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eV-scale sterile neutrino: A window open to non-unitarity?
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Abstract
An excess observed in the accelerator neutrino experiments in the channel at high confidence level (CL) has been interpreted as due to eV-scale sterile neutrino(s). But, it has been suffered from the problem of ``appearance - disappearance tension'' at the similarly high CL because the measurements of the channel do not observe the expected event number depletion corresponding to the sterile contribution in the appearance channel. We suggest non-unitarity as a simple and natural way of resolving the tension, which leads us to construct the non-unitary (3+1) model. With reasonable estimation of the parameters governing non-unitarity, we perform an illustrative analysis to know if the tension is resolved in this model. At the best fit of the appearance signature we have found the unique solution with , which is consistent with the (reactors + Ga) data combined fit. Unexpectedly, our tension-easing mechanism bridges between the two high CL signatures, the BEST and LSND-MiniBooNE anomalies.
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A bstract
In the ν SM extended by adding an eV-scale sterile state, the (3 + 1) model, the sterile-active level crossing entails the MSW resonance, here referred as the sterile-active (SA) resonance. In this paper, we construct an effective theory of SA resonance which involves only the sterile-active mixing angles and ∆ m 41 2 , thanks to the given environment of high matter potential which freezes the ν SM oscillations. We give our first attempt at an analytic treatment of the effective theory to illuminate the global picture of the SA resonance at a glance. We formulate a perturbative framework in which the structure of “texture zeros” of the S matrix in the flavor space and the suppression by the small parameters sin θ j 4 ( j = 1 , 2 , 3) allows us to reveal the flavor–event-type hierarchy of the resonance-effect strength in the probabilities. We have shown that the cascade events dominantly comes from the three paths through P ( ν e → ν e ), P ( ν ¯ e → ν ¯ e ), and P ( ν ¯ μ → ν ¯ τ ), and a three-component fit is suggested to disentangle the SA resonance generation mechanisms.
We provide supporting details for the search for a 3 + 1 sterile neutrino using data collected over 10.7 years at the IceCube Neutrino Observatory. The analysis uses atmospheric muon-flavored neutrinos from 0.5 to 100 TeV that traverse Earth to reach the IceCube detector and finds a best-fit point at sin 2 ( 2 θ 24 ) = 0.16 and Δ m 41 2 = 3.5 eV 2 with a goodness-of-fit p value of 12% and consistency with the null hypothesis of no oscillations to sterile neutrinos with a p value of 3.1%. Several improvements were made over past analyses, which are reviewed in this article, including upgrades to the reconstruction and the study of sources of systematic uncertainty. We provide details of the fit quality and discuss stability tests that split the data for separate samples, comparing results. We find that the fits are consistent between split datasets.
Published by the American Physical Society 2024
This Letter presents the result of a 3 + 1 sterile neutrino search using 10.7 yr of IceCube data. We analyze atmospheric muon neutrinos that traverse the Earth with energies ranging from 0.5 to 100 TeV, incorporating significant improvements in modeling neutrino flux and detector response compared to earlier studies. Notably, for the first time, we categorize data into starting and throughgoing events, distinguishing neutrino interactions with vertices inside or outside the instrumented volume, to improve energy resolution. The best-fit point for a 3 + 1 model is found to be at sin 2 ( 2 θ 24 ) = 0.16 and Δ m 41 2 = 3.5 eV 2 , which agrees with previous iterations of this Letter. The result is consistent with the null hypothesis of no sterile neutrinos with a p value of 3.1%.
Published by the American Physical Society 2024
Recent results from the DANSS experiment on the searches for sterile neutrinos are presented. They are based on approximately 5.5 million inverse beta decay events collected at 10.9, 11.9 and 12.9m from the reactor core of the 3.1 GW Kalinin Nuclear Power Plant in Russia. We do not see any statistically significant sign for the ν̄e oscillations. The exclusion area in the sterile neutrino oscillation parameters Δm412, sin22θee goes up to sin22θee<0.005 in the most sensitive region in the reactor model independent study. A comparison of the absolute neutrino event rate and reactor models predictions allowed to exclude practically the whole sterile neutrino parameter space preferred by the BEST experiment as well as the best fit point of the Neutrino-4 experiment. Studies of the positron energy spectrum in the antineutrino inverse beta decay reaction and its dependence on the fuel composition are also presented. The ratio of the cross sections for 235U and 239Pu was measured. Reactor power was measured remotely using antineutrinos with 1.3% accuracy in 3 days during 6.5 years.
A bstract
This article reports global fits of short-baseline neutrino data to oscillation models involving light sterile neutrinos. In the commonly-used 3+1 plane wave model, there is a well-known 4.9 σ tension between data sets sensitive to appearance versus disappearance of neutrinos. We find that models that damp the oscillation prediction for the reactor data sets, especially at low energy, substantially improve the fits and reduce the tension. We consider two such scenarios. The first scenario introduces the quantum mechanical wavepacket effect that accounts for the source size in reactor experiments into the 3+1 model. We find that inclusion of the wavepacket effect greatly improves the overall fit compared to a three-neutrino model by ∆ χ ² / dof = 61 . 1 / 4 (7 . 1 σ improvement) with best-fit ∆ m ² = 1 . 4 eV ² and wavepacket length of 67 fm. The internal tension is reduced to 3.4 σ . If reactor-data only is fit, then the wavepacket preferred length is 91 fm ( > 20 fm at 99% CL). The second model introduces oscillations involving sterile flavor and allows the decay of the heaviest, mostly sterile mass state, ν 4 . This model introduces a damping term similar to the wavepacket effect, but across all experiments. Compared to a three-neutrino fit, this has a ∆ χ ² / dof = 60 . 6 / 4 (7 σ improvement) with preferred ∆ m ² = 1 . 4 eV ² and decay Γ = 0 . 35 eV. The internal tension is reduced to 3.7 σ .
