Atmospheric Neutrino Experiments Research Articles

Neutrino physics: Experimental and theoretical challenges

The existence of massive neutrinos is the first solid evidence of physics beyond the standard model of particle physics. A remarkable progress has been achieved in solar, atmospheric, reactor, and accelerator neutrino experiments during the last decades. On the theoretical side, several questions are being addressed, namely the Dirac or Majorana nature of neutrinos, the mechanisms for neutrino mass generation, and the relation between neutrinos and the matter–antimatter asymmetry observed in the Universe, among others. This article provides a brief overview on some of the current experimental and theoretical aspects in neutrino physics.

Discriminating between Lorentz violation and non-standard interactions using core-passing atmospheric neutrinos at INO-ICAL

Precision measurements of neutrino oscillation parameters have provided a tremendous boost to the search for sub-leading effects due to several beyond the Standard Model scenarios in neutrino oscillation experiments. Among these, two of the well-studied scenarios are Lorentz violation (LV) and non-standard interactions (NSI), both of which can affect neutrino oscillations significantly. We point out that, at a long-baseline experiment where the neutrino oscillation probabilities can be well-approximated by using the line-averaged constant matter density, the effects of these two scenarios can mimic each other. This would allow the limits obtained at such an experiment on one of the above scenarios to be directly translated to the limits on the other scenario. However, for the same reason, it would be difficult to distinguish between LV and NSI at a long-baseline experiment. We show that the observations of atmospheric neutrinos, which travel a wide range of baselines and may encounter sharp density changes at the core-mantle boundary, can break this degeneracy. We observe that identifying neutrinos and antineutrinos separately, as can be done at INO-ICAL, can enhance the capability of atmospheric neutrino experiments to discriminate between these two new-physics scenarios.

Charged current neutrino and antineutrino induced eta production off the nucleon

The charged current (anti)neutrino induced eta production from the nucleons is studied in a model based on the effective Lagrangians to evaluate the contribution from the nonresonant and resonant diagrams. The contribution from the nonresonant background terms has been obtained using a microscopic model based on the SU(3) chiral Lagrangians. The contribution from the resonant diagrams due to the low lying ${S}_{11}(1535)$, ${S}_{11}(1650)$, and ${P}_{11}(1710)$ resonances has been evaluated using an effective phenomenological Lagrangian with its parameters determined from the experimental values of their branching ratios and decay widths to the $N\ensuremath{\eta}$ channel. The model is first used to reproduce satisfactorily the experimental data from the MAINZ and JLab on the total cross sections for the photo- and electro- production of $\ensuremath{\eta}$ mesons, which fixes the model parameters in the vector current interaction. The partially conserved axial vector current (PCAC) hypothesis and generalized Goldberger-Treiman relation are used to fix the parameters of the axial vector current interaction. The model is then applied to study the weak production of eta mesons induced by the neutrinos and antineutrinos, and predicts the numerical values for the ${Q}^{2}$-distribution, $\ensuremath{\eta}$-momentum distribution, and the total cross section for the reactions ${\ensuremath{\nu}}_{\ensuremath{\mu}}+n\ensuremath{\rightarrow}{\ensuremath{\mu}}^{\ensuremath{-}}+p+\ensuremath{\eta}$ and ${\overline{\ensuremath{\nu}}}_{\ensuremath{\mu}}+p\ensuremath{\rightarrow}{\ensuremath{\mu}}^{+}+n+\ensuremath{\eta}$ in the energy region up to 2 GeV. It is found that the photo, electro, and (anti)neutrino induced production of eta mesons is dominated by the contribution from the ${S}_{11}(1535)$ resonance. The results discussed in this work are relevant for the present and future accelerator experiments like MicroBooNE, T2K, NOvA, MINERvA, T2-HyperK, and DUNE as well as for the atmospheric neutrino experiments.

