Bill Flaherty with Glenn Starkman

Accounting for the sun’s gravity in dark matter detection

Poster Paper

Dark matter is thought to compose a large fraction of the energy density of the universe, and in particular of our galaxy. The favored candidate for dark matter is weakly interacting massive particles (WIMPs), particles tens to hundreds of times the mass of ordinary protons and neutrons and that would be streaming through our solar system. Scientists have built and are building detectors to look for such WIMPs passing through the Earth. An important factor in determining the signal of the WIMPs is their velocity distribution. Various models for the velocity distribution of WIMPs in the Galaxy have been used. It has also been noted that as the earth moves around the sun, the velocity of the Earth through the galaxy either adds to the sun’s or subtracts from it, depending on the time of year. This modulates the actual velocity distribution of the WIMPS passing through the Earth, and our detectors. Surprisingly, the fact that as the WIMPs approach the Earth they must fall deep into the Sun’s gravitational potential well has not apparently been taken into account in determining the WIMPs’ velocity distribution. Since that should cause an approximately few to ten percent change in the WIMP velocity it is not a negligible effect. In particular since it will increase the velocity of the fastest WIMPs, it should increase the detectability of WIMPs. It may also increase the detectability of the annual modulation of the WIMP signal as the Earth moves around the sun. Our plan is first to calculate the effect of falling into the Sun’s gravitational well on the WIMP velocity distribution as seen by an observer on the Earth, and then, to determine the influence on dark matter detector signals.