For many years, the reactor event rates have been observed to have structure that deviates from prediction. Community discussion has focused on an excess compared to prediction observed at 5 MeV; however, other deviations are apparent. This structure has L dependence that is well-fit by the damped models. Before assuming this points to new physics, we urge closer examination of systematic effects that could lead to this L dependence.
The LUX-ZEPLIN experiment is a dark matter detector centered on a dual-phase xenon time projection chamber operating at the Sanford Underground Research Facility in Lead, South Dakota, USA. This Letter reports results from LUX-ZEPLIN’s first search for weakly interacting massive particles (WIMPs) with an exposure of 60 live days using a fiducial mass of 5.5 t. A profile-likelihood ratio analysis shows the data to be consistent with a background-only hypothesis, setting new limits on spin-independent WIMP-nucleon, spin-dependent WIMP-neutron, and spin-dependent WIMP-proton cross sections for WIMP masses above 9 GeV/c2. The most stringent limit is set for spin-independent scattering at 36 GeV/c2, rejecting cross sections above 9.2×10−48 cm at the 90% confidence level.
We report on the first search for nuclear recoils from dark matter in the form of weakly interacting massive particles (WIMPs) with the XENONnT experiment, which is based on a two-phase time projection chamber with a sensitive liquid xenon mass of 5.9 ton. During the (1.09±0.03) ton yr exposure used for this search, the intrinsic Kr85 and Rn222 concentrations in the liquid target are reduced to unprecedentedly low levels, giving an electronic recoil background rate of (15.8±1.3) events/ton yr keV in the region of interest. A blind analysis of nuclear recoil events with energies between 3.3 and 60.5 keV finds no significant excess. This leads to a minimum upper limit on the spin-independent WIMP-nucleon cross section of 2.58×1047 cm2 for a WIMP mass of 28 GeV/c2 at 90% confidence level. Limits for spin-dependent interactions are also provided. Both the limit and the sensitivity for the full range of WIMP masses analyzed here improve on previous results obtained with the XENON1T experiment for the same exposure.
We study the online detection of gallium capture of monoenergetic neutrinos produced by a Cr51 radioactive source in a scintillation experiment. We find that cerium-doped gadolinium aluminum gallium garnet (Ce:GAGG) is a suitable scintillator which contains about 21% of gallium per weight and has a high mass density and light yield. Combined with a highly efficient light detection system this allows tagging of the subsequent germanium decay and thus a clean distinction of gallium capture and elastic neutrino electron scattering events. With 1.5 tons of scintillator and 10 source runs of 3.4 MCi, each, we obtain about 1700 gallium capture events with a purity of 90% and 680,000 neutrino electron scattering events, where the latter provide a precise normalization independent of any nuclear physics. We include a detailed discussion of backgrounds and find that this configuration would allow us to test the gallium anomaly at more than 5σ in an independent way.
A bstract
The significance of the Gallium Anomaly, from the BEST, GALLEX, and SAGE radioactive source experiments, is quantified using different theoretical calculations of the neutrino detection cross section, and its explanation due to neutrino oscillations is compared with the bounds from the analyses of reactor rate and spectral ratio data, β -decay data, and solar neutrino data. In the 3+1 active-sterile neutrino mixing scheme, the Gallium Anomaly is in strong tension with the individual and combined bounds of these data sets. In the combined scenario with all available data, the parameter goodness of fit is below 0.042%, corresponding to a severe tension of 4–5 σ , or stronger. Therefore, we conclude that one should pursue other possible solutions beyond short-baseline oscillations for the Gallium Anomaly. We also present a new global fit of ν e and ν ¯ e disappearance data, showing that there is a 2.6–3.3 σ preference in favor of short-baseline oscillations, which is driven by an updated analysis of reactor spectral ratio data.
We present a measurement of ν_{e} interactions from the Fermilab Booster Neutrino Beam using the MicroBooNE liquid argon time projection chamber to address the nature of the excess of low energy interactions observed by the MiniBooNE Collaboration. Three independent ν_{e} searches are performed across multiple single electron final states, including an exclusive search for two-body scattering events with a single proton, a semi-inclusive search for pionless events, and a fully inclusive search for events containing all hadronic final states. With differing signal topologies, statistics, backgrounds, reconstruction algorithms, and analysis approaches, the results are found to be either consistent with or modestly lower than the nominal ν_{e} rate expectations from the Booster Neutrino Beam and no excess of ν_{e} events is observed.
We discuss correlations between the νSM CP phase δ and the phases that originate from new physics which causes neutrino-sector unitarity violation (UV) at low energies. This study is motivated to provide one of the building pieces for a machinery to diagnose non-unitarity, our ultimate goal. We extend the perturbation theory of neutrino oscillation in matter proposed by Denton et al. (DMP) to include the UV effect expressed by the α parametrization. By analyzing the DMP-UV perturbation theory to first order, we are able to draw a completed picture of the δ - UV phase correlations in the whole kinematical region covered by the terrestrial neutrino experiments. There exist the two regions with the characteristically different patterns of the correlations: (1) the chiral-type [e−iδαμe, e−iδατe, ατμ] (PDG convention) correlation in the entire high-energy region |ρE| ≳ 6 (g/cm3) GeV, and (2) (blobs of the α parameters) - e±iδ correlation in anywhere else. Some relevant aspects for measurement of the UV parameters, such as the necessity of determining all the αβγ elements at once, are also pointed out.
