Accelerator Neutrino Research Articles

High-energy and ultra-high-energy neutrinos: A Snowmass white paper

Astrophysical neutrinos are excellent probes of astroparticle physics and high-energy physics. With energies far beyond solar, supernovae, atmospheric, and accelerator neutrinos, high-energy and ultra-high-energy neutrinos probe fundamental physics from the TeV scale to the EeV scale and beyond. They are sensitive to physics both within and beyond the Standard Model through their production mechanisms and in their propagation over cosmological distances. They carry unique information about their extreme non-thermal sources by giving insight into regions that are opaque to electromagnetic radiation. This white paper describes the opportunities astrophysical neutrino observations offer for astrophysics and high-energy physics, today and in coming years.

Design and performance of a scintillation tracker for track matching in nuclear-emulsion-based neutrino interaction measurement

Precise measurement of neutrino–nucleus interactions with an accelerator neutrino beam is highly important for current and future neutrino oscillation experiments. To measure muon-neutrino charged-current interactions with nuclear-emulsion-based hybrid detector, muon track matching among the detectors are essential. We describe the design and performance of a newly developed scintillation tracker for the muon track matching in the neutrino–nucleus interaction measurement with nuclear emulsion detectors. The muon tracks are reconstructed using the scintillation tracker and another detector called Baby MIND, then, they are matched with the tracks in nuclear emulsion detectors.The scintillation tracker consists of four layers of horizontally and vertically aligned scintillator bars, covering an area of 1m×1m. In the layer, 24mm-wide plastic scintillator bars are specially arranged with deliberate gaps between each other. By recognizing the hit pattern of the four layers, a precise positional resolution of 2.5mm is achieved while keeping the number of readout channels as small as 256. The efficiency of the track matching is evaluated to be more than 97% for forward-going muons, and the positional and angular resolutions of the scintillation tracker are 2.5mm and 20–40mrad respectively. The results demonstrate the usefulness of the design of the scintillation tracker for the muon track matching in the nuclear-emulsion-based neutrino–nucleus interaction measurements.

A Review of the Tension between the T2K and NOνA Appearance Data and Hints to New Physics

In this article, we review the status of the tension between the long-baseline accelerator neutrino experiments T2K and NOνA. The tension arises mostly due to the mismatch in the apappearance data of the two experiments. We explain how this tension arises based on νμ→νe and ν¯μ→ν¯e oscillation probabilities. We define the reference point of vacuum oscillation, maximal θ23 and δCP and compute the νe/ν¯e appearance events for each experiment. We then study the effects of deviating the unknown parameters from the reference point and the compatibility of any given set of values of unknown parameters with the data from T2K and NOνA. T2K observes a large excess in the νe appearance event sample compared to the expected νe events at the reference point, whereas NOνA observes a moderate excess. The large excess in T2K dictates that δCP be anchored at −90° and that θ23 > π/4 with a preference for normal hierarchy. The moderate excess at NOνA leads to two degenerate solutions: (a) NH, 0 < δCP < 180°, and θ23 > π/4; (b) IH, −180° < δCP < 0, and θ23 > π/4. This is the main cause of tension between the two experiments. We review the status of three beyond standard model (BSM) physics scenarios, (a) non-unitary mixing, (b) Lorentz invariance violation, and (c) non-standard neutrino interactions, to resolve the tension.

Search for sterile neutrinos by shower events at a future neutrino telescope

Abstract It is pointed out that searching for sterile neutrinos in high energy regions at a future IceCube-like facility has advantages compared with reactor or short baseline accelerator neutrino experiments, in which the size of the detector and energy resolution make it difficult to gain good sensitivity for a large mass-squared difference. In this study we show that it is possible to improve constraints on sterile neutrino mixing θ14 for 1 eV$^2 \lesssim \Delta m_{41}^2\lesssim$ 100 eV2 by looking for a dip in the shower events at an IceCube-like neutrino telescope whose volume is at least 10 times as large as that of IceCube and whose duration is 10 years. We also give an analytic expression for the oscillation probabilities in two cases where the condition of one mass scale dominance is satisfied.

