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Kastanas, Konstatinos A.ORCID iD iconorcid.org/0000-0001-6945-1916
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Publications (10 of 191) Show all publications
Aaboud, M., Kastanas, K. A., Jensen, B., Ohm, C., Ripellino, G., Sidebo, P. E., . . . Zwalinski, L. (2019). A strategy for a general search for new phenomena using data-derived signal regions and its application within the ATLAS experiment. European Physical Journal C, 79(2), Article ID 120.
Open this publication in new window or tab >>A strategy for a general search for new phenomena using data-derived signal regions and its application within the ATLAS experiment
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2019 (English)In: European Physical Journal C, ISSN 1434-6044, E-ISSN 1434-6052, Vol. 79, no 2, article id 120Article in journal (Refereed) Published
Abstract [en]

This paper describes a strategy for a general search used by the ATLAS Collaboration to find potential indications of new physics. Events are classified according to their final state into many event classes. For each event class an automated search algorithm tests whether the data are compatible with the Monte Carlo simulated expectation in several distributions sensitive to the effects of new physics. The significance of a deviation is quantified using pseudo-experiments. A data selection with a significant deviation defines a signal region for a dedicated follow-up analysis with an improved background expectation. The analysis of the data-derived signal regions on a new dataset allows a statistical interpretation without the large look-elsewhere effect. The sensitivity of the approach is discussed using Standard Model processes and benchmark signals of new physics. As an example, results are shown for 3.2fb-1 of proton-proton collision data at a centre-of-mass energy of 13TeV collected with the ATLAS detector at the LHC in 2015, in which more than 700 event classes and more than 105 regions have been analysed. No significant deviations are found and consequently no data-derived signal regions for a follow-up analysis have been defined.

Place, publisher, year, edition, pages
SPRINGER, 2019
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:kth:diva-244519 (URN)10.1140/epjc/s10052-019-6540-y (DOI)000457998500007 ()2-s2.0-85061187220 (Scopus ID)
Note

QC 20190403

Available from: 2019-04-03 Created: 2019-04-03 Last updated: 2019-04-04Bibliographically approved
Aaboud, M., Kastanas, K. A., Lund-Jensen, B., Ohm, C., Ripellino, G., Sidebo, P. E., . . . et al., . (2019). Comparison of Fragmentation Functions for Jets Dominated by Light Quarks and Gluons from pp and Pb plus Pb Collisions in ATLAS. Physical Review Letters, 123(4), Article ID 042001.
Open this publication in new window or tab >>Comparison of Fragmentation Functions for Jets Dominated by Light Quarks and Gluons from pp and Pb plus Pb Collisions in ATLAS
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2019 (English)In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 123, no 4, article id 042001Article in journal (Refereed) Published
Abstract [en]

Charged-particle fragmentation functions for jets azimuthally balanced by a high-transverse-momentum, prompt, isolated photon are measured in 25 pb(-1) of pp and 0.49 nb(-1) of Pb + Pb collision data at 5.02 TeV per nucleon pair recorded with the ATLAS detector at the Large Hadron Collider. The measurements are compared to predictions of Monte Carlo generators and to measurements of inclusively selected jets. In pp collisions, a different jet fragmentation function in photon-tagged events from that in inclusive jet events arises from the difference in fragmentation between light quarks and gluons. The ratios of the fragmentation functions in Pb + Pb events to that in pp events are used to explore the parton color-charge dependence of jet quenching in the hot medium. In relatively peripheral collisions, fragmentation functions exhibit a similar modification pattern for photon-tagged and inclusive jets. However, photon-tagged jets are observed to have larger modifications than inclusive jets in central Pb + Pb events.

Place, publisher, year, edition, pages
American Physical Society, 2019
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-256030 (URN)10.1103/PhysRevLett.123.042001 (DOI)000476696300004 ()2-s2.0-85071896619 (Scopus ID)
Note

