Session J14: Dark Matter, Gamma-ray Experiments, and Elementary Particles

Fermi LAT Collaboration Update on Dark Matter Gamma--ray Line Search

A number of studies of publicly available data from the Fermi Large Area Telescope (LAT) have presented indications of a narrow spectral feature at 130 GeV in the direction of the center of the Milky Way. We present an update of results from the Fermi --LAT Collaboration on our gamma --ray line search, including recent results in the region of 130 GeV. Our analysis searches for spectral lines from 5 GeV to 500 GeV using 4 years of Fermi-LAT data. We parameterize the energy redistribution [or resolution] function of the LAT as a two-dimensional probability distribution function that incorporates the quality of the gamma-ray energy measurement. In addition, given the many uncertainties associated with the Galactic dark matter density distribution, we search in several different selection regions on the sky optimized for various dark matter density profiles. Our results include 95{\%} CL limits on the presence of gamma-ray lines as well as studies of systematic uncertainties and an evaluation of the robustness of the analysis method.   

The VERITAS Dark Matter Program

In the cosmological paradigm, Cold Dark Matter (DM) dominates the mass content of the Universe and is present at every scale. Candidates for DM include many extensions of the standard model, with a Weakly Interacting Massive Particle (WIMP) in the mass range from 50 GeV to greater than 10 TeV. The self-annihilation of WIMPs in astrophysical regions of high DM density can produce secondary particles including Very High Energy (VHE) gamma rays with energies up to the DM particle mass. The VERITAS array of Cherenkov telescopes, designed for the detection of VHE gamma rays in the 100 GeV-10 TeV energy range, is an appropriate instrument for the indirect detection of DM. Among the possible astrophysical objects considered to be candidates for indirect DM detection, VERITAS has focused on observations of dwarf spheroidal galaxies (dSphs) of the Local Group, the Milky Way galactic center, Fermi-LAT unidentified GeV sources and the local group galaxy M31. This presentation reports on our extensive observations of these targets and our present exclusion regions obtained on the thermally averaged annihilation cross section of the WIMP derived from these observations.   

Searching for Q-balls with HAWC

The High Altitude Water Cherenkov (HAWC) Observatory is a gamma-ray experiment currently under construction at Sierra Negra in the state of Puebla, Mexico. Once completed, it will consist of a 20,000 square meter array of 300 water Cherenkov detectors. Although the HAWC Observatory is designed to study gamma rays from galactic and extra-galactic sources, the large instrumented volume of water gives the opportunity to search for more exotic species. One such target, predicted by several varieties of supersymmetric theory, is the Q-ball. Q-balls are very massive, sub-relativistic particles that can have a large baryon number, and can be stable since their creation in the early universe. They are also an appealing candidate for the dark matter of the universe, but their large masses must mean that their flux is very low. The data acquisition system of the HAWC Observatory is flexible enough that, with a dedicated trigger algorithm for non-relativistic particles, it allows the search for Q-balls traversing the detector. The trigger algorithm and preliminary analysis will be presented.   

Enhancing the Sensitivity of HAWC to sub-Tev Transients

The High Altitude Water Cherenkov (HAWC) Observatory, currently being built 4100 meters above sea level near Pico de Orizaba, Mexico, is well-suited for observing transient phenomena above 1 TeV due to its large field of view (2 sr) and high uptime ($\sim$100\%). However, sub-TeV transient events are also of physical interest due to the overlap in energy with satellite experiments such as the Fermi gamma-ray space telescope. This presents a challenge since the sub-TeV primary particles observed with HAWC tend to be difficult to distinguish from noise. To address this problem, we propose a method in which particle arrival directions are fit to triplets of triggered PMTs in a short sliding trigger window (100 ns). The resulting arrival directions are then summed in a coarsely binned significance map of the sky with a time window of one to several seconds. This algorithm is simple enough to be applied online, and can localize the positions of transient sources to within 8 degrees. We run the method over HAWC30 detector data to estimate the noise rate and use simulated events to calculate the sensitivity to transients.   

