1434-6044, E-ISSN 1434-6052, Vol. 72, nr 9, s. 2151-Artikkel i tidsskrift (Fagfellevurdert)

The results of a search for the production of second generation scalar leptoquarks are presented for final states consisting of either two muons and at least two jets or a muon plus missing transverse momentum and at least two jets. A total of 1.03 fb(-1) integrated luminosity of proton-proton collision data produced by the Large Hadron Collider at root s = 7 TeV and recorded by the ATLAS detector is used for the search. The event yields in the signal regions are found to be consistent with the Standard Model background expectations. The production of second generation leptoquarks is excluded for a leptoquark mass m(LQ) < 594 (685) GeV at 95 % confidence level, for a branching ratio of 0.5 (1.0) for leptoquark decay to a muon and a quark.

• 154. Aad, G.
KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik. KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik. KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik. KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik.
Search for supersymmetry in events with large missing transverse momentum, jets, and at least one tau lepton in 7 TeV proton-proton collision data with the ATLAS detector2012Inngår i: European Physical Journal C, ISSN 1434-6044, E-ISSN 1434-6052, Vol. 72, nr 11, s. 2215-Artikkel i tidsskrift (Fagfellevurdert)

A search for supersymmetry (SUSY) in events with large missing transverse momentum, jets, and at least one hadronically decaying tau lepton, with zero or one additional light lepton (e/mu), has been performed using 4.7 fb(-1) of proton-proton collision data at root s = 7 TeV recorded with the ATLAS detector at the Large Hadron Collider. No excess above the Standard Model background expectation is observed and a 95 % confidence level visible cross-section upper limit for new phenomena is set. In the framework of gauge-mediated SUSY-breaking models, lower limits on the mass scale. are set at 54 TeV in the regions where the (tau) over tilde (1) is the next-to-lightest SUSY particle (tan beta > 20). These limits provide the most stringent tests to date of GMSB models in a large part of the parameter space considered.

• 155. Aad, G.
KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik. KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik. KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik. KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik.
Search for top and bottom squarks from gluino pair production in final states with missing transverse energy and at least three b-jets with the ATLAS detector2012Inngår i: European Physical Journal C, ISSN 1434-6044, E-ISSN 1434-6052, Vol. 72, nr 10, s. 2174-Artikkel i tidsskrift (Fagfellevurdert)

This letter reports the results of a search for top and bottom squarks from gluino pair production in 4.7 fb(-1) of pp collisions at root s = 7 TeV using the ATLAS detector at the LHC. The search is performed in events with large missing transverse momentum and at least three jets identified as originating from a b-quark. Exclusion limits are presented for a variety of gluino-mediated models with gluino masses up to 1 TeV excluded.

• 156. Aad, G.
KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik. KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik. KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik. KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik.
Single hadron response measurement and calorimeter jet energy scale uncertainty with the ATLAS detector at the LHC2013Inngår i: European Physical Journal C, ISSN 1434-6044, E-ISSN 1434-6052, Vol. 73, nr 3, s. 2305-Artikkel i tidsskrift (Fagfellevurdert)

The uncertainty on the calorimeter energy response to jets of particles is derived for the ATLAS experiment at the Large Hadron Collider (LHC). First, the calorimeter response to single isolated charged hadrons is measured and compared to the Monte Carlo simulation using proton-proton collisions at centre-of-mass energies of root s = 900 GeV and 7 TeV collected during 2009 and 2010. Then, using the decay of K-s and Lambda particles, the calorimeter response to specific types of particles (positively and negatively charged pions, protons, and anti-protons) is measured and compared to the Monte Carlo predictions. Finally, the jet energy scale uncertainty is determined by propagating the response uncertainty for single charged and neutral particles to jets. The response uncertainty is 2-5 % for central isolated hadrons and 1-3 % for the final calorimeter jet energy scale.

• 157. Aad, G.
KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik. KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik. KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik. KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik.
The differential production cross section of the phi(1020) meson in root s=7 TeV pp collisions measured with the ATLAS detector2014Inngår i: European Physical Journal C, ISSN 1434-6044, E-ISSN 1434-6052, Vol. 74, nr 7, s. 2895-Artikkel i tidsskrift (Fagfellevurdert)

A measurement is presented of the phi x BR(phi -> K+ K-) production cross section at root s = 7 TeV using pp collision data corresponding to an integrated luminosity of 383 mu b(-1), collected with the ATLAS experiment at the LHC. Selection of phi(1020) mesons is based on the identification of charged kaons by their energy loss in the pixel detector. The differential cross section is measured as a function of the transverse momentum, pT, phi, and rapidity, y(phi), of the phi(1020) meson in the fiducial region 500 < pT,phi < 1200MeV, vertical bar y phi| < 0.8, kaon p(T), (K) > 230 MeV and kaon momentum p(K) < 800 MeV. The integrated phi(1020)-meson production cross section in this fiducial range is measured to be sigma(phi) x BR(phi -> K+ K-) = 570 +/- 8 (stat) +/- 66 (syst) +/- 20 (lumi) mu b.

• 158. Aad, G.
KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik. KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik. KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik. KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik. KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik.
Search for resonant diboson production in the llq(q)over-bar final state in pp collisions at root s=8 TeV with the ATLAS detector2015Inngår i: European Physical Journal C, ISSN 1434-6044, E-ISSN 1434-6052, Vol. 75, nr 2, artikkel-id 69Artikkel i tidsskrift (Fagfellevurdert)

This paper reports on a search for narrow resonances in diboson production in the llq (q) over bar final state using pp collision data corresponding to an integrated luminosity of 20 fb(-1) collected at root s = 8 TeV with the ATLAS detector at the Large Hadron Collider. No significant excess of data events over the Standard Model expectation is observed. Upper limits at the 95 % confidence level are set on the production cross section times branching ratio for Kaluza-Klein gravitons predicted by the Randall-Sundrum model and for Extended Gauge Model W' bosons. These results lead to the exclusion of mass values below 740 and 1590 GeV for the graviton and W' boson respectively.

• 159. Aad, G.
KTH, Skolan för teknikvetenskap (SCI), Fysik. KTH, Skolan för teknikvetenskap (SCI), Fysik. KTH, Skolan för teknikvetenskap (SCI), Fysik. KTH, Skolan för teknikvetenskap (SCI), Fysik.
Search for the Standard Model Higgs boson produced in association with top quarks and decaying into in collisions at with the ATLAS detector2015Inngår i: European Physical Journal C, ISSN 1434-6044, E-ISSN 1434-6052, Vol. 75, nr 7, artikkel-id 349Artikkel i tidsskrift (Fagfellevurdert)

A search for the Standard Model Higgs boson produced in association with a top-quark pair, , is presented. The analysis uses 20.3 fb(-1) of pp collision data at , collected with the ATLAS detector at the Large Hadron Collider during 2012. The search is designed for the decay mode and uses events containing one or two electrons or muons. In order to improve the sensitivity of the search, events are categorised according to their jet and b-tagged jet multiplicities. A neural network is used to discriminate between signal and background events, the latter being dominated by +jets production. In the single-lepton channel, variables calculated using a matrix element method are included as inputs to the neural network to improve discrimination of the irreducible background. No significant excess of events above the background expectation is found and an observed (expected) limit of 3.4 (2.2) times the Standard Model cross section is obtained at 95 % confidence level. The ratio of the measured signal cross section to the Standard Model expectation is found to be assuming a Higgs boson mass of 125 Gev.