QC 20190816

Available from: 2019-08-16 Created: 2019-08-16 Last updated: 2019-10-17Bibliographically approved
Aaboud, M., Kastanas, K. A., Lund-Jensen, B., Ohm, C., Ripellino, G., Sidebo, P. E., . . . et al., . (2019). Correlated long-range mixed-harmonic fluctuations measured in pp, p plus Pb and low-multiplicity Pb plus Pb collisions with the ATLAS detector. Physics Letters B, 789, 444-471
Open this publication in new window or tab >>Correlated long-range mixed-harmonic fluctuations measured in pp, p plus Pb and low-multiplicity Pb plus Pb collisions with the ATLAS detector
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2019 (English)In: Physics Letters B, ISSN 0370-2693, E-ISSN 1873-2445, Vol. 789, p. 444-471Article in journal (Refereed) Published
Abstract [en]

Correlations of two flow harmonics v(n) and v(m) via three- and four-particle cumulants are measured in 13 TeV pp, 5.02 TeV p+Pb, and 2.76 TeV peripheral Pb+Pb collisions with the ATLAS detector at the LHC. The goal is to understand the multi-particle nature of the long-range collective phenomenon in these collision systems. The large non-flow background from dijet production present in the standard cumulant method is suppressed using a method of subevent cumulants involving two, three and four subevents separated in pseudorapidity. The results show a negative correlation between v(2) and v(3) and a positive correlation between v(2) and v(4) for all collision systems and over the full multiplicity range. However, the magnitudes of the correlations are found to depend on the event multiplicity, the choice of transverse momentum range and collision system. The relative correlation strength, obtained by normalisation of the cumulants with the < v(n)(2)> from a two-particle correlation analysis, is similar in the three collision systems and depends weakly on the event multiplicity and transverse momentum. These results based on the subevent methods provide strong evidence of a similar long-range multi-particle collectivity in pp, p+Pb and peripheral Pb+Pb collisions. Elsevier B.V.

Place, publisher, year, edition, pages
ELSEVIER SCIENCE BV, 2019
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-244090 (URN)10.1016/j.physletb.2018.11.065 (DOI)000457165400062 ()2-s2.0-85059588295 (Scopus ID)
Note

QC 20190219

Available from: 2019-02-19 Created: 2019-02-19 Last updated: 2019-02-19Bibliographically approved
Aaboud, M., Kastanas, K. A., Jensen, B., Ohm, C., Ripellino, G., Sidebo, P. E., . . . et al., . (2019). Cross-section measurements of the Higgs boson decaying into a pair of tau-leptons in proton-proton collisions at root s=13 TeV with the ATLAS detector. Physical Review D: covering particles, fields, gravitation, and cosmology, 99(7), Article ID 072001.
Open this publication in new window or tab >>Cross-section measurements of the Higgs boson decaying into a pair of tau-leptons in proton-proton collisions at root s=13 TeV with the ATLAS detector
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2019 (English)In: Physical Review D: covering particles, fields, gravitation, and cosmology, ISSN 2470-0010, E-ISSN 2470-0029, Vol. 99, no 7, article id 072001Article in journal (Refereed) Published
Abstract [en]

A measurement of production cross sections of the Higgs boson in proton-proton collisions is presented in the H -> tau tau decay channel. The analysis is performed using 36.1 fb(-1) of data recorded by the ATLAS experiment at the Large Hadron Collider at a center-of-mass energy of root s = 13 TeV. All combinations of leptonic (tau -> l v (v) over bar with l = e, mu) and hadronic (tau -> hadrons v) tau decays arc considered. The H -> tau tau signal over the expected background from other Standard Model processes is established with an observed (expected) significance of 4.4 (4.1) standard deviations. Combined with results obtained using data taken at 7 and 8 TeV center-of-mass energies, the observed (expected) significance amounts to 6.4 (5.4) standard deviations and constitutes an observation of H -> tau tau decays. Using the data taken at root s = 13 TeV, the total cross section in the H -> tau tau decay channel is measured to be 3.77(-0.59)(+0.60)(stat)(-0.74)(+0.87) (syst) pb, for a Higgs boson of mass 125 GeV assuming the relative contributions of its production modes as predicted by the Standard Model. Total cross sections in the H -> tau tau decay channel are determined separately for vector-boson-fusion production and gluon-gluon-fusion production to be sigma(VBF)(H -> tau tau) = 0.28 +/- 0.09 (stat)(-0.09)(+0.11) (syst) pb and sigma(ggF)(H -> tau tau) = 3.1 +/- 1.0 (stat)(-1.3)(+1.6) (syst) pb, respectively. Similarly, results of a fit are reported in the framework of simplified template cross sections. All measurements are in agreement with Standard Model expectations.

