Search For Sterile Neutrinos Research Articles

Characterization of the correlated background for a sterile neutrino search using the first dataset of the JSNS^2 experiment

JSNS^2 (J-PARC Sterile Neutrino Search at J-PARC Spallation Neutron Source) is an experiment that is searching for sterile neutrinos via the observation of {bar{nu }}_{mu } rightarrow {bar{nu }}_{e} appearance oscillations using muon decay-at-rest neutrinos. Before dedicated data taking in the first-half of 2021, we performed a commissioning run for 10 days in June 2020. Using the data obtained in this commissioning run, in this paper, we present an estimate of the correlated background which imitates the {bar{nu }}_{e} signal in a sterile neutrino search. In addition, in order to demonstrate future prospects of the JSNS^2 experiment, possible pulse shape discrimination improvements towards reducing cosmic ray induced fast neutron background are described.

Optimized scintillation strip design for the DANSS upgrade

DANSS is a one cubic meter plastic scintillator detector with a primary goal of sterile neutrino searches at a commercial nuclear reactor. Due to its highly advantageous location, fine segmentation and ability to change the distance to the neutrino production origin, DANSS is ahead of many similar experiments around the world in terms of the counting rate, signal to background ratio and sterile neutrino exclusion regions. Yet a moderate energy resolution of the detector prevents further progress in the physics program. The main challenge of the planned upgrade is to achieve an energy resolution of 12% at 1 MeV. The new design of the main sensitive element — the plastic scintillation strip — is the most important step forward. The strip prototypes were manufactured and tested at the pion beam of the PNPI synchrocyclotron. More than twice higher light output together with fairly flat detector response uniformity, longitudinal timing information and other optimizations will help to reach the upgrade goal. This paper discusses the drawbacks of the current strip version, outlines the new features of the proposed upgrade, describes the beam test procedure and presents the test results reflecting the advantages of the new strip design in comparison with the current version.

Charged B mesogenesis

We leverage the CP violation in charged $B$ meson decays to generate the observed baryon asymmetry and dark matter at $\mathcal{O}(10 \text{ MeV})$ temperatures. We realize this in two scenarios: $B_c^+$ Mesogenesis and $B^+$ Mesogenesis. In the first, CP violating $B_c^\pm$ decays to $B^\pm$ mesons are followed by decays to dark and Standard Model baryons. In the second, CP violating $B^\pm$ decays to lighter charged mesons are accompanied by the latter's decays to dark and Standard Model leptons, which then scatter into the baryon asymmetry. $B_c^+$ Mesogenesis is actively being probed at Belle and LHCb, while $B^+$ Mesogenesis can be tested at colliders and sterile neutrino searches.

BEST impact on sterile neutrino hypothesis

Recently the Baksan Experiment on Sterile Transitions (BEST) has presented results confirming the gallium anomaly -- a lack of electron neutrinos $\nu_e$ at calibrations of SAGE and GALLEX -- at the statistical significance exceeding 5$\,\sigma$. This result is consistent with explanation of the gallium anomaly as electron neutrino oscillations into sterile neutrino, $\nu_s$. Within this explanation the BEST experiment itself provides the strongest evidence for the sterile neutrino among all the previous anomalous results in the neutrino sector. We combine the results of gallium experiments with searches for sterile neutrinos in reactor antineutrino experiments (assuming CPT-conservation in the $3+1$ neutrino sector). While the "gallium" best fit point in the model parameter space (sterile neutrino mass squared $m_{\nu_s}^2\approx 1.25\,$eV$^2$, sterile-electron neutrino mixing $\sin^22\theta\approx 0.34$) is excluded by these searches, a part of the BEST-favored 2$\,\sigma$ region with $m^2_{\nu_s}>5\,$eV$^2$ is consistent with all of them. Remarkably, the regions advertised by anomalous results of the NEUTRINO-4 experiment overlap with those of the BEST experiment: the best fit point of the joint analysis is $\sin^22\theta\approx 0.38$, $m_{\nu_s}^2\approx7.3\,$eV$^2$, the favored region will be explored by the KATRIN experiment. The sterile neutrino explanation of the BEST results would suggest not only the extension of the Standard Model of particle physics, but also either serious modifications of the Standard Cosmological Model and Solar Model, or a specific modification of the sterile sector needed to suppress the sterile neutrino production in the early Universe and in the Sun.

