Abstract
If Dark Matter is made of Weakly Interacting Massive Particles (WIMPs) with masses below {sim }20 GeV, the corresponding nuclear recoils in mainstream WIMP experiments are of energies too close, or below, the experimental threshold. Gas Time Projection Chambers (TPCs) can be operated with a variety of target elements, offer good tracking capabilities and, on account of the amplification in gas, very low thresholds are achievable. Recent advances in electronics and in novel radiopure TPC readouts, especially micro-mesh gas structure (Micromegas), are improving the scalability and low-background prospects of gaseous TPCs. Here we present TREX-DM, a prototype to test the concept of a Micromegas-based TPC to search for low-mass WIMPs. The detector is designed to host an active mass of {sim }0.300 kg of Ar at 10 bar, or alternatively {sim }0.160 kg of Ne at 10 bar, with an energy threshold below 0.4 keVee, and is fully built with radiopure materials. We will describe the detector in detail, the results from the commissioning phase on surface, as well as a preliminary background model. The anticipated sensitivity of this technique may go beyond current experimental limits for WIMPs of masses of 2–8 GeV.
Highlights
Standard Model, in particular those including SuperSymmetry (SUSY) [2].If our galactic Dark Matter (DM) halo is made of Weakly Interacting Massive Particles (WIMPs), they could interact with nuclei and produce detectable nuclear recoils in the target material of underground terrestrial experiments
If Dark Matter is made of Weakly Interacting Massive Particles (WIMPs) with masses below ∼20 GeV, the corresponding nuclear recoils in mainstream WIMP experiments are of energies too close, or below, the experimental threshold
Sensitivity projections for low WIMP masses should be treated with great caution because such low-mass WIMP interactions produce recoil energy deposits that are mostly below the energy threshold of experiments based on heavy target nuclei like Xe or Ge. This means that the exclusion limits derived for lowmass WIMPs by these experiments rely on detecting the interactions of a very small (1 % or lower) fraction of the incident WIMP velocity distribution, corresponding to the WIMPs with kinetic energies high enough to produce a nuclear recoil above the detector energy threshold
Summary
Standard Model, in particular those including SuperSymmetry (SUSY) [2]. If our galactic DM halo is made of WIMPs, they could interact with nuclei and produce detectable nuclear recoils in the target material of underground terrestrial experiments. The leading experiments in the “WIMP race” are those using relatively heavy target nuclei (e.g. Xe or Ge) – to exploit the A2 dependence of the coherent WIMP-nucleus interaction – and using detection techniques that provide nuclear recoil discrimination This is the case, e.g. of liquid Xe double-phase detectors (e.g. LUX [3] or XENON [4]) or hybrid Ge bolometers (like CDMS/SuperCDMS [5,6,7]). SuperCDMS [7] has operated ∼9 kg of Ge target mass observing 11 nuclear-recoil candidate events in 577 kg-days, while plans for the 100 kg scale are ongoing; LUX [3] has operated 118 kg of liquid Xe fiducial mass, observing a background level that effectively limits a possible WIMP nuclear recoil signal to 2–5 events (depending on the mass) in a run of 85.3 live days This corresponds to the current most stringent upper limit on the WIMP-nucleon cross-section of 7.6 × 10−46 cm at a WIMP mass of 33 GeV. With these facts in mind, recent theoretical and phenomenological efforts have focused on the study of less conventional SUSY models, or even non-SUSY WIMP models (like e.g. Asymmetric DM models [13])
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