Place, publisher, year, edition, pages
AMER PHYSICAL SOC, 2019
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:kth:diva-251493 (URN)10.1103/PhysRevD.99.072001 (DOI)000464740700002 ()
Note

QC 20190515

Available from: 2019-05-15 Created: 2019-05-15 Last updated: 2019-05-15Bibliographically approved
Aaboud, M., Kastanas, K. A., Jensen, B., Ohm, C., Ripellino, G., Sidebo, P. E., . . . Zwalinski, L. (2019). In situ calibration of large-radius jet energy and mass in 13TeVproton-proton collisions with the ATLAS detector. European Physical Journal C, 79(2), Article ID 135.
Open this publication in new window or tab >>In situ calibration of large-radius jet energy and mass in 13TeVproton-proton collisions with the ATLAS detector
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2019 (English)In: European Physical Journal C, ISSN 1434-6044, E-ISSN 1434-6052, Vol. 79, no 2, article id 135Article in journal (Refereed) Published
Abstract [en]

The response of the ATLAS detector to large-radius jets is measured in situ using 36.2 fb(-1) of root s = 13TeV proton-proton collisions provided by the LHC and recorded by the ATLAS experiment during 2015 and 2016. The jet energy scale is measured in events where the jet recoils against a reference object, which can be either a calibrated photon, a reconstructed Z boson, or a system of well-measured small-radius jets. The jet energy resolution and a calibration of forward jets are derived using dijet balance measurements. The jet mass response is measured with two methods: using mass peaks formed by W bosons and top quarks with large transverse momenta and by comparing the jet mass measured using the energy deposited in the calorimeter with that using the momenta of charged-particle tracks. The transverse momentum and mass responses in simulations are found to be about 2-3% higher than in data. This difference is adjusted for with a correction factor. The results of the different methods are combined to yield a calibration over a large range of transverse momenta (p(T)). The precision of the relative jet energy scale is 1-2% for 200 GeV < p(T) < TeV, while that of the mass scale is 2-10%. The ratio of the energy resolutions in data and simulation is measured to a precision of 10-15% over the same p(T) range.

Place, publisher, year, edition, pages
SPRINGER, 2019
National Category
Subatomic Physics
Identifiers
urn:nbn:se:kth:diva-245136 (URN)10.1140/epjc/s10052-019-6632-8 (DOI)000458691000004 ()2-s2.0-85061476087 (Scopus ID)
Note

QC 20190307

Available from: 2019-03-07 Created: 2019-03-07 Last updated: 2019-03-07Bibliographically approved
Aaboud, M., Kastanas, K. A., Jensen, B., Ohm, C., Ripellino, G., Sidebo, P. E., . . . et al., . (2019). Measurement of colour flow using jet-pull observables in tt¯ events with the ATLAS experiment at √s=13TeV. European Physical Journal C, 78(10)
Open this publication in new window or tab >>Measurement of colour flow using jet-pull observables in tt¯ events with the ATLAS experiment at √s=13TeV
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2019 (English)In: European Physical Journal C, ISSN 1434-6044, E-ISSN 1434-6052, Vol. 78, no 10Article in journal (Refereed) Published
Abstract [en]

Previous studies have shown that weighted angular moments derived from jet constituents encode the colour connections between partons that seed the jets. This paper presents measurements of two such distributions, the jet-pull angle and jet-pull magnitude, both of which are derived from the jet-pull angular moment. The measurement is performed in tt¯ events with one leptonically decaying W boson and one hadronically decaying W boson, using 36.1fb-1 of pp collision data recorded by the ATLAS detector at s=13TeV delivered by the Large Hadron Collider. The observables are measured for two dijet systems, corresponding to the colour-connected daughters of the W boson and the two b-jets from the top-quark decays, which are not expected to be colour connected. To allow the comparison of the measured distributions to colour model predictions, the measured distributions are unfolded to particle level, after correcting for experimental effects introduced by the detector. While good agreement can be found for some combinations of predictions and observables, none of the predictions describes the data well across all observables

Place, publisher, year, edition, pages
Springer, 2019
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-250804 (URN)10.1140/epjc/s10052-018-6290-2 (DOI)000448080300001 ()2-s2.0-85055343357 (Scopus ID)
Note