Determining neutrino mass ordering with ICAL, JUNO and T2HK

In this paper we study the synergy among the future accelerator (T2HK), future atmospheric (ICAL) and future reactor (JUNO) neutrino experiments to determine the neutrino mass ordering. T2HK can measure the mass ordering only for favorable values of $\delta_{\rm CP}$, whereas the mass ordering sensitivity of JUNO is dependent on the energy resolution. Our results show that with a combination of T2HK, ICAL and JUNO one can have a mass ordering sensitivity of 7.2 $\sigma$ even for the unfavorable value of $\delta_{\rm CP} = 0^\circ$ for T2HK and most conservative value of JUNO energy resolution of 5$\%/\sqrt{E(MeV)}$. The synergy mainly comes because different oscillation channels prefer different values of $|\Delta m_{31}^2|$ in the fit when the mass-ordering $\chi^2$ is minimized. In this context we also study: (i) effect of varying energy resolution of JUNO, (ii) the effect of longer run-time of ICAL, (iii) effect of different true values of $\theta_{23}$ and (iv) effect of octant degeneracy in the determination of neutrino mass ordering.

Analytic treatment of 3-flavor neutrino oscillation and decay in matter

We present compact analytic expressions for 3-flavor neutrino oscillation probabilities with invisible neutrino decay, where matter effects have been explicitly included. We take into account the possibility that the oscillation and decay components of the effective Hamiltonian do not commute. This is achieved by employing the techniques of inverse Baker-Campbell-Hausdorff (BCH) expansion and the Cayley-Hamilton theorem applied in the 3-flavor framework. If only the vacuum mass eigenstate ν3 decays, we show that the treatment of neutrino propagation may be reduced to an effective 2-flavor analysis in the One Mass Scale Dominance (OMSD) approximation. The oscillation probabilities for Pμμ, Pee, Peμ and Pμe — relevant for reactor, long baseline and atmospheric neutrino experiments — are obtained as perturbative expansions for the case of only ν3 decay, as well as for the more general scenario where all components of the decay matrix are non-zero. The analytic results thus obtained match the exact numerical results for constant density matter to a high precision and provide physical insights into possible effects of the decay of neutrinos as they propagate through Earth matter. We find that the effects of neutrino decay are most likely to be observable in Pμμ. We also point out that at any long baseline, the oscillation dips in Pμμ can show higher survival probabilities in the case with decay than without decay, and explain this feature using our analytic approximations.

Synergies and prospects for early resolution of the neutrino mass ordering

The measurement of neutrino mass ordering (MO) is a fundamental element for the understanding of leptonic flavour sector of the Standard Model of Particle Physics. Its determination relies on the precise measurement of Delta m^2_{31} and Delta m^2_{32} using either neutrino vacuum oscillations, such as the ones studied by medium baseline reactor experiments, or matter effect modified oscillations such as those manifesting in long-baseline neutrino beams (LBnuB) or atmospheric neutrino experiments. Despite existing MO indication today, a fully resolved MO measurement (ge 5sigma) is most likely to await for the next generation of neutrino experiments: JUNO, whose stand-alone sensitivity is sim 3sigma, or LBnuB experiments (DUNE and Hyper-Kamiokande). Upcoming atmospheric neutrino experiments are also expected to provide precious information. In this work, we study the possible context for the earliest full MO resolution. A firm resolution is possible even before 2028, exploiting mainly vacuum oscillation, upon the combination of JUNO and the current generation of LBnuB experiments (NOvA and T2K). This opportunity is possible thanks to a powerful synergy boosting the overall sensitivity where the sub-percent precision of Delta m^2_{32} by LBnuB experiments is found to be the leading order term for the MO earliest discovery. We also found that the comparison between matter and vacuum driven oscillation results enables unique discovery potential for physics beyond the Standard Model.