• 160.
Aix Marseille Univ, CPPM, Marseille, France.;CNRS, IN2P3, Marseille, France..
KTH, Skolan för teknikvetenskap (SCI), Fysik. KTH, Skolan för teknikvetenskap (SCI), Fysik. KTH, Skolan för teknikvetenskap (SCI), Fysik. KTH, Skolan för teknikvetenskap (SCI), Fysik. CERN, Geneva, Switzerland..
Search for supersymmetry in events containing a same-flavour opposite-sign dilepton pair, jets, and large missing transverse momentum in root s = 8 TeV pp collisions with the ATLAS detector (vol 75, 318, 2015)2015Inngår i: European Physical Journal C, ISSN 1434-6044, E-ISSN 1434-6052, Vol. 75, nr 10, s. 1-15, artikkel-id 463Artikkel i tidsskrift (Fagfellevurdert)
• 161. Aad, G.
KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik. KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik. KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik. KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik.
Constraints on the off-shell Higgs boson signal strength in the high-mass ZZ and WW final states with the ATLAS detector2015Inngår i: European Physical Journal C, ISSN 1434-6044, E-ISSN 1434-6052, Vol. 75, nr 7, artikkel-id 335Artikkel i tidsskrift (Fagfellevurdert)

Measurements of the ZZ and WW final states in the mass range above the 2m(Z) and 2m(W) thresholds provide a unique opportunity to measure the off-shell coupling strength of the Higgs boson. This paper presents constraints on the off-shell Higgs boson event yields normalised to the Standard Model prediction (signal strength) in the ZZ -> 4l, ZZ -> 2l2 nu and WW -> e nu mu nu final states. The result is based on pp collision data collected by the ATLAS experiment at the LHC, corresponding to an integrated luminosity of 20.3 fb(-1) at a collision energy of root s = 8 TeV. Using the CLs method, the observed 95 % confidence level (CL) upper limit on the off-shell signal strength is in the range 5.1-8.6, with an expected range of 6.7-11.0. In each case the range is determined by varying the unknown gg -> ZZ and gg -> WW background K-factor from higher-order quantum chromodynamics corrections between half and twice the value of the known signal K-factor. Assuming the relevant Higgs boson couplings are independent of the energy scale of the Higgs boson production, a combination with the on-shell measurements yields an observed (expected) 95 % CL upper limit on Gamma(H)/Gamma(SM)(H) in the range 4.5-7.5 (6.5-11.2) using the same variations of the background K-factor. Assuming that the unknown gg -> VV background K-factor is equal to the signal K-factor, this translates into an observed (expected) 95 % CL upper limit on the Higgs boson total width of 22.7 (33.0) MeV.

• 162. Aad, G.
KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik. KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik. KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik. KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik.
Identification and energy calibration of hadronically decaying tau leptons with the ATLAS experiment in pp collisions at root s=8 TeV2015Inngår i: European Physical Journal C, ISSN 1434-6044, E-ISSN 1434-6052, Vol. 57, nr 7, artikkel-id 303Artikkel i tidsskrift (Fagfellevurdert)

This paper describes the trigger and offline reconstruction, identification and energy calibration algorithms for hadronic decays of tau leptons employed for the data collected from pp collisions in 2012 with the ATLAS detector at the LHC center-of-mass energy root s = 8 TeV. The performance of these algorithms is measured in most cases with Z decays to tau leptons using the full 2012 dataset, corresponding to an integrated luminosity of 20.3 fb(-1). An uncertainty on the offline reconstructed tau energy scale of 2-4%, depending on transverse energy and pseudorapidity, is achieved using two independent methods. The offline tau identification efficiency is measured with a precision of 2.5% for hadronically decaying tau leptons with one associated track, and of 4% for the case of three associated tracks, inclusive in pseudorapidity and for a visible transverse energy greater than 20 GeV. For hadronic tau lepton decays selected by offline algorithms, the tau trigger identification efficiency is measured with a precision of 2-8%, depending on the transverse energy. The performance of the tau algorithms, both offline and at the trigger level, is found to be stable with respect to the number of concurrent proton-proton interactions and has supported a variety of physics results using hadronically decaying tau leptons at ATLAS.

• 163. Aad, G.
KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik. KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik. KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik. KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik.
Search for supersymmetry in events containing a same-flavour opposite-sign dilepton pair, jets, and large missing transverse momentum in root s=8TeV pp collisions with the ATLAS detector2015Inngår i: European Physical Journal C, ISSN 1434-6044, E-ISSN 1434-6052, Vol. 75, nr 7, artikkel-id 318Artikkel i tidsskrift (Fagfellevurdert)

Two searches for supersymmetric particles in final states containing a same-flavour opposite-sign lepton pair, jets and large missing transverse momentum are presented. The proton-proton collision data used in these searches were collected at a centre-of-mass energy root s = 8TeV by the ATLAS detector at the Large Hadron Collider and corresponds to an integrated luminosity of 20.3 fb(-1). Two leptonic production mechanisms are considered: decays of squarks and gluinos with Z bosons in the final state, resulting in a peak in the dilepton invariant mass distribution around the Z-boson mass; and decays of neutralinos (e.g.. (chi) over tilde (0)(2) -> l(+)l(-) (chi) over tilde (0)(1)), resulting in a kinematic endpoint in the dilepton invariant mass distribution. For the former, an excess of events above the expected Standard Model background is observed, with a significance of three standard deviations. In the latter case, the data are well-described by the expected Standard Model background. The results from each channel are interpreted in the context of several supersymmetric models involving the production of squarks and gluinos.

• 164. Aad, G.
KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik. KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik. KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik.
Measurement of the centrality and pseudorapidity dependence of the integrated elliptic flow in lead-lead collisions at root S-NN=2.76 TeV with the ATLAS detector2014Inngår i: European Physical Journal C, ISSN 1434-6044, E-ISSN 1434-6052, Vol. 74, nr 8, s. 2982-Artikkel i tidsskrift (Fagfellevurdert)

The integrated elliptic flow of charged particles produced in Pb+Pb collisions at root S-NN = 2.76 TeV has been measured with the ATLAS detector using data collected at the Large Hadron Collider. The anisotropy parameter, upsilon(2), was measured in the pseudorapidity range |eta| <= 2.5 with the event-plane method. In order to include tracks with very low transverse momentum p(T), thus reducing the uncertainty in upsilon(2) integrated over p(T), a 1 mu b(-1) data sample recorded without a magnetic field in the tracking detectors is used. The centrality dependence of the integrated upsilon(2) is compared to other measurements obtained with higher PT thresholds. The integrated elliptic flow is weakly decreasing with |eta|. The integrated upsilon 2 transformed to the rest frame of one of the colliding nuclei is compared to the lower-energy RHIC data.

• 165. Aad, G.
KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik. KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik. KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik.
Measurement of the charge asymmetry in top quark pair production in pp collisions at root s=7 TeV using the ATLAS detector2012Inngår i: European Physical Journal C, ISSN 1434-6044, E-ISSN 1434-6052, Vol. 72, nr 6, s. 2039-Artikkel i tidsskrift (Fagfellevurdert)

A measurement of the top-antitop production charge asymmetry A(C) is presented using data corresponding to an integrated luminosity of 1.04 fb(-1) of pp collisions at root s = 7 TeV collected by the ATLAS detector at the LHC. Events are selected with a single lepton (electron or muon), missing transverse momentum and at least four jets of which at least one jet is identified as coming from a b-quark. A kinematic fit is used to reconstruct the t (t) over bar event topology. After background subtraction, a Bayesian unfolding procedure is performed to correct for acceptance and detector effects. The measured value of A(C) is A(C) =-0.019+/-0.028 (stat.)+/-0.024 (syst.), consistent with the prediction from the MC@NLO Monte Carlo generator of A(C) = 0.006+/-0.002. Measurements of AC in two ranges of invariant mass of the top-antitop pair are also shown.

