Search for a Light Sterile Neutrino at Daya Bay

Abstract

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 GW 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 eV 2 < |∆m 2 41 | < 0.3 eV 2 range. The relative spectral distortion due to electron antineutrino disappearance was found to be consistent with that of the three-flavor oscillation model. The derived limits on sin 2 2θ14 cover the 10 −3 eV 2 |∆m 2 41 | 0.1 eV 2 region, which was largely unexplored.

Cosmological implications of light sterile neutrinos produced after the QCD phase transition

“…The latest limits on sterile neutrino mixing from atmospheric neutrino data have been set by the Super Kamiokande experiment [31] which sets the limits |U µ4 | 2 < 0.041. Similar bounds have been by the Daya Bay collaboration [32] and the analysis in [33,34] examines the global fits for various light sterile neutrino scenarios (3+1,3+2,3+1+1). A summary of the light sterile neutrino bounds for active-sterile mixing from accelerators, cosmology and other experiments are summarized concisely in figs 1-3 of ref [35] while those for heavy steriles can be found in [36].…”

“…A sterile species that is still relativistic at the time of matter-radiation equality will contribute to N ef f and, since this type of sterile neutrino becomes non-relativistic at T ∼ m, the contributions to N ef f are only valid for m ν 1eV . Parameterizing the contribution to dark radiation as N ef f = N 0 ef f + ∆N ef f where N 0 ef f = 3.046 is the standard model contribution [77], the sterile neutrinos we consider here contribute Combining with a recent analysis [31,32] we find that ∆N ef f 4, suggesting that this mechanism could provide a significant contribution to N ef f although severe tensions remain between accelerator/reactor fits and CMB observations.…”

“…We find (today) If the mass of sterile neutrinos is m νs < 1eV they may contribute to the radiation component between matter radiation equality and photon decoupling thereby contributing to the effective number of relativistic degrees of freedom N ef f . The most recent accelerator and astrophysical bounds on the masses and mixing angles of sterile neutrinos in 3 + 1 or 3 + 2 schemes [31][32][33][34] combined with the result for the frozen distribution function suggest substantial contributions from this species to N ef f although severe tensions remain between accelerator data and Planck bounds from the CMB Further Questions While we focused on "light" sterile neutrinos with m νs < 1MeV , there are potentially important aspects to be studied for the case of 10MeV m νs 140MeV , a range of masses kinematically available in π → eν s . These "heavier" species may actually contribute as a CDM component since freeze-out still occurs at a scale T f ∼ 10 − 15MeV therefore this species will be non-relativistic and non-thermal upon freeze out.…”

“…Kamland and Daya Bay [31,32] recently reported upper bounds of |U µs | 2 < 0.01 for the mass squared difference 10 −3 eV 2 < |∆m 1s | 2 < 0.1eV 2 . Taking the upper bound leads to ∆N ef f < 4 suggesting that π → µν s can contribute significantly to N ef f for a ∼ 1eV sterile.…”

New mixing schemes for (3+1) neutrinos

“…Super-Kamiokande has provided upper bounds on sterile neutrino parameters |U µ4 | 2 < 0.041 and |U τ 4 | 2 < 0.18 at 90% CL [13]. The recent data from reactor and other short and long baseline neutrino experiments such as MINOS [14], Daya Bay [15] etc. provide new bounds on active-sterile mixing and ∆m 2 41 .…”