New limits on spin-independent and spin-dependent couplings of low-mass WIMP dark matter with a germanium detector at a threshold of 220 eV

Abstract

An energy threshold of (220±10) eV was achieved at an efficiency of 50% with a four-channel ultra-low-energy germanium detector each with an active mass of 5 g. This provides a unique probe to WIMP dark matter with mass below 10 GeV. With a data acquisition live time of 0.338 kg-day at the Kuo-Sheng Laboratory, constraints on WIMPs in the galactic halo were derived. The limits improve over previous results on both spin-independent WIMP-nucleon and spin-dependent WIMPneutron cross-sections for WIMP mass between 3−6 GeV. Sensitivities for full-scale experiments are projected. This detector technique makes the unexplored sub-keV energy window accessible for new neutrino and dark matter experiments.PACS numbers: 95.35.+d, 98.70.Vc There is compelling evidence from cosmological and astrophysical observations that about one quarter of the energy density of the universe can be attributed to Cold Dark Matter(CDM), whose nature and properties are still unknown [1]. Weakly Interacting Massive Particles (WIMP, denoted by χ) are the leading candidates for CDM. There are intense experimental efforts[2] to look for WIMPs through direct detection of nuclear recoils in χN→χN elastic scattering or in the studies of the possible products through χχ annihilations. The Kuo-Sheng(KS) Laboratory[12] is located at 28 m from a 2.9 GW reactor core with an overburden of about 30 meter-water-equivalence. Limits on neutrino magnetic moments(µ ν )[13] with a 1.06-kg germanium detector(HPGe) at a threshold of 5 keV were reported [14]. These data also allowed the studies of reactor electron neutrinos [15] and reactor axions [16]. A background level of ∼ 1 event kg −1 keV −1 day −1 (cpkkd) at 20 keV, comparable with those of underground CDM experiments, was achieved. The current goal is to develop detectors with kg-scale target mass, 100 eV-range threshold and lowbackground specifications for the studies of WIMPs, µ ν and neutrino-nucleus coherent scatterings [17].Ultra-low-energy germanium detectors(ULEGe) is a matured technique for sub-keV soft X-rays measurements. They typically have modular mass of 5−10 g while detector arrays of up to 30 elements have been constructed. Compared with Al 2 O 3 , Ge provides enhancement in χN spin-independent couplings(σ SI χN ) due to the A 2 dependence [1,18], where A is the mass number of the target isotopes. The isotope 73 Ge (natural isotopic abundance of 7.73%) comprises an unpaired neutron such that it can provide additional probe to the spin-dependent couplings of WIMPs with the neutrons(σ SD χn ). The nuclear recoils from χN interactions in ULEGe only give rise to ∼20% of the observable ionizations compared with electron recoils at the same energy. The suppression ratio is called the quenching factor(QF) [19]. For clarity, all ULEGe measurements discussed hereafter in this article are electron-equivalent-energy, unless otherwise stated.The ULEGe array consists of 4-element each having an active mass of 5 g [20]. Standard ultra-low-background specifications were adopted in its construction and ch...

Consistent dark matter interpretation for CoGeNT and DAMA/LIBRA

“…For the measurements used, see Ref. [24]. Additional measurements by the CoGeNT Collaboration span down to E Recoil ¼ 0:7 keV [25].…”

Exothermic dark matter

“…The TEXONO experiment uses a Ge crystal detector with a very low threshold energy of 200 eV [42]. Given its relatively small number of kg-keV days this experiment does not provide a stringent constraint on exoDM parameter space.…”

Galactic searches for dark matter

“…• TEXONO-TEXONO is a germanium detector similar to CoGeNT, which is located in the Kuo-Sheng Laboratory (Lin et al, 2009). From an exposure of 0.338 kg-day, TEXONO has reported limits on the spin-independent cross section down to 2 GeV, constraining the cross section to 10 −38 cm 2 at this mass.…”