A bstract
We evaluate the statistical significance of the 3+1 sterile-neutrino hypothesis using ν e and ν ¯ e disappearance data from reactor, solar and gallium radioactive source experiments. Concerning the latter, we investigate the implications of the recent BEST results. For reactor data we focus on relative measurements independent of flux predictions. For the problem at hand, the usual χ ² -approximation to hypothesis testing based on Wilks’ theorem has been shown in the literature to be inaccurate. We therefore present results based on Monte Carlo simulations, and find that this typically reduces the significance by roughly 1 σ with respect to the naïve expectation. We find no significant indication in favor of sterile-neutrino oscillations from reactor data. On the other hand, gallium data (dominated by the BEST result) show more than 5 σ of evidence supporting the sterile-neutrino hypothesis, favoring oscillation parameters in agreement with constraints from reactor data. This explanation is, however, in significant tension (∼ 3 σ ) with solar neutrino experiments. In order to assess the robustness of the signal for gallium experiments we present a discussion of the impact of cross-section uncertainties on the results.
We investigate the sensitivity of solar neutrino data to mixing of sterile neutrinos with masses ≳ eV. For current data, we perform a Feldman–Cousins analysis to derive a robust limit on the sterile neutrino mixing. The solar neutrino limit excludes significant regions of the parameter space relevant to hints from reactor and radioactive gallium source experiments. We then study the sensitivity of upcoming solar neutrino data, most notably elastic neutrino-electron scattering in the DARWIN and DUNE experiments as well as coherent neutrino-nucleus scattering in DARWIN. These high precision measurements will increase the sensitivity to sterile neutrino mixing by about a factor of 4.5 compared to present limits. As a by-product, we introduce a simplified solar neutrino analysis using only four data points: the low- and high-energy ν e survival and transition probabilities. We show that this simplified analysis is in excellent agreement with a full solar neutrino analysis; it is very easy to handle numerically and can be applied to any new physics model in which the energy dependence of the ν e transition probabilities is not significantly modified.
We report the first dark matter search results using the commissioning data from PandaX-4T. Using a time projection chamber with 3.7 tonne of liquid xenon target and an exposure of 0.63 tonne·year, 1058 candidate events are identified within an approximate nuclear recoil energy window between 5 and 100 keV. No significant excess over background is observed. Our data set a stringent limit to the dark matter–nucleon spin-independent interactions, with a lowest excluded cross section (90% C.L.) of 3.8×10−47 cm2 at a dark matter mass of 40 GeV/c2.
Nonunitary neutrino mixing in the light neutrino sector is a direct consequence of type-I seesaw neutrino mass models. In these models, light neutrino mixing is described by a submatrix of the full lepton mixing matrix and, then, it is not unitary in general. In consequence, neutrino oscillations are characterized by additional parameters, including new sources of violation. Here we perform a combined analysis of short and long-baseline neutrino oscillation data in this extended mixing scenario. We did not find a significant deviation from unitary mixing, and the complementary data sets have been used to constrain the nonunitarity parameters. We have also found that the T2K and NOvA tension in the determination of the Dirac -phase is not alleviated in the context of nonunitary neutrino mixing.
The experiment Neutrino-4 started in 2014 with a detector model and continued with a full-scale detector in 2016–2021. In this article, we describe all the steps of the preparatory work on this experiment. We present all results of the Neutrino-4 experiment with increased statistical accuracy provided to date. The experimental setup is constructed to measure the flux and spectrum of the reactor antineutrinos as a function of distance to the center of the active zone of the SM-3 reactor (Dimitrovgrad, Russia) in the range of 6–12 meters. Using all the collected data, we performed a model-independent analysis to determine the oscillation parameters Δm142 and sin22θ14. The method of coherent summation of measurement results allows us to directly demonstrate the oscillation effect. We present the analysis of possible systematic errors and the MC model of the experiment, which considers the possibility of the effect manifestation at the present precision level. As a result of the analysis, we can conclude that at currently available statistical accuracy, we observe the oscillations at the 2.9σ level with the parameters Δm142=(7.3±0.13st±1.16syst) eV2=(7.3±1.17) eV2 and sin22θ=0.36±0.12stat(2.9σ). Monte Carlo based statistical analysis gave an estimation of the confidence level at 2.7σ. We plan to improve the currently working experimental setup and create a completely new setup in order to increase the accuracy of the experiment by 3 times. We also provide a brief analysis of the general experimental situation in the search for sterile neutrinos.
A bstract
We study the capabilities of the DUNE near detector to probe deviations from unitarity of the leptonic mixing matrix, the 3+1 sterile formalism and Non-Standard Interactions affecting neutrino production and detection. We clarify the relation and possible mappings among the three formalisms at short-baseline experiments, and we add to current analyses in the literature the study of the ν μ → ν τ appearance channel. We study in detail the impact of spectral uncertainties on the sensitivity to new physics using the DUNE near detector, which has been widely overlooked in the literature. Our analysis shows that this plays an important role on the results and, in particular, that it can lead to a strong reduction in the sensitivity to sterile neutrinos from ν μ → ν e transitions, by more than two orders of magnitude. This stresses the importance of a joint experimental and theoretical effort to improve our understanding of neutrino nucleus cross sections, as well as hadron production uncertainties and beam focusing effects. Nevertheless, even with our conservative and more realistic implementation of systematic uncertainties, we find that an improvement over current bounds in the new physics frameworks considered is generally expected if spectral uncertainties are below the 5% level.