• 166. Aad, G.
KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik. KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik. KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik.
Measurement of the top quark mass with the template method in the t(t)over-bar -> lepton plus jets channel using ATLAS data2012Inngår i: European Physical Journal C, ISSN 1434-6044, E-ISSN 1434-6052, Vol. 72, nr 6, s. 2046-Artikkel i tidsskrift (Fagfellevurdert)

The top quark mass has been measured using the template method in the t (t) over bar -> lepton + jets channel based on data recorded in 2011 with the ATLAS detector at the LHC. The data were taken at a proton-proton centre-of-mass energy of root s = 7 TeV and correspond to an integrated luminosity of 1.04 fb(-1). The analyses in the e + jets and mu + jets decay channels yield consistent results. The top quark mass is measured to be m(top) = 174.5 +/- 0.6(stat) +/- 2.3(syst) GeV.

• 167. Aad, G.
KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik. KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik. KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik.
Search for lepton flavour violation in the e mu continuum with the ATLAS detector in root s=7 TeV pp collisions at the LHC2012Inngår i: European Physical Journal C, ISSN 1434-6044, E-ISSN 1434-6052, Vol. 72, nr 6, s. 2040-Artikkel i tidsskrift (Fagfellevurdert)

This paper presents a search for the t-channel exchange of an R-parity violating scalar top quark ((t) over tilde) in the e(+/-)mu(-/+) continuum using 2.1 fb(-1) of data collected by the ATLAS detector in root s = 7 TeV pp collisions at the Large Hadron Collider. Data are found to be consistent with the expectation from the Standard Model backgrounds. Limits on R-parity-violating couplings at 95 % C.L. are calculated as a function of the scalar top mass (m((t) over tilde)). The upper limits on the production cross section for pp -> e mu X, through the t-channel exchange of a scalar top quark, ranges from 170 fb for m((t) over tilde) = 95 GeV to 30 fb for m((t) over tilde) = 1000 GeV.

• 168. Aad, G.
KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik. KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik. KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik.
Rapidity gap cross sections measured with the ATLAS detector in pp collisions at root s=7 TeV2012Inngår i: European Physical Journal C, ISSN 1434-6044, E-ISSN 1434-6052, Vol. 72, nr 3, s. 1926-Artikkel i tidsskrift (Fagfellevurdert)

Pseudorapidity gap distributions in proton-proton collisions at root s = 7 TeV are studied using a minimum bias data sample with an integrated luminosity of 7.1 mu b(-1). Cross sections are measured differentially in terms of Delta eta(F), the larger of the pseudorapidity regions extending to the limits of the ATLAS sensitivity, at eta = +/- 4.9, in which no final state particles are produced above a transverse momentum threshold p(T)(cut). The measurements span the region 0 < Delta eta(F) < 8 for 200 MeV < p(T)(cut) < 800 MeV. At small Delta eta(F), the data test the reliability of hadronisation models in describing rapidity and transverse momentum fluctuations in final state particle production. The measurements at larger gap sizes are dominated by contributions from the single diffractive dissociation process (pp -> Xp), enhanced by double dissociation (pp -> XY) where the invariant mass of the lighter of the two dissociation systems satisfies M-Y less than or similar to 7 GeV. The resulting cross section is ds sigma/d Delta eta(F) approximate to 1 mb for Delta eta(F) greater than or similar to 3. The large rapidity gap data are used to constrain the value of the Pomeron intercept appropriate to triple Regge models of soft diffraction. The cross section integrated over all gap sizes is compared with other LHC inelastic cross section measurements.

• 169. Aad, G.
KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik. KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik.
Measurement of the inclusive and dijet cross-sections of b-jets in pp collisions at root s=7 TeV with the ATLAS detector2011Inngår i: European Physical Journal C, ISSN 1434-6044, E-ISSN 1434-6052, Vol. 71, nr 12, s. 1846-Artikkel i tidsskrift (Fagfellevurdert)

The inclusive and dijet production cross-sections have been measured for jets containing b-hadrons (b-jets) in proton-proton collisions at a centre-of-mass energy of root s = 7 TeV, using the ATLAS detector at the LHC. The measurements use data corresponding to an integrated luminosity of 34 pb(-1). The b-jets are identified using either a lifetime-based method, where secondary decay vertices of b-hadrons in jets are reconstructed using information from the tracking detectors, or a muon-based method where the presence of a muon is used to identify semileptonic decays of b-hadrons inside jets. The inclusive b-jet cross-section is measured as a function of transverse momentum in the range 20 < p(T) < 400 GeV and rapidity in the range vertical bar y vertical bar < 2.1. The b<(b)over bar>-dijet cross-section is measured as a function of the dijet invariant mass in the range 110 < m(jj) < 760 GeV, the azimuthal angle difference between the two jets and the angular variable chi in two dijet mass regions. The results are compared with next-to-leading-order QCD predictions. Good agreement is observed between the measured cross-sections and the predictions obtained using POWHEG + Pythia. MC@NLO + Herwig shows good agreement with the measured b (b) over bar -dijet cross-section. However, it does not reproduce the measured inclusive cross-section well, particularly for central b-jets with large transverse momenta.

• 170. Aad, G
KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik. KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik. KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik.
Study of the B-c(+) -> J/psi D-s(+) and Bc(+) -> J/psi D-s*(+) decays with the ATLAS detector2016Inngår i: European Physical Journal C, ISSN 1434-6044, E-ISSN 1434-6052, Vol. 76, nr 1, artikkel-id 4Artikkel i tidsskrift (Fagfellevurdert)

The decays B-c(+) -> J/psi D-s(+) and B-c(+) -> J/psi D-s*(+) are studied with the ATLAS detector at the LHC using a dataset corresponding to integrated luminosities of 4.9 and 20.6 fb(-1) of pp collisions collected at centre-of-mass energies root s = 7 TeV and 8 TeV, respectively. Signal candidates are identified through J/psi -> mu(+)mu(-) and D-s(()*()+) -> phi pi(+)(gamma/pi(0)) decays. With a two-dimensional likelihood fit involving the B-c(+) reconstructed invariant mass and an angle between the mu(+) and D-s(+) candidate momenta in the muon pair rest frame, the yields of B-c(+) -> J/psi D-s(+) and B-c(+) -> J/psi D-s*(+), and the transverse polarisation fraction in B-c(+) -> J/psi D-s*(+) decay are measured. The transverse polarisation fraction is determined to be Gamma +/-+/-(B-c(+) -> J/psi D-s*(+))/Gamma(B-c(+) -> J/psi D-s*(+)) = 0.38 +/- 0.23 +/- 0.07, and the derived ratio of the branching fractions of the two modes is B-Bc+ -> J/psi D-s*+/B-Bc+ -> J/psi D-s(+) = 2.8(-0.8)(+1.2) +/- 0.3, where the first error is statistical and the second is systematic. Finally, a sample of B-c(+) -> J/psi pi(+) decays is used to derive the ratios of branching fractions B-Bc+ -> J/psi D-s*+/B-Bc+ -> J/psi pi(+) = 3.8 +/- 1.1 +/- 0.4 +/- 0.2 and B-Bc+ -> J/psi D-s*+/B-Bc+ -> J/psi pi(+) = 10.4 +/- 3.1 +/- 1.5 +/- 0.6, where the third error corresponds to the uncertainty of the branching fraction of D-s(+) -> phi(K+ K-)pi(+) decay. The available theoretical predictions are generally consistent with the measurement.

