High Energy Physics

I will describe DM-Ice, a direct detection dark matter experiment at the South Pole. The aim of the experiment is to test the claim for an observation of dark matter by the DAMA collaboration by carrying out an experiment with the same detector technology, but in the southern hemisphere. By going to the opposite hemisphere, many of the suspected backgrounds would produce annual modulation with the opposite phase whereas the dark matter signature should stay the same. DMIce-17, a 17-kg detector was installed in the Antarctic ice at the South Pole in December 2010 at the depth of ~2200 m.w.e. and is currently taking data. An experiment that can test DAMA's claim is currently being designed. I will report on the status of DMIce-17 and the plans for the full-scale experiment.

The XENON100 detector, currently taking data at the Laboratori Nazionali del Gran Sasso in Italy, is a dual-phase xenon time projection chamber used to search for dark matter by simultaneously measuring the scintillation and ionization signals produced by nuclear recoils. These two signals allow the three-dimensional localization of events with millimeter precision and the ability to fiducialize the target volume, yielding an inner core with a very low background. As the energy scale is based on the scintillation signal of nuclear recoils, the precise knowledge of the scintillation efficiency of nuclear recoils is of prime importance. I will briefly discuss the results of a new measurement of the relative scintillation efficiency of nuclear recoils in LXe, Leff, performed with a new single phase detector, designed and built specifically for this purpose. Finally, I will present the recent XENON100 results obtained from 100 live days of data acquired in 2010 and discuss the current status of the experiment and its evolution into XENON1T.

Departing from the context of CoGeNT and COUPP, two direct searches for WIMP dark matter, we will inspect the recent landscape of anomalies observed by these and several other detectors. The aim of this talk is to communicate an appreciation for the subtleties inherent to experimental efforts in this field, and for the considerable difficulties that await for those trying to make sense of WIMP search observations (or lack thereof).

The particular properties of different dark matter particle candidates can lead to different properties and distributions of sub-structure within galaxies; structure that may uniquely be probed through specific state of the art observations of galaxy-scale dark matter halos that happen to be acting as strong gravitational lenses. I will discuss how the matter power spectrum and non-linear evolution within galaxies depend on the specific properties of dark matter particle candidates, develop the types of strong gravitational lenses that lend themselves to probing substructure, and give both the current state of the art and the prospects for quantitative constraints in the near future. Throughout, I will emphasize what cross-germination opportunities there are between such astrophysical structure measurements, and other exciting avenues of insight into the nature of dark matter.

The expansion history of the Universe before big bang nucleosynthesis is unknown; in many models, the Universe was effectively matter-dominated between the end of inflation and the onset of radiation domination. I will show how an early matter-dominated era leaves an imprint on the small-scale matter power spectrum. This imprint depends on the origin of dark matter. If dark matter originates from the radiation bath after reheating, then small-scale density perturbations are suppressed, leading to a cut-off in the matter power spectrum. Conversely, small-scale density perturbations are significantly enhanced if the dark matter was created nonthermally during reheating. These enhanced perturbations trigger the formation of numerous dark matter microhalos during the cosmic dark ages. The abundance of dark matter microhalos is therefore a new window on the Universe before nucleosynthesis.

Models of dark matter with Sommerfeld-enhanced annihilation have been proposed to explain the CR excess observed by the PAMELA and Fermi experiments. In such models, the local annihilation signal can easily be dominated by small, dense, cold subhalos, instead of by the smooth DM halo as usually assumed. I will discuss how such a "substructure+Sommerfeld" scenario modifies constraints from the CMB, limits on DM self-interaction, and bounds from measurements of inner-Galaxy gamma rays and the extragalactic diffuse background. These constraints provide stringent limits on the usual smooth-halo scenario, robustly ruling out force carrier masses below ~200 MeV (in the context of explaining the PAMELA/Fermi signals) and causing tension for higher mediator masses, but in the presence of a modest amount of local substructure, force carrier masses down to 20 MeV or even lower can still be consistent with these bounds.

Dark matter models with an annihilation cross section enhanced by a Sommerfeld mechanism have been proposed in the past years to explain a number of observed anomalies, such as the excess of high energy positrons in cosmic rays reported by PAMELA. However, this enhancement can not be arbitrarily large without violating a number of astrophysical measurements. In this talk, I will discuss the degree to which these measurements can constrain Sommerfeld-enhanced models. In particular, I will talk about constraints coming from the observed abundance of dark matter and the extragalactic background light measured at multiple wavelengths.

No known astrophysical mechanisms are expected to generate a significant high-energy flux of cosmic-ray electrons and positrons (CREs) from the Sun. However, some recently considered classes of dark matter models predict such a signal. We analyzed the CRE events collected by the Fermi-LAT during its first year of operation to search for a flux excess correlated with the Sun's direction, and found no evidence of a significant signal. I will discuss the constraints these results place on secluded dark matter models and inelastic dark matter models.

One of the most exciting, albeit slightly speculative, components of the Fermi mission is to search for evidence of energetic events related to dark matter decay or annihilation. The best targets for this search a regions where we suspect there is dark matter, but see few conventional gamma-ray sources such as molecular clouds, cosmic ray sources, or compact objects. Much emphasis has been placed on local dwarf satellites in particular, since many of these systems show evidence for relatively deep potential wells, but have few stars and no recent star formation. In this talk I will propose another possible target for indirect dark matter searches, among our nearby galactic neighbours.

The existence of dark matter is hardly in doubt, yet astrophysicists continue to search in vain for any non-gravitational signals of it. In the case of weakly interacting massive particle (WIMP) models, ongoing annihilation or decay of WIMPs to Standard Model particles could provide observable signals, e.g. as excess gamma rays in the center of the Milky Way or as excess ionization at high redshift. I will present our latest results on the most rigorous constraint from astrophysics: the effect of WIMP annihilation on the ionization history of the Universe, as recorded in the CMB.