Search

The Daya Bay Reactor Neutrino Experiment has measured a nonzero value for the neutrino mixing angle ${\theta }_{13}$ with a significance of 5.2 standard deviations. Antineutrinos from six 2.9 $\mathrm{G}{\mathrm{W}}_{\mathrm{th}}$ reactors were detected in six antineutrino detectors deployed in two near (flux-weighted baseline 470 m and 576 m) and one far (1648 m) underground experimental halls. With a $43000\mathrm{ton}–\mathrm{G}{\mathrm{W}}_{\mathrm{th}}–\mathrm{day}$ live-time exposure in 55 days, 10 416 (80 376) electron-antineutrino candidates were detected at the far hall (near halls). The ratio of the observed to expected number of antineutrinos at the far hall is $R=0.940\pm 0.011(\mathrm{stat}.)\pm 0.004(\mathrm{syst}.)$. A rate-only analysis finds ${sin}^{2}2{\theta }_{13}=0.092\pm 0.016(\mathrm{stat}.)\pm 0.005(\mathrm{syst}.)$ in a three-neutrino framework.

Neutrons produced by cosmic ray muons are an important background for underground experiments studying neutrino oscillations, neutrinoless double beta decay, dark matter, and other rare-event signals. A measurement of the neutron yield in the three different experimental halls of the Daya Bay Reactor Neutrino Experiment at varying depth is reported. The neutron yield in Daya Bay’s liquid scintillator is measured to be $Y_n=(10.26\pm 0.86)\times {10}^{-5}$, $(10.22\pm 0.87)\times {10}^{-5}$, and $(17.03\pm 1.22)\times {10}^{-5}{\mu }^{-1}{\mathrm{g}}^{-1}{\mathrm{cm}}^2$ at depths of 250, 265, and 860 meters-water-equivalent. These results are compared to other measurements and the simulated neutron yield in Fluka and Geant4. A global fit including the Daya Bay measurements yields a power law coefficient of $0.77\pm 0.03$ for the dependence of the neutron yield on muon energy.

This Letter reports an improved search for light sterile neutrino mixing in the electron antineutrino disappearance channel with the full configuration of the Daya Bay Reactor Neutrino Experiment. With an additional 404 days of data collected in eight antineutrino detectors, this search benefits from 3.6 times the statistics available to the previous publication, as well as from improvements in energy calibration and background reduction. A relative comparison of the rate and energy spectrum of reactor antineutrinos in the three experimental halls yields no evidence of sterile neutrino mixing in the $2\times {10}^{-4}\lesssim |\mathrm{\Delta }{m}_{41}^2|\lesssim 0.3{\mathrm{eV}}^2$ mass range. The resulting limits on ${\sin }^{2}2{\theta }_{14}$ are improved by approx imately a factor of 2 over previous results and constitute the most stringent constraints to date in the $|\mathrm{\Delta }{m}_{41}^2|\lesssim 0.2{\mathrm{eV}}^2$ region.

This Letter reports a measurement of the flux and energy spectrum of electron antineutrinos from six 2.9 ${\mathrm{GW}}_{\mathrm{t}\mathrm{h}}$ nuclear reactors with six detectors deployed in two near (effective baselines 512 and 561 m) and one far (1579 m) underground experimental halls in the Daya Bay experiment. Using 217 days of data, 296 721 and 41 589 inverse $\beta$ decay (IBD) candidates were detected in the near and far halls, respectively. The measured IBD yield is ($1.55\pm 0.04$) $\times {10}^{-18}{\mathrm{cm}}^2{\mathrm{GW}}^{-1}{\text{day}}^{-1}$ or ($5.92\pm 0.14$) $\times {10}^{-43}{\mathrm{cm}}^2{\text{fission}}^{-1}$. This flux measurement is consistent with previous short-baseline reactor antineutrino experiments and is $0.946\pm 0.022$ ($0.991\pm 0.023$) relative to the flux predicted with the $\mathrm{Huber}\text{−}\mathrm{Mueller}$ ($\mathrm{ILL}\text{-Vogel}$) fissile antineutrino model. The measured IBD positron energy spectrum deviates from both spectral predictions by more than $2\sigma$ over the full energy range with a local significance of up to $\sim 4\sigma$ between 4–6 MeV. A reactor antineutrino spectrum of IBD reactions is extracted from the measured positron energy spectrum for model-independent predictions.