• 171. Aad, G
KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik. KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik. KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik. KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik.
Study of the spin and parity of the Higgs boson in diboson decays with the ATLAS detector2015Inngår i: European Physical Journal C, ISSN 1434-6044, E-ISSN 1434-6052, Vol. 75, nr 10, artikkel-id 476Artikkel i tidsskrift (Fagfellevurdert)

Studies of the spin, parity and tensor couplings of the Higgs boson in the H -> ZZ* -> 4l, H -> WW* -> e nu mu nu and H -> gamma gamma decay processes at the LHC are presented. The investigations are based on 25 fb(-1) of pp collision data collected by the ATLAS experiment at root s = 7 TeV and root s = 8 TeV. The Standard Model (SM) Higgs boson hypothesis, corresponding to the quantum numbers J (P) = 0(+), is tested against several alternative spin scenarios, including non-SM spin-0 and spin-2 models with universal and non-universal couplings to fermions and vector bosons. All tested alternative models are excluded in favour of the SM Higgs boson hypothesis at more than 99.9% confidence level. Using the H -> ZZ* -> 4l and H -> WW* -> e nu mu nu. decays, the tensor structure of the interaction between the spin-0 boson and the SM vector bosons is also investigated. The observed distributions of variables sensitive to the non-SM tensor couplings are compatible with the SM predictions and constraints on the non-SM couplings are derived.

• 172. Aad, G.
KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik. KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik. KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik.
Measurement of the top quark mass in the t(t)over-bar -> lepton plus jets and t(t)over-bar -> dilepton channels using root s=7 TeV ATLAS data2015Inngår i: European Physical Journal C, ISSN 1434-6044, E-ISSN 1434-6052, Vol. 75, nr 7, artikkel-id 330Artikkel i tidsskrift (Fagfellevurdert)

The top quark mass was measured in the channels t (t) over bar -> lepton+jets and t (t) over bar -> dilepton (lepton = e, mu) based on ATLAS data recorded in 2011. The data were taken at the LHC with a proton-proton centre-of-mass energy of root s = 7 TeV and correspond to an integrated luminosity of 4.6 fb(-1). The t (t) over bar -> lepton+jets analysis uses a three-dimensional template technique which determines the top quark mass together with a global jet energy scale factor (JSF), and a relative b-to-light-jet energy scale factor (bJSF), where the terms b-jets and light-jets refer to jets originating from b-quarks and u,d,c, s-quarks or gluons, respectively. The analysis of the t (t) over bar -> dilepton channel exploits a one-dimensional template method using the m(lb) observable, defined as the average invariant mass of the two lepton+b-jet pairs in each event. The top quark mass is measured to be 172.33 +/- 0.75(stat + JSF + bJSF) +/- 1.02(syst) GeV, and 173.79 +/- 0.54(stat) +/- 1.30(syst) GeV in the t (t) over bar -> lepton+jets and t (t) over bar -> dilepton channels, respectively. The combination of the two results yields m(top) = 172.99 +/- 0.48(stat) +/- 0.78(syst) GeV, with a total uncertainty of 0.91 GeV.

• 173. Aad, G
KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik. KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik. KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik.
Measurements of four-lepton production in pp collisions at √s = 8 TeV with the ATLAS detector2016Inngår i: European Physical Journal C, ISSN 1434-6044, E-ISSN 1434-6052, Vol. 76, nr 1, s. 1-51, artikkel-id 6Artikkel i tidsskrift (Fagfellevurdert)

Combined analyses of the Higgs boson production and decay rates as well as its coupling strengths to vector bosons and fermions are presented. The combinations include the results of the analyses of the H → γγ, ZZ, WW, Zγ, bb, ττ and μμ decay modes, and the constraints on the associated production with a pair of top quarks and on the off-shell coupling strengths of the Higgs boson. The results are based on the LHC proton-proton collision datasets, with integrated luminosities of up to 4.7 fb-1 at √S =7 TeV and 20.3 fb-1 at √S = 8 TeV, recorded by the ATLAS detector in 2011 and 2012. Combining all production modes and decay channels, the measured signal yield, normalised to the Standard Model expectation, is 1.18+0.15 -0.14. The observed Higgs boson production and decay rates are interpreted in a leading-order coupling framework, exploring a wide range of benchmark coupling models both with and without assumptions on the Higgs boson width and on the Standard Model particle content in loop processes. The data are found to be compatible with the Standard Model expectations for a Higgs boson at a mass of 125.36 GeV for all models considered.

• 174. Aad, G
KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik. KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik. KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik.
Search for a new resonance decaying to a W or Z boson and a Higgs boson in the ll/lv/vv plus b(b)over-bar final states with the ATLAS detector2015Inngår i: European Physical Journal C, ISSN 1434-6044, E-ISSN 1434-6052, Vol. 75, nr 6, artikkel-id 263Artikkel i tidsskrift (Fagfellevurdert)

A search for a new resonance decaying to a W or Z boson and a Higgs boson in the ll/lv/vv + b (b) over bar final states is performed using 20.3 fb(-1) of pp collision data recorded at root s = 8 TeV with the ATLAS detector at the Large Hadron Collider. The search is conducted by examining the WH/ZH invariant mass distribution for a localized excess. No significant deviation from the Standard Model background prediction is observed. The results are interpreted in terms of constraints on the Minimal Walking Technicolor model and on a simplified approach based on a phenomenological Lagrangian of Heavy Vector Triplets.

• 175. Aad, G.
KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik. KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik. KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik.
Search for heavy long-lived multi-charged particles in pp collisions at root s=8 TeV using the ATLAS detector2015Inngår i: European Physical Journal C, ISSN 1434-6044, E-ISSN 1434-6052, Vol. 75, nr 8, artikkel-id 362Artikkel i tidsskrift (Fagfellevurdert)

Asearch for heavy long-lived multi-charged particles is performed using the ATLAS detector at the LHC. Data collected in 2012 at root s = 8TeV from pp collisions corresponding to an integrated luminosity of 20.3 fb(-1) are examined. Particles producing anomalously high ionisation, consistent with long-livedmassive particleswith electric charges from vertical bar q vertical bar = 2e to vertical bar q vertical bar = 6e are searched for. No signal candidate events are observed, and 95% confidence level crosssection upper limits are interpreted as lower mass limits for a Drell-Yan production model. The mass limits range between 660 and 785 GeV.

• 176. Aad, G
KTH, Skolan för teknikvetenskap (SCI), Fysik. KTH, Skolan för teknikvetenskap (SCI), Fysik. KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik.
Search for Higgs boson pair production in the b(b)over-barb(b)over-bar final state from pp collisions at root s=8 TeVwith the ATLAS detector2015Inngår i: European Physical Journal C, ISSN 1434-6044, E-ISSN 1434-6052, Vol. 75, nr 9, artikkel-id 412Artikkel i tidsskrift (Fagfellevurdert)

A search for Higgs boson pair production pp -> hh is performed with 19.5 fb(-1) of proton-proton collision data at root s = 8TeV, which were recorded by the ATLAS detector at the Large Hadron Collider in 2012. The decay products of each Higgs boson are reconstructed as a high-momentum b (b) over bar system with either a pair of small-radius jets or a single large-radius jet, the latter exploiting jet substructure techniques and associated b-tagged track-jets. No evidence for resonant or non-resonant Higgs boson pair production is observed. The data are interpreted in the context of the Randall-Sundrum model with a warped extra dimension as well as the two-Higgs-doublet model. An upper limit on the cross-section for pp -> G(KK)* -> hh -> b (b) over barb (b) over bar of 3.2 (2.3) fb is set for a Kaluza-Klein graviton G(KK)* mass of 1.0 (1.5) TeV, at the 95 % confidence level. The search for non-resonant Standard Model hh production sets an observed 95 % confidence level upper limit on the production cross-section sigma(pp -> hh -> b (b) over barb (b) over bar) of 202 fb, compared to a Standard Model prediction of s(pp -> hh -> b (b) over barb (b) over bar) = 3.6 +/- 0.5fb.