Future searches for light sterile neutrinos at nuclear reactors

We study the optimization of a green-field, two-baseline reactor experiment with respect to the sensitivity for electron antineutrino disappearance in search of a light sterile neutrino. We consider both commercial and research reactors and identify as key factors the distance of closest approach and detector energy resolution. We find that a total of 5 tons of detectors deployed at a commercial reactor with a closest approach of 25 m can probe the mixing angle $\sin^22\theta$ down to $\sim5\times10^{-3}$ around $\Delta m^2\sim 1$ eV$^2$. The same detector mass deployed at a research reactor can be sensitive up to $\Delta m^2\sim20-30$ eV$^2$ assuming a closest approach of 3 m and excellent energy resolution, such as that projected for the Taishan Antineutrino Observatory. We also find that lithium doping of the reactor could be effective in increasing the sensitivity for higher $\Delta m^2$ values.

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.

Reactor experiments review

Reactors are a copious source of anti-neutrinos and for this reason they have been critical tools to pin down neutrino fundamental properties since their discovery. Depending on the distance between reactor cores and experiments different oscillation parameter regions can be probed: in this review the regimes of very short, short and medium baselines are discussed with highlights on the current results and prospectives. Furthermore the high resolution which can be nowadays achieved allows to understand better the contributions from different fissile elements in the anti-neutrino spectrum. Fuel composition analysis as well as sterile neutrino search are also discussed.

Antineutrino spectrometer DANSS — 5 years of running

Solid scintillator antineutrino detector DANSS is placed on a movable platform below 3.1 GWth industrial reactor of the Kalininskaya Nuclear Power Plant. The distance between the detector and the center of the reactor core is frequently changed in the range 10.9 - 12.9 m. The reactor materials provide overburden of about 50 m.w.e. The one-cubic-meter sensitive volume of the detector is assembled from 2500 polystyrene scintillation strips and surrounded by a multilayer passive and active shielding. A dual readout by silicon photo multipliers (individually) and by convention PMTs (in groups of 50 strips) is used to achieve both high detector granularity and low noise trigger.In the position closest to the reactor core up to 5000 inverse beta-decay events per day are collected with cosmic muons induced background below 2%. The talk covers 5 years of the detector operation, observation of fuel evolution and results of our search for light sterile neutrinos. Spectrum and counting rate dependence on the fission fuel composition is clearly observed and agrees with H-M model. No statistically significant signal of short baseline neutrino oscillations observed so far with large portion of oscillation phase space excluded taking into account the improved analysis of systematic errors. The exclusion region at 90% confidence level reaches sin22θ < 8. 10−3 around Δm 2 ∼ 0.9 eV2.

A Geant4-based model for the TRISTAN detector

The TRISTAN project is the upgrade of the KATRIN experiment designed for the search of sterile neutrinos by replacing the current KATRIN detector with a multipixel SDD (Silicon Drift Detector) matrix. We have characterized SDDs response to electrons using a SEM (Scanning Electron Microscope) as an electron source and a Geant4-based simulation whose output is processed with an empirical function to reproduce data. We have crosschecked this model by reconstructing backscattering measurements obtained using a radioactive electron source.

Short-Baseline neutrino oscillation searches with the ICARUS detector

The ICARUS collaboration employed the 760-ton T600 detector in a successful three-year physics run at the underground LNGS laboratories studying neutrino oscillations with the CNGS neutrino beam from CERN, and searching for atmospheric neutrino interactions. ICARUS initially performed a sensitive search for LSND-like anomalous υ e appearance in the CNGS beam constraining the allowed parameters to a narrow region around 1eV 2. After a significant overhauling phase at CERN, the T600 detector was installed at Fermilab and activated in 2020 in view of the forthcoming sensitive search for sterile neutrinos using the Booster beam within the Short Baseline Neutrino (SBN) program. During the initial commissioning phase ICARUS already collected the first neutrino events from the Booster Neutrino Beam and the NuMI off-axis. The proposed contribution will address ICARUS achievements, its status and plans for the new run at Fermilab.

Separation of the 235U and 239Pu Prompt Energy Spectra in NEOS-II

The NEOS searches for sterile neutrinos by detecting reactor antineutrinos at a very short baseline in Korea. The NEOS detector (1-ton Gd-LS) is deployed at the tendon gallery of the Hanbit reactor unit 5 (2.8 GW thermal power), 23.7 m away from the reactor core. In NEOS-I, we measured the prompt energy spectrum from inverse beta decay using 180 days of reactor-on data and observed the “5 MeV excess”. To understand the origin of the “5 MeV excess”, NEOS-II has taken 368 (120) days of reactor-on (-off) data from September 2018 to October 2020, covering a whole burnup cycle of the reactor. We tested a method for the extraction of the prompt energy spectra for 235U and 239Pu from the whole burnup cycle using pseudo data.

