New Results from the Daya Bay Reactor Neutrino Experiment

Abstract

Some recent results obtained by Daya Bay Collaboration and presented at Neutrino Telescopes 2017 are briefly reviewed. The most precise measurement of neutrino mixing $\sin^2 2 \theta_{13} =0.0841\pm0.0027\textrm{ (stat.) }\pm0.0019\textrm{ (syst.) }$ and mass squared difference $\left|{\Delta}m^2_{\mathrm{ee}}\right| =\left[2.50\pm0.06\textrm{ (stat.) }\pm0.06\textrm{ (syst.) }\right]\times 10^{-3}\,\mathrm{eV}^2$ was obtained using nGd data set. These results agree with an independent analysis of nH data sample. The data was analized also within the model of neutrino wave packet. For the first time an upper limit on the intrinsic relative dispersion of neutrino momentum \UpperLimit{} at a 95\% C.L. was obtained. The allowed decoherence effect due to the wave packet nature of neutrino oscillation is found to be insignificant for reactor antineutrinos detected by the Daya Bay experiment thus ensuring an unbiased measurement of the oscillation parameters $\sin^22\theta_{13}$ and $\Delta m^2_{32}$ within the plane wave model. The flux of reactor $\overline{\nu}_e$ was measured and found to be in agreement with old reactor experiments observing $\overline{\nu}_e$ flux smaller than expectation in Huber+Muller model. Comparing observed and predicted energy spectra we find an almost flat deficit of measured events for $E_\nu<5$ MeV and an agreement with the Huber+Muller model for $5.5 \textrm{ MeV}\lesssim E_\nu\lesssim 6.5$ MeV. A possible hypothesis of the observed flux deficit invokes $\overline{\nu}_e$ oscillation into a sterile state. This hypothesis was excluded for a large range of $\Delta m^2_{41}$. A combined analysis of Daya Bay, Bugey-3 and MINOS data excludes most of the allowed by LSND and MiniBooNE experiments sterile-neutrino phase space for $\Delta m^2_{41}<0.8$ eV${}^2$ at 90\% C.L.