We present a detailed report on sterile neutrino oscillation and U235 ν¯e energy spectrum measurement results from the PROSPECT experiment at the highly enriched High Flux Isotope Reactor (HFIR) at Oak Ridge National Laboratory. In 96 calendar days of data taken at an average baseline distance of 7.9 m from the center of the 85 MW HFIR core, the PROSPECT detector has observed more than 50,000 interactions of ν¯e produced in beta decays of U235 fission products. New limits on the oscillation of ν¯e to light sterile neutrinos have been set by comparing the detected energy spectra of ten reactor-detector baselines between 6.7 and 9.2 meters. Measured differences in energy spectra between baselines show no statistically significant indication of ν¯e to sterile neutrino oscillation and disfavor the reactor antineutrino anomaly best-fit point at the 2.5σ confidence level. The reported U235 ν¯e energy spectrum measurement shows excellent agreement with energy spectrum models generated via conversion of the measured U235 beta spectrum, with a χ2/d.o.f. of 31/31. PROSPECT is able to disfavor at 2.4σ confidence level the hypothesis that U235 ν¯e are solely responsible for spectrum discrepancies between model and data obtained at commercial reactor cores. A data-model deviation in PROSPECT similar to that observed by commercial core experiments is preferred with respect to no observed deviation, at a 2.2σ confidence level.
A bstract
The unitarity of the lepton mixing matrix is a critical assumption underlying the standard neutrino-mixing paradigm. However, many models seeking to explain the as-yet-unknown origin of neutrino masses predict deviations from unitarity in the mixing of the active neutrino states. Motivated by the prospect that future experiments may provide a precise measurement of the lepton mixing matrix, we revisit current constraints on unitarity violation from oscillation measurements and project how next-generation experiments will improve our current knowledge. With the next-generation data, the normalizations of all rows and columns of the lepton mixing matrix will be constrained to ≲10% precision, with the e -row best measured at ≲1% and the τ -row worst measured at ∼ 10% precision. The measurements of the mixing matrix elements themselves will be improved on average by a factor of 3. We highlight the complementarity of DUNE, T2HK, JUNO, and IceCube Upgrade for these improvements, as well as the importance of ν τ appearance measurements and sterile neutrino searches for tests of leptonic unitarity.
We formulate a perturbative framework for the flavor transformation of the standard active three neutrinos but with a non-unitary flavor mixing matrix, a system which may be relevant for the leptonic unitarity test. We use the parametrization of the non-unitary matrix and take its elements () and the ratio as the small expansion parameters. Two qualitatively new features that hold in all the oscillation channels are uncovered in the probability formula obtained to first order in the expansion: (1) The phases of the complex elements always come into the observable in the particular combination with the SM CP phase in the form under the Particle Data Group convention of a unitary SM mixing matrix. (2) The diagonal parameters appear in particular combinations and , where a and b denote, respectively, the matter potential due to charged current and neutral current reactions. This property holds only in the unitary evolution part of the probability, and there is no such feature in the genuine non-unitary part, while the – parameter phase correlation exists for both. The reason for such remarkable stability of the phase correlation is discussed.
The STEREO experiment is a very short baseline reactor antineutrino experiment. It is designed to test the hypothesis of light sterile neutrinos being the cause of a deficit of the observed antineutrino interaction rate at short baselines with respect to the predicted rate, known as the reactor antineutrino anomaly. The STEREO experiment measures the antineutrino energy spectrum in six identical detector cells covering baselines between 9 and 11 m from the compact core of the ILL research reactor. In this article, results from 179 days of reactor turned on and 235 days of reactor turned off are reported at a high degree of detail. The current results include improvements in the modelling of detector optical properties and the γ-cascade after neutron captures by gadolinium, the treatment of backgrounds, and the statistical method of the oscillation analysis. Using a direct comparison between antineutrino spectra of all cells, largely independent of any flux prediction, we find the data compatible with the null oscillation hypothesis. The best-fit point of the reactor antineutrino anomaly is rejected at more than 99.9% C.L.
The MiniBooNE experiment at Fermilab reports results from an analysis of νe appearance data from 12.84×1020 protons on target in neutrino mode, an increase of approximately a factor of 2 over previously reported results. A νe charged-current quasielastic event excess of 381.2±85.2 events (4.5σ) is observed in the energy range 200<EνQE<1250 MeV. Combining these data with the ν¯e appearance data from 11.27×1020 protons on target in antineutrino mode, a total νe plus ν¯e charged-current quasielastic event excess of 460.5±99.0 events (4.7σ) is observed. If interpreted in a two-neutrino oscillation model, νμ→νe, the best oscillation fit to the excess has a probability of 21.1%, while the background-only fit has a χ2 probability of 6×10−7 relative to the best fit. The MiniBooNE data are consistent in energy and magnitude with the excess of events reported by the Liquid Scintillator Neutrino Detector (LSND), and the significance of the combined LSND and MiniBooNE excesses is 6.0σ. A two-neutrino oscillation interpretation of the data would require at least four neutrino types and indicate physics beyond the three neutrino paradigm. Although the data are fit with a two-neutrino oscillation model, other models may provide better fits to the data.