Light Sterile Neutrinos

We review the status of light sterile neutrino searches, motivated by the original Short BaseLine (SBL) anomalies. Here, we discuss how sterile neutrino properties can be constrained by different types of neutrino oscillation experiments (considering appearance or disappearance probes in different oscillation channels) and non-oscillation measurements. These latter include experiments aiming at obtaining a value for the absolute scale of neutrino masses (β decay probes) and the indirect constraints that we could obtain from cosmological observations.

Sterile Neutrinos with Neutrino Telescopes

Searches for light sterile neutrinos are motivated by the unexpected observation of an electron neutrino appearance in short-baseline experiments, such as the Liquid Scintillator Neutrino Detector (LSND) and the Mini Booster Neutrino Experiment (MiniBooNE). In light of these unexpected results, a campaign using natural and anthropogenic sources to find the light (mass-squared-difference around 1 eV2) sterile neutrinos is underway. Among the natural sources, atmospheric neutrinos provide a unique gateway to search for sterile neutrinos due to the broad range of baseline-to-energy ratios, L/E, and the presence of significant matter effects. Since the atmospheric neutrino flux rapidly falls with energy, studying its highest energy component requires gigaton-scale neutrino detectors. These detectors—often known as neutrino telescopes since they are designed to observe tiny astrophysical neutrino fluxes—have been used to perform searches for light sterile neutrinos, and researchers have found no significant signal to date. This brief review summarizes the current status of searches for light sterile neutrinos with neutrino telescopes deployed in solid and liquid water.

144Ce - 144Pr spectrum measurement with 4π semiconductor β-spectrometer

Precision β-spectra measurement always had a great importance in some fundamental physics problems including neutrino physics. Magnetic and electrostatic spectrometers have high resolution, but at the same time usage of such kinds of equipment involves the size and cost issues. Since electron mean free path at the energy of 3 MeV (which is basically the maximum energy of a β-transition for the long-lived nuclei) does not exceed 2 g/crn2 , electron registration could be effectively performed with the solid state scintillators and semiconductors. A strong probability of backscattering from detector surface is present in case of semiconductor detectors and is dependent upon the detector material. Such problem can be solved with 4π geometry detector development, which fully covers the radioactive source and is able to register the backscattered electrons. In this work we present the newly developed technology of 4π geometry β-spectrometer based on two semiconductor detectors. This spectrometer was used for measurement of the 144Ce - 144Pr spectrum, that is the perspective anti-neutrino source due to endpoint energy at 3 MeV and can be used for the sterile neutrino search experiments. The form-factor parameters that were obtained are: C(W) = 1 + (-0.02877 ± 0.00028)W + (-0.11722 ± 0.00297)W-1. The measurement accuracy was sufficiently enhanced with respect to the previous results.

Status of Anomalies and Sterile Neutrino Searches at Nuclear Reactors

Two anomalies at nuclear reactors, one related to the absolute antineutrino flux, one related to the antineutrino spectral shape, have drawn special attention to the field of reactor neutrino physics during the past decade. Numerous experimental efforts have been launched to investigate the reliability of flux models and to explore whether sterile neutrino oscillations are at the base of the experimental findings. This review aims to provide an overview on the status of experimental searches at reactors for sterile neutrino oscillations and measurements of the antineutrino spectral shape in mid-2021. The individual experimental approaches and results are reviewed. Moreover, global and joint oscillation and spectral shape analyses are discussed. Many experiments allow setting of constraints on sterile oscillation parameters, but cannot yet cover the entire relevant parameter space. Others find evidence in favour of certain parameter space regions. In contrast, findings on the spectral shape appear to give an overall consistent picture across experiments and allow narrowing down of contributions of certain isotopes.

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.