• 177. Aad, G.
KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik. KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik. KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik.
Search for invisible decays of the Higgs boson produced in association with a hadronically decaying vector boson in pp collisions at, root s=8 TeV with the ATLAS detector2015Inngår i: European Physical Journal C, ISSN 1434-6044, E-ISSN 1434-6052, Vol. 75, nr 7, artikkel-id 337Artikkel i tidsskrift (Fagfellevurdert)

A search for Higgs boson decays to invisible particles is performed using 20,3 of fb(-1) collision data at a centre-of-mass energy of 8 TeV recorded by the ArL As detector at the Large IHIadron Collider. The process considered is Higgs boson production in association with a vector boson (V = W or Z) that decays hadronically, resulting in events with two or more jets and large missing transverse momentum. No excess of candidates is observed in the data over the background expectation. The results are used to constrain V H production followed by H decaying to invisible particles for the Higgs boson mass range 115 < m(H) < 300 GeV. The 95 % confidence-level observed upper limit on sigma vH x BR(H -> inv.) varies from 1.6 pb at 115 GeV to 0.13 ph at 300 GeV. Assuming Standard Model production and including the gg -> H contribution as signal, the results also lead to an observed upper limit of 78 c/c at 95 % confidence level on the branching ratio of Higgs bosons decays to invisible particles at a mass of 125 GeV.

• 178. Aad, G.
KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik. KTH, Skolan för teknikvetenskap (SCI), Fysik. KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik.
Search for anomaly-mediated supersymmetry breaking with the ATLAS detector based on a disappearing-track signature in pp collisions at root s=7 TeV2012Inngår i: European Physical Journal C, ISSN 1434-6044, E-ISSN 1434-6052, Vol. 72, nr 4, s. 1993-Artikkel i tidsskrift (Fagfellevurdert)

In models of anomaly-mediated supersymmetry breaking (AMSB), the lightest chargino is predicted to have a lifetime long enough to be detected in collider experiments. This letter explores AMSB scenarios in pp collisions at root s = 7 TeV by attempting to identify decaying charginos which result in tracks that appear to have few associated hits in the outer region of the tracking system. The search was based on data corresponding to an integrated luminosity of 1.02 fb(-1) collected with the ATLAS detector in 2011. The p(T) spectrum of candidate tracks is found to be consistent with the expectation from Standard Model background processes and constraints on the lifetime and the production cross section were obtained. In the minimal AMSB framework with m(3/2) < 32 TeV, m(0) < 1.5 TeV, tan beta = 5 and mu > 0, a chargino having mass below 92 GeV and a lifetime between 0.5 ns and 2 ns is excluded at 95 % confidence level.

• 179. Aad, G.
KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik. KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik. KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik.
ATLAS Run 1 searches for direct pair production of third-generation squarks at the Large Hadron Collider2015Inngår i: European Physical Journal C, ISSN 1434-6044, E-ISSN 1434-6052, Vol. 75, nr 10, s. 1-48, artikkel-id 510Artikkel i tidsskrift (Fagfellevurdert)

This paper reviews and extends searches for the direct pair production of the scalar supersymmetric partners of the top and bottom quarks in proton–proton collisions collected by the ATLAS collaboration during the LHC Run 1. Most of the analyses use 20 fb-1 of collisions at a centre-of-mass energy of √s = 8 TeV, although in some case an additional 4.7 fb-1 of collision data at √s = 7 TeV are used. New analyses are introduced to improve the sensitivity to specific regions of the model parameter space. Since no evidence of third-generation squarks is found, exclusion limits are derived by combining several analyses and are presented in both a simplified model framework, assuming simple decay chains, as well as within the context of more elaborate phenomenological supersymmetric models.

• 180. Aad, G.
KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik. KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik. KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik.
Charged-particle distributions in pp interactions at root s=8TeV measured with the ATLAS detector2016Inngår i: European Physical Journal C, ISSN 1434-6044, E-ISSN 1434-6052, Vol. 76, nr 7, artikkel-id 403Artikkel i tidsskrift (Fagfellevurdert)

This paper presents measurements of distributions of charged particles which are produced in proton-proton collisions at a centre-of-mass energy of root s = 8TeV and recorded by the ATLAS detector at the LHC. A special dataset recorded in 2012 with a small number of interactions per beam crossing (below 0.004) and corresponding to an integrated luminosity of 160 mu b(-1) was used. A minimumbias trigger was utilised to select a data sample of more than 9 million collision events. The multiplicity, pseudorapidity, and transverse momentum distributions of charged particles are shown in different regions of kinematics and charged-particle multiplicity, including measurements of final states at high multiplicity. The results are corrected for detector effects and are compared to the predictions of various Monte Carlo event generator models which simulate the full hadronic final state.

• 181. Aad, G.
KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik. KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik. KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik.
Identification of boosted, hadronically decaying W bosons and comparisons with ATLAS data taken at √s = 8 TeV2016Inngår i: European Physical Journal C, ISSN 1434-6044, E-ISSN 1434-6052, Vol. 76, nr 3, artikkel-id 154Artikkel i tidsskrift (Fagfellevurdert)

This paper reports a detailed study of techniques for identifying boosted, hadronically decaying W bosons using 20.3 fb (Formula presented.) of proton–proton collision data collected by the ATLAS detector at the LHC at a centre-of-mass energy (Formula presented.). A range of techniques for optimising the signal jet mass resolution are combined with various jet substructure variables. The results of these studies in Monte Carlo simulations show that a simple pairwise combination of groomed jet mass and one substructure variable can provide a 50 % efficiency for identifying W bosons with transverse momenta larger than 200 GeV while maintaining multijet background efficiencies of 2–4 % for jets with the same transverse momentum. These signal and background efficiencies are confirmed in data for a selection of tagging techniques.

• 182. Aad, G.
KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik. KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik. KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik.
Measurement of the charge asymmetry in top-quark pair production in the lepton-plus-jets final state in pp collision data at root s=8 TeV with the ATLAS detector2016Inngår i: European Physical Journal C, ISSN 1434-6044, E-ISSN 1434-6052, Vol. 76, nr 2, artikkel-id 87Artikkel i tidsskrift (Fagfellevurdert)

This paper reports inclusive and differential measurements of the t (t) over bar charge asymmetry A(C) in 20.3 fb(-1) of root s = 8 TeV pp collisions recorded by the ATLAS experiment at the Large Hadron Collider at CERN. Three differential measurements are performed as a function of the invariant mass, transverse momentum and longitudinal boost of the t (t) over bar system. The t (t) over bar pairs are selected in the single-lepton channels (e or mu) with at least four jets, and a likelihood fit is used to reconstruct the t (t) over bar event kinematics. A Bayesian unfolding procedure is performed to infer the asymmetry at parton level from the observed data distribution. The inclusive t (t) over bar charge asymmetry is measured to be A(C) = 0.009 +/- 0.005 (stat. + syst.). The inclusive and differential measurements are compatible with the values predicted by the Standard Model.