A precision measurement of the nitrogen flux with rigidity (momentum per unit charge) from 2.2 GV to 3.3 TV based on $2.2\times {10}^6$ events is presented. The detailed rigidity dependence of the nitrogen flux spectral index is presented for the first time. The spectral index rapidly hardens at high rigidities and becomes identical to the spectral indices of primary He, C, and O cosmic rays above $\sim 700\mathrm{GV}$. We observed that the nitrogen flux ${\mathrm{\Phi }}_{\mathrm{N}}$ can be presented as the sum of its primary component ${\mathrm{\Phi }}_{\mathrm{N}}^P$ and secondary component ${\mathrm{\Phi }}_{\mathrm{N}}^S$, ${\mathrm{\Phi }}_{\mathrm{N}}={\mathrm{\Phi }}_{\mathrm{N}}^P+{\mathrm{\Phi }}_{\mathrm{N}}^S$, and we found ${\mathrm{\Phi }}_{\mathrm{N}}$ is well described by the weighted sum of the oxygen flux ${\mathrm{\Phi }}_{\mathrm{O}}$ (primary cosmic rays) and the boron flux ${\mathrm{\Phi }}_{\mathrm{B}}$ (secondary cosmic rays), with ${\mathrm{\Phi }}_{\mathrm{N}}^P=(0.090\pm 0.002)\times {\mathrm{\Phi }}_{\mathrm{O}}$ and ${\mathrm{\Phi }}_{\mathrm{N}}^S=(0.62\pm 0.02)\times {\mathrm{\Phi }}_{\mathrm{B}}$ over the entire rigidity range. This corresponds to a change of the contribution of the secondary cosmic ray component in the nitrogen flux from 70% at a few GV to $<30\%$ above 1 TV.

A precision measurement by the Alpha Magnetic Spectrometer on the International Space Station of the positron fraction in primary cosmic rays in the energy range from 0.5 to 350 GeV based on $6.8\times {10}^6$ positron and electron events is presented. The very accurate data show that the positron fraction is steadily increasing from 10 to $\sim 250\mathrm{GeV}$, but, from 20 to 250 GeV, the slope decreases by an order of magnitude. The positron fraction spectrum shows no fine structure, and the positron to electron ratio shows no observable anisotropy. Together, these features show the existence of new physical phenomena.

The Daya Bay experiment has observed correlations between reactor core fuel evolution and changes in the reactor antineutrino flux and energy spectrum. Four antineutrino detectors in two experimental halls were used to identify 2.2 million inverse beta decays (IBDs) over 1230 days spanning multiple fuel cycles for each of six 2.9 $G{W}_{\mathrm{th}}$ reactor cores at the Daya Bay and Ling Ao nuclear power plants. Using detector data spanning effective ${}^{239}\mathrm{Pu}$ fission fractions $F_{239}$ from 0.25 to 0.35, Daya Bay measures an average IBD yield ${\overline{\sigma }}_f$ of $(5.90\pm 0.13)\times {10}^{-43}{\mathrm{cm}}^2/\mathrm{fission}$ and a fuel-dependent variation in the IBD yield, $d{\sigma }_f/d{F}_{239}$, of $(-1.86\pm 0.18)\times {10}^{-43}{\mathrm{cm}}^2/\mathrm{fission}$. This observation rejects the hypothesis of a constant antineutrino flux as a function of the ${}^{239}\mathrm{Pu}$ fission fraction at 10 standard deviations. The variation in IBD yield is found to be energy dependent, rejecting the hypothesis of a constant antineutrino energy spectrum at 5.1 standard deviations. While measurements of the evolution in the IBD spectrum show general agreement with predictions from recent reactor models, the measured evolution in total IBD yield disagrees with recent predictions at $3.1\sigma$. This discrepancy indicates that an overall deficit in the measured flux with respect to predictions does not result from equal fractional deficits from the primary fission isotopes ${}^{235}\mathrm{U}$, ${}^{239}\mathrm{Pu}$, ${}^{238}\mathrm{U}$, and ${}^{241}\mathrm{Pu}$. Based on measured IBD yield variations, yields of $(6.17\pm 0.17)$ and $(4.27\pm 0.26)\times {10}^{-43}{\mathrm{cm}}^2/\mathrm{fission}$ have been determined for the two dominant fission parent isotopes ${}^{235}\mathrm{U}$ and ${}^{239}\mathrm{Pu}$. A 7.8% discrepancy between the observed and predicted ${}^{235}\mathrm{U}$ yields suggests that this isotope may be the primary contributor to the reactor antineutrino anomaly.

