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The Daya Bay Reactor Neutrino Experiment is designed to determine precisely the neutrino mixing angle θ 13 with a sensitivity better than 0.01 in the parameter sin 2 2θ 13 at the 90% confidence level. To achieve this goal, the collaboration will build eight functionally identical antineutrino detectors. The first two detectors have been constructed, installed and commissioned in Experimental Hall 1, with steady data-taking beginning September 23, 2011. A comparison of the data collected over the subsequent three months indicates that the detectors are functionally identical, and that detector-related systematic uncertainties exceed requirements.

We present a detailed report of the method, setup, analysis and results of a precision measurement of the positive muon lifetime. The experiment was conducted at the Paul Scherrer Institute using a time-structured, nearly 100%-polarized, surface muon beam and a segmented, fast-timing, plastic scintillator array. The measurement employed two target arrangements; a magnetized ferromagnetic target with a ∼4 kG internal magnetic field and a crystal quartz target in a 130 G external magnetic field. Approximately 1.6 × 10 12 positrons were accumulated and together the data yield a muon lifetime of τµ(MuLan) = 2 196 980.3(2.2) ps (1.0 ppm), thirty times more precise than previous generations of lifetime experiments. The lifetime measurement yields the most accurate value of the Fermi constant GF (MuLan) = 1.166 378 7(6) × 10 −5 GeV −2 (0.5 ppm). It also enables new precision studies of weak interactions via lifetime measurements of muonic atoms.

The Daya Bay experiment was the first to report simultaneous measurements of reactor antineutrinos at multiple baselines leading to the discovery ofν e oscillations over km-baselines. Subsequent data has provided the world's most precise measurement of sin 2 2θ 13 and the effective mass splitting ∆m 2 ee . The experiment is located in Daya Bay, China where the cluster of six nuclear reactors is among the world's most prolific sources of electron antineutrinos. Multiple antineutrino detectors are deployed in three underground water pools at different distances from the reactor cores to search for deviations in the antineutrino rate and energy spectrum due to neutrino mixing. Instrumented with photomultiplier tubes, the water pools serve as shielding against natural radioactivity from the surrounding rock and provide efficient muon tagging. Arrays of resistive plate chambers over the top of each pool provide additional muon detection. The antineutrino detectors were specifically designed for measurements of the antineutrino flux with minimal systematic uncertainty. Relative detector efficiencies between the near and far detectors are known to better than 0.2%. With the unblinding of the final two detectors' baselines and target masses, a complete description and comparison of the eight antineutrino detectors can now be presented. This paper describes the Daya Bay detector systems, consisting of eight antineutrino detectors in three instrumented water pools in three underground halls, and their operation through the first year of eight detector data-taking.

We report a measurement of the positive muon lifetime to a precision of 1.0 parts per million (ppm); it is the most precise particle lifetime ever measured. The experiment used a time-structured, low-energy muon beam and a segmented plastic scintillator array to record more than 2 × 10 12 decays. Two different stopping target configurations were employed in independent data-taking periods. The combined results give τ µ + (MuLan) = 2196980.3(2.2) ps, more than 15 times as precise as any previous experiment. The muon lifetime gives the most precise value for the Fermi constant: GF (MuLan) = 1.1663788(7) × 10 −5 GeV −2 (0.6 ppm). It is also used to extract the µ − p singlet capture rate, which determines the proton's weak induced pseudoscalar coupling g P .A measurement of the positive muon lifetime, τ µ + , to high precision determines the Fermi constant, G F , according to the relationHere, ∆q represents well-known phase space and both QED and hadronic radiative corrections [1], and we assume that G F is universal for weak interactions. Strictly speaking, τ µ + determines a muon-decay-specific coupling, denoted G µ , which could be compared to other G F determinations as a test of the standard model [2]. Prior to 1999, the limitation on the precision of G F was dominated by the uncertainty on ∆q. Van Ritbergen and Stuart were the first to compute the secondorder QED radiative corrections in the massless electron limit, reducing the theoretical uncertainty to below 0.3 ppm [3], and well below the then-current experimental uncertainty. This development motivated a new generation of precision muon lifetime measurements, MuLan [4] and FAST [5]. More recently, Pak and Czarnecki extended the result in [3] to finite electron mass, which shifts the predicted decay rate 1/τ µ by -0.43 ppm; alternatively, it decreases G F by 0.21 ppm [6].In Ref.[4], we reported an 11 ppm measurement of τ µ + based on a relatively short commissioning run. This Letter reports the results from a 100 times larger data set, accumulated using the final setup of the experiment.The experiment is designed to stop muons in a target during a beam-on accumulation interval and measure the decay positrons-primarily from the µ + → e + ν eνµ decay mode-during a beam-off measurement period. The two running periods, R06 and R07, used different targets. More than 10 12 decays were recorded in each period.The experiment used the πE3 beamline at the Paul Scherrer Institute (PSI). During the run, positive muons from at-rest pion decay near the surface of the production target are directed to the experiment through two opposing vertical dipole magnets and a series of 15 magnetic quadrupole lenses. A velocity-selecting E × B separator is tuned to pass muons and reject positrons. A special feature of the beamline is a custom, 60-ns switching, 25-kV kicker [8]. When energized, the electric field across the 120-mm vertical gap and 1500-mm length displaces the muon beam by 46 mm at the exit and deflects it by 45 mrad onto a downstream collimator. The muon flux of ∼ 1...

This Letter presents results of a search for the mixing of a sub-eV sterile neutrino with three active neutrinos based on the full data sample of the Daya Bay Reactor Neutrino Experiment, collected during 3158 days of detector operation, which contains 5.55 × 10 6 reactor νe candidates identified as inverse beta-decay interactions followed by neutron-capture on gadolinium. The analysis benefits from a doubling of the statistics of our previous result and from improvements of several important systematic uncertainties. No significant oscillation due to mixing of a sub-eV sterile neutrino with active neutrinos was found. Exclusion limits are set by both Feldman-Cousins and CLs methods. Light sterile neutrino mixing with sin 2 2θ14 ≳ 0.01 can be excluded at 95% confidence level in the region of 0.01 eV 2 ≲ |∆m 2 41 | ≲ 0.1 eV 2 . This result represents the world-leading constraints in the region of 2 × 10 −4 eV 2 ≲ |∆m 2 41 | ≲ 0.2 eV 2 .