A bstract
We present a comprehensive study of the three-active plus N sterile neutrino model as a framework for constraining leptonic unitarity violation induced at energy scales much lower than the electroweak scale. We formulate a perturbation theory with expansion in small unitarity violating matrix element W while keeping (non- W suppressed) matter effect to all orders. We show that under the same condition of sterile state masses 0.1 eV ² ≲ m J ² ≲ (1–10) GeV ² as in vacuum, assuming typical accelerator based long-baseline neutrino oscillation experiment, one can derive a very simple form of the oscillation probability which consists only of zeroth-order terms with the unique exception of probability leaking term C αβ of O ( W ⁴ ). We argue, based on our explicit computation to fourth-order in W , that all the other terms are negligibly small after taking into account the suppression due to the mass condition for sterile states, rendering the oscillation probability sterile - sector model independent . Then, we identify a limited energy region in which this suppression is evaded and the effects of order W ² corrections may be observable. Its detection would provide another way, in addition to detecting C αβ , to distinguish between low-scale and high-scale unitarity violation. We also solve analytically the zeroth-order system in matter with uniform density to provide a basis for numerical evaluation of non-unitary neutrino evolution.
A search for mixing between active neutrinos and light sterile neutrinos has been performed by looking for muon neutrino disappearance in two detectors at baselines of 1.04 km and 735 km, using a combined MINOS and MINOS+ exposure of protons-on-target. A simultaneous fit to the charged-current muon neutrino and neutral-current neutrino energy spectra in the two detectors yields no evidence for sterile neutrino mixing using a 3+1 model. The most stringent limit to date is set on the mixing parameter for most values of the sterile neutrino mass-splitting eV.
The simplest Standard Model extension to explain neutrino masses involves the addition of right-handed neutrinos. At some level, this extension will impact neutrino oscillation searches. In this work we explore the differences and similarities between the case in which these neutrinos are kinematically accessible (sterile neutrinos) or not (mixing matrix non-unitarity). We clarify apparent inconsistencies in the present literature when using different parametrizations to describe these effects and recast both limits in the popular neutrino non-standard interaction (NSI) formal- ism. We find that, in the limit in which sterile oscillations are averaged out at the near detector, their effects at the far detector coincide with non-unitarity at leading order, even in presence of a matter potential. We also summarize the present bounds existing in both limits and compare them with the expected sensitivities of near-future facilities taking the DUNE proposal as a bench- mark. We conclude that non-unitarity effects are too constrained to impact present or near future neutrino oscillation facilities but that sterile neutrinos can play an important role at long baseline experiments. The role of the near detector is also discussed in detail.
If leptonic unitarity is violated by new physics at an energy scale much lower than the electroweak scale, which we call low-scale unitarity violation, it has different characteristic features from those expected in unitarity violation at high-energy scales. They include maintaining flavor universality and absence of zero-distance flavor transition. We present a framework for testing such unitarity violation at low energies by neutrino oscillation experiments. Starting from the unitary 3 active plus N (arbitrary integer) sterile neutrino model we show that by restricting the active-sterile and sterile-sterile neutrino mass squared differences to 0.1 eV the oscillation probability in the (3+N) model becomes insensitive to details of the sterile sector, providing a nearly model-independent framework for testing low-scale unitarity violation. Yet, the presence of the sterile sector leaves trace as a constant probability leaking term, which distinguishes low-scale unitarity violation from the high-scale one. The non-unitary mixing matrix in the active neutrino subspace is common for the both cases. We analyze how severely the unitarity violation can be constrained in -row by taking a JUNO-like setting to simulate medium baseline reactor experiments. Possible modification of the features of the (3+N) model due to matter effect is discussed to first order in the matter potential.
The main objectives of the KM3NeT Collaboration are (i) the discovery and subsequent observation of high-energy neutrino sources in the Universe and (ii) the determination of the mass hierarchy of neutrinos. These objectives are strongly motivated by two recent important discoveries, namely: (1) the high-energy astrophysical neutrino signal reported by IceCube and (2) the sizable contribution of electron neutrinos to the third neutrino mass eigenstate as reported by Daya Bay, Reno and others. To meet these objectives, the KM3NeT Collaboration plans to build a new Research Infrastructure consisting of a network of deep-sea neutrino telescopes in the Mediterranean Sea. A phased and distributed implementation is pursued which maximises the access to regional funds, the availability of human resources and the synergistic opportunities for the Earth and sea sciences community. Three suitable deep-sea sites are selected, namely off-shore Toulon (France), Capo Passero (Sicily, Italy) and Pylos (Peloponnese, Greece). The infrastructure will consist of three so-called building blocks. A building block comprises 115 strings, each string comprises 18 optical modules and each optical module comprises 31 photo-multiplier tubes. Each building block thus constitutes a three-dimensional array of photo sensors that can be used to detect the Cherenkov light produced by relativistic particles emerging from neutrino interactions. Two building blocks will be sparsely configured to fully explore the IceCube signal with similar instrumented volume, different methodology, improved resolution and complementary field of view, including the galactic plane. One building block will be densely configured to precisely measure atmospheric neutrino oscillations.
Neutrino oscillations are well established and the relevant parameters
determined with good precision, except for the CP phase, in terms of a unitary
lepton mixing matrix. Seesaw extensions of the Standard Model predict unitarity
deviations due to the admixture of heavy isosinglet neutrinos. We provide a
complete description of the unitarity and universality deviations in the light
neutrino sector. Neutrino oscillation experiments involving electron or muon
neutrinos and anti-neutrinos are fully described in terms of just three new
real parameters and a new CP phase, in addition to the ones describing
oscillations with unitary mixing. Using this formalism we describe the
implications of non-unitarity for neutrino oscillations and summarize the
model-independent constraints on heavy neutrino couplings that arise from
current experiments.