New prospects for iodine detector and Solar neutrinos registration

The neutrino capture cross section σ(E) by 127I nucleus is studied with proper consideration of the nuclear strength function S(E) and its resonance structure. The calculations of giant Gamow-Teller, analog and pigmy resonances are realized based on the framework of the self-consistent finite Fermi systems theory. The results are compared with known experimental (p,n) reaction data that confirmed the significant influence of high-lying resonances on the cross section values. In the case when the energy of incident neutrino is higher than the sum of neutron emission threshold Sn and reaction threshold the 126Xe isotope is produced. Comparison of experimental yields for 127Xe and 126Xe isotopes can give the sensitive indicator of the metallicity for different Solar models. The results allow to confirm the advantages of the iodine detector for detecting hard astrophysical and accelerator neutrinos.

Event reconstruction performance with new retro-reflector based designs for water Cherenkov detectors

We have proposed the possibility of a cost-efficient way to improve the detector performance for water Cherenkov detectors, by reflecting the usually lost light falling between photo-detectors onto the other side of the tank with retro-reflectors. Using a detector simulation based on optical measurements of retro-reflectors, we developed a convolutional neural network-based reconstruction algorithm. Here we report on the reconstruction performance for ring events in the energy scale expected for atmospheric and accelerator neutrinos under various candidate detector configurations.

The Hyper-Kamiokande Experiment

The Hyper-Kamiokande experiment consists of a 260 kt underground water Cherenkov detector with a fiducial volume more than 8 times larger than that of Super-Kamiokande. It will serve both as a far detector of a long-baseline neutrino experiment and an observatory for astrophysical neutrinos and rare decays. The long-baseline neutrino experiment will detect neutrinos originating from the upgraded 1.3 MW neutrino beam produced at the J-PARC accelerator 295 km away. A near detector suite, close to the accelerator, will help characterise the beam and minimise systematic errors. The experiment is now under construction and due to start data taking in 2027. The experiment will investigate neutrino oscillation phenomena (including CP-violation and mass ordering) by studying accelerator, solar and atmospheric neutrinos, neutrino astronomy (solar, supernova, supernova relic neutrinos) and nucleon decays. This paper gives an overview of the Hyper-Kamiokande experiment, its physics goals and the current status.

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.

Unraveling the flux-averaged neutrino–nucleus cross section

The interpretation of the nuclear cross sections measured using accelerator neutrino beams involve severe difficulties, arising primarily from the average over the incoming neutrino flux. The broad energy distribution of the beam particles hampers the determination of the energy transfer to the nuclear target, the knowledge of which is needed to pin down the dominant reaction mechanism. Overcoming this problem requires the development of a theoretical approach suitable to describe neutrino interactions at energies ranging from hundreds of MeV to few GeV. In this paper, it is argued that the approach based on the factorisation of the nuclear cross section provides a consistent framework for the calculation of neutrino-nucleus interactions in both the quasi elastic and inelastic channels. The near-degeneracy between theoretical models based on different assumptions, and the use of electron scattering data to advance the understanding of neutrino-nucleus cross sections are also discussed.

Looking for Lorentz invariance violation (LIV) in the latest long baseline accelerator neutrino oscillation data

In this paper, we have analysed the latest data from NOnu A and T2K with the Lorentz invariance violation along with the standard oscillation hypothesis. We have found that the NOnu A data cannot distinguish between the two hypotheses at 1, sigma confidence level. T2K data and the combined data analysis excludes standard oscillation at 1, sigma . All three cases do not have any hierarchy sensitivity when analysed with LIV. There is a mild tension between the two experiments, when analysed with LIV, as {theta _{23}} at NOnu A best-fit is at higher octant but the same for T2K is at lower octant. The present data from accelerator neutrino long baseline experiments lose octant determination sensitivity when analysed with LIV. The tension between the two experiments is also reduced when the data are analysed with LIV.