We present high-statistics, precision measurements of the detailed time and energy dependence of the primary cosmic-ray electron flux and positron flux over 79 Bartels rotations from May 2011 to May 2017 in the energy range from 1 to 50 GeV. For the first time, the charge-sign dependent modulation during solar maximum has been investigated in detail by leptons alone. Based on $23.5\times {10}^6$ events, we report the observation of short-term structures on the timescale of months coincident in both the electron flux and the positron flux. These structures are not visible in the $e^{+}/e^{-}$ flux ratio. The precision measurements across the solar polarity reversal show that the ratio exhibits a smooth transition over $830\pm 30$ days from one value to another. The midpoint of the transition shows an energy dependent delay relative to the reversal and changes by $260\pm 30$ days from 1 to 6 GeV.

We present the precision measurement of the daily proton fluxes in cosmic rays from May 20, 2011 to October 29, 2019 (a total of 2824 days or 114 Bartels rotations) in the rigidity interval from 1 to 100 GV based on $5.5\times {10}^9$ protons collected with the Alpha Magnetic Spectrometer aboard the International Space Station. The proton fluxes exhibit variations on multiple timescales. From 2014 to 2018, we observed recurrent flux variations with a period of 27 days. Shorter periods of 9 days and 13.5 days are observed in 2016. The strength of all three periodicities changes with time and rigidity. The rigidity dependence of the 27-day periodicity is different from the rigidity dependences of 9-day and 13.5-day periods. Unexpectedly, the strength of 9-day and 13.5-day periodicities increases with increasing rigidities up to $\sim 10\mathrm{GV}$ and $\sim 20\mathrm{GV}$, respectively. Then the strength of the periodicities decreases with increasing rigidity up to 100 GV.

A search for a time-varying ${\overline{\nu }}_e$ signal was performed with 621 days of data acquired by the Daya Bay Reactor Neutrino Experiment over 704 calendar days. The time spectrum of the measured ${\overline{\nu }}_e$ flux normalized to its prediction was analyzed with a Lomb-Scargle periodogram, which yielded no significant signal for periods ranging from 2 hours to nearly 2 years. The normalized time spectrum was also fit for a sidereal modulation under the Standard Model Extension (SME) framework to search for Lorentz and $CPT$ violation (LV-CPTV). Limits were obtained for all six flavor pairs $\overline{e}\overline{\mu },\overline{e}\overline{\tau }$, $\overline{\mu }\overline{\tau }$, $\overline{e}\overline{e},\overline{\mu }\overline{\mu }$ and $\overline{\tau }\overline{\tau }$ by fitting them one at a time, constituting the first experimental constraints on the latter three. Daya Bay’s high statistics and unique layout of multiple directions from three pairs of reactors to three experimental halls allowed the simultaneous constraint of individual SME LV-CPTV coefficients without assuming others contribute negligibly, a first for a neutrino experiment.

A measurement of the energy dependence of antineutrino disappearance at the Daya Bay reactor neutrino experiment is reported. Electron antineutrinos (${\overline{\nu }}_e$) from six $2.9{\mathrm{GW}}_{\mathrm{th}}$ reactors were detected with six detectors deployed in two near (effective baselines 512 and 561 m) and one far (1579 m) underground experimental halls. Using 217 days of data, 41 589 (203 809 and 92 912) antineutrino candidates were detected in the far hall (near halls). An improved measurement of the oscillation amplitude ${\sin }^{2}2{\theta }_{13}=0.090_{-0.009}^{+0.008}$ and the first direct measurement of the ${\overline{\nu }}_e$ mass-squared difference $|\mathrm{\Delta }{m}_{ee}^2|=(2.59_{-0.20}^{+0.19})\times {10}^{-3}{\mathrm{e}\mathrm{V}}^2$ is obtained using the observed ${\overline{\nu }}_e$ rates and energy spectra in a three-neutrino framework. This value of $|\mathrm{\Delta }{m}_{ee}^2|$ is consistent with $|\mathrm{\Delta }{m}_{\mu \mu }^2|$ measured by muon neutrino disappearance, supporting the three-flavor oscillation model.

