Neutrino mass hierarchy, CP-violation, and octant of θ23 are the fundamental unknowns in neutrino oscillations. In order to address all these three unknowns, we study the physics reach of a setup, where we replace the antineutrino run of T2HK with antineutrinos from muon decay at rest (μ-DAR). This approach has the advantages of having higher statistics in both neutrino and antineutrino modes, and lower beam-on backgrounds for antineutrino run with reduced systematics. We find that a hybrid setup consisting of T2HK (ν) and μ-DAR (ν¯) in conjunction with full exposure from T2K and NOνA can resolve the issue of mass hierarchy at greater than 3σ C.L. irrespective of the choices of hierarchy, δCP, and θ23. This hybrid setup can also establish the CP-violation at 5σ C.L. for ∼ 55% choices of δCP, whereas the same for conventional T2HK (ν+ν¯) setup along with T2K and NOνA is around 30%. As far as the octant of θ23 is concerned, this hybrid setup can exclude the wrong octant at 5σ C.L. if θ23 is at least 3° away from maximal mixing for any δCP.

The T2K experiment has provided the first hint for the best-fit value for the leptonic CP phase delta(CP) similar to -90 degrees from neutrino data. This is now corroborated by the NO nu A neutrino runs. We study the implications for neutrino mass hierarchy and octant of theta(23) in the context of this data assuming that the true value of delta(CP) in nature is -90 degrees. Based on simple arguments on degeneracies in the probabilities, we show that a clear signal of delta(CP) = -90 degrees coming from T2K neutrino (antineutrino) data is only possible if the true hierarchy is normal and the true octant is higher (lower). Thus, if the T2K neutrino and antineutrino data are fitted separately and both give the true value of delta(CP) = -90 degrees, this will imply that nature has chosen the true hierarchy to be normal and theta(23) approximate to 45 degrees. However, we find that the combined fit of neutrino and antineutrino data will still point to true hierarchy as normal but the octant of theta(23) will remain undetermined. We do our analysis for both, the current projected exposure (7.8 x 10(21) pot) and planned extended exposure (20 x 10(21) pot). We also present the CP discovery potential of T2K emphasizing on the role of antineutrinos. We find that one of the main contributions of the antineutrino data is to remove the degenerate solutions with the wrong octant. Thus, the antineutrino run plays a more significant role for those hierarchy-octant combinations for which this degeneracy is present. If this degeneracy is absent, then only neutrino run gives a better result for fixed theta(13). However, if we marginalize over theta(13) then, sensitivity corresponding to mixed run can be better than pure neutrino run.

We simultaneously investigate source, detector and matter non-standard neutrino interactions at the proposed DUNE experiment. Our analysis is performed using a Markov Chain Monte Carlo exploring the full parameter space. We find that the sensitivity of DUNE to the standard oscillation parameters is worsened due to the presence of non-standard neutrino interactions. In particular, there are degenerate solutions in the leptonic mixing angle θ23 and the Dirac CP-violating phase δ. We also compute the expected sensitivities at DUNE to the non-standard interaction parameters. We find that the sensitivities to the matter non-standard interaction parameters are substantially stronger than the current bounds (up to a factor of about 15). Furthermore, we discuss correlations between the source/detector and matter non-standard interaction parameters and find a degenerate solution in θ23. Finally, we explore the effect of statistics on our results.

The presence of a zero texture in the neutrino mass matrix can indicate the presence of an underlying symmetry which can generate neutrino mass and mixing. In this paper, for the first time, we study the four-zero textures of the low energy neutrino mass matrix in the presence of an extra light-sterile neutrino, i.e., the 3 + 1 neutrino scheme. In our analysis, we find that out of the 210 possible four-zero textures only 15 textures are allowed. We divide the allowed four-zero textures into two classes-A in which the value of mass matrix element M-ee is zero and class B in which M-ee is nonzero. In this way, we obtain ten possible four-zero textures in class A and five possible four-zero textures in class B. In our analysis, we find that for normal hierarchy the allowed number of textures in class A (B) is nine (three). For the case of inverted hierarchy, we find that two textures in class A are disallowed, and these textures are different from the disallowed textures for normal hierarchy in class A. However, we find that all the five textures in class B are allowed for the inverted hierarchy. Based on analytic expressions for the elements M-alpha beta, we discuss the reasons for certain textures being disallowed. We also discuss the correlations between the different parameters of the allowed textures. Finally, we present the implications of our study on experimental searches for neutrinoless double beta decay.

The T2K neutrino data has already given a hint for the best-fit value of the leptonic CP phase δCP as −90◦. In this paper we ask the question that if this hint is confirmed by the subsequent neutrino and anti-neutrino runs of T2K, then can it also give any information about the other two remaining unknown oscillation parameters—the neutrino mass hierarchy and octant of θ23. We find that if T2K runs in only neutrino mode with its full targeted exposure, then δCP = −90◦ would indicate the true hierarchy as normal and the true octant as higher. On the other hand if T2K runs in equal neutrino and anti-neutrino mode then the true hierarchy can be confirmed as normal but the octant will remain undetermined. We have also studied the effect of anti-neutrino runs on CP sensitivity of T2K.

