Dark matter detection with cryogenic detectors

Direct detection of dark matter in the form of Weakly-Interacting Massive Particles (WIMPS) is an active field of research. Cryogenic detectors have been in the forefront of the field, due to their exquisite ability to reject backgrounds from interactions of normal matter. In this paper, I describe the current status and future prospects for such experiments. 1. The landscape of dark matter direct detection Observations of galaxies, superclusters, distant supernovae and the cosmic microwave background radiation, tell us that ~85% of the matter in the universe is not made of particles that emit or absorb electromagnetic radiation. A leading hypothesis is that it is comprised of Weakly Interacting Massive Particles [1], or WIMPs, that were produced moments after the Big Bang. Particle physics theories provide possible WIMP candidates with masses ~10-1000 GeV/c. For example, supersymmetry contains a lightest supersymmetric partner (LSP) that is stable and interacts at roughly the weakinteraction rate, allowing it to decouple from ordinary matter in the early universe with a relic density comparable to the dark matter density [2]. Similarly, many models involving extra dimensions predict the lightest Kaluza Klein excitation is stable, with weak-scale mass and interaction cross sections [3]. If WIMPs are indeed the dark matter, their density in the galactic halo may allow them to be detected via elastic scattering from atomic nuclei in a suitable terrestrial target. The energy depositions and interaction rates are low, requiring that this type of experiment be located deep underground for protection from cosmic rays and requiring the use of radio-pure materials to shield against radioactivity in the environment. The current generation of direct detection experiments is now reaching the level of sensitivity needed to probe theoretical predictions in a way that is quite complementary to accelerator searches. The combination of LHC and WIMP-nucleus elasticscattering experiments would check the consistency of the models and provide powerful constraints on the parameters. More generally, the low mass of the Higgs inferred from electroweak measurements point to a WIMP-nucleon cross section in the range10->10 cm [4]. The WIMP detection field has grown over the last several years, with the introduction of new technologies. There are now approximately fourteen operating experiments (DAMA/LIBRA, KIMS, CaF2-Kamioka, WARP, XENON-10, ZEPLIN-II, DEAP-I, XMASS, CDMS, CRESST, EDELWEISS, COUPP, PICASSO, DRIFT) and at least another three active R&D programs (ArDM, LUX and DEAP/CLEAN). In this paper, I will focus on the status and prospects for cryogenic direct-detection experiments. A separate contribution to these proceedings will survey non-cryogenic techniques [5]. 10th Int. Conf. on Topics in Astroparticle and Underground Physics (TAUP2007) IOP Publishing Journal of Physics: Conference Series 120 (2008) 042002 doi:10.1088/1742-6596/120/4/042002 c © 2008 IOP Publishing Ltd 1 1.1. WIMP detection When considering the scattering of WIMPs on nuclei, two types of coupling between the WIMP and a nucleon must be considered: spin-dependent and spin-independent [6]. The balance between the two types of coupling in supersymmetry depends on the flavor composition of the lightest particle and can strongly favor one or the other coupling. However, in the case of spin-dependent couplings, there is a cancellation between opposite-aligned spins in the target nucleus, while in the spin-independent case, all nucleons add coherently. This amplifies the sensitivity of experiments using large-A nuclei to search for spin-independent scattering by a factor A. For this reason, the experimental community has focused most attention and resources on attempts to discover WIMPs via spin-independent interactions. Note that this coherence is lost if the WIMP mass becomes much heavier than the mass of the target nucleus. Use of several different target materials can help to distinguish whether any nuclear recoil events seen are due to WIMP interactions, or backgrounds. If no events are seen, upper limits on the cross section are presented as WIMP-nucleon cross sections versus WIMP mass. Even a signal of a few events would constrain the cross section and WIMP mass to within about an order of magnitude. 1.2. Backgrounds Direct detection dark matter experiments have currently reached a level of sensitivity corresponding to a few events per kilogram of target mass per year. This has already required a large effort to shield the detectors against gammas, electrons and neutrons from radioactivity. In addition, the experiments must be located deep underground to avoid neutrons produced in cosmic ray interactions. Finally, the experiments must have either significant event-by-event discrimination between electron recoils (from electromagnetic background sources) and nuclear recoils (from either WIMP or neutron interactions), or exploit some other expected characteristic of WIMPS such as annual modulation or directionality. It is crucial that direct detection experiments strive for zero background. This clearly maximizes discovery potential, and allows WIMP sensitivity to improve linearly with increase in target mass and running time. If a background arises, the sensitivity improvement will initially degrade as the square root of exposure (the product of target mass and running time) and then plateau at an irreducible level until the background can be removed or rejected. 1.2.1. Sources of backgrounds The primary sources of radioactivity in the natural environment are long-lived isotopes of Uranium and Thorium, which yield alpha, beta and gamma particles from their decay chains. Also troublesome are the long-lived isotopes K and Pb, the latter resulting from decays of ubiquitous Radon gas. It is possible to screen materials for radioactive contamination by detecting either the gamma or alpha particles emitted from surfaces. However, the sensitivity required for direct detection experiments is beginning to challenge the best screening technologies. A limiting background for all direct detection experiments is the presence of neutrons, since they produce the same nuclear recoil signature as WIMPS. Mounting the experiments far enough underground is sufficient to reduce neutrons from cosmic ray showers. However, neutrons also come from (alpha,n) reactions and fission decays, both resulting from the presence of small residues of radioactivity surrounding the detectors. This background will likely begin to dominate in the next generation of experiments without extreme efforts to further reduce contamination. Use of multiple target materials with different atomic weights can exploit the likelihood that WIMP scattering scales as A, whereas neutron cross sections are relatively insensitive to A, to allow statistical rejection of a neutron background. Similarly, the ability to recognize multiple scattering of neutrons, extremely unlikely for WIMPS, can also allow an experiment to recognize a neutron background. However, the presence of a neutron background would still limit discovery potential, since it would require a much larger sample of events to measure and subtract a neutron background, so as to extract a WIMP signal. 10th Int. Conf. on Topics in Astroparticle and Underground Physics (TAUP2007) IOP Publishing Journal of Physics: Conference Series 120 (2008) 042002 doi:10.1088/1742-6596/120/4/042002