Projects: Cryogenic Dark Matter Search (CDMS)
The headframe that caps the mine shaft is one of the most distinctive features of the Soudan Underground Mine State Park — home to