First Probe of Sub-GeV Dark Matter beyond the Cosmological Expectation with the COHERENT CsI Detector at the SNS.

The COHERENT Collaboration searched for scalar dark matter particles produced at the Spallation Neutron Source with masses between 1 and 220 MeV/c^{2} using a CsI[Na] scintillation detector sensitive to nuclear recoils above 9 keV_{nr}. No evidence for dark matter is found and we thus place limits on allowed parameter space. With this low-threshold detector, we are sensitive to coherent elastic scattering between dark matter and nuclei. The cross section for this process is orders of magnitude higher than for other processes historically used for accelerator-based direct-detection searches so that our small, 14.6 kg detector significantly improves on past constraints. At peak sensitivity, we reject the flux consistent with the cosmologically observed dark-matter concentration for all coupling constants α_{D}<0.64, assuming a scalar dark-matter particle. We also calculate the sensitivity of future COHERENT detectors to dark-matter signals which will ambitiously test multiple dark-matter spin scenarios.

Measurement of Electron-Neutrino Charged-Current Cross Sections on

Using an 185-kg NaI[Tl] array, COHERENT has measured the inclusive electron-neutrino charged-current cross section on ^{127}I with pion decay-at-rest neutrinos produced by the Spallation Neutron Source at Oak Ridge National Laboratory. Iodine is one the heaviest targets for which low-energy (≤50 MeV) inelastic neutrino-nucleus processes have been measured, and this is the first measurement of its inclusive cross section. After a five-year detector exposure, COHERENT reports a flux-averaged cross section for electron neutrinos of 9.2_{-1.8}^{+2.1}×10^{-40} cm^{2}. This corresponds to a value that is ∼41% lower than predicted using the MARLEY event generator with a measured Gamow-Teller strength distribution. In addition, the observed visible spectrum from charged-current scattering on ^{127}I has been measured between 10 and 55 MeV, and the exclusive zero-neutron and one-or-more-neutron emission cross sections are measured to be 5.2_{-3.1}^{+3.4}×10^{-40} and 2.2_{-0.5}^{+0.4}×10^{-40} cm^{2}, respectively.

Proton Compton Scattering from Linearly Polarized Gamma Rays.

Differential cross sections for Compton scattering from the proton have been measured at scattering angles of 55°, 90°, and 125° in the laboratory frame using quasimonoenergetic linearly (circularly) polarized photon beams with a weighted mean energy value of 83.4 MeV (81.3 MeV). These measurements were performed at the High Intensity Gamma-Ray Source facility at the Triangle Universities Nuclear Laboratory. The results are compared to previous measurements and are interpreted in the chiral effective field theory framework to extract the electromagnetic dipole polarizabilities of the proton, which gives α_{E1}^{p}=13.8±1.2_{stat}±0.1_{BSR}±0.3_{theo},ß_{M1}^{p}=0.2∓1.2_{stat}±0.1_{BSR}∓0.3_{theo} in units of 10^{-4} fm^{3}.

Measurement of the Coherent Elastic Neutrino-Nucleus Scattering Cross Section on CsI by COHERENT.

We measured the cross section of coherent elastic neutrino-nucleus scattering (CEvNS) using a CsI[Na] scintillating crystal in a high flux of neutrinos produced at the Spallation Neutron Source at Oak Ridge National Laboratory. New data collected before detector decommissioning have more than doubled the dataset since the first observation of CEvNS, achieved with this detector. Systematic uncertainties have also been reduced with an updated quenching model, allowing for improved precision. With these analysis improvements, the COHERENT Collaboration determined the cross section to be (165_{-25}^{+30})×10^{-40} cm^{2}, consistent with the standard model, giving the most precise measurement of CEvNS yet. The timing structure of the neutrino beam has been exploited to compare the CEvNS cross section from scattering of different neutrino flavors. This result places leading constraints on neutrino nonstandard interactions while testing lepton flavor universality and measures the weak mixing angle as sin^{2}θ_{W}=0.220_{-0.026}^{+0.028} at Q^{2}≈(50 MeV)^{2}.

First Measurement of Coherent Elastic Neutrino-Nucleus Scattering on Argon.

We report the first measurement of coherent elastic neutrino-nucleus scattering (CEvNS) on argon using a liquid argon detector at the Oak Ridge National Laboratory Spallation Neutron Source. Two independent analyses prefer CEvNS over the background-only null hypothesis with greater than 3σ significance. The measured cross section, averaged over the incident neutrino flux, is (2.2±0.7)×10^{-39} cm^{2}-consistent with the standard model prediction. The neutron-number dependence of this result, together with that from our previous measurement on CsI, confirms the existence of the CEvNS process and provides improved constraints on nonstandard neutrino interactions.

Precision Analysis of the

We present a precision analysis of the ^{136}Xe two-neutrino ßß electron spectrum above 0.8 MeV, based on high-statistics data obtained with the KamLAND-Zen experiment. An improved formalism for the two-neutrino ßß rate allows us to measure the ratio of the leading and subleading 2νßß nuclear matrix elements (NMEs), ξ_{31}^{2ν}=-0.26_{-0.25}^{+0.31}. Theoretical predictions from the nuclear shell model and the majority of the quasiparticle random-phase approximation (QRPA) calculations are consistent with the experimental limit. However, part of the ξ_{31}^{2ν} range allowed by the QRPA is excluded by the present measurement at the 90% confidence level. Our analysis reveals that predicted ξ_{31}^{2ν} values are sensitive to the quenching of NMEs and the competing contributions from low- and high-energy states in the intermediate nucleus. Because these aspects are also at play in neutrinoless ßß decay, ξ_{31}^{2ν} provides new insights toward reliable neutrinoless ßß NMEs.