A search for light sterile neutrino mixing was performed with the first 217 days of data from the Daya Bay Reactor Antineutrino Experiment. The experiment’s unique configuration of multiple baselines from six 2.9 ${\mathrm{GW}}_{\mathrm{th}}$ nuclear reactors to six antineutrino detectors deployed in two near (effective baselines 512 m and 561 m) and one far (1579 m) underground experimental halls makes it possible to test for oscillations to a fourth (sterile) neutrino in the ${10}^{-3}{\mathrm{eV}}^2<|\mathrm{\Delta }{m}_{41}^2|<0.3{\mathrm{eV}}^2$ range. The relative spectral distortion due to the disappearance of electron antineutrinos was found to be consistent with that of the three-flavor oscillation model. The derived limits on ${\sin }^{2}2{\theta }_{14}$ cover the ${10}^{-3}{\mathrm{eV}}^2\lesssim |\mathrm{\Delta }{m}_{41}^2|\lesssim 0.1{\mathrm{eV}}^2$ region, which was largely unexplored.

Precision measurements by the Alpha Magnetic Spectrometer on the International Space Station of the primary cosmic-ray electron flux in the range 0.5 to 700 GeV and the positron flux in the range 0.5 to 500 GeV are presented. The electron flux and the positron flux each require a description beyond a single power-law spectrum. Both the electron flux and the positron flux change their behavior at $\sim 30\mathrm{GeV}$ but the fluxes are significantly different in their magnitude and energy dependence. Between 20 and 200 GeV the positron spectral index is significantly harder than the electron spectral index. The determination of the differing behavior of the spectral indices versus energy is a new observation and provides important information on the origins of cosmic-ray electrons and positrons.

A precision measurement by AMS of the antiproton flux and the antiproton-to-proton flux ratio in primary cosmic rays in the absolute rigidity range from 1 to 450 GV is presented based on $3.49\times 10^5$ antiproton events and $2.42\times 10^9$ proton events. The fluxes and flux ratios of charged elementary particles in cosmic rays are also presented. In the absolute rigidity range $\sim 60$ to $\sim 500\mathrm{GV}$, the antiproton $\overline{p}$, proton $p$, and positron $e^{+}$ fluxes are found to have nearly identical rigidity dependence and the electron $e^{-}$ flux exhibits a different rigidity dependence. Below 60 GV, the ($\overline{p}/p$), ($\overline{p}/e^{+}$), and ($p/e^{+}$) flux ratios each reaches a maximum. From $\sim 60$ to $\sim 500\mathrm{GV}$, the ($\overline{p}/p$), ($\overline{p}/e^{+}$), and ($p/e^{+}$) flux ratios show no rigidity dependence. These are new observations of the properties of elementary particles in the cosmos.

Searches for electron antineutrino, muon neutrino, and muon antineutrino disappearance driven by sterile neutrino mixing have been carried out by the Daya Bay and $\mathrm{MINOS}+$ collaborations. This Letter presents the combined results of these searches, along with exclusion results from the Bugey-3 reactor experiment, framed in a minimally extended four-neutrino scenario. Significantly improved constraints on the ${\theta }_{\mu e}$ mixing angle are derived that constitute the most constraining limits to date over five orders of magnitude in the mass-squared splitting $\mathrm{\Delta }{m}_{41}^2$, excluding the 90% C.L. sterile-neutrino parameter space allowed by the LSND and MiniBooNE observations at 90% ${\mathrm{CL}}_s$ for $\mathrm{\Delta }{m}_{41}^2<13{\mathrm{eV}}^2$. Furthermore, the LSND and MiniBooNE 99% C.L. allowed regions are excluded at 99% ${\mathrm{CL}}_s$ for $\mathrm{\Delta }{m}_{41}^2<1.6{\mathrm{eV}}^2$.

This Letter reports the first measurement of the oscillation amplitude and frequency of reactor antineutrinos at Daya Bay via neutron capture on hydrogen using 1958 days of data. With over 3.6 million signal candidates, an optimized candidate selection, improved treatment of backgrounds and efficiencies, refined energy calibration, and an energy response model for the capture-on-hydrogen sensitive region, the relative ${\overline{\nu }}_e$ rates and energy spectra variation among the near and far detectors gives ${\sin }^{2}2{\theta }_{13}=0.0759_{-0.0049}^{+0.0050}$ and $\mathrm{\Delta }{m}_{32}^2=(2.72_{-0.15}^{+0.14})\times {10}^{-3}{\mathrm{eV}}^2$ assuming the normal neutrino mass ordering, and $\mathrm{\Delta }{m}_{32}^2=(-2.83_{-0.14}^{+0.15})\times {10}^{-3}{\mathrm{eV}}^2$ for the inverted neutrino mass ordering. This estimate of ${\sin }^{2}2{\theta }_{13}$ is consistent with and essentially independent from the one obtained using the capture-on-gadolinium sample at Daya Bay. The combination of these two results yields ${\sin }^{2}2{\theta }_{13}=0.0833\pm 0.0022$, which represents an 8% relative improvement in precision regarding the Daya Bay full 3158-day capture-on-gadolinium result.