The standard three-neutrino oscillation paradigm, associated with small
squared mass splittings << 0.1 eV^2, has been successfully built up over the
last 15 years using solar, atmospheric, long baseline accelerator and reactor
neutrino experiments. However, this well-established picture might suffer from
anomalous results reported at very short baselines in some of these
experiments. If not experimental artifacts, such results could possibly be
interpreted as the existence of at least an additional fourth sterile neutrino
species, mixing with the known active flavors with an associated mass splitting
>> 0.01 eV^2, and being insensitive to standard weak interactions. Precision
measurements at very short baselines (5 to 15 m) with intense MeV electronic
antineutrino emitters can be used to probe these anomalies. In this article,
the expected antineutrino signal and backgrounds of a generic experiment which
consists of deploying an intense beta minus radioactive source inside or in the
vicinity of a large liquid scintillator detector are studied. The technical
challenges to perform such an experiment are identified, along with quantifying
the possible source and detector induced systematics, and their impact on the
sensitivity to the observation of neutrino oscillations at short baselines.
We present limits on sterile neutrino mixing using 4,438 live-days of
atmospheric neutrino data from the Super-Kamiokande experiment. We search for
fast oscillations driven by an eV-scale mass splitting and for oscillations
into sterile neutrinos instead of tau neutrinos at the atmospheric mass
splitting. When performing both these searches we assume that the sterile mass
splitting is large, allowing to be approximated as
0.5, and we assume that there is no mixing between electron neutrinos and
sterile neutrinos (). No evidence of sterile oscillations is
seen and we limit to less than 0.041 and to less
than 0.18 for eV at the 90% C.L. in a 3+1 framework. The
approximations that can be made with atmospheric neutrinos allow these limits
to be easily applied to 3+N models, and we provide our results in a generic
format to allow comparisons with other sterile neutrino models.
Neutrinos are one of the most promising messengers for signals of new physics Beyond the Standard Model (BSM). On the theoretical side, their elusive nature, combined with their unknown mass mechanism, seems to indicate that the neutrino sector is indeed opening a window to new physics. On the experimental side, several long-standing anomalies have been reported in the past decades, providing a strong motivation to thoroughly test the standard three-neutrino oscillation paradigm. In this Snowmass21 white paper, we explore the potential of current and future neutrino experiments to explore BSM effects on neutrino flavor during the next decade.
The Baksan Experiment on Sterile Transitions (BEST) was designed to investigate the deficit of electron neutrinos ν_{e} observed in previous gallium-based radiochemical measurements with high-intensity neutrino sources, commonly referred to as the "gallium anomaly," which could be interpreted as evidence for oscillations between ν_{e} and sterile neutrino (ν_{s}) states. A 3.414-MCi ^{51}Cr ν_{e} source was placed at the center of two nested Ga volumes and measurements were made of the production of ^{71}Ge through the charged current reaction, ^{71}Ga(ν_{e},e^{-})^{71}Ge, at two average distances. The measured production rates for the inner and the outer targets, respectively, are [54.9_{-2.4}^{+2.5}(stat)±1.4(syst)] and [55.6_{-2.6}^{+2.7}(stat)±1.4(syst)] atoms of ^{71}Ge/d. The ratio (R) of the measured rate of ^{71}Ge production at each distance to the expected rate from the known cross section and experimental efficiencies are R_{in}=0.79±0.05 and R_{out}=0.77±0.05. The ratio of the outer to the inner result is 0.97±0.07, which is consistent with unity within uncertainty. The rates at each distance were found to be similar, but 20%-24% lower than expected, thus reaffirming the anomaly. These results are consistent with ν_{e}→ν_{s} oscillations with a relatively large Δm^{2} (>0.5 eV^{2}) and mixing sin^{2}2θ (≈0.4).
The Baksan Experiment on Sterile Transitions (BEST) probes the gallium anomaly and its possible connections to oscillations between active and sterile neutrinos. Based on the Gallium-Germanium Neutrino Telescope (GGNT) technology of the SAGE experiment, BEST employs two zones of liquid Ga target to explore neutrino oscillations on the meter scale. Oscillations on this short scale could produce deficits in the Ge71 production rates within the two zones, as well as a possible rate difference between the zones. From July 5th to October 13th 2019, the two-zone target was exposed to a primarily monoenergetic, 3.4-MCi Cr51 neutrino source 10 times for a total of 20 independent Ge71 extractions from the two Ga targets. The Ge71 production rates from the neutrino source were measured from July 2019 to March 2020. At the end of these measurements, the counters were filled with Ge71 doped gas and calibrated during November 2020. In this paper, results from the BEST sterile neutrino oscillation experiment are presented in details. The ratio of the measured Ge71 production rates to the predicted rates for the inner and the outer target volumes are calculated from the known neutrino capture cross section. Comparable deficits in the measured ratios relative to predicted values are found for both zones, with the 4σ deviations from unity consistent with the previously reported gallium anomaly. If interpreted in the context of neutrino oscillations, the deficits give best-fit oscillation parameters of Δm2=3.3-2.3+∞eV2 and sin22θ=0.42-0.17+0.15, consistent with νe→νs oscillations governed by a surprisingly large mixing angle.
We study the status of the reactor antineutrino anomaly in light of recent reactor flux models obtained with the conversion and summation methods. We present a new improved calculation of the IBD yields of the standard Huber-Mueller (HM) model and those of the new models. We show that the reactor rates and the fuel evolution data are consistent with the predictions of the Kurchatov Institute (KI) conversion model and with those of the Estienne-Fallot (EF) summation model, leading to a plausible robust demise of the reactor antineutrino anomaly. We show that the results of several goodness of fit tests favor the KI and EF models over other models that we considered. We also discuss the implications of the new reactor flux models for short-baseline neutrino oscillations due to active-sterile mixing. We show that reactor data give upper bounds on active-sterile neutrino mixing that are not very different for the reactor flux models under consideration and are in tension with the large mixing required by the Gallium anomaly that has been refreshed by the recent results of the BEST experiment.
