Particle Physics

The particle physics community is bubbling with excitement since the recent discovery in the cosmic radiation of a positron and electron excess at high energy. This may be the first indirect hint that dark matter particles wander in the halo of the Milky Way. However, these species do not seem to have the expected properties. I will review the various pieces of that puzzle and present a status report of the current developments in that fast moving field.

Strongly warped regions, or throats, are a common feature of string theory compactifications. In the early, hot universe, energy will be transferred between these throats and between throats and the standard model. Using the gauge-gravity duality, we calculate the rate of this energy transfer. Due to the warping, the resulting decay rate of throat-localized Kaluza-Klein states to other throats or the standard model is strongly suppressed. If their lifetime is longer than the current age of the universe, these states are an interesting dark matter candidate. We discuss a scenario along these lines.

Light hidden sectors are a generic possibility for new physics, and recent astrophysical signals motivate hidden sector dark matter. I will discuss probes of a minimal secluded U(1) hidden sector scenario with high luminosity particle physics experiments.

The XENON project pursues the goal of directly detecting nuclear recoils resulting from scattering interactions with Weakly Interacting Massive Particles (WIMPs), using a phased approach of increasingly more sensitive experiments. The detector consists of a dual-phase liquid/gas xenon time projection chamber, which can measure down to ~2 keV(ee) energy threshold and discriminates against background using both the primary scintillation light and the charge signal resulting from interactions in the noble liquid. The current exeriment XENON100 is the successor of the highly successful XENON10 detector, featuring 10 times greater sensitive mass and 100 times lower background. Its sensitivity with an ultimate exposure of 6000 kg days will be 2 times 10^{-45} cm^2 for spin-independent interactions at 100 GeV/c^2. XENON100 has been installed and is operating. I will report on the present status and discuss its physics reach along with future prospects of detectors at the ton scale.

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.