Title: The first result on 76Ge neutrinoless double beta decay from CDEX-1 experiment,
Abstract: We report the first result on Ge-76 neutrinoless double beta decay from
CDEX-1 experiment at China Jinping Underground Laboratory. A mass of 994 g
p-type point-contact high purity germanium detector has been installed to
search the neutrinoless double beta decay events, as well as to directly detect
dark matter particles. An exposure of 304 kg*day has been analyzed. The
wideband spectrum from 500 keV to 3 MeV was obtained and the average event rate
at the 2.039 MeV energy range is about 0.012 count per keV per kg per day. The
half-life of Ge-76 neutrinoless double beta decay has been derived based on
this result as: T 1/2 > 6.4*10^22 yr (90% C.L.). An upper limit on the
effective Majorana-neutrino mass of 5.0 eV has been achieved. The possible
methods to further decrease the background level have been discussed and will
be pursued in the next stage of CDEX experiment.