QC 20190507

Available from: 2019-05-07 Created: 2019-05-07 Last updated: 2019-05-07Bibliographically approved
Aaboud, M., Kastanas, K. A., Lund-Jensen, B., Ohm, C., Ripellino, G., Sidebo, P. E., . . . et al., . (2019). Measurement of photon-jet transverse momentum correlations in 5.02 TeV Pb + Pb and pp collisions with ATLAS. Physics Letters B, 789, 167-190
Open this publication in new window or tab >>Measurement of photon-jet transverse momentum correlations in 5.02 TeV Pb + Pb and pp collisions with ATLAS
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2019 (English)In: Physics Letters B, ISSN 0370-2693, E-ISSN 1873-2445, Vol. 789, p. 167-190Article in journal (Refereed) Published
Abstract [en]

Jets created in association with a photon can be used as a calibrated probe to study energy loss in the medium created in nuclear collisions. Measurements of the transverse momentum balance between isolated photons and inclusive jets are presented using integrated luminosities of 0.49 nb(-1) of Pb + Pb collision data at root(NN)-N-s = 5.02 TeV and 25 pb(-1) of pp collision data at. root s= 5.02 TeV recorded with the ATLAS detector at the LHC. Photons with transverse momentum 63.1 < p(T)(gamma) < 200 GeV and vertical bar eta(gamma vertical bar) < 2.37 are paired with all jets in the event that have p(T)(jet) > 31.6 GeV and pseudorapidity vertical bar eta(Jet)vertical bar < 2.8. The transverse momentum balance given by the jet-to-photon p(T) ratio, x(j gamma), is measured for pairs with azimuthal opening angle Delta phi > 7 pi/8. Distributions of the per-photon jet yield as a function of x(j gamma), (1/N-gamma)(dN/dx(j gamma)), are corrected for detector effects via a two-dimensional unfolding procedure and reported at the particle level. In pp collisions, the distributions are well described by Monte Carlo event generators. In Pb + Pb collisions, the x(j gamma) distribution is modified from that observed in pp collisions with increasing centrality, consistent with the picture of parton energy loss in the hot nuclear medium. The data are compared with a suite of energy-loss models and calculations.

Place, publisher, year, edition, pages
ELSEVIER SCIENCE BV, 2019
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-244092 (URN)10.1016/j.physletb.2018.12.023 (DOI)000457165400023 ()2-s2.0-85058927090 (Scopus ID)
Note

QC 20190219

Available from: 2019-02-19 Created: 2019-02-19 Last updated: 2019-02-19Bibliographically approved
Aaboud, M., Kastanas, K. A., Jensen, B., Ohm, C., Ripellino, G., Sidebo, P. E. & Strandberg, J. (2019). Measurement of the four-lepton invariant mass spectrum in 13 TeV proton-proton collisions with the ATLAS detector. Journal of High Energy Physics (JHEP) (4), Article ID 048.
Open this publication in new window or tab >>Measurement of the four-lepton invariant mass spectrum in 13 TeV proton-proton collisions with the ATLAS detector
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2019 (English)In: Journal of High Energy Physics (JHEP), ISSN 1126-6708, E-ISSN 1029-8479, no 4, article id 048Article in journal (Refereed) Published
Abstract [en]

A measurement of the four-lepton invariant mass spectrum is made with the ATLAS detector, using an integrated luminosity of 36.1 fb(-1) of proton-proton collisions at root s = 13 TeV delivered by the Large Hadron Collider. The differential cross-section is measured for events containing two same-flavour opposite-sign lepton pairs. It exhibits a rich structure, with different mass regions dominated in the Standard Model by single Z boson production, Higgs boson production, and Z boson pair production, and non-negligible interference effects at high invariant masses. The measurement is compared with state-of-the-art Standard Model calculations, which are found to be consistent with the data. These calculations are used to interpret the data in terms of gg -> ZZ -> 4l and Z -> 4l subprocesses, and to place constraints on a possible contribution from physics beyond the Standard Model.

