Session B7: Dark Matter: Direct Searches and Theory

Study of Light Collection Efficiency for a Neutron Veto for the DarkSide-50 Dark Matter Detector

DarkSide-50 is a 50-kg liquid argon dark matter detector that will use a borated liquid scintillator to veto neutron-induced background events. The liquid scintillator consists of a mixture of tri-methyl-borate and pseudocumene with wavelength shifters. The scintillator is contained in a 4-meter diameter sphere with an array of photomultiplier tubes on the inside surface. To enhance the detection efficiency of low energy products in the neutron capture reaction n+$^{10}$B$\rightarrow ^4$He+$^7$Li, the inside surface of the sphere is covered with a reflector. A small test chamber has been built to measure the light collection efficiency produced by the borated scintillator with various combination of reflectors, and primary and secondary wavelength shifters. In addition, we have attempted to generalize our measured results to the proposed detector through a Monte Carlo simulation of the light collection. In this talk, the results from these studies will be presented.   

Preliminary Analysis of Electroluminescence from DarkSide-10 Dark Matter Detector

DarkSide-10 is the first, fully operational, liquid argon time projection (TPC) prototype detector for the DarkSide Collaboration. We employ a dual-phase argon TPC with the goal of achieving the highest possible discrimination of nuclear recoils from background radiation. The first studies from DarkSide-10 in an underground shielded environment will be reported here. Specifically a detailed analysis of the pulse shape and light yield of electroluminescence from extracted ionization charge drifting in the gas phase will be presented.   

Drift Field Effects on Prompt Scintillation in the DarkSide-10 Liquid Argon TPC

The DarkSide-10 dark matter detector is a dual-phase argon time projection chamber (TPC). Its dual-phase design allows for particle discrimination based on parameters from both the liquid and gas phases. The primary discrimination parameter is the prompt scintillation fraction observed in the liquid phase, defined as the fraction of light collected in the first 90 ns of the primary scintillation signal. In the current detector run, we have looked at signals from various gamma sources with various applied electric fields in order to better characterize the detector response. In this talk the effect of the applied electric field on the prompt scintillation fraction will be discussed.   

Cross Section Measurements for Elastic and Inelastic Scattering of Neutrons from Argon and Neon

Neutron backgrounds are a significant concern to experiments that attempt to directly detect Weakly Interacting Massive Particle (WIMP) dark matter. Recoil nuclei produced by neutron elastic scattering can mimic WIMP signatures. There is insufficient experimental data available for the scattering cross sections of neutrons with noble gases (Ne, Ar, Xe), which are candidate target materials for such experiments. Neutron elastic scattering from argon and neon of natural abundance was investigated at the Triangle Universities Nuclear Laboratory at neutron energies relevant to ($\alpha$,n) and low-energy spallation neutron backgrounds in these experiments. The differential cross sections were measured using a time-of-flight technique. Partial $\gamma$-ray production cross sections for ($n,xn\gamma$) reactions from 1--30 MeV were also measured at the Los Alamos Neutron Science Center. Details of the experimental techniques and results will be presented.   

Axion Dark Matter and Cosmological Parameters

The excellent agreement between observations and the big bang nucleosynthesis (BBN) predictions for the primordial abundances of light elements is often touted as a triumph of the standard $\Lambda$-CDM cosmological model. There remains, however, one anomaly: the abundance of $^7$Li is approximately two to three times less than what the theory predicts. This anomaly can be alleviated or removed altogether if photons have been cooled after BBN and before recombination. Such cooling is ordinarily difficult to achieve, though it may be realized if dark matter axions form a Bose-Einstein condensate after BBN and before decoupling. In this case, Bose enhancement helps facilitate energy exchange between the photons and axions, so that the baryon-to-photon ratio during BBN is smaller than originally thought. This scenario predicts a high effective number of neutrinos, as measured by the cosmic microwave anisotropy spectrum.   

Axion Bose-Einstein Condensation: a model beyond Cold Dark Matter

Cold dark matter axions form a Bose-Einstein condensate if the axions thermalize. Recently, it was realized that they do thermalize when the photon temperature reaches approximately 500eV. We discuss the differences between axion BEC and CDM in the linear regime and the non-linear regime of evolution of density perturbations. We find that axion BEC provides a mechanism for the production of net overall rotation in dark matter halos.   

Dark Matter from Binary Tetrahedral Flavor Symmetry

Binary Tetrahedral Flavor Symmetry, originally developed as a quark family symmetry and later adapted to leptons, has proved both resilient and versatile over the past decade. In 2008 a minimal T' model was developed to accommodate quark and lepton masses and mixings using a family symmetry of (T'xZ2). We examine an expansion of this earlier model using an additional Z2 group that facilitates predictions of WIMP dark matter, the Cabibbo angle, and deviations from Tribimaximal Mixing, while giving hints at the nature of leptogenesis.   

Dark Matter Direct Search Rates in Self-Consistent Models of the Milky Way and Sagittarius Stream

We analyze self-consistent N-body models of the Milky Way disk and the ongoing disruption of the Sagittarius dwarf, in order to determine how strongly these components of the Galactic environment affect scattering rates in experiments designed to detect dark matter via nuclear recoil events. We find that the accidental proximity of the solar neighborhood to the leading tidal arm of Sagittarius induces significant deviations from the standard halo model with respect to the phase and amplitude of the predicted annual modulation signal, and that the standard halo model is radically insufficient to describe event rates arising from the non-Maxwellian velocity distribution of the Milky Way host halo as it is globally contracted by the influence of the stellar disk. In the regime of light dark matter and for energy ranges similar to those probed by DAMA/LIBRA and CoGeNT, we conclude that these and other experiments may be significantly more sensitive to dark matter scattering than prior interpretations have found, and also that current constraints may already be distinguishing the presence of Sagittarian dark matter on Earth.   

Magnetic Moment of Leptons at Finite Temperature and Density

Magnetic moment is related to the perturbative nature of QED. We study the magnetic moment of electron and the magnetic moment of neutrinos at finite temperature and density. The change in the magnetic moment of leptons is used to study the electromagnetic properties of the QED medium at finite temperature and density. Some of the applications of these calculations in astrophysics and cosmology are also mentioned.