New Limits on the Coherent Neutrino-Nucleus Elastic Scattering Cross Section at the Kuo-Sheng Reactor-Neutrino Laboratory

Neutrino-nucleus elastic scattering (νAₑₗ) with partial quantum-mechanical coherency has been observed in several nuclei with neutrinos from decay-at-rest pions. This interaction with reactor neutrinos where coherency is >95% has not yet been experimentally observed. We present new results on the studies of the νAₑₗ cross section with an electrocooled p-type point-contact germanium detector at the Kuo-Sheng Reactor Neutrino Laboratory. A total of 242(357) kg-days of Reactor ON(OFF) data at a detector threshold of 200 eV in ionization energy are analyzed. The Lindhard model parametrized by a single variable k which characterizes the quenching function was used. A limit at 90% confidence level of ρ<4.7 at the predicted value of k=0.162 is derived, where ρ is the ratio of the measured to standard model (SM) cross section. Conversely, k<0.288 at the SM value of ρ=1.