Cosmology

The DEAP/CLEAN collaboration will be constructing a 3600-kg single-phase liquid-argon dark matter detector at SNOLAB with sensitivity to 10-46 cm2 for a 100 GeV WIMP. We are currently operating a 7-kg liquid-argon detector (DEAP-1) at SNOLAB. Using DEAP-1 we have made measurements of alpha surface activity and radon levels in the detector. We have also performed studies of pulse-shape discrimination to separate electromagnetic interactions in the liquid argon from nuclear recoils. Recently published data from surface at Queen’s University showed no contamination in the WIMP signal region from 16.7 Million tagged gamma events in WIMP the region of interest. A further 22 M events have been accumulated at SNOLAB with no contamination. The design of the DEAP-3600 detector will be presented with emphasis on reduction of backgrounds, including design of a resurfacer to remove radon daughters which plate out on acrylic and the design of the acrylic container to plate shield against neutron activity from the PMTs and steel outer vessel.

Dark sectors with multi-component WIMP states, with small MeV- to GeV-scale splittings, can lead to more complex signatures in direct detection experiments. I'll discuss some scenarios with excited states charged under either the Standard Model or hidden sector gauge groups, and the ensuing constraints.

The ZEPLIN-III liquid xenon dark matter detector has completed its first underground science run, with a final exposure after cuts of 128kg.days of data. This has led to a limit on the spin-independent cross section of 7.8e-8pb for a 60GeV mass WIMP. The required techniques to derive this limit will be outlined, including data stability, detector calibrations, analysis techniques and selection efficiencies. Future plans for ZEPLIN-III will be Outlined. In addition, as a reflection of a new position, the current status of the SNOLab facility will be described, outlining the construction progress, current status of the first experimental suite and future plans and opportunities.

I consider a the dark matter relic abundance computation in a model where the dark matter annihilates into a light mediator rather than directly into the standard model. Obtaining the correct relic abundance in such a model may imply a different annihilation cross section than is implied by the usual WIMP decoupling computation. I show that the maximum annihilation cross section is obtained when the hidden sector decouples from the standard model before the dark matter annihilates into the mediator particles, and may be as much as a factor of 5 larger than the standard WIMP value.

KeV-MeV scale dark matter particles with integer spin, very weakly unstable and super-weakly interacting, can produce an observable ionization signal in direct detection experiments. I zoom in on some sensible models and discuss their observational consequences.

The highly radiopure about 250 kg NaI(Tl) DAMA/LIBRA set-up is running at the Gran Sasso National Laboratory of the I.N.F.N.. Results exploiting the model independent annual modulation signature for Dark Matter particles in the galactic halo are presented (exposure of 0.53 ton x yr). The DAMA/LIBRA data confirm the evidence for the presence of Dark Matter particles in the galactic halo as observed by the former DAMA/NaI experiment. The combined analysis of the data of the two experiments (total exposure 0.82 ton x yr) gives a C.L. at 8.2 sigma.

The Cryogenic Dark Matter Search (CDMS) experiment employs cryogenic ionization detectors to search for nuclear recoils induced by Weakly Interacting assive dark matter particles (WIMPs). A fast readout of the thermal energy deposition and the simultaneous measurement of an ionization signal provide an excellent handle for rejection of electron recoil background events from environmental radiation. This unique technology together with passive and active shielding makes CDMS the only background free experiment in the field. The recently published data based on the full complement of 30 individual detector modules operated in the Soudan Underground Laboratory in Minnesota give the best sensitivity for spin-independent WIMP-nucleon scattering for the most interesting mass range above about 40 GeV/c². The experiment is in a transition to the next phase, SuperCDMS, with increased total target mass and larger individual detector modules with improved sensor technology. SuperCDMS plans to install a total of 100-200 kg of cryogenic germanium detectors in the new SNOLAB facility near Sudbury ON, which, as the deepest large underground laboratory, provides the best conditions for direct dark matter search experiments.

LUX (Large Underground Xenon) is a two-phase Time Projection Chamber that will instrument 350 kg of Xenon, 100 kg of which will form a fiducially active target for WIMP interactions. It will be deployed at the Sanford Underground Science and Engineering Lab at the Homestake Mine in Lead, South Dakota. The Early Implementation Program of Sanford Lab is providing space at the 4850 feet level for LUX. The first detector with 120 photomultiplier tubes is being constructed and is projected to start collecting data in late 2009. Estimated background rates and LUX sensitivity to WIMP like Dark Matter particles will be presented. At the same time, we are engaged in planning for future detectors of this kind. Besides scaling to larger target masses, several new technological avenues are also being pursued. Status of LUX and plans for a roadmap for the future will be presented.

The PICASSO experiment searches for cold dark matter through the direct detection of weakly interacting massive particles (WIMPs) via their spin-dependent interactions with fluorine at SNOLAB, Sudbury - ON, Canada. The detection principle is based on the superheated droplet technique; the detectors consist of a gel matrix with millions of droplets of superheated fluorocarbon (C4F10) dispersed in it. The previous phase of the experiment, which employed 1-litre detector modules (for a total of about 20g of active mass), ended in 2005. The present phase of the PICASSO experiment consists of 32 4.5-litre detector modules for a total of approximately 1,795 g of active mass. In this talk, I will give an overview of the experiment, discuss the progress in background mitigation, which includes improved purification and fabrication techniques, as well as a background discrimination technique that we have recently discovered.

Dark matter (DM) annihilation around the redshift of last scattering can alter the recombination history of the universe, broaden the last scattering surface, and influence the observed temperature and polarization fluctuations of the cosmic microwave background (CMB). Unlike other indirect astrophysical signals of DM annihilation, these CMB signatures are free of the significant uncertainties inherent in modeling galactic physics, and provide an independent method to test and constrain models of dark matter. Recently measured anomalous excesses of 10-1000 GeV electron and positron cosmic rays have motivated DM models with large annihilation cross sections when the relative velocity of the annihilating particles is low. We have calculated in detail the efficiency with which energy from DM annihilation is deposited into the photon-baryon plasma around the redshift of last scattering, for an array of annihilation channels, allowing precise predictions of the effect of DM annihilation on the CMB. I will discuss CMB constraints for specific annihilation channels, which can strongly limit the allowed parameter space for DM models fitting the excesses measured by PAMELA and/or Fermi. I will also describe degeneracies between the effect of DM annihilation and changes to the cosmological parameters, and their implications. In particular, DM annihilation could alter the apparent value of the scalar spectral index n_s as measured by WMAP.