Sensitivity of a search for eV-scale sterile neutrinos with 8 years of IceCube DeepCore data

Several anomalies in reactor and accelerator neutrino experiments motivate the search for sterile neutrinos with squared mass splittings at the eV-scale. We present an analysis that probes the elements Uμ4 and Uτ4 of the extended (3+1) neutrino mixing matrix using an 8 year data sample from IceCube's DeepCore sub-array containing atmospheric neutrinos between 5 and 300 GeV. We show the expected sensitivities of the analysis to |Uμ4|2 and |Uτ4|2 under the assumption of mass splittings between 1 and 100 eV2, and to |Uμ4|2 for mass splittings below 1 eV2. We specifically discuss how we overcame the challenges of efficiently calculating neutrino oscillation probabilities in the presence of very fast neutrino oscillations involving large Δ m2.

Simulating low-energy neutrino interactions with MARLEY

Monte Carlo event generators are a critical tool for the interpretation of data obtained by neutrino experiments. Several modern event generators are available which are well-suited to the GeV energy scale used in studies of accelerator neutrinos. However, theoretical modeling differences make their immediate application to lower energies difficult. In this paper, I present a new event generator, MARLEY, which is designed to better address the simulation needs of the low-energy (tens of MeV and below) neutrino community. The code is written in C++14 with an optional interface to the popular ROOT data analysis framework. The current release of MARLEY (version 1.2.0) emphasizes simulations of the reaction 40Ar(νe,e−)40K⁎ but is extensible to other channels with suitable user input. This paper provides detailed documentation of MARLEY's implementation and usage, including guidance on how generated events may be analyzed and how MARLEY may be interfaced with external codes such as Geant4. Further information about MARLEY is available on the official website at http://www.marleygen.org. Program summaryProgram title:MARLEY 1.2.0CPC Library link to program files:https://doi.org/10.17632/4v7zxnc8j3.1Developer's respository link:http://github.com/MARLEY-MC/marleyCode Ocean capsule:https://codeocean.com/capsule/9868179Licensing provisions: GNU General Public License 3.0Programming language: C++14External routines/libraries used: GNU Scientific Library [1,2] (required), ROOT [3,4] (optional)Nature of problem: Simulation of neutrino-nucleus scattering events at energies of tens-of-MeV and belowSolution method: Initial two-to-two scattering kinematics are sampled using the allowed approximation differential cross section and tables of precomputed nuclear matrix elements. Subsequent de-excitations of the remnant nucleus are simulated using a Monte Carlo implementation of the Hauser-Feshbach statistical model and tabulated γ-ray decay schemes for discrete nuclear levels.Additional comments including restrictions and unusual features: Input data are provided with the code that are suitable for producing simulations of the charged-current reaction 40Ar(νe,e−)40K⁎, coherent elastic neutrino-nucleus scattering on spin-zero target nuclei, and neutrino-electron elastic scattering on any atomic target. Preparation of new reaction input files (whose format is documented in Appendix B) would enable other reaction channels and nuclear targets to be handled by the existing code framework. Although there is no maximum neutrino energy enforced by the code itself, realistic neutrino-nucleus scattering events may be generated up to roughly 50 MeV. Above this energy, the effects of forbidden nuclear transitions, which are neglected in the current treatment of the cross sections (see section 2.1), become increasingly important.

Probing source and detector nonstandard interaction parameters at the DUNE near detector

We investigate the capability of the DUNE near detector (ND) to constrain nonstandard interaction parameters (NSIs) describing the production of neutrinos $({ϵ}_{\ensuremath{\alpha}\ensuremath{\beta}}^{s})$ and their detection $({ϵ}_{\ensuremath{\alpha}\ensuremath{\beta}}^{d})$. We show that the DUNE ND is able to reject a large portion of the parameter space allowed by DUNE far detector analyses and to set the most stringent bounds from accelerator neutrino experiments on $|{ϵ}_{\ensuremath{\mu}e}^{s,d}|$ for wide intervals of the related phases. We also provide simple analytic understanding of our results as well as a numerical study of their dependence on the systematic errors, showing that the DUNE ND offers a clean environment to study source and detector NSIs.

