Studies of Neutrino-Nucleus Elastic Scattering with Point-Contact Germanium Detectors at the Kuo-Sheng Reactor Neutrino Laboratory

Abstract

The low energy and intense flux of electron anti-neutrinos from nuclear reactors provide the perfect stage to study elastic neutrino-nucleus scattering ($\hat{I}½A_{el}$) in the fully coherent regime. We report results from the TEXONO experiment using electro-cooled $p$-type point-contact Germanium detectors with masses of 523~g and 1434~g at the Kuo-Sheng Reactor Neutrino Laboratory. We report improved constraints on the $\hat{I}½A_{el}$ cross section with a combined exposure of 404(813.7)~kg-days of Reactor ON(OFF) data at an electron-equivalent threshold of 200~eV${}_{ee}$. The Lindhard model, in which the quenching factor is parameterized by a single parameter k, is adopted to describe the suppression of ionization yield. At the benchmark value of k=0.162, a limit of $\ddot{I}<$2.0 at 90\% confidence level (CL) is derived, where $\ddot{I}$ represents the ratio of the observed to the predicted Standard Model cross section. Moreover the region k$>$0.205 is excluded at 90\% CL using the SM-predicted $\hat{I}½A_{el}$ rate. A bound on the neutrino magnetic moment from $\hat{I}½A_{el}$ at $\hat{I}{¼}_{\hat{I}}½<5.9\times 10^{-10}\hat{I}{¼}_B$ at 90\% CL is also derived.

21 pages, 19 figures