We report a reanalysis of the reactor antineutrino energy spectra based on the new relative measurements of the ratio R=Se5/Se9 between cumulative β spectra from U235 and Pu239, performed at a research reactor in National Research Centre Kurchatov Institute (KI). A discrepancy with the β spectra measured at Institut Laue-Langevin (ILL) was observed, indicating a steady excess of the ILL ratio by the factor of 1.054±0.002. We find a value of the ratio between inverse beta decay cross section per fission for U235 and Pu239: (5σf/9σf)KI=1.45±0.03, and then we reevaluate the converted antineutrino spectra for U235 and U238. We conclude that the new predictions are consistent with the results of Daya Bay and STEREO experiments.
We review the status of searches for sterile neutrinos in the ∼1eV range, with an emphasis on the latest results from short baseline oscillation experiments and how they fit within sterile neutrino oscillation models. We present global fit results to a three-active-flavor plus one-sterile-flavor model (3+1), where we find an improvement of Δχ2=35 for 3 additional parameters compared to a model with no sterile neutrino. This is a 5σ improvement, indicating that an effect that is like that of a sterile neutrino is highly preferred by the data. However we note that separate fits to the appearance and disappearance oscillation data sets within a 3+1 model do not show the expected overlapping allowed regions in parameter space. This “tension” leads us to explore two options: 3+2, where a second additional mass state is introduced, and a 3+1+decay model, where the ν4 state can decay to invisible particles. The 3+1+decay model, which is also motivated by improving compatibility with cosmological observations, yields the larger improvement, with a Δχ2=8 for 1 additional parameter beyond the 3+1 model, which is a 2.6σ improvement. Moreover the tension between appearance and disappearance experiments is reduced compared to 3+1, although disagreement remains. In these studies, we use a frequentist approach and also a Bayesian method of finding credible regions.
With respect to this tension, we review possible problems with the global fitting method. We note multiple issues, including problems with reproducing the experimental results, especially in the case of experiments that do not provide adequate data releases. We discuss an unexpected 5 MeV excess, observed in the reactor flux energy spectrum, that may be affecting the oscillation interpretation of the short baseline reactor data. We emphasize the care that must be taken in mapping to the true neutrino energy in the case of oscillation experiments that are subject to multiple interaction modes and nuclear effects. We point to problems with the “Parameter-Goodness-of-Fit test” that is used to quantify the tension. Lastly, we point out that analyses presenting limits often receive less scrutiny that signals.
While we provide a snapshot of the status of sterile neutrino searches today and global fits to their interpretation, we emphasize that this is a fast-moving field. We briefly review experiments that are expected to report new data in the immediate future. Lastly, we consider the 5-year horizon, where we propose that decay-at-rest neutrino sources are the best method of finally resolving the confusing situation.
A number of anomalous results in short-baseline oscillation may hint at the existence of one or more light sterile neutrino states in the eV mass range and have triggered a wave of new experimental efforts to search for a definite signature of oscillations between active and sterile neutrino states. The present paper aims to provide a comprehensive review on the status of light sterile neutrino searches in mid-2019: we discuss not only the basic experimental approaches and sensitivities of reactor, source, atmospheric, and accelerator neutrino oscillation experiments but also the complementary bounds arising from direct neutrino mass experiments and cosmological observations. Moreover, we review current results from global oscillation analyses that include the constraints set by running reactor and atmospheric neutrino experiments. They permit to set tighter bounds on the active-sterile oscillation parameters but as yet are not able to provide a definite conclusion on the existence of eV-scale sterile neutrinos.
The Short-Baseline Neutrino (SBN) program consists of three liquid argon time-projection chamber detectors located along the Booster Neutrino Beam at Fermi National Accelerator Laboratory. Its main goals include searches for New Physics—particularly eV-scale sterile neutrinos, detailed studies of neutrino–nucleus interactions at the GeV energy scale, and the advancement of the liquid argon detector technology that will also be used in the DUNE/LBNF long-baseline neutrino experiment in the next decade. We review these science goals and the current experimental status of SBN.
An experiment to search for light sterile neutrinos was performed at a reactor with a thermal power of 2.8 GW located at the Hanbit nuclear power complex. The search was done with a detector consisting of a ton of Gd-loaded liquid scintillator in a tendon gallery approximately 24 m from the reactor core. The measured antineutrino event rate is 1965 per day with a signal to background ratio of about 23. The shape of the antineutrino energy spectrum obtained from eight-month data-taking period is compared with a hypothesis of oscillations due to active-sterile antineutrino mixing. It is found to be consistent with no oscillation. An excess around 5 MeV prompt energy range is observed as seen in existing longer baseline experiments. Most of the allowed parameter space of eV range for a previously reported reactor antineutrino anomaly, is excluded with a confidence level higher than 95%.
We report results of a search for oscillations involving a light sterile neutrino over distances of 1.04 and 735 km in a νμ-dominated beam with a peak energy of 3 GeV. The data, from an exposure of 10.56×1020 protons on target, are analyzed using a phenomenological model with one sterile neutrino. We constrain the mixing parameters θ24 and Δm412 and set limits on parameters of the four-dimensional Pontecorvo-Maki-Nakagawa-Sakata matrix, |Uμ4|2 and |Uτ4|2, under the assumption that mixing between νe and νs is negligible (|Ue4|2=0). No evidence for νμ→νs transitions is found and we set a world-leading limit on θ24 for values of Δm4121 eV2.