Exploring NSI using oscillation dip and valley in atmospheric neutrino experiments

We present a new approach to probe flavor-changing neutral-current non-standard neutrino interaction parameter εμτ using the oscillation dip and valley features. We use the simulated up/down ratio of reconstructed muons events at the proposed Iron Calorimeter (ICAL) detector at the India-based Neutrino Observatory to demonstrate that in presence of NSI parameter ε μτ, the oscillation dip location shifts and oscillation valley bends. Utilizing the charge identification capability of ICAL, the opposite shifts in the oscillation dip locations and the contrast in the oscillation valley curvatures for μ− and μ+ can be used to place 90% C.L. bounds of about 2% on |εμτ| using 500 kt• yr exposure. In our procedure, we take care of statistical fluctuations, systematic errors, and uncertainties in neutrino oscillation parameters.

NuFIT: Three-Flavour Global Analyses of Neutrino Oscillation Experiments

In this contribution, we summarise the determination of neutrino masses and mixing arising from global analysis of data from atmospheric, solar, reactor, and accelerator neutrino experiments performed in the framework of three-neutrino mixing and obtained in the context of the NuFIT collaboration. Apart from presenting the latest status as of autumn 2021, we discuss the evolution of global-fit results over the last 10 years, and mention various pending issues (and their resolution) that occurred during that period in the global analyses.

Future prospects in neutrino physics

Neutrino physics has come a long way and made great strides in the past decades. We discuss the prospects of what more can be learned in this field in the forthcoming neutrino oscillation facilities. We will mostly focus on the potential of the long-baseline experiments and the atmospheric neutrino experiments. Sensitivity of these experiments to standard neutrino oscillation parameters will be presented. We will also discuss the prospects of new physics searches at these facilities.

A new approach to probe non-standard interactions in atmospheric neutrino experiments

We propose a new approach to explore the neutral-current non-standard neutrino interactions (NSI) in atmospheric neutrino experiments using oscillation dips and valleys in reconstructed muon observables, at a detector like ICAL that can identify the muon charge. We focus on the flavor-changing NSI parameter εμτ, which has the maximum impact on the muon survival probability in these experiments. We show that non-zero εμτ shifts the oscillation dip locations in L/E distributions of the up/down event ratios of reconstructed μ− and μ+ in opposite directions. We introduce a new variable ∆d representing the difference of dip locations in μ− and μ+, which is sensitive to the magnitude as well as the sign of εμτ, and is independent of the value of Delta {m}_{32}^2 . We further note that the oscillation valley in the (E, cos θ) plane of the reconstructed muon observables bends in the presence of NSI, its curvature having opposite signs for μ− and μ+. We demonstrate the identification of NSI with this curvature, which is feasible for detectors like ICAL having excellent muon energy and direction resolutions. We illustrate how the measurement of contrast in the curvatures of valleys in μ− and μ+ can be used to estimate εμτ. Using these proposed oscillation dip and valley measurements, the achievable precision on |εμτ| at 90% C.L. is about 2% with 500 kt·yr exposure. The effects of statistical fluctuations, systematic errors, and uncertainties in oscillation parameters have been incorporated using multiple sets of simulated data. Our method would provide a direct and robust measurement of εμτ in the multi-GeV energy range.

From oscillation dip to oscillation valley in atmospheric neutrino experiments

Atmospheric neutrino experiments can show the “oscillation dip” feature in data, due to their sensitivity over a large L/E range. In experiments that can distinguish between neutrinos and antineutrinos, like INO, oscillation dips can be observed in both these channels separately. We present the dip-identification algorithm employing a data-driven approach – one that uses the asymmetry in the upward-going and downward-going events, binned in the reconstructed L/E of muons – to demonstrate the dip, which would confirm the oscillation hypothesis. We further propose, for the first time, the identification of an “oscillation valley” in the reconstructed (E_mu ,,cos theta _mu ) plane, feasible for detectors like ICAL having excellent muon energy and direction resolutions. We illustrate how this two-dimensional valley would offer a clear visual representation and test of the L/E dependence, the alignment of the valley quantifying the atmospheric mass-squared difference. Owing to the charge identification capability of the ICAL detector at INO, we always present our results using mu ^{-} and mu ^{+} events separately. Taking into account the statistical fluctuations and systematic errors, and varying oscillation parameters over their currently allowed ranges, we estimate the precision to which atmospheric neutrino oscillation parameters would be determined with the 10-year simulated data at ICAL using our procedure.