Place, publisher, year, edition, pages
SPRINGER, 2019
Keywords
Hadron-Hadron scattering (experiments)
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:kth:diva-251499 (URN)10.1007/JHEP04(2019)048 (DOI)000464201100001 ()2-s2.0-85064268443 (Scopus ID)
Note

QC 20190515

Available from: 2019-05-15 Created: 2019-05-15 Last updated: 2019-05-15Bibliographically approved
Aaboud, M., Kastanas, K. A., Jensen, B., Ohm, C., Ripellino, G., Sidebo, P. E., . . . et al., . (2019). Measurement of the nuclear modification factor for inclusive jets in Pb plus Pb collisions at root s(NN)=5.02 TeV with the ATLAS detector. Physics Letters B, 790
Open this publication in new window or tab >>Measurement of the nuclear modification factor for inclusive jets in Pb plus Pb collisions at root s(NN)=5.02 TeV with the ATLAS detector
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2019 (English)In: Physics Letters B, ISSN 0370-2693, E-ISSN 1873-2445, Vol. 790Article in journal (Refereed) Published
Abstract [en]

Measurements of the yield and nuclear modification factor, R-AA, for inclusive jet production are performed using 0.49 nb(-1) of Pb+Pb data at root s(NN) = 5.02 TeV and 25 pb(-1) of Pb+Pb data at root s = 5.02 TeV with the ATLAS detector at the LHC. Jets are reconstructed with the anti-k(t) algorithm with radius parameter R = 0.4 and are measured over the transverse momentum range of 40-1000 GeV in six rapidity intervals covering vertical bar y vertical bar < 2.8. The magnitude of R-AA increases with increasing jet transverse momentum, reaching a value of approximately 0.6 at 1 TeV in the most central collisions. The magnitude of R-AA also increases towards peripheral collisions. The value of R-AA is independent of rapidity at low jet transverse momenta, but it is observed to decrease with increasing rapidity at high transverse momenta. 

Place, publisher, year, edition, pages
Elsevier, 2019
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-247991 (URN)10.1016/j.physletb.2018.10.076 (DOI)000460118200012 ()2-s2.0-85060207264 (Scopus ID)
Note

QC 20190401

Available from: 2019-04-01 Created: 2019-04-01 Last updated: 2019-04-01Bibliographically approved
Aaboud, M., Kastanas, K. A., Lund-Jensen, B., Ohm, C., Ripellino, G., Sidebo, P. E., . . . et al., . (2019). Measurement of the photon identification efficiencies with the ATLAS detector using LHC Run 2 data collected in 2015 and 2016. European Physical Journal C, 79(3)
Open this publication in new window or tab >>Measurement of the photon identification efficiencies with the ATLAS detector using LHC Run 2 data collected in 2015 and 2016
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2019 (English)In: European Physical Journal C, ISSN 1434-6044, E-ISSN 1434-6052, Vol. 79, no 3Article in journal (Refereed) Published
Abstract [en]

The efficiency of the photon identification criteria in the ATLAS detector is measured using 36.1 fb1 to 36.7 fb1 of pp collision data at v s = 13 TeV collected in 2015 and 2016. The efficiencies are measured separately for converted and unconverted isolated photons, in four different pseudorapidity regions, for transverse momenta between 10 GeV and 1.5 TeV. The results from the combination of three data-driven techniques are compared with the predictions from simulation after correcting the variables describing the shape of electromagnetic showers in simulation for the average differences observed relative to data. Data-tosimulation efficiency ratios are determined to account for the small residual efficiency differences. These factors are measured with uncertainties between 0.5% and 5% depending on the photon transverse momentum and pseudorapidity. The impact of the isolation criteria on the photon identification efficiency, and that of additional soft pp interactions, are also discussed. The probability of reconstructing an electron as a photon candidate ismeasured in data, and compared with the predictions from simulation. The efficiency of the reconstruction of photon conversions is measured using a sample of photon candidates from Z. mu mu. events, exploiting the properties of the ratio of the energies deposited in the first and second longitudinal layers of the ATLAS electromagnetic calorimeter.

Place, publisher, year, edition, pages
Springer, 2019
Keywords
ROOT-S=13 TEV; PP COLLISIONS; MASS
National Category
Subatomic Physics
Identifiers
urn:nbn:se:kth:diva-251265 (URN)10.1140/epjc/s10052-019-6650-6 (DOI)000460674700008 ()2-s2.0-85062611658 (Scopus ID)
Note

QC 20190509

Available from: 2019-05-09 Created: 2019-05-09 Last updated: 2019-05-09Bibliographically approved
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ORCID iD: ORCID iD iconorcid.org/0000-0001-6945-1916

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