The phenomenon of neutrino oscillation is now well understood from the solar, atmospheric, reactor and accelerator neutrino experiments. This oscillation is characterized by a unitary PMNS matrix which is parametrized by three mixing angles (oee integral (12), oee integral (23) and oee integral (13)) and one phase (delta (CP)) known as the leptonic CP phase. Neutrino oscillation also involves two mass squared differences: the solar mass square difference () and the atmospheric mass square difference (). Though there is already significant amount of information about the three mixing angles, the CP phase is still unknown. Apart from the CP phase, one should also know what is the true nature of the neutrino mass hierarchy, i.e., normal (m (3)> m (1): NH) or inverted (m (1)> m (3): IH) and what is the true octant of oee integral (23), i.e., lower (oee integral (23)< 45(a similar to): LO) or higher (oee integral (23)> 45(a similar to): HO). The long-baseline experiments (LBL) have CP sensitivity coming from the appearance channel (). On the other hand, atmospheric neutrinos are known to have negligible CP sensitivity. In this work, we study the synergy between the LBL experiment NO nu A, T2K and the atmospheric neutrino experiment ICAL@INO for obtaining the first hint of CP violation in the lepton sector. We find that due to the lack of knowledge of hierarchy and octant, CP sensitivity of NO nu A/T2K is poorer for some parameter ranges. Addition of ICAL data to T2K and NO nu A can exclude these spurious wrong-hierarchy and /or wrong-octant solutions and cause a significant increase in the range of delta (CP) values for which a hint of CP violation can be achieved. Similarly, the precision with which delta (CP) can be measured also improves with the inclusion of ICAL data.

The deep underground neutrino experiment (DUNE) is a proposed next generation superbeam experiment at Fermilab. Its aims include measuring the unknown neutrino oscillation parameters—the neutrino mass hierarchy, the octant of the mixing angle θ23, and the CP-violating phase δCP. The current and upcoming experiments T2K, NOνA, and ICAL@INO will also be collecting data for the same measurements. In this paper, we explore the sensitivity reach of DUNE in combination with these other experiments. We evaluate the least exposure required by DUNE to determine the above three unknown parameters with reasonable confidence. We find that for each case, the inclusion of data from T2K, NOνA, and ICAL@INO help to achieve the same sensitivity with a reduced exposure from DUNE thereby helping to economize the configuration. Further, we quantify the effect of the proposed near detector on systematic errors and study the consequent improvement in sensitivity. We also examine the role played by the second oscillation cycle in furthering the physics reach of DUNE. Finally, we present an optimization study of the neutrino–antineutrino running of DUNE.

The three major unknown neutrino oscillation parameters at the present juncture are the mass hierarchy, the octant of the mixing angle theta(23) and the CP phase delta(CP). It is well known that the presence of hierarchy-delta(CP) and octant degeneracies affects the unambiguous determination of these parameters. In this paper, we show that a comprehensive way to study the remaining parameter degeneracies is in the form of a generalized hierarchy -theta(23) - delta(CP) degeneracy. This is best depicted as contours in the test (theta(23) - delta(CP)) plane for different representative true values of parameters. We show that the wrong-hierarchy and/or wrong-octant solutions can be further classified into eight different solutions depending on whether they occur with the wrong or right value of delta(CP). These eight solutions are different from the original eightfold degenerate solutions and can exist, in principle, even if theta(13) is known. These multiple solutions, apart from affecting the determination of the true hierarchy and octant, also affect the accurate estimation of delta(CP). We identify which of these eight different degenerate solutions can occur in the test (theta(23) - delta(CP)) parameter space, taking the long-baseline experiment NO nu A running in the neutrino mode as an example. The inclusion of the NO nu A antineutrino run removes the wrong-octant solutions appearing with both right and wrong hierarchy. Adding T2K data to this resolves the wrong hierarchy-right octant solutions to a large extent. The remaining wrong-hierarchy solutions can be removed by combining NO nu A + T2K with atmospheric neutrino data. We demonstrate this using ICAL@INO as the prototype atmospheric neutrino detector. We find that the degeneracies can be resolved at the 2 sigma level by the combined data set, for the true parameter space considered in the study.

We investigate source and detector non-standard neutrino interactions at the proposed ESS nu SB experiment. We analyze the effect of non-standard physics at the probability level, the event-rate level and by a full computation of the ESS nu SB setup. We find that the precision measurement of the leptonic mixing angle theta(23) at ESS nu SB is robust in the presence of non-standard interactions, whereas that of the leptonic CP-violating phase delta is worsened at most by a factor of two. We compute sensitivities to all the relevant source and decector non-standard interaction parameters and find that the sensitivities to the parameters epsilon(s)(mu e) and epsilon(d)(mu e) are comparable to the existing limits in a realistic scenario, while they improve by a factor of two in an optimistic scenario. Finally, we show that the absence of a near detector compromises the sensitivity of ESS nu SB to non-standard interactions.