Flavors of astrophysical neutrinos with active-sterile mixing

We revisit the flavor composition of high-energy astrophysical neutrinos observed at neutrino telescopes. Assuming unitary time evolution of the neutrino flavor states, the flavor composition observable at Earth is related to the initial composition at their sources via oscillation-averaged flavor transitions. In a previous study we derived general bounds on the flavor composition of TeV–PeV astrophysical neutrinos assuming three-flavor unitary mixing. We extend these bounds to the case of active-sterile neutrino mixing. Our bounds are analytical, derived based only on the unitarity of the mixing, and do not require sampling over the values of the unknown active-sterile mixing parameters. These bounds apply to any extended active-sterile neutrino mixing scenario where energy-dependent nonstandard flavor mixing dominates over the standard mixing observed in accelerator, reactor, and atmospheric neutrino oscillations.

Matter versus vacuum oscillations at long-baseline accelerator neutrino experiments

The neutrino oscillation probabilities at the long-baseline accelerator neutrino experiments are expected to be modified by matter effects. We search for evidence of such modification in the data of T2K and NO[Formula: see text]A, by fitting the data to the hypothesis of (a) matter modified oscillations and (b) vacuum oscillations. We find that vacuum oscillations provide as good a fit to the data as matter modified oscillations. Even extended runs of T2K and NO[Formula: see text]A, with five years in neutrino mode [Formula: see text] and five years in anti-neutrino mode [Formula: see text], cannot make a [Formula: see text] distinction between vacuum and matter modified oscillations. The future experiment DUNE, with neutrino and anti-neutrino runs of five years each [Formula: see text], can rule out vacuum oscillations by itself at [Formula: see text] if the hierarchy is normal. If the hierarchy is inverted, a [Formula: see text] discrimination against vacuum oscillations requires the combination of [Formula: see text] runs of T2K, NO[Formula: see text]A and DUNE.

Non-standard interactions in SMEFT confronted with terrestrial neutrino experiments

The Standard Model Effective Field Theory (SMEFT) provides a systematic and model-independent framework to study neutrino non-standard interactions (NSIs). We study the constraining power of the on-going neutrino oscillation experiments T2K, NOνA, Daya Bay, Double Chooz and RENO in the SMEFT framework. A full consideration of matching is provided between different effective field theories and the renormalization group running at different scales, filling the gap between the low-energy neutrino oscillation experiments and SMEFT at the UV scale. We first illustrate our method with a top- down approach in a simplified scalar leptoquark model, showing more stringent constraints from the neutrino oscillation experiments compared to collider studies. We then provide a bottom-up study on individual dimension-6 SMEFT operators and find NSIs in neutrino experiments already sensitive to new physics at ∼20 TeV when the Wilson coefficients are fixed at unity. We also investigate the correlation among multiple operators at the UV scale and find it could change the constraints on SMEFT operators by several orders of magnitude compared with when only one operator is considered. Furthermore, we find that accelerator and reactor neutrino experiments are sensitive to different SMEFT operators, which highlights the complementarity of the two experiment types.

