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Register a new userInvisible neutrino decay : First vs second oscillation maximum
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We study the physics potential of the long-baseline experiments T2HK, T2HKK and ESS$ u$SB in the context of invisible neutrino decay. We consider normal mass ordering and assume that the state $ u_{3}$ as unstable, decaying into sterile states during the flight and obtain constraints on the neutrino decay lifetime ($tau_3$). We find that T2HK, T2HKK and ESS$ u$SB are sensitive to the decay-rate of $ u_{3}$ for $tau_{3}/m_{3} leq 2.72times10^{-11}$s/eV, $tau_{3}/m_{3} leq 4.36times10^{-11}$s/eV and $tau_{3}/m_{3} leq 2.43times10^{-11}$s/eV respectively at 3$sigma$ C.L. We compare and contrast the sensitivities of the three experiments and specially investigate the role played by the mixing angle $theta_{23}$. It is seen that for experiments with flux peak near the second oscillation maxima, the poorer sensitivity to $theta_{23}$ results in weaker constraints on the decay lifetime. Although, T2HKK has one detector close to the second oscillation maxima, having another detector at the first oscillation maxima results in superior sensitivity to decay. In addition, we find a synergy between the two baselines of the T2HKK experiment which helps in giving a better sensitivity for $theta_{23}$ in the higher octant. We discuss the octant sensitivity in presence of decay and show that there is an enhancement in sensitivity which occurs due to the contribution from the survival probability $P_{mumu}$ which is more pronounced for the experiments at the second oscillation maxima. We also obtain the combined sensitivity of T2HK+ESS$ u$SB and T2HKK+ESS$ u$SB as $tau_{3}/m_{3} leq 4.36times10^{-11}$s/eV and $tau_{3}/m_{3} leq 5.53times10^{-11}$s/eV respectively at 3$sigma$ C.L.
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Current long-baseline neutrino-oscillation experiments such as NO$ u$A and T2K are mainly sensitive to physics in the neighbourhood of the first oscillation maximum of the $ u_mu to u_e$ oscillation probability. The future Deep Underground Neutrino Experiment (DUNE) utilizes a wide-band beam tune optimized for CP violation sensitivity that fully covers the region of the first maxima and part of the second. In the present study, we elucidate the role of second oscillation maximum in addressing issues pertaining to unknowns in the standard three flavour paradigm. We consider a new DUNE beam tune optimized for coverage of the region of the second oscillation maxima which could be realized using proposed accelerator upgrades that provide multi-MW of power at proton energies of 8 GeV. We find that addition of the multi-MW 8 GeV beam to DUNE wide-band running leads to modest improvement in sensitivity to CP violation, mass hierarchy, the octant of $theta_{23}$ as well as the resolution of $delta$ and the Jarlskog invariant. Significant improvements to the DUNE neutrino energy resolution yield a much larger improvement in performance. We conclude that the standard DUNE wide-band beam when coupled with excellent detector resolution capabilities is sufficient to resolve $delta$ to better than $sim 12^circ$ for all values of $delta$ in a decade of running. For second maxima (8 GeV 3MW) beam running concurrently with the standard wide-band (80 GeV 2.2 MW) beam for 5 of the 10 years, it is found that $delta$ can be further resolved better than $sim 10^circ$ for all values of $delta$.
Several theories of particle physics beyond the Standard Model consider that neutrinos can decay. In this work we assume that the standard mechanism of neutrino oscillations is altered by the decay of the heaviest neutrino mass state into a sterile neutrino and, depending on the model, a scalar or a Majoron. We study the sensitivity of the forthcoming KM3NeT-ORCA experiment to this scenario and find that it could improve the current bounds coming from oscillation experiments, where three-neutrino oscillations have been considered, by roughly two orders of magnitude. We also study how the presence of this neutrino decay can affect the determination of the atmospheric oscillation parameters $sin^2theta_{23}$ and $Delta m_{31}^2$, as well as the sensitivity to the neutrino mass ordering.
The hypothesis of the decay of neutrino mass eigenstates leads to a substantial modification of the appearance and disappearance probabilities of flavor eigenstates. We investigate the impact on the standard oscillation scenario caused by the decay of the heaviest mass eigenstate $ u_3$ (with a mass $m_3$ and a mean life $tau_3$) to a sterile state in DUNE. We find that the lower bound of $5.1 times 10^{-11}~s/eV$ at 90% CL on the decay parameter $tau_3/m_3$ can be set if the Neutral Current data are included in the analysis, thus providing the best long-baseline expected limit so far. We also show that the $ u_tau$ appearance channel would give only a negligible contribution to the decay parameter constraints. Our numerical results are corroborated by analytical formulae for the appearance and disappearance probabilities in vacuum (which is a useful approximation for the study of the invisible decay model) that we have developed up to the second order in the solar mass splitting and to all orders in the decay factor $t/tau_3$.
The experimental bound on lifetime of nu_3, the neutrino mass eigenstate with the smallest nu_e component, is much weaker than those of nu_1 and nu_2 by many orders of magnitude to which the astrophysical constraints apply. We argue that the future reactor neutrino oscillation experiments with medium-baseline (~ 50 km), such as JUNO or RENO-50, has the best chance of placing the most stringent constraint on nu_3 lifetime among all neutrino experiments which utilize the artificial source neutrinos. Assuming decay into invisible states, we show by a detailed chi^2 analysis that the nu_3 lifetime divided by its mass, tau_3/m_3, can be constrained to be tau_3/m_3 > 7.5 (5.5) x 10^{-11} s/eV at 95% (99%) C.L. by 100 kt.years exposure by JUNO. It may be further improved to the level comparable to the atmospheric neutrino bound by its longer run. We also discuss to what extent nu_3 decay affects mass-ordering determination and precision measurements of the mixing parameters.
Taking account of possible CP violation, we discuss about the constraints on the lepton mixing angles from the neutrinoless double beta decay and from the neutrino oscillation for the three flavour Majorana neutrinos. From the CHORUS oscillation experiment, combined with the data of neutrinoless double beta decay, we show that the large angle solution of (theta_{23}) is improbable if the neutrino mass (m_3) of the third generation is a candidate of hot dark matters.
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