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Register a new userNew look at the degeneracies in the neutrino oscillation parameters, and their resolution by T2K, NO$ u$A and ICAL
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At present the three major unknowns in neutrino oscillation parameters are the mass hierarchy, the octant of $theta_{23}$ 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. 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$ u$A running in the neutrino mode as an example. The inclusion of the NO$ u$A antineutrino run removes the wrong-octant solutions appearing with both right and wrong hierarchy. Adding T2K data to this resolves the wrong hierarchy -- right octant solutions to a large extent. The remaining wrong hierarchy solutions can be removed by combining NO$ u$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 $2sigma$ level by the combined data set, for the true parameter space considered in the study.
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The main aim of the ESS$ u$SB proposal is the discovery of the leptonic CP phase $delta_{CP}$ with a high significance ($5sigma$ for 50% values of $delta_{CP}$) by utilizing the physics at the second oscillation maxima of the $P_{mu e}$ channel. It can achieve $3sigma$ sensitivity to hierarchy for all values of $delta_{CP}$. In this work, we concentrate on the hierarchy and octant sensitivity of the ESS$ u$SB experiment. We show that combining the ESS$ u$SB experiment with the atmospheric neutrino data from the proposed India-based Neutrino Observatory(INO) experiment can result in an increased sensitivity to mass hierarchy. In addition, we also combine the results from the ongoing experiments T2K and NO$ u$A assuming their full runtime and present the combined sensitivity of ESS$ u$SB + ICAL@INO + T2K + NO$ u$A. We show that while by itself ESS$ u$SB can have up to $3sigma$ hierarchy sensitivity, the combination of all the experiments can give up to $5sigma$ sensitivity depending on the true hierarchy-octant combination. The octant sensitivity of ESS$ u$SB is low by itself. However the combined sensitivity of all the above experiments can give up to $3sigma$ sensitivity depending on the choice of true hierarchy and octant. We discuss the various degeneracies and the synergies that lead to the enhanced sensitivity when combining different experimental data.
The combined analysis of $ u_mu$ disappearance and $ u_e$ appearance data of NO$ u$A experiment leads to three nearly degenerate solutions. This degeneracy can be understood in terms of deviations in $ u_e$ appearance signal, caused by unknown effects, with respect to the signal expected for a reference set of oscillations parameters. We define the reference set to be vacuum oscillations in the limit of maximal $theta_{23}$ and no CP-violation. We then calculate the deviations induced in the $ u_e$ appearance signal event rate by three unknown effects: (a) matter effects, due to normal or inverted hierarchy (b) octant effects, due to $theta_{23}$ being in higher or lower octant and (c) CP-violation, whether $delta_{CP} sim - pi/2$ or $delta_{CP} sim pi/2$. We find that the deviation caused by each of these effects is the same for NO$ u$A. The observed number of $ u_e$ events in NO$ u$A is equivalent to the increase caused by one of the effects. Therefore, the observed number of $ u_e$ appearance events of NO$ u$A is the net result of the increase caused by two of the unknown effects and the decrease caused by the third. Thus we get the three degenerate solutions. We also find that further data by NO$ u$A can not distinguish between these degenerate solutions but addition of one year of neutrino run of DUNE can make a distinction between all three solutions. The distinction between the two NH solutions and the IH solution becomes possible because of the larger matter effect in DUNE. The distinction between the two NH solutions with different octants is a result of the synergy between the anti-neutrino data of NO$ u$A and the neutrino data of DUNE.
We study in detail the impact of a light sterile neutrino in the interpretation of the latest data of the long baseline experiments NO$ u$A and T2K, assessing the robustness/fragility of the estimates of the standard 3-flavor parameters with respect to the perturbations induced in the 3+1 scheme. We find that all the basic features of the 3-flavor analysis, including the weak indication ($sim$1.4$sigma$) in favor of the inverted neutrino mass ordering, the preference for values of the CP-phase $delta_{13} sim 1.2pi$, and the substantial degeneracy of the two octants of $theta_{23}$, all remain basically unaltered in the 4-flavor scheme. Our analysis also demonstrates that it is possible to attain some constraints on the new CP-phase $delta_{14}$. Finally, we point out that, differently from non-standard neutrino interactions, light sterile neutrinos are not capable to alleviate the tension recently emerged between NO$ u$A and T2K in the appearance channel.
Precise and adequate knowledge of neutrino scattering cross-sections and nuclear effects in them is very important to reduce the systematic uncertainties in neutrino beam oscillation experiments. The insufficiency in our present understanding of these effects inflicts the precision measurements of yet unknown neutrino oscillation parameters and some other experimentally observed anomalies in the neutrino sector. Another source of uncertainty is the energy dependence of neutrino oscillation probability which is a nontrivial function of the true incoming neutrino energy. This energy is reconstructed using different methods, which in turn is used in the analysis leading to the extraction of various neutrino oscillation parameters. The extraction of still unknown parameters like the leptonic CP violation phase demands the precision level in these measurements to be very high. In this work, we use the kinematic method of reconstruction of the incoming neutrino energy, both at the near and far detectors of NO$ u$A (USA) experiment, and investigate the role of multinucleon (MN) effects on the sensitivity measurement of various neutrino oscillation parameters. We use the values of various light neutrino oscillation parameters from their latest global fit values in our analysis.
The observation of the recent electron neutrino appearance in a muon neutrino beam and the high-precision measurement of the mixing angle $theta_{13}$ have led to a re-evaluation of the physics potential of the T2K long-baseline neutrino oscillation experiment. Sensitivities are explored for CP violation in neutrinos, non-maximal $sin^22theta_{23}$, the octant of $theta_{23}$, and the mass hierarchy, in addition to the measurements of $delta_{CP}$, $sin^2theta_{23}$, and $Delta m^2_{32}$, for various combinations of $ u$-mode and (bar{ u})-mode data-taking. With an exposure of $7.8times10^{21}$~protons-on-target, T2K can achieve 1-$sigma$ resolution of 0.050(0.054) on $sin^2theta_{23}$ and $0.040(0.045)times10^{-3}~rm{eV}^2$ on $Delta m^2_{32}$ for 100%(50%) neutrino beam mode running assuming $sin^2theta_{23}=0.5$ and $Delta m^2_{32} = 2.4times10^{-3}$ eV$^2$. T2K will have sensitivity to the CP-violating phase $delta_{rm{CP}}$ at 90% C.L. or better over a significant range. For example, if $sin^22theta_{23}$ is maximal (i.e $theta_{23}$=$45^circ$) the range is $-115^circ<delta_{rm{CP}}<-60^circ$ for normal hierarchy and $+50^circ<delta_{rm{CP}}<+130^circ$ for inverted hierarchy. When T2K data is combined with data from the NO$ u$A experiment, the region of oscillation parameter space where there is sensitivity to observe a non-zero $delta_{CP}$ is substantially increased compared to if each experiment is analyzed alone.
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