Publisher's Note: Search for Majorana Neutrinos Near the Inverted Mass Hierarchy Region with KamLAND-Zen [Phys. Rev. Lett. 117, 082503 (2016)].

This corrects the article DOI: 10.1103/PhysRevLett.117.082503.

Search for Majorana Neutrinos Near the Inverted Mass Hierarchy Region with KamLAND-Zen.

We present an improved search for neutrinoless double-beta (0νßß) decay of ^{136}Xe in the KamLAND-Zen experiment. Owing to purification of the xenon-loaded liquid scintillator, we achieved a significant reduction of the ^{110m}Ag contaminant identified in previous searches. Combining the results from the first and second phase, we obtain a lower limit for the 0νßß decay half-life of T_{1/2}^{0ν}>1.07×10^{26} yr at 90% C.L., an almost sixfold improvement over previous limits. Using commonly adopted nuclear matrix element calculations, the corresponding upper limits on the effective Majorana neutrino mass are in the range 61-165 meV. For the most optimistic nuclear matrix elements, this limit reaches the bottom of the quasidegenerate neutrino mass region.

Limit on neutrinoless ßß decay of 136Xe from the first phase of KamLAND-Zen and comparison with the positive claim in 76Ge.

We present results from the first phase of the KamLAND-Zen double-beta decay experiment, corresponding to an exposure of 89.5 kg yr of (136)Xe. We obtain a lower limit for the neutrinoless double-beta decay half-life of T(1/2)(0ν)>1.9×10(25) yr at 90% C.L. The combined results from KamLAND-Zen and EXO-200 give T(1/2)(0ν)>3.4×10(25) yr at 90% C.L., which corresponds to a Majorana neutrino mass limit of <(120-250) meV based on a representative range of available matrix element calculations. Using those calculations, this result excludes the Majorana neutrino mass range expected from the neutrinoless double-beta decay detection claim in (76)Ge, reported by a part of the Heidelberg-Moscow Collaboration, at more than 97.5% C.L.

Experimental upper bound and theoretical expectations for parity-violating neutron spin rotation in 4He.

Neutron spin rotation is expected from quark-quark weak interactions in the standard model, which induce weak interactions among nucleons that violate parity. We present the results from an experiment searching for the effect of parity violation via the spin rotation of polarized neutrons in a liquid 4He medium. The value for the neutron spin rotation angle per unit length in 4He, d ϕ / d z = [ + 2.1 ± 8.3 (stat.) - 0.2 + 2.9 (sys.) ] × 10 - 7 rad/m, is consistent with zero. The result agrees with the best current theoretical estimates of the size of nucleon-nucleon weak amplitudes from other experiments and with the expectations from recent theoretical approaches to weak nucleon-nucleon interactions. In this paper we review the theoretical status of parity violation in the n → + 4He system and discuss details of the data analysis leading to the quoted result. Analysis tools are presented that quantify systematic uncertainties in this measurement and that are expected to be essential for future measurements.

Observation of coherent elastic neutrino-nucleus scattering.

The coherent elastic scattering of neutrinos off nuclei has eluded detection for four decades, even though its predicted cross section is by far the largest of all low-energy neutrino couplings. This mode of interaction offers new opportunities to study neutrino properties and leads to a miniaturization of detector size, with potential technological applications. We observed this process at a 6.7σ confidence level, using a low-background, 14.6-kilogram CsI[Na] scintillator exposed to the neutrino emissions from the Spallation Neutron Source at Oak Ridge National Laboratory. Characteristic signatures in energy and time, predicted by the standard model for this process, were observed in high signal-to-background conditions. Improved constraints on nonstandard neutrino interactions with quarks are derived from this initial data set.

Measurement of the Parity-Violating Neutron Spin Rotation in (4) He.

In the meson exchange model of weak nucleon-nucleon (NN) interactions, the exchange of virtual mesons between the nucleons is parameterized by a set of weak meson exchange amplitudes. The strengths of these amplitudes from theoretical calculations are not well known, and experimental measurements of parity-violating (PV) observables in different nuclear systems have not constrained their values. Transversely polarized cold neutrons traveling through liquid helium experience a PV spin rotation due to the weak interaction with an angle proportional to a linear combination of these weak meson exchange amplitudes. A measurement of the PV neutron spin rotation in helium (φ PV ( n ,α)) would provide information about the relative strengths of the weak meson exchange amplitudes, and with the longitudinal analyzing power measurement in the p + α system, allow the first comparison between isospin mirror systems in weak NN interaction. An earlier experiment performed at NIST obtained a result consistent with zero: φ PV ( n ,α) = (8.0 ±14(stat) ±2.2(syst)) ×10(-7) rad / m[1]. We describe a modified apparatus using a superfluid helium target to increase statistics and reduce systematic effects in an effort to reach a sensitivity goal of 10(-7) rad/m.

A slow neutron polarimeter for the measurement of parity-odd neutron rotary power.

We present the design, description, calibration procedure, and an analysis of systematic effects for an apparatus designed to measure the rotation of the plane of polarization of a transversely polarized slow neutron beam as it passes through unpolarized matter. This device is the neutron optical equivalent of a crossed polarizer/analyzer pair familiar from light optics. This apparatus has been used to search for parity violation in the interaction of polarized slow neutrons in matter. Given the brightness of existing slow neutron sources, this apparatus is capable of measuring a neutron rotary power of dϕ/dz = 1 × 10(-7) rad/m.