A measurement of electron antineutrino oscillation by the Daya Bay Reactor Neutrino Experiment is described in detail. Six 2.9-GWth nuclear power reactors of the Daya Bay and Ling Ao nuclear power facilities served as intense sources of ${\overline{\nu }}_e$’s. Comparison of the ${\overline{\nu }}_e$ rate and energy spectrum measured by antineutrino detectors far from the nuclear reactors ($\sim 1500–1950\mathrm{m}$) relative to detectors near the reactors ($\sim 350–600\mathrm{m}$) allowed a precise measurement of ${\overline{\nu }}_e$ disappearance. More than 2.5 million ${\overline{\nu }}_e$ inverse beta-decay interactions were observed, based on the combination of 217 days of operation of six antineutrino detectors (December, 2011–July, 2012) with a subsequent 1013 days using the complete configuration of eight detectors (October, 2012–July, 2015). The ${\overline{\nu }}_e$ rate observed at the far detectors relative to the near detectors showed a significant deficit, $R=0.949\pm 0.002(\text{stat})\pm 0.002(\text{syst})$. The energy dependence of ${\overline{\nu }}_e$ disappearance showed the distinct variation predicted by neutrino oscillation. Analysis using an approximation for the three-flavor oscillation probability yielded the flavor-mixing angle ${\sin }^{2}2{\theta }_{13}=0.0841\pm 0.0027(\text{stat})\pm 0.0019(\text{syst})$ and the effective neutrino mass-squared difference of $|\mathrm{\Delta }{m}_{\mathrm{ee}}^2|=(2.50\pm 0.06(\text{stat})\pm 0.06(\text{syst}))\times {10}^{-3}{\mathrm{eV}}^2$. Analysis using the exact three-flavor probability found $\mathrm{\Delta }{m}_{32}^2=(2.45\pm 0.06(\text{stat})\pm 0.06(\text{syst}))\times {10}^{-3}{\mathrm{eV}}^2$ assuming the normal neutrino mass hierarchy and $\mathrm{\Delta }{m}_{32}^2=(-2.56\pm 0.06(\text{stat})\pm 0.06(\text{syst}))\times {10}^{-3}{\mathrm{eV}}^2$ for the inverted hierarchy.

Precision measurements by the Alpha Magnetic Spectrometer (AMS) on the International Space Station of ${}^3\mathrm{He}$ and ${}^4\mathrm{He}$ fluxes are presented. The measurements are based on 100 million ${}^4\mathrm{He}$ nuclei in the rigidity range from 2.1 to 21 GV and 18 million ${}^3\mathrm{He}$ from 1.9 to 15 GV collected from May 2011 to November 2017. We observed that the ${}^3\mathrm{He}$ and ${}^4\mathrm{He}$ fluxes exhibit nearly identical variations with time. The relative magnitude of the variations decreases with increasing rigidity. The rigidity dependence of the ${}^3\mathrm{He}/{}^4\mathrm{He}$ flux ratio is measured for the first time. Below 4 GV, the ${}^3\mathrm{He}/{}^4\mathrm{He}$ flux ratio was found to have a significant long-term time dependence. Above 4 GV, the ${}^3\mathrm{He}/{}^4\mathrm{He}$ flux ratio was found to be time independent, and its rigidity dependence is well described by a single power law $\propto R^{\mathrm{\Delta }}$ with $\mathrm{\Delta }=-0.294\pm 0.004$. Unexpectedly, this value is in agreement with the B/O and B/C spectral indices at high energies.

Knowledge of the precise rigidity dependence of the helium flux is important in understanding the origin, acceleration, and propagation of cosmic rays. A precise measurement of the helium flux in primary cosmic rays with rigidity (momentum/charge) from 1.9 GV to 3 TV based on 50 million events is presented and compared to the proton flux. The detailed variation with rigidity of the helium flux spectral index is presented for the first time. The spectral index progressively hardens at rigidities larger than 100 GV. The rigidity dependence of the helium flux spectral index is similar to that of the proton spectral index though the magnitudes are different. Remarkably, the spectral index of the proton to helium flux ratio increases with rigidity up to 45 GV and then becomes constant; the flux ratio above 45 GV is well described by a single power law.