• 183. Aad, G
KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik. KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik. KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik.
Measurements of fiducial cross-sections for (Formula presented.) production with one or two additional b-jets in pp collisions at √s = 8 TeV using the ATLAS detector2016Inngår i: European Physical Journal C, ISSN 1434-6044, E-ISSN 1434-6052, Vol. 76, nr 1, s. 1-37, artikkel-id 11Artikkel i tidsskrift (Fagfellevurdert)

Fiducial cross-sections for tt¯ production with one or two additional b-jets are reported, using an integrated luminosity of 20.3 fb-1 of proton–proton collisions at a centre-of-mass energy of 8 TeV at the Large Hadron Collider, collected with the ATLAS detector. The cross-section times branching ratio for tt¯ events with at least one additional b-jet is measured to be 950 ± 70 (stat.) -190 +240 (syst.) fb in the lepton-plus-jets channel and 50 ± 10 (stat.) -10 +15 (syst.) fb in the eμ channel. The cross-section times branching ratio for events with at least two additional b-jets is measured to be 19.3 ± 3.5 (stat.) ± 5.7 (syst.) fb in the dilepton channel (eμ, μμ, and  ee) using a method based on tight selection criteria, and 13.5 ± 3.3 (stat.) ± 3.6 (syst.) fb using a looser selection that allows the background normalisation to be extracted from data. The latter method also measures a value of 1.30 ± 0.33 (stat.) ± 0.28 (syst.)% for the ratio of tt¯ production with two additional b-jets to tt¯ production with any two additional jets. All measurements are in good agreement with recent theory predictions.

• 184. Aad, G.
KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik. KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik. KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik.
Probing lepton flavour violation via neutrinoless tau -> 3 mu decays with the ATLAS detector2016Inngår i: European Physical Journal C, ISSN 1434-6044, E-ISSN 1434-6052, Vol. 76, nr 5, artikkel-id 232Artikkel i tidsskrift (Fagfellevurdert)

This article presents the sensitivity of the ATLAS experiment to the lepton-flavour-violating decays of tau -> 3 mu. A method utilising the production of t leptons via W -> tau nu decays is used. This method is applied to the sample of 20.3 fb(-1) of pp collision data at a centre-of-mass energy of 8 TeV collected by the ATLAS experiment at the LHC in 2012. No event is observed passing the selection criteria, and the observed (expected) upper limit on the tau lepton branching fraction into three muons, Br(tau -> 3 mu), is 3.76 x 10(-7) (3.94 x 10(-7)) at 90 % confidence level.

• 185. Aad, G
KTH, Skolan för teknikvetenskap (SCI), Fysik. KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik. KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik.
Search for an additional, heavy Higgs boson in the decay channel at in collision data with the ATLAS detector2016Inngår i: European Physical Journal C, ISSN 1434-6044, E-ISSN 1434-6052, Vol. 76, nr 1, artikkel-id 45Artikkel i tidsskrift (Fagfellevurdert)

A search is presented for a high-mass Higgs boson in the , , , and decay modes using the ATLAS detector at the CERN Large Hadron Collider. The search uses proton-proton collision data at a centre-of-mass energy of 8 TeV corresponding to an integrated luminosity of 20.3 fb. The results of the search are interpreted in the scenario of a heavy Higgs boson with a width that is small compared with the experimental mass resolution. The Higgs boson mass range considered extends up to for all four decay modes and down to as low as 140 , depending on the decay mode. No significant excess of events over the Standard Model prediction is found. A simultaneous fit to the four decay modes yields upper limits on the production cross-section of a heavy Higgs boson times the branching ratio to boson pairs. 95 % confidence level upper limits range from 0.53 pb at GeV to 0.008 pb at GeV for the gluon-fusion production mode and from 0.31 pb at GeV to 0.009 pb at GeV for the vector-boson-fusion production mode. The results are also interpreted in the context of Type-I and Type-II two-Higgs-doublet models.

• 186. Aad, G.
KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik. KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik. KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik.
Search for direct top squark pair production in final states with two tau leptons in pp collisions at root s=8 TeV with the ATLAS detector2016Inngår i: European Physical Journal C, ISSN 1434-6044, E-ISSN 1434-6052, Vol. 76, nr 2, artikkel-id 81Artikkel i tidsskrift (Fagfellevurdert)

A search for direct pair production of the supersymmetric partner of the top quark, decaying via a scalar tau to a nearly massless gravitino, has been performed using 20 fb(-1) of proton-proton collision data at root s = 8 TeV. The data were collected by the ATLAS experiment at the LHC in 2012. Top squark candidates are searched for in events with either two hadronically decaying tau leptons, one hadronically decaying tau and one light lepton, or two light leptons. No significant excess over the Standard Model expectation is found. Exclusion limits at 95 % confidence level are set as a function of the top squark and scalar tau masses. Depending on the scalar tau mass, ranging from the 87 GeV LEP limit to the top squark mass, lower limits between 490 and 650 GeV are placed on the top squark mass within the model considered.

• 187. Aad, G
KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik. KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik. KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik.
Search for gluinos in events with an isolated lepton, jets and missing transverse momentum at √s = 13 Te V with the ATLAS detector2016Inngår i: European Physical Journal C, ISSN 1434-6044, E-ISSN 1434-6052, Vol. 76, nr 10, artikkel-id 565Artikkel i tidsskrift (Fagfellevurdert)

The results of a search for gluinos in final states with an isolated electron or muon, multiple jets and large missing transverse momentum using proton–proton collision data at a centre-of-mass energy of s=13TeV are presented. The dataset used was recorded in 2015 by the ATLAS experiment at the Large Hadron Collider and corresponds to an integrated luminosity of 3.2 fb- 1. Six signal selections are defined that best exploit the signal characteristics. The data agree with the Standard Model background expectation in all six signal selections, and the largest deviation is a 2.1 standard deviation excess. The results are interpreted in a simplified model where pair-produced gluinos decay via the lightest chargino to the lightest neutralino. In this model, gluinos are excluded up to masses of approximately 1.6 Te V depending on the mass spectrum of the simplified model, thus surpassing the limits of previous searches.

• 188. Aad, G.
KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik. KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik. KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik.
Search for lepton-flavour-violating decays of the Higgs and Z bosons with the ATLAS detector2017Inngår i: European Physical Journal C, ISSN 1434-6044, E-ISSN 1434-6052, Vol. 77, nr 2, artikkel-id 70Artikkel i tidsskrift (Fagfellevurdert)

Direct searches for lepton flavour violation in decays of the Higgs and Z bosons with the ATLAS detector at the LHC are presented. The following three decays are considered: H -> e tau, H -> mu tau, and Z -> mu tau. The searches are based on the data sample of proton-proton collisions collected by the ATLAS detector corresponding to an integrated luminosity of 20.3 fb(-1) at a centre-of-mass energy of root s = 8 TeV. No significant excess is observed, and upper limits on the lepton-flavour-violating branching ratios are set at the 95% confidence level: Br(H -> e tau) < 1.04%, Br(H -> mu tau) < 1.43%, and Br(Z -> mu tau) < 1.69 x 10(-5).

• 189. Aad, G.
KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik. KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik. KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik.
Search for metastable heavy charged particles with large ionisation energy loss in pp collisions at root s=8 TeV using the ATLAS experiment2015Inngår i: European Physical Journal C, ISSN 1434-6044, E-ISSN 1434-6052, Vol. 75, nr 9, artikkel-id 407Artikkel i tidsskrift (Fagfellevurdert)

Many extensions of the Standard Model predict the existence of charged heavy long-lived particles, such as R-hadrons or charginos. These particles, if produced at the Large Hadron Collider, should bemoving non-relativistically and are therefore identifiable through the measurement of an anomalously large specific energy loss in the ATLAS pixel detector. Measuring heavy long-lived particles through their track parameters in the vicinity of the interaction vertex provides sensitivity to metastable particles with lifetimes from 0.6 ns to 30 ns. A search for such particles with the ATLAS detector at the Large Hadron Collider is presented, based on a data sample corresponding to an integrated luminosity of 18.4 fb(-1) of pp collisions at root s = 8 TeV. No significant deviation from the Standard Model background expectation is observed, and lifetime-dependent upper limits on R-hadrons and chargino production are set. Gluino R-hadrons with 10 ns lifetime and masses up to 1185 GeV are excluded at 95 % confidence level, and so are charginos with 15 ns lifetime and masses up to 482 GeV.

