First results on low-mass WIMPs from the CDEX-1 experiment at the China Jinping underground laboratory

Abstract

The China Dark Matter Experiment Collaboration reports the first experimental limit on weakly interacting massive particles (WIMPs) dark matter from 14.6 kg-days of data taken with a 994 g p-type point-contact germanium detector at the China Jinping underground laboratory where the rock overburden is more than 2400 m. The energy threshold achieved was 400 eVee. According to the 14.6 kg-day live data, we placed the limit of N ¼ 1:75 Â 10 À40 cm 2 at a 90% confidence level on the spin-independent cross section at a WIMP mass of 7 GeV before differentiating bulk signals from the surface backgrounds.There are many pieces of evidence from astroparticle physics and cosmology which indicate that about one quarter of the mass of our Universe is composed of dark matter [1]. The nature of dark matter is unknown, except that it is coupled with matter via gravity. One of the possible candidates for dark matter is weakly interacting massive particles (WIMPs, denoted by ), as motivated by many new theories beyond the standard model [2]. Direct detection of WIMP dark matter has been attempted with different detector technologies in the particle physics domain [3].In recent years, several experiments have expanded their coverage down to low-mass WIMPs with m < 10 GeV [4][5][6][7][8][9]. A point-contact germanium detector can reach an energy threshold of hundreds of eV while keeping almost the same energy resolution as the traditional coaxial germanium detector [10]. Thus, it can be a good choice for a low-mass dark matter search. Based on our previous work [11], the China Dark Matter Experiment (CDEX) Collaboration has formally started a program aimed at the direct detection of low-mass WIMPs using a ton-scale germanium array detector system [12]. As the first step, in 2011 the CDEX phase I experiment (CDEX-1) started to test and run its first prototype p-type point-contact germanium (PPCGe) detector with a crystal mass of 994 g. The experiment took place at the China Jinping underground laboratory (CJPL), which was established at the end of 2010. With 2400 m of rock overburden, CJPL is the deepest operational underground laboratory for particle physics in the world. The cosmic ray flux in CJPL is down to 61:7 y À1 m À2 [13], and this makes it a very good site for ultra-low-background experiments such as dark matter search, double beta decay, and so on.The point-contact germanium detectors have also been used by several experiments [4,9] to directly search for low-mass WIMPs. Due to the relative shallow cosmic ray shielding, CoGeNT and TEXONO used muon veto detectors to decrease the direct and indirect background contributions from cosmic ray muons. The muon veto method can decrease the background contribution of the * Corresponding author. yueq@mail.tsinghua.edu.cn † Participating as a member of the TEXONO Collaboration.PHYSICAL REVIEW D 88, 052004 (2013) 1550-7998= 2013=88(5)=052004(5) 052004-1

New Results from the Search for Low-Mass Weakly Interacting Massive Particles with the CDMS Low Ionization Threshold Experiment

“…In this case, the number densities of the two are also related, suggesting searches for particles with masses of a few GeV/c 2 . Hints of WIMP signals near detector thresholds [17][18][19][20][21], and an excess of gamma-ray emission from the Galactic Center [22], have also contributed to interest in the low-mass WIMP region [5,[23][24][25][26][27][28][29][30][31][32].…”

Production Yield of Muon-Induced Neutrons in Lead

“…First data are obtained with a PPC-Ge of 994 g total mass [324]. However, as the rejection of Compton and surface background is not yet applied, the obtained exclusion limit [324] is slightly worse than the latest TEXONO result [229].…”

“…14 CDMS II (Si) [31], CoGeNT [7,8], CRESST-II [63] . 15 CDEX [324], CDMS II (Ge) [37], CDMS II (Ge) + EDELWEISS II [38], CDMS II (Ge)* [39], CDMS (SUF)* [41], COUPP [91], EDELWEISS-II [76], EDELWEISS-II* [77], KIMS [213], LUX [42], SIMPLE [172], TEXONO [229], WArP [101], XENON 10* [60], XENON 100 [68], XMASS* [20], ZEPLIN III [47] . 16 The data for CDMS II (Ge) [37], CDMS II (Ge)* [39], CDMS SUF* [41], CDMS II (Ge) + EDELWEISS II [38], CRESST-II [63], CoGeNT [7,8], COUPP [91], DAMA/LIBRA [282], EDELWEISS-II [76], KIMS [213], LUX [42], SIMPLE [172], WArP [101], XENON 100 [68], XENON 10* [60], ZEPLIN-III [47], and the prediction for the coherent neutrino background [123] were obtained from the DMTools (http://dmtools.brown.edu).…”

“…16 The data for CDMS II (Ge) [37], CDMS II (Ge)* [39], CDMS SUF* [41], CDMS II (Ge) + EDELWEISS II [38], CRESST-II [63], CoGeNT [7,8], COUPP [91], DAMA/LIBRA [282], EDELWEISS-II [76], KIMS [213], LUX [42], SIMPLE [172], WArP [101], XENON 100 [68], XENON 10* [60], ZEPLIN-III [47], and the prediction for the coherent neutrino background [123] were obtained from the DMTools (http://dmtools.brown.edu). The data for CDMS II (Si) [31], CDEX [324], DAMA/LIBRA [98, Fig. 1,rightpanel], EDELWEISS-II* [77], IceCube [12], TEX-ONO [229], XMASS [20], and the theoretical model [137] were copied directly from the original publication.…”

“…Experimental upper limits at 90 % CL on the spin-independent cross section for elastic scattering of WIMPs off various targets, σ SI χ,N , normalized per nucleon, as function of the WIMP mass m χ (CDEX [324], CDMS II (Ge) [37], CDMS II (Si) [31], CDSM II + EDELWEISS II [38], COUPP [91], EDELWEISS-II [76], KIMS [213], IceCube for W + W − , b b final states [12], LUX [42], SIMPLE [172], TEXONO [229], WArP [101], XENON100 [68], ZEPLIN-III [47]). Dedicated analyses for low WIMP masses are indicated by an asterisk (CDMS II (Ge) [39], CDMS (SUF) [41], EDELWEISS-II [77], XENON10 [60], XMASS [20]).…”