Cosmology

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.

The spectra of cosmic ray electrons and positrons should have contributions from known sources such as particles accelerated in supernova remnants and from the cosmic rays interactions. Besides these guaranteed contributions, any evidence for an additional component may carry hints of a new phenomenon. Most recently PAMELA and ATIC experiments hinted an overabundance of these particles as compared to model expectations and generated much interest on astrophysical and exotic explanations. I will first examine the implications of the recent detection of extended, multi-TeV gamma-ray emission from Geminga pulsar wind nebula, which reveals the existence of an ancient/nearby cosmic ray accelerator that can also plausibly account for the observed excess. Next, I will focus on a possibility that these particles might be produced through dark matter decays/annihilations within the halo of our Galaxy. I will conclude by reviewing implications of these scenarios for several categories of upcoming Gamma Ray/Neutrino observatories including Fermi and IceCube.

The positron excess measured by PAMELA may be the long waited hint of the presence of dark matter particles in the Milky Way halo. But before we rejoice, we need to examine the other Possible astrophysical explanations. Whatever the sources -- DM or conventional -- a crucial ingredient is the transport of cosmic rays in the magnetic fields of the Galaxy to which I will pay particular attention in this presentation.

The cosmic-ray excess observed by PAMELA in the positron fraction and by FERMI and HESS in the electron + positron flux can be interpreted in terms of DM annihilations or decays into leptonic final states. Final states into tau's or 4mu give the best fit to the excess. However, in the annihilation scenario, they are incompatible with photon and neutrino constraints, unless DM has a quasi-constant density profile. Final states involving electrons are less constrained but poorly fit the excess, unless hidden sector radiation makes their energy spectrum smoother, allowing a fit to all the data with a combination of leptonic modes. In general, DM lighter than about a TeV cannot fit the excesses, so PAMELA should find a greater positron fraction at higher energies. The DM interpretation can be tested by FERMI gamma observations above 10 GeV: if the electronic excess is everywhere in the DM halo, inverse Compton scattering on ambient light produces a well-predicted gamma excess that FERMI should soon detect.