• 190. Aad, G.
KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik. KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik. KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik.
Search for single top-quark production via flavour-changing neutral currents at 8TeV with the ATLAS detector2016Inngår i: European Physical Journal C, ISSN 1434-6044, E-ISSN 1434-6052, Vol. 76, nr 2, artikkel-id 55Artikkel i tidsskrift (Fagfellevurdert)

A search for single top-quark production via flavour-changing neutral current processes from gluon plus up- or charm-quark initial states in proton-proton collisions at the LHC is presented. Data collected with the ATLAS detector in 2012 at a centre-of-mass energy of 8TeV and corresponding to an integrated luminosity of 20.3 fb(-1) are used. Candidate events for a top quark decaying into a lepton, a neutrino and a jet are selected and classified into signal- and background-like candidates using a neural network. No signal is observed and an upper limit on the production cross-section multiplied by the t -> Wb branching fraction is set. The observed 95% CL limit is sigma(qg -> t) x B(t -> Wb) < 3.4 pb and the expected 95% CL limit is sigma(qg -> t) x B(t -> Wb) < 2.9 pb. The observed limit can be interpreted as upper limits on the coupling constants of the flavour-changing neutral current interactions divided by the scale of new physics kappa(ugt)/Lambda < 5.8 x 10(-3) TeV-1 and kappa(cgt)/Lambda < 13x10(-3) TeV and on the branching fractions B(t -> ug) < 4.0 x 10(-5) and B(t -> cg) < 20 x 10(-5).

• 191. Aad, G
KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik. KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik. KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik.
Searches for scalar leptoquarks in pp collisions at √s = 8 TeV with the ATLAS detector2016Inngår i: European Physical Journal C, ISSN 1434-6044, E-ISSN 1434-6052, Vol. 76, nr 1, s. 1-28, artikkel-id 5Artikkel i tidsskrift (Fagfellevurdert)

Searches for pair-produced scalar leptoquarks are performed using 20 fb$$^{-1}$$-1 of proton–proton collision data provided by the LHC and recorded by the ATLAS detector at $$\sqrt{s}=8$$s=8 TeV. Events with two electrons (muons) and two or more jets in the final state are used to search for first (second)-generation leptoquarks. The results from two previously published ATLAS analyses are interpreted in terms of third-generation leptoquarks decaying to $$b\nu _{\tau }\bar{b}\bar{\nu _{\tau }}$$bντb¯ντ¯ and $$t\nu _{\tau }\bar{t}\bar{\nu _{\tau }}$$tντt¯ντ¯ final states. No statistically significant excess above the Standard Model expectation is observed in any channel and scalar leptoquarks are excluded at 95 % CL with masses up to $$m_{\mathrm {LQ1}}<$$mLQ1< 1050 GeV for first-generation leptoquarks, $$m_{\mathrm {LQ2}}<$$mLQ2< 1000 GeV for second-generation leptoquarks, $$m_{\mathrm {LQ3}}<$$mLQ3< 625 GeV for third-generation leptoquarks in the $$b\nu _{\tau }\bar{b}\bar{\nu _{\tau }}$$bντb¯ντ¯ channel, and 200 $$< m_{\mathrm {LQ3}}<$$<mLQ3< 640 GeV in the $$t\nu _{\tau }\bar{t}\bar{\nu _{\tau }}$$tντt¯ντ¯ channel.

• 192. Aad, G
KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik. KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik. KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik.
Search for flavour-changing neutral current top-quark decays to q Z in pp collision data collected with the ATLAS detector at root s=8 TeV2016Inngår i: European Physical Journal C, ISSN 1434-6044, E-ISSN 1434-6052, Vol. 76, nr 1, artikkel-id 12Artikkel i tidsskrift (Fagfellevurdert)

A search for the flavour-changing neutral-current decay is presented. Data collected by the ATLAS detector during 2012 from proton-proton collisions at the Large Hadron Collider at a centre-of-mass energy of root s = 8 TeV, corresponding to an integrated luminosity of 20.3 fb(-1), are analysed. Top-quark pair-production events with one top quark decaying through the t -> qZ (q = u,c) channel and the other through the dominant Standard Model mode t -> bW are considered as signal. Only the decays of the Z boson to charged leptons and leptonic W boson decays are used. No evidence for a signal is found and an observed (expected) upper limit on the t -> qZ branching ratio of 7 x 10(-4) (8 x 10(-4)) is set at the 95 % confidence level.

• 193. Aad, G.
KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik. KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik. KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik.
Measurement of event-shape observables in Z -> l(+)l(-) events in pp collisions at root s=7 TeV with the ATLAS detector at the LHC2016Inngår i: European Physical Journal C, ISSN 1434-6044, E-ISSN 1434-6052, Vol. 76, nr 7, artikkel-id 375Artikkel i tidsskrift (Fagfellevurdert)

Event-shape observables measured using charged particles in inclusive Z-boson events are presented, using the electron and muon decay modes of the Z bosons. The measurements are based on an integrated luminosity of 1.1 fb(-1) of proton-proton collisions recorded by the ATLAS detector at the LHC at a centre-of-mass energy root s = 7 TeV. Charged-particle distributions, excluding the lepton-antilepton pair from the Z-boson decay, are measured in different ranges of transverse momentum of the Z boson. Distributions include multiplicity, scalar sum of transverse momenta, beam thrust, transverse thrust, spherocity, and F-parameter, which are in particular sensitive to properties of the underlying event at small values of the Z-boson transverse momentum. The measured observables are compared with predictions from PYTHIA 8, Sherpa, and HERWIG 7. Typically, all three Monte Carlo generators provide predictions that are in better agreement with the data at high Z-boson transverse momenta than at low Z-boson transverse momenta, and for the observables that are less sensitive to the number of charged particles in the event.

• 194. Aad, G.
KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik. KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik. KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik.
Measurement of the transverse momentum and phi(eta)*. distributions of Drell-Yan lepton pairs in proton-proton collisions at root s=8 TeV with the ATLAS detector2016Inngår i: European Physical Journal C, ISSN 1434-6044, E-ISSN 1434-6052, Vol. 76, nr 5, artikkel-id 291Artikkel i tidsskrift (Fagfellevurdert)

Distributions of transverse momentum p(T)(ll) and the related angular variable phi(eta)*. of Drell-Yan lepton pairs are measured in 20.3 fb(-1) of proton-proton collisions at root s = 8 TeV with the ATLAS detector at the LHC. Measurements in electron-pair and muon-pair final states are corrected for detector effects and combined. Compared to previous measurements in proton-proton collisions at root s = 7 TeV, these new measurements benefit from a larger data sample and improved control of systematic uncertainties. Measurements are performed in bins of lepton-pair mass above, around and below the Z-boson mass peak. The data are compared to predictions from perturbative and resummed QCD calculations. For values of phi(eta)*. < 1 the predictions from the Monte Carlo generator ResBos are generally consistent with the data within the theoretical uncertainties. However, at larger values of phi(eta)*. this is not the case. Monte Carlo generators based on the parton-shower approach are unable to describe the data over the full range of p(T)(ll) while the fixed-order prediction of Dynnlo falls below the data at high values of p(T)(ll). ResBos and the parton-shower Monte Carlo generators provide a much better description of the evolution of the phi(eta)*. and p(T)(ll) distributions as a function of lepton-pair mass and rapidity than the basic shape of the data.

