First Search for Short-Baseline Neutrino Oscillations at HFIR with PROSPECT

Final Measurement of the ^{235}U Antineutrino Energy Spectrum with the PROSPECT-I Detector at HFIR

This Letter reports one of the most precise measurements to date of the antineutrino spectrum from a purely ^{235}U-fueled reactor, made with the final dataset from the PROSPECT-I detector at the High Flux Isotope Reactor. By extracting information from previously unused detector segments, this analysis effectively doubles the statistics of the previous PROSPECT measurement. The reconstructed energy spectrum is unfolded into antineutrino energy and compared with both the Huber-Mueller model and a spectrum from a commercial reactor burning multiple fuel isotopes. A local excess over the model is observed in the 5-7 MeV energy region. Comparison of the PROSPECT results with those from commercial reactors provides new constraints on the origin of this excess, disfavoring at 2.0 and 3.7 standard deviations the hypotheses that antineutrinos from ^{235}U are solely responsible and noncontributors to the excess observed at commercial reactors, respectively.

Cerium Ruthenium Low-Energy Antineutrino Measurements for Safeguarding Military Naval Reactors

The recent agreement to transfer nuclear submarine reactors and technology from two nuclear-weapon states to a non-nuclear-weapon state (AUKUS deal) highlights an unsolved problem in international safeguards: how to safeguard naval reactor fuel while it is on board an operational nuclear submarine. Proposals to extend existing safeguards technologies and practices are complicated by the need for civilian international inspectors to gain access to the interior of the submarine and the reactor compartment, which raises national security concerns. In this Letter we show that implementing safeguards on submarine propulsion reactors using a low-energy antineutrino reactor-off method, between submarine patrols, can by-pass the need for onboard access all together. We find that, using inverse beta decay, detectors can achieve a timely and high level of assurance that a submarine's nuclear core has not been diverted (detector mass of around 100 kg) nor its enrichment level changed (detector mass of around 10 tons).

Joint Determination of Reactor Antineutrino Spectra from ^{235}U and ^{239}Pu Fission by Daya Bay and PROSPECT

A joint determination of the reactor antineutrino spectra resulting from the fission of ^{235}U and ^{239}Pu has been carried out by the Daya Bay and PROSPECT Collaborations. This Letter reports the level of consistency of ^{235}U spectrum measurements from the two experiments and presents new results from a joint analysis of both data sets. The measurements are found to be consistent. The combined analysis reduces the degeneracy between the dominant ^{235}U and ^{239}Pu isotopes and improves the uncertainty of the ^{235}U spectral shape to about 3%. The ^{235}U and ^{239}Pu antineutrino energy spectra are unfolded from the jointly deconvolved reactor spectra using the Wiener-SVD unfolding method, providing a data-based reference for other reactor antineutrino experiments and other applications. This is the first measurement of the ^{235}U and ^{239}Pu spectra based on the combination of experiments at low- and highly enriched uranium reactors.

Searching for Hidden Neutrons with a Reactor Neutrino Experiment: Constraints from the STEREO Experiment

Different extensions of the standard model of particle physics, such as braneworld or mirror matter models, predict the existence of a neutron sterile state, possibly as a dark matter candidate. This Letter reports a new experimental constraint on the probability p for neutron conversion into a hidden neutron, set by the STEREO experiment at the high flux reactor of the Institut Laue-Langevin. The limit is p<3.1×10^{-11} at 95% C.L. improving the previous limit by a factor of 13. This result demonstrates that short-baseline neutrino experiments can be used as competitive passing-through-walls neutron experiments to search for hidden neutrons.

Borexino Results on Neutrinos from the Sun and Earth

Borexino is a 280-ton liquid scintillator detector located at the Laboratori Nazionali del Gran Sasso in Italy. Since the start of its data-taking in May 2007, it has provided several measurements of low-energy neutrinos from various sources. At the base of its success lie unprecedented levels of radio-purity and extensive thermal stabilization, both resulting from a years-long effort of the collaboration. Solar neutrinos, emitted in the Hydrogen-to-Helium fusion in the solar core, are important for the understanding of our star, as well as neutrino properties. Borexino is the only experiment that has performed a complete spectroscopy of the pp chain solar neutrinos (with the exception of the hep neutrinos contributing to the total flux at 10−5 level), through the detection of pp, 7Be, pep, and 8B solar neutrinos and has experimentally confirmed the existence of the CNO fusion cycle in the Sun. Borexino has also detected geoneutrinos, antineutrinos from the decays of long-lived radioactive elements inside the Earth, that can be exploited as a new and unique tool to study our planet. This paper reviews the most recent Borexino results on solar and geoneutrinos, from highlighting the key elements of the analyses up to the discussion and interpretation of the results for neutrino, solar, and geophysics.