Unitarity is a fundamental property of any theory required to ensure we work
in a theoretically consistent framework. In comparison with the quark sector,
experimental tests of unitarity for the 3x3 neutrino mixing matrix are
considerably weaker. It must be remembered that the vast majority of our
information on the neutrino mixing angles originates from
and disappearance experiments, with the assumption of unitarity being
invoked to constrain the remaining elements. New physics can invalidate this
assumption for the 3x3 subset and thus modify our precision measurements. We
perform a reanalysis to see how global knowledge is altered when one refits
oscillation results without assuming unitarity, and present ranges
for allowed elements consistent with all observed phenomena. We
calculate the bounds on the closure of the six neutrino unitarity triangles,
with the closure of the triangle being constrained to be
0.03, while the remaining triangles are significantly less constrained to be
0.1 - 0.2. Similarly for the row and column normalization, we find their
deviation from unity is constrained to be 0.2 - 0.4, for four out of six
such normalisations, while for the and row normalisation the
deviations are constrained to be 0.07, all at the CL. We
emphasise that there is significant room for new low energy physics, especially
in the sector which very few current experiments constrain directly.
Sterile neutrinos in the electronvolt mass range are hinted at by a number of
terrestrial neutrino experiments. However, such neutrinos are highly
incompatible with data from the Cosmic Microwave Background and large scale
structure. This paper discusses how charging sterile neutrinos under a new
pseudoscalar interaction can reconcile eV sterile neutrinos with terrestrial
neutrino data. We show that this model can reconcile eV sterile neutrinos in
cosmology, providing a fit to all available data which is way better than the
standard CDM model with one additional fully thermalized sterile
neutrino. In particular it also prefers a value of the Hubble parameter much
closer to the locally measured value.
Assuming invariance of theory under three-dimensional unitary group, various consequences have been investigated. Both Sakata's
and Gell-Mann's scheme can be treated in the same fashion and in a simpler way. Mass formula for particles belonging to the
same irreducible representation has been derived and compared with experiments.
A particle mixture theory of neutrino is proposed assuming the existence of two kinds of neutrinos. Based on the neutrino-mixture
theory, a possible unified model of elementary particles is constructed by generalizing the Sakata-Nagoya model. Our scheme
gives a natural explanation of smallness of leptonic decay rate of hyperons as well as the subtle difference of Gν's between µ-e and β-decay.
Starting with this scheme, the possibility of Ke3 mode with ΔS/ΔQ = −1 is also examined, and some bearings on the dynamical role of the B-matter, a fundamental constituent of baryons in the Nagoya model, are clarified.
The MiniBooNE experiment at Fermilab reports results from an analysis of ν[over ¯]_{e} appearance data from 11.27×10^{20} protons on target in the antineutrino mode, an increase of approximately a factor of 2 over the previously reported results. An event excess of 78.4±28.5 events (2.8σ) is observed in the energy range 200<E_{ν}^{QE}<1250 MeV. If interpreted in a two-neutrino oscillation model, ν[over ¯]_{μ}→ν[over ¯]_{e}, the best oscillation fit to the excess has a probability of 66% while the background-only fit has a χ^{2} probability of 0.5% relative to the best fit. The data are consistent with antineutrino oscillations in the 0.01<Δm^{2}<1.0 eV^{2} range and have some overlap with the evidence for antineutrino oscillations from the Liquid Scintillator Neutrino Detector. All of the major backgrounds are constrained by in situ event measurements so nonoscillation explanations would need to invoke new anomalous background processes. The neutrino mode running also shows an excess at low energy of 162.0±47.8 events (3.4σ) but the energy distribution of the excess is marginally compatible with a simple two neutrino oscillation formalism. Expanded models with several sterile neutrinos can reduce the incompatibility by allowing for CP violating effects between neutrino and antineutrino oscillations.
Short baseline neutrino oscillation experiments have shown hints of the
existence of additional sterile neutrinos in the eV mass range. Such sterile
neutrinos are incompatible with cosmology because they suppress structure
formation unless they can be prevented from thermalising in the early Universe.
Here we present a novel scenario in which both sterile neutrinos and dark
matter are coupled to a new, light pseudoscalar. This can prevent
thermalisation of sterile neutrinos and make dark matter sufficiently
self-interacting to have an impact on galactic dynamics and possibly resolve
some of the known problems with the standard cold dark matter scenario. Our
model singles out a dimensionless coupling strength for both sterile neutrinos
and dark matter in the range and predicts a dark
matter particle mass in the MeV range.
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We show that sterile neutrinos with masses ≳1 eV, as motivated by several short baseline oscillation anomalies, can be consistent with cosmological constraints if they are charged under a hidden sector force mediated by a light boson. In this case, sterile neutrinos experience a large thermal potential that suppresses mixing between active and sterile neutrinos in the early Universe, even if vacuum mixing angles are large. Thus, the abundance of sterile neutrinos in the Universe remains very small, and their impact on big bang nucleosynthesis, cosmic microwave background, and large-scale structure formation is negligible. It is conceivable that the new gauge force also couples to dark matter, possibly ameliorating some of the small-scale structure problems associated with cold dark matter.
Short baseline neutrino oscillation experiments have shown hints of the existence of additional sterile neutrinos in the eV mass range. However, such neutrinos seem incompatible with cosmology because they have too large of an impact on cosmic structure formation. Here we show that new interactions in the sterile neutrino sector can prevent their production in the early Universe and reconcile short baseline oscillation experiments with cosmology.