Form factors of the nucleon axial current

A symmetry-preserving Poincaré-covariant quark+diquark Faddeev equation treatment of the nucleon is used to deliver parameter-free predictions for the nucleon's axial and induced pseudoscalar form factors, GA and GP, respectively. The result for GA can reliably be represented by a dipole form factor characterised by an axial charge gA=GA(0)=1.25(3) and a mass-scale MA=1.23(3)mN, where mN is the nucleon mass; and regarding GP, the induced pseudoscalar charge gp⁎=8.80(23), the ratio gp⁎/gA=7.04(22), and the pion pole dominance Ansatz is found to provide a reliable estimate of the directly computed result. The ratio of flavour-separated quark axial charges is also calculated: gAd/gAu=−0.16(2). This value expresses a marked suppression of the size of the d-quark component relative to that found in nonrelativistic quark models and owes to the presence of strong diquark correlations in the nucleon Faddeev wave function – both scalar and axial-vector, with the scalar diquark being dominant. The predicted form for GA should provide a sound foundation for analyses of the neutrino-nucleus and antineutrino-nucleus cross-sections that are relevant to modern accelerator neutrino experiments.

Design and Diagnostics of High-Precision Accelerator Neutrino Beams

Neutrino oscillation physics has entered a new precision era, which poses major challenges to the level of control and diagnostics of the neutrino beams. In this paper, we review the design of high-precision beams, their current limitations, and the latest techniques envisaged to overcome such limits. We put emphasis on “monitored neutrino beams” and advanced diagnostics to determine the flux and flavor of the neutrinos produced at the source at the per-cent level. We also discuss ab-initio measurements of the neutrino energy–i.e., measurements performed without relying on the event reconstruction at the ν detector–to remove any flux induced bias in the determination of the cross sections.

Neutrino oscillation constraints on U(1)′ models: from non-standard interactions to long-range forces

We quantify the effect of gauge bosons from a weakly coupled lepton flavor dependent U(1)′ interaction on the matter background in the evolution of solar, atmospheric, reactor and long-baseline accelerator neutrinos in the global analysis of oscillation data. The analysis is performed for interaction lengths ranging from the Sun-Earth distance to effective contact neutrino interactions. We survey ∼ 10000 set of models characterized by the six relevant fermion U(1)′ charges and find that in all cases, constraints on the coupling and mass of the Z′ can be derived. We also find that about 5% of the U(1)′ model charges lead to a viable LMA-D solution but this is only possible in the contact interaction limit. We explicitly quantify the constraints for a variety of models including mathrm{U}{(1)}_{B-3{L}_e} , mathrm{U}{(1)}_{B-3{L}_{mu }} , mathrm{U}{(1)}_{B-3{L}_{tau }} , mathrm{U}{(1)}_{B-frac{3}{2}left({L}_{mu }+{L}_{tau}right)} , mathrm{U}{(1)}_{L_e-{L}_{mu }} , mathrm{U}{(1)}_{L_e-{L}_{tau }} , mathrm{U}{(1)}_{L_e-frac{1}{2}left({L}_{mu }+{L}_{tau}right)} . We compare the constraints imposed by our oscillation analysis with the strongest bounds from fifth force searches, violation of equivalence principle as well as bounds from scattering experiments and white dwarf cooling. Our results show that generically, the oscillation analysis improves over the existing bounds from gravity tests for Z′ lighter than ∼ 10−8→ 10−11 eV depending on the specific couplings. In the contact interaction limit, we find that for most models listed above there are values of g′ and MZ′ for which the oscillation analysis provides constraints beyond those imposed by laboratory experiments. Finally we illustrate the range of Z′ and couplings leading to a viable LMA-D solution for two sets of models.

Axial and pseudoscalar form factors from charged current quasielastic neutrino-nucleon scattering

We study the scattering of neutrinos on polarized and unpolarized free nucleons, and also the polarization of recoil particles in these scatters. In contrast to electromagnetic processes, the parity-violating weak interaction gives rise to large spin asymmetries at leading order. Future polarization measurements could provide independent access to the proton axial structure and allow the first extraction of the pseudoscalar form factor from neutrino data without the conventional partially conserved axial current (PCAC) ansatz and assumptions about the pion-pole dominance. The pseudoscalar form factor can be accessed with precise measurements with muon (anti)neutrinos of a few hundreds $\mathrm{MeV}$ of energy or with tau (anti)neutrinos. The axial form factor can be extracted from scattering measurements using accelerator neutrinos of all energies.