Bound on 3+1 Active-Sterile Neutrino Mixing from the First Four-Week Science Run of KATRIN

We report on the light sterile neutrino search from the first four-week science run of the KATRIN experiment in 2019. Beta-decay electrons from a high-purity gaseous molecular tritium source are analyzed by a high-resolution MAC-E filter down to 40 eV below the endpoint at 18.57 keV. We consider the framework with three active neutrinos and one sterile neutrino. The analysis is sensitive to the mass, m_{4}, of the fourth mass state for m_{4}^{2}≲1000  eV^{2} and to active-to-sterile neutrino mixing down to |U_{e4}|^{2}≳2×10^{-2}. No significant spectral distortion is observed and exclusion bounds on the sterile mass and mixing are reported. These new limits supersede the Mainz results for m_{4}^{2}≲1000  eV^{2} and improve the Troitsk bound for m_{4}^{2}<30  eV^{2}. The reactor and gallium anomalies are constrained for 100<Δm_{41}^{2}<1000  eV^{2}.

Accurate Measurement of the Electron Antineutrino Yield of ^{235}U Fissions from the STEREO Experiment with 119 Days of Reactor-On Data

We report a measurement of the antineutrino rate from the fission of ^{235}U with the STEREO detector using 119 days of reactor turned on. In our analysis, we perform several detailed corrections and achieve the most precise single measurement at reactors with highly enriched ^{235}U fuel. We measure an IBD cross section per fission of σ_{f}=(6.34±0.06[stat]±0.15[sys]±0.15[model])×10^{-43}  cm^{2}/fission and observe a rate deficit of (5.2±0.8[stat]±2.3[sys]±2.3[model])% compared to the model, consistent with the deficit of the world average. Testing ^{235}U as the sole source of the deficit, we find a tension between the results of lowly and highly enriched ^{235}U fuel of 2.1 standard deviations.

eV-Scale Sterile Neutrino Search Using Eight Years of Atmospheric Muon Neutrino Data from the IceCube Neutrino Observatory

The results of a 3+1 sterile neutrino search using eight years of data from the IceCube Neutrino Observatory are presented. A total of 305 735 muon neutrino events are analyzed in reconstructed energy-zenith space to test for signatures of a matter-enhanced oscillation that would occur given a sterile neutrino state with a mass-squared differences between 0.01 and 100  eV^{2}. The best-fit point is found to be at sin^{2}(2θ_{24})=0.10 and Δm_{41}^{2}=4.5  eV^{2}, which is consistent with the no sterile neutrino hypothesis with a p value of 8.0%.

New Opportunities at the Next-Generation Neutrino Experiments (Part 1: BSM Neutrino Physics and Dark Matter

With the advent of a new generation of neutrino experiments which leverage high-intensity neutrino beams for precision neutrino oscillation parameter and for CP violation phase measurements, it is timely to explore physics topics beyond the standard neutrino-related physics. Given that beyond the standard model (BSM) physics phenomena have been mostly sought at high-energy regimes, such as the LHC at CERN, the exploration of BSM physics in neutrino experiments will enable complementary measurements at the energy regimes that balance that of the LHC. This is in concert with new ideas for high-intensity beams for fixed target and beam-dump experiments world-wide. The combination of the high intensity beam facilities and large mass detectors with highly precise track and energy measurements, excellent timing resolution, and low energy thresholds will help make BSM physics reachable even in low energy regimes in accelerator-based experiments and searches for BSM phenomena from cosmogenic origin. Therefore, it is conceivable that BSM topics could be the dominant physics topics in the foreseeable future. In this spirit, this paper provides a review of the current theory landscape theory in neutrino experiments in two selected areas of the BSM topics - dark matter and neutrino related BSM - and summarizes the current results from existing neutrino experiments for benchmark. This paper then provides a review of upcoming neutrino experiments and their capabilities to set the foundation for potential reach in BSM physics in the two themes. One of the most important outcomes of this paper is to ensure theoretical and simulation tools exist to perform studies of these new areas of physics from the first day of the experiments, such as DUNE and Hyper-K. Tasks to accomplish this goal, and the time line for them to be completed and tested to become reliable tools in a timely fashion are also discussed.