The JSNS[formula omitted] detector

The JSNS2 (J-PARC Sterile Neutrino Search at J-PARC Spallation Neutron Source) experiment aims to search for oscillations involving a sterile neutrino in the eV2 mass-splitting range. The experiment will search for the appearance of electron antineutrinos oscillated from muon antineutrinos. The electron antineutrinos are detected via the inverse beta decay process using a liquid scintillator detector. A 1MW beam of 3GeV protons incident on a spallation neutron target produces an intense and pulsed neutrino source from pion, muon, and kaon decay at rest. The JSNS2 detector is located 24m away from the neutrino source and began operation from June 2020. The detector contains 17 tonnes of gadolinium (Gd) loaded liquid scintillator (LS) in an acrylic vessel, as a neutrino target. It is surrounded by 31 tonnes of unloaded LS in a stainless steel tank. Optical photons produced in LS are viewed by 120 R7081 Hamamatsu 10-inch Photomultiplier Tubes (PMTs). In this paper, we describe the JSNS2 detector design, construction, and operation.

Search for sterile neutrinos with the Neutrino-4 experiment and measurement results

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 $\mathrm{\ensuremath{\Delta}}{\mathrm{m}}_{14}^{2}$ and ${\mathrm{sin}}^{2}2{\ensuremath{\theta}}_{14}$. 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 $2.9\ensuremath{\sigma}$ level with the parameters $\mathrm{\ensuremath{\Delta}}{m}_{14}^{2}=(7.3\ifmmode\pm\else\textpm\fi{}{0.13}_{\mathrm{st}}\ifmmode\pm\else\textpm\fi{}{1.16}_{\mathrm{syst}})\text{ }\text{ }{\mathrm{eV}}^{2}=(7.3\ifmmode\pm\else\textpm\fi{}1.17)\text{ }\text{ }{\mathrm{eV}}^{2}$ and ${\mathrm{sin}}^{2}2\ensuremath{\theta}=0.36\ifmmode\pm\else\textpm\fi{}{0.12}_{\mathrm{stat}}(2.9\ensuremath{\sigma})$. Monte Carlo based statistical analysis gave an estimation of the confidence level at $2.7\ensuremath{\sigma}$. 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.

Design and optimization of the RF input-coupler for a low-frequency, split-coaxial RFQ

The Isotope Decay-At-Rest experiment (IsoDAR) is a proposed underground experiment which is expected to be a definitive search for sterile neutrinos. In order to be decisive within 5 years, high rates of neutrinos must be produced, byimpinging a 10 mA continuous wave proton beam at 60 MeV on a high power target.Due to space restrictions, a compact cyclotron was chosen as an accelerator to producethis driver beam.To overcome space charge limitations during injection, H2 + ions are accelerated andlater stripped into protons by means of a carbon foil.IsoDAR uses an especially designed low-frequency (32.8 MHz) split-coaxial Radio-FrequencyQuadrupole (RFQ) to effectively bunch H2 + ions before injecting them into this cyclotron.The RFQ will be embedded vertically in the cyclotron yoke,facilitating a very compact design. This puts stringent limits on RFQ size, type, andaccessibility.Here, we present the design and optimization of the low-frequency (32.8 MHz) RF input-couplerfor the IsoDAR RFQ.The design is challenging due to the necessarily small diameter of the RFQ (28 cm)and the split-coaxial type, as well as limited access to the RFQ.We have determined the optimal position and shape for the coupler, leading to a low powerconsumption of < 6 kW for an inter-vane voltage of 22 kV. The highest calculated fieldsare safely below the Kilpatrick limit for this structure.

Searches for sterile neutrinos and axionlike particles from the Galactic halo with eROSITA

Dark matter might be made of "warm" particles, such as sterile neutrinos in the keV mass range, which can decay into photons through mixing and are consequently detectable by X-ray telescopes. Axionlike particles (ALPs) are detectable by X-ray telescopes too when coupled to standard model particles and decay into photons in the keV range. Both particles could explain the unidentified 3.5 keV line and, interestingly, XENON1T observed an excess of electron recoil events most prominent at 2-3 keV. One explanation could be an ALPs origin, which is not yet excluded by X-ray constraints in an anomaly-free symmetry model in which the photon production is suppressed. We study the diffuse emission coming from the Galactic halo, and calculate the sensitivity of all-sky X-ray survey performed by eROSITA to identify a sterile neutrino or ALP dark matter. We estimate bounds on the mixing angle of the sterile neutrinos and coupling strength of the ALPs. After four years of data-taking by eROSITA, we expect to set stringent constraints, and in particular, we expect to firmly probe mixing angle $\sin^2(2\theta)$ up to nearly two orders magnitude below the best-fit value for explaining the unidentified 3.5 keV line. Moreover, with eROSITA, we will be able to probe the ALP parameter space of couplings to photons and electrons, and potentially confirm an ALP origin of the XENON1T excess.