Matter vs vacuum oscillations in atmospheric neutrinos

At present, both vacuum oscillations as well as matter modified oscillations can explain the data of atmospheric neutrino experiments and the long baseline accelerator neutrino experiments equally well. Given the important role the matter effects play in the determination of CP violation in neutrino oscillations, it is imperative to establish the signal for matter effects unambiguously. In this work, we study the ability of ICAL at INO to make a distinction between vacuum oscillations and matter modified oscillations. We find that it is possible to make a 3σ discrimination in ten years, whether the atmospheric mass-squared difference is positive or negative.

Model-independent test for CPT violation using long-baseline and atmospheric neutrino experiments

Charge-Parity-Time (CPT) symmetry governs that the oscillation parameters for neutrinos and anti-neutrinos are to be identical. Different mass and mixing parameters for these particles may give us a possible hint for CPT violation in the neutrino sector. Using this approach, we discuss the ability of long-baseline and atmospheric neutrino experiments to determine the difference between mass squared splittings ($\Delta m^{2}_{32}-\Delta\bar{m}^{2}_{32}$) and atmospheric mixing angles ($\sin^{2}\theta_{23}-\sin^{2}\bar{\theta}_{23}$) of neutrinos and anti-neutrinos. We show the joint sensitivity of the T2K, NOvA and INO experiments to such CPT violating observables in different possible combinations of octant for neutrinos and anti-neutrinos.

Recent highlights on neutrino-nucleus interactions

The recent developments on neutrino-nucleus interactions at low and intermediate energies are reviewed and discussed in conjunction with the recent data of atmospheric, solar, and accelerator neutrino experiments. The theoretical nuclear physics approaches used to interpret and predict phenomena for which neutrinos play a crucial role are also investigated. We emphasize on the implications of neutrino reactions and properties into the astrophysical phenomena and atmospheric neutrino problems.

The AQUA-RICH project

The AQUA-RICH project, a long baseline and atmospheric neutrino experiment proposed by Tom Ypsilantis during the last decade of the last century, is briefly reviewed. The detector proposal is discussed within the context of the searches for neutrino oscillations at that time and as part of the longstanding seminal ideas and key contributions provided by Tom Ypsilantis in the development of the RICH technique.

Icecube/DeepCore tests for novel explanations of the MiniBooNE anomaly

While the low-energy excess observed at MiniBooNE remains unchallenged, it has become increasingly difficult to reconcile it with the results from other sterile neutrino searches and cosmology. Recently, it has been shown that non-minimal models with new particles in a hidden sector could provide a better fit to the data. As their main ingredients they require a GeV-scale Z', kinetically mixed with the photon, and an unstable heavy neutrino with a mass in the 150 MeV range that mixes with the light neutrinos. In this letter we point out that atmospheric neutrino experiments (and, in particular, IceCube/DeepCore) could probe a significant fraction of the parameter space of such models by looking for an excess of “double-bang” events at low energies, as proposed in our previous work (Coloma et al., Phys Rev Lett 119(20):201804, https://doi.org/10.1103/PhysRevLett.119.20180, 2017). Such a search would probe exactly the same production and decay mechanisms required to explain the anomaly.