Exploring Light Sterile Neutrinos at Long Baseline Experiments: A Review

Several anomalies observed in short-baseline neutrino experiments suggest the existence of new light sterile neutrino species. In this review, we describe the potential role of long-baseline experiments in the searches of sterile neutrino properties and, in particular, the new CP-violation phases that appear in the enlarged 3 + 1 scheme. We also assess the impact of light sterile states on the discovery potential of long-baseline experiments of important targets such as the standard 3-flavor CP violation, the neutrino mass hierarchy, and the octant of θ 23 .

Search for light sterile neutrinos with the STEREO experiment

The stereo experiment is searching for a non-standard oscillation in the propagation of anti-neutrinos produced by a nuclear reactor which could be the sign for the existence of a sterile state of eV mass and the origin of the reactor anti-neutrino anomaly. In this paper, results from 66 days of reactor turned on and 138 days of reactor turned off are reported excluding large amplitude oscillations. A special focus is put on the data analysis and studies of correlated backgrounds. In particular the origin of the correlated background measured in reactor turned off periods is discussed. This background mainly originates from neutrons produced by cosmic radiation.

Measurement of the Antineutrino Spectrum from ^{235}U Fission at HFIR with PROSPECT

This Letter reports the first measurement of the ^{235}U ν[over ¯]_{e} energy spectrum by PROSPECT, the Precision Reactor Oscillation and Spectrum experiment, operating 7.9 m from the 85  MW_{th} highly enriched uranium (HEU) High Flux Isotope Reactor. With a surface-based, segmented detector, PROSPECT has observed 31678±304(stat) ν[over ¯]_{e}-induced inverse beta decays, the largest sample from HEU fission to date, 99% of which are attributed to ^{235}U. Despite broad agreement, comparison of the Huber ^{235}U model to the measured spectrum produces a χ^{2}/ndf=51.4/31, driven primarily by deviations in two localized energy regions. The measured ^{235}U spectrum shape is consistent with a deviation relative to prediction equal in size to that observed at low-enriched uranium power reactors in the ν[over ¯]_{e} energy region of 5-7 MeV.

Physics with reactor neutrinos

Neutrinos produced by nuclear reactors have played a major role in advancing our knowledge of the properties of neutrinos. The first direct detection of the neutrino, confirming its existence, was performed using reactor neutrinos. More recent experiments utilizing reactor neutrinos have also found clear evidence for neutrino oscillation, providing unique input for the determination of neutrino mass and mixing. Ongoing and future reactor neutrino experiments will explore other important issues, including the neutrino mass hierarchy and the search for sterile neutrinos and other new physics beyond the standard model. In this article, we review the recent progress in physics using reactor neutrinos and the opportunities they offer for future discoveries.

Searches for sterile neutrinos at the DANSS experiment

DANSS is a highly segmented 1 m3 plastic scintillator detector. The DANSS detector is placed under an industrial 3.1 GWth reactor of the Kalinin Nuclear Power Plant 350 km NW from Moscow. The distance to the core is varied on-line from 10.7 m to 12.7 m. The reactor building provides about 50 m water-equivalent shielding against the cosmic background. DANSS detects almost 5000 νe per day at the closest position with the cosmic background less than 3%. The inverse beta decay process is used to detect νe. Sterile neutrinos are searched for assuming the 4ν model (3 active and 1 sterile ν). The exclusion area in the Δm142,sin22θ14 plane is obtained using a ratio of positron energy spectra collected at different distances. Therefore results do not depend on the shape and normalization of the reactor νe spectrum nor the detector efficiency. Results are based on 966 thousand antineutrino events collected at three different distances from the reactor core. The excluded area covers a wide range of the sterile neutrino parameters down to sin22θ14 < 0.01 in the most sensitive region.

New results and perspectives in neutrino physics

A brief review of new results and perspectives in neutrino physics is presented. An emphasis on a search for CP violation in neutrino oscillations and a search for sterile neutrinos is given. Status of measurement of the direct neutrino mass measurement and searches for neutrinoless double beta decay are also discussed.