• 195. Aad, G.
KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik. KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik. KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik.
Measurements of top-quark pair differential cross-sections in the lepton+jets channel in pp collisions at √s=8TeV using the ATLAS detector2016Inngår i: European Physical Journal C, ISSN 1434-6044, E-ISSN 1434-6052, Vol. 76, nr 10, artikkel-id 538Artikkel i tidsskrift (Fagfellevurdert)

Measurements of normalized differential cross-sections of top-quark pair production are presented as a function of the top-quark, tt¯ system and event-level kinematic observables in proton–proton collisions at a centre-of-mass energy of s=8TeV. The observables have been chosen to emphasize the tt¯ production process and to be sensitive to effects of initial- and final-state radiation, to the different parton distribution functions, and to non-resonant processes and higher-order corrections. The dataset corresponds to an integrated luminosity of 20.3 fb- 1, recorded in 2012 with the ATLAS detector at the CERN Large Hadron Collider. Events are selected in the lepton+jets channel, requiring exactly one charged lepton and at least four jets with at least two of the jets tagged as originating from a b-quark. The measured spectra are corrected for detector effects and are compared to several Monte Carlo simulations. The results are in fair agreement with the predictions over a wide kinematic range. Nevertheless, most generators predict a harder top-quark transverse momentum distribution at high values than what is observed in the data. Predictions beyond NLO accuracy improve the agreement with data at high top-quark transverse momenta. Using the current settings and parton distribution functions, the rapidity distributions are not well modelled by any generator under consideration. However, the level of agreement is improved when more recent sets of parton distribution functions are used.

• 196. Aad, G.
KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik. KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik. KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik.
Muon reconstruction performance of the ATLAS detector in proton-proton collision data at root s=13 TeV2016Inngår i: European Physical Journal C, ISSN 1434-6044, E-ISSN 1434-6052, Vol. 76, nr 5, artikkel-id 292Artikkel i tidsskrift (Fagfellevurdert)

This article documents the performance of the ATLAS muon identification and reconstruction using the LHC dataset recorded at root s = 13 TeVin 2015. Using a large sample of J/.psi -> mu mu and Z -> mu mu decays from 3.2 fb(-1) of pp collision data, measurements of the reconstruction efficiency, as well as of the momentum scale and resolution, are presented and compared to Monte Carlo simulations. The reconstruction efficiency is measured to be close to 99 % over most of the covered phase space (vertical bar eta vertical bar < 2.5 and 5 < p(T) < 100 GeV). The isolation efficiency varies between 93 and 100 % depending on the selection applied and on the momentum of the muon. Both efficiencies are well reproduced in simulation. In the central region of the detector, the momentum resolution is measured to be 1.7 % (2.3 %) for muons from J/psi -> mu mu(Z -> mu mu) decays, and the momentum scale is known with an uncertainty of 0.05 %. In the region vertical bar eta vertical bar > 2.2, the p(T) resolution for muons from Z -> mu decays is 2.9 % while the precision of the momentum scale for low-p(T) muons from J/psi -> mu mu decays is about 0.2 %.

• 197. Aad, G.
KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik. KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik. KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik.
Performance of algorithms that reconstruct missing transverse momentum in root s=8 TeV proton-proton collisions in the ATLAS detector2017Inngår i: European Physical Journal C, ISSN 1434-6044, E-ISSN 1434-6052, Vol. 77, nr 4, artikkel-id 241Artikkel i tidsskrift (Fagfellevurdert)

The reconstruction and calibration algorithms used to calculate missing transverse momentum (E-T(miss)) with the ATLAS detector exploit energy deposits in the calorimeter and tracks reconstructed in the inner detector as well as the muon spectrometer. Various strategies are used to suppress effects arising from additional proton-proton interactions, called pileup, concurrent with the hard-scatter processes. Tracking information is used to distinguish contributions from the pileup interactions using their vertex separation along the beam axis. The performance of the E-T(miss) reconstruction algorithms, especially with respect to the amount of pileup, is evaluated using data collected in proton-proton collisions at a centre-of-mass energy of 8 TeV during 2012, and results are shown for a data sample corresponding to an integrated luminosity of 20.3 fb(-1). The simulation and modelling of E-T(miss) in events containing a Z boson decaying to two charged leptons (electrons or muons) or a W boson decaying to a charged lepton and a neutrino are compared to data. The acceptance for different event topologies, with and without high transverse momentum neutrinos, is shown for a range of threshold criteria for E-T(miss), and estimates of the systematic uncertainties in the E-T(miss) measurements are presented.

• 198. Aad, G.
KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik. KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik. KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik.
Performance of pile-up mitigation techniques for jets in pp collisions at √s=8 TeV using the ATLAS detector2016Inngår i: European Physical Journal C, ISSN 1434-6044, E-ISSN 1434-6052, Vol. 76, nr 11Artikkel i tidsskrift (Fagfellevurdert)

The large rate of multiple simultaneous proton–proton interactions, or pile-up, generated by the Large Hadron Collider in Run 1 required the development of many new techniques to mitigate the adverse effects of these conditions. This paper describes the methods employed in the ATLAS experiment to correct for the impact of pile-up on jet energy and jet shapes, and for the presence of spurious additional jets, with a primary focus on the large 20.3 fb- 1data sample collected at a centre-of-mass energy of s=8TeV. The energy correction techniques that incorporate sophisticated estimates of the average pile-up energy density and tracking information are presented. Jet-to-vertex association techniques are discussed and projections of performance for the future are considered. Lastly, the extension of these techniques to mitigate the effect of pile-up on jet shapes using subtraction and grooming procedures is presented.

• 199. Aad, G.
KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik. KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik. KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik.
Reconstruction of hadronic decay products of tau leptons with the ATLAS experiment2016Inngår i: European Physical Journal C, ISSN 1434-6044, E-ISSN 1434-6052, Vol. 76, nr 5Artikkel i tidsskrift (Fagfellevurdert)

This paper presents a new method of reconstructing the individual charged and neutral hadrons in tau decays with the ATLAS detector. The reconstructed hadrons are used to classify the decay mode and to calculate the visible four-momentum of reconstructed tau candidates, significantly improving the resolution with respect to the calibration in the existing tau reconstruction. The performance of the reconstruction algorithm is optimised and evaluated using simulation and validated using samples of Z -> tau tau and Z(-> mu mu)+jets events selected from proton-proton collisions at a centre-of-mass energy root s = 8 TeV, corresponding to an integrated luminosity of 5 fb(-1).

• 200. Aad, G.
KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik. KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik. KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik.
Search for new phenomena in events with at least three photons collected in pp collisions at root s=8 TeV with the ATLAS detector2016Inngår i: European Physical Journal C, ISSN 1434-6044, E-ISSN 1434-6052, Vol. 76, nr 4, artikkel-id 210Artikkel i tidsskrift (Fagfellevurdert)

Results of a search for new phenomena in events with at least three photons are reported. Data from proton-proton collisions at a centre-of-mass energy of 8 TeV, corresponding to an integrated luminosity of 20.3 fb(-1), were collected with the ATLAS detector at the LHC. The observed data are well described by the Standard Model. Limits at the 95 % confidence level on new phenomena are presented based on the rate of events in an inclusive signal region and a restricted signal region targeting the rare decay Z -> 3 gamma, as well as di-photon and tri-photon resonance searches. For a Standard Model Higgs boson decaying to four photons via a pair of intermediate pseudoscalar particles (a), limits are found to be sigma x BR(h -> aa) x BR(a -> gamma gamma)(2) < 10(-3) sigma(SM) for 10 GeV < m(a) < 62 GeV. Limits are also presented for Higgs boson-like scalars (H) for m(H) > 125 GeV, and for a Z' decaying to three photons via Z' -> a + gamma -> 3 gamma. Additionally,the observed limit on the branching ratio of the Z boson decay to three photons is found to be BR (Z -> 3 gamma) < 2.2 x 10(-6), a result five times stronger than the previous result from LEP.

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