Weak Quasielastic Hyperon Production Leading to Pions in the Antineutrino-Nucleus Reactions

In this review, we have studied the quasielastic production cross sections and polarization components of $\Lambda$, $\Sigma^0$ and $\Sigma^-$ hyperons induced by the weak charged currents in the antineutrino reactions on the nucleon and the nuclear targets like $^{12}$C, $^{16}$O, $^{40}$Ar and $^{208}$Pb. It is shown that the energy and the $Q^2$ dependence of the cross sections and the various polarization components can be effectively used to determine the axial vector transition form factors in the strangeness sector and test the validity of various symmetry properties of the weak hadronic currents like G-invariance, T-invariance and SU(3) symmetry. In particular, the energy and the $Q^2$ dependence of the polarization components of the hyperons is found to be sensitive enough to determine the presence of the second class current with or without T-invariance. These hyperons decay dominantly into pions giving an additional contribution to the weak pion production induced by the antineutrinos. This contribution is shown to be quantitatively significant as compared to the pion production by the $\Delta$ excitation in the nuclear targets in the sub-GeV energy region relevant for the $\bar{\nu}_\mu$ cross section measurements in the oscillation experiments. We have also included a few new results, based on our earlier works, which are in the kinematic region of the present and future (anti)neutrino experiments being done with the accelerator (anti)neutrinos at T2K, MicroBooNE, MiniBooNE, NO$\nu$A, MINER$\nu$A and DUNE, as well as for the atmospheric (anti)neutrino experiments in this energy region.

Can INO be sensitive to flavor-dependent long-range forces?

Flavor-dependent long-range leptonic forces mediated by the ultra-light and neutral bosons associated with gauged Le − Lμ or Le − Lτ symmetry constitute a minimal extension of the Standard Model. In presence of these new anomaly free abelian symmetries, the SM remains invariant and renormalizable, and can lead to interesting phenomenological consequences. For an example, the electrons inside the Sun can generate a flavor-dependent long-range potential at the Earth surface, which can enhance νμ and {overline{nu}}_{mu } survival probabilities over a wide range of energies and baselines in atmospheric neutrino experiments. In this paper, we explore in detail the possible impacts of these long-range flavor-diagonal neutral current interactions due to Le − Lμ and Le − Lτ symmetries (one at-a-time) in the context of proposed 50 kt magnetized ICAL detector at INO. Combining the information on muon momentum and hadron energy on an event-by-event basis, ICAL will be sensitive to long-range forces at 90% (3σ) C.L. with 500 kt·yr exposure if the effective gauge coupling αeμ/eτ> 1.2 × 10−53 (1.75 × 10−53). The sensitivity of ICAL towards αeμ (αeτ) is ∼ 46 (53) times better than the existing bound from the Super-Kamiokande experiment at 90% C.L., and at 3σ, the sensitivity of ICAL is comparable to the bounds obtained from the combined solar and KamLAND data.

Improving the hierarchy sensitivity of ICAL using neural network

Atmospheric neutrino experiments can determine the neutrino mass hierarchy for any value of δCP. The Iron Calorimeter (ICAL) detector at the India-based Neutrino Observatory can distinguish between the charged current interactions of νμ and by determining the charge of the produced muon. Hence it is particularly well suited to determine the hierarchy. The hierarchy signature is more prominent in neutrinos with energy of a few GeV and with pathlength of a few thousand kilometers, i.e. neutrinos whose direction is not close to horizontal. We use adaptive neural networks to identify such events with good efficiency and good purity. The hierarchy sensitivity, calculated from these selected events, is above 3σ level.

Erratum: Generalized mass ordering degeneracy in neutrino oscillation experiments [Phys. Rev. D 94 , 055005 (2016)

We consider the impact of neutral-current (NC) non-standard neutrino interactions (NSI) on the determination of the neutrino mass ordering. We show that in presence of NSI there is an exact degeneracy which makes it impossible to determine the neutrino mass ordering and the octant of the solar mixing angle $\theta_{12}$ at oscillation experiments. The degeneracy holds at the probability level and for arbitrary matter density profiles, and hence, solar, atmospheric, reactor, and accelerator neutrino experiments are affected simultaneously. The degeneracy requires order-one corrections from NSI to the NC neutrino--quark interaction and can be tested in neutrino--nucleus NC scattering experiments.