Life In the Dark Matter Fast Lane

May 5, 2015

A new prediction of the density and velocity distribution of dark matter particles at our position in the Galaxy has provided a revised estimate of the likely detection rates for dark matter in particle physics experiments.

Typical conception of the halo of dark matter surrounding the Galaxy.

Looking at all of the bright stars in the sky, it is easy to forget that it is dark matter that dominates the mass density of the Universe, and is the most abundant form of matter in our Galaxy. In fact, in our Milky Way, there is over 10 times more dark matter than than luminous matter. The dark matter is believed by most to be in the form of an as yet undetected elementary particle of nature, and experimental 'direct detection' searches are underway to find it, such as the CDMS experiment which has significant DOE participation. In many models the dark matter interacts weakly with ordinary matter - such as with an atomic nucleus in a detection experiment like CDMS - with the resulting energy deposit recorded as evidence of the interaction with dark matter.
The rate of dark matter scattering events at these detectors depends not only on the microphysics of the weak interaction between dark matter and ordinary matter, but also importantly on the density and velocity of the dark matter at our position in the Galaxy. According to estimates, the density of dark matter at our location corresponds roughly to one particle in the volume of a coffee cup. In order to better predict the theoretical event rates for dark matter direct detection experiments, this density estimate should be more precise, along with estimates of the velocities.
In a recent paper, KIPAC postdoc Louie Strigari and professor Risa Wechsler, along with Stanford particle theorist Jay Wacker and Mariangela Lisanti of Princeton University, undertook a detailed study of the theoretical distribution of particle dark matter in our Galaxy. The analysis was guided by results from observations of the motions of stars in the Galaxy, which help determine the overall matter density as a function of position, and by simulations on computers, which can allow a system such as the Galaxy to evolve according to the laws of physics and track the behavior of particles.
The authors concluded, from the likely distributions of velocities at our position, that the dark matter particles that we are most likely to detect directly via scattering events in Earth-bound detectors are those particles on very long timescale orbits that spend most of their lives in the far reaches of the Milky Way, away from the center. Some of these particles would be moving at high velocities as they pass us on their long journey, and these high velocity particles, which pack a lot of energy, would be much easier to detect than their slower moving but more abundant partners that spend most of their lives on orbits closer to the Galactic center. Based on the authors' determined dark matter particle velocity distribution, they conclude that the detection rate for various of the direct detection experiments should be slightly lower than many previous estimates. In the case of an eventual direct detection of particle dark matter, studies like these will guide us into a new era of 'dark matter astrophysics', in which we can learn about the detailed structure of our Galaxy from dark matter detection experiments here on Earth.
This work is based in part on a paper published in Phys. Rev. D (2010, 83:023519).
Science Contact:
Dr. Louie Strigari

Tidbit author: Louie Strigari and Jack Singal