Muon data from a water Cherenkov detector prototype at Colorado State University

The High Altitude Water Cherenkov (HAWC) Observatory is a very high energy gamma-ray experiment currently under construction in Sierra Negra in the state of Puebla, Mexico, at an altitude of 4,100 m a.s.l. The HAWC Observatory will consist of 300 water Cherenkov detectors (WCDs), each instrumented with three 8'' photomultiplier tubes (PMTs) and one 10'' high efficiency (HE) PMT. The PMTs are upward facing, anchored to the bottom of a 5 m deep by 7.3 m diameter steel tank, containing a multilayer hermetic plastic bag holding 200,000 L of purified water. The only full size WCD prototype outside of the HAWC site is located at Colorado State University (CSU) in Fort Collins, CO at an altitude of 1,525 m a.s.l. This prototype is instrumented with six 8'' PMTs, one 10'' HE PMT, and the same laser calibration system, electronics, and data acquisition system as the WCDs at the HAWC site. The CSU prototype is additionally equipped with scintillator paddles both under and above the volume of water, temperature probes (in the water, outside, and in the DAQ room), and one covered PMT. Preliminary results for muon rates and their temperature dependance using data collected with the CSU prototype will be presented.   

Sensitivity of XENON detectors to Galactic/Solar Axions and Bosonic Super-WIMPs based on the Axio-electric Effect

We investigate the sensitivity of the XENON detectors to hypothetical Galactic/Solar axions and Bosonic Super-WIMPs using axio-electric effect (analogue of the photoelectric effect) on xenon atoms. Since liquid xenon detectors also provide an extremely low radioactivity environment for electron recoils, very weakly-interacting low-mass particles (\textless 100 keV/c$^{2})$, such as the hypothetical axion (and Bosonic Super-WIMP), could be detected using the axio-electric effect. The limits for the axion-electron coupling constant g$_{Ae}$ were calculated from the analysis of 224.6 live days X 40 kg of exposure of XENON100 Dark Matter Detector. For the future ton-scale liquid Xe detectors (e.g. XENON1T) assuming one ton-year of exposure, galactic axions (as non-relativistic dark matter) could be detected if the axio-electric coupling g$_{Ae}$ is greater than 10$^{-14}$ at 1 keV/c$^{2}$ (or 10$^{-13}$ at 100 keV/c$^{2})$. Below a few keV/c$^{2}$, and independent of the mass, a solar axion search would be sensitive to a coupling g$_{Ae}$ $\sim$10$^{-12}$.   

A Two-Dimensional Analysis of XENON100 Data Using the Ionization Channel as the Energy Estimator

The recent results from 225 days of XENON100 data have culminated in the world's best WIMP limits across essentially the full mass range probed. The analysis presented here goes a step further and demonstrates the utility gained from parametrizing the ionization energy scale for use as the energy estimator instead of the primary scintillation. This has the effect of increasing detector resolution and allowing a lower energy threshold, thereby improving the limits significantly at low WIMP masses.   

Preliminary Results From the pGAPS Test Flight

The General Anti-Particle Spectrometer (GAPS) experiment is a proposed dark matter indirect-detection experiment intended to fly on an Antarctic balloon later this decade. A number of theoretical WIMP dark matter candidates are predicted to produce enhancements in the flux of anti-matter particles in Galactic cosmic rays. Searches for excesses in the anti-proton and positron spectra are ongoing but are difficult to interpret because of large backgrounds from secondary production. GAPS will instead look for low-energy cosmic ray anti-deuterons with a much smaller expected background. In June of 2012 a prototype GAPS experiment (pGAPS) was flown from Hokkaido, Japan to validate the crucial components and techniques needed to build and operate a full-scale GAPS experiment. The flight lasted for 8 hours (over 3 hours at float altitude), with over 1 million cosmic ray triggers recorded. Preliminary analysis of the performance of the different detector systems will be presented as well as the status of preparations for a full-scale GAPS science payload.   

Recent Progress on the Directional Dark Matter Detector (D$^{3})$

We present recent progress towards a direction-sensitive WIMP dark matter search based on gas-filled Time Projection Chambers (TPCs) where the drift charge is amplified with Gas Electron Multipliers (GEMs) and detected with pixel electronics. The advantage of this approach is that nuclear recoils can be reconstructed in three dimensions with low energy threshold and low noise, at room temperature. We present performance measurements of small prototypes and a development path towards an affordable, m$^{3}$-scale Directional Dark Matter Detector; D$^{3}$. Such a detector may be particularly suitable for low-mass WIMP searches, and perhaps sufficiently sensitive to clearly determine whether the signals seen by DAMA, CoGeNT, and CRESST-II are due to low-mass WIMPs or background.