Abstract

1. Constraining Inelastic Dark Matter with X-Rays Alexander Wijangco, Department of Physics Advisor: Glenn Starkman, Department of Physics, CERCA/ISO Abstract Dark matter is a generic term for an exotic class of particles that might provide sufficient gravity to explain the observed movements of stars and galaxies. However, despite the best efforts of several experiments, dark matter particles have so far eluded direct detection. There have been experiments attempting to indirectly detect Weakly Interacting Massive Particles (WIMPs) – the leading dark matter candidates -- but the results of these experiments cannot be explained with traditional notions of these particles. To accommodate this data, inelastic models of dark matter have been proposed as a new theory for dark matter. This research concerns itself with the inelastic model used to explain the results of a particular indirect detection result: the DAMA signal. If these models are correct, then there should exist other detectable evidence for these particles from the byproducts, principally X-Rays, of excited dark matter decays. This research examines the theoretical signal one would see off massive gravitational bodies and whether this signal falls within the realm of what can be experimentally verified. Motivation The DAMA project postulated that the Earth translated through an isotropic background of dark matter particles. However, since the Earth’s velocity changes throughout the year, the number of dark matter interactions should also change along with it. So, the DAMA project measured the background rate of interactions and found a periodic signal corresponding to the orbit of the Earth. This signal is explainable by the inelastic dark matter model, in which dark matter particles are able to reach an excited state by scattering off normal matter. If that were so, then the excited dark matter particles should eventually decay into detectable by products, such as 100 keV photons. This project looks at the possibility of detecting such byproducts from dark matter particles scattering off bodies such as the moon. This is the background reaction measurements of the DAMA Project, showing the periodic signal. [1] Analytic Derivations Numerical Results and Conclusions Shown are the numerical results for the moon. Certain parameters, such as the lifetime of the excited dark matter state and the cross section of excitation, are dependant upon which inelastic dark matter model one is considering. The independent axis is the lifetime of the dark matter particle, and the dependent axis is accurate to within a factor of the cross section of interaction and density of dark matter particles. The blue line is the signal one would expect looking three moon radii off the moon. The red line is the observed background of 100 keV photons. Dark matter particles should excite off massive bodies. The rate of this for some small volume, dV, depends on the density of normal and dark matter, nm and nx, and the velocity and excitation. Or: Excited dark matter particles should decay exponentially as they travel from their source. With a lifetime τ and velocity v, the number at a distance R away should scale as: R The number of decays one would expect to see in some small volume, dv, a detector would see depends on the solid angle subtended from the source and the rate of decay, which is approximately: The number of photons one would expect a detector would see depends on the solid angle subtended from the source, which from a distance r away is approximately: r The maximal value of the photon signal is approximately 5.5 • 10-5, which is well below the background rate of about 1 photon a second. The parameters for this calculation have the cross section of excitation to be on the order of 10-40, so a proportional increase of this parameter would proportionally increase this signal. However, it is likely that if the lifetime is less then 100 seconds the particle will not be detectable by photon observation. However, initial examination beyond this domain indicates the signal may peak sharply, allowing possible detection. The results of this are pending future work. The quantity of interest is the number of photons a detector would see from the decay of excited dark matter particles. By approximating that the cross section of interaction for excitation is very low, assuming an isotropic ground state background, and approximating the expression for the subtended solid angle, the number of photons can be found by integrating over the volume of the massive body, the Moon, and the cone the detector views. This is expressed by: Bibliography [1] R. Bernabei et al., First results from DAMA/LIBRA and the combined results with DAMA/NaI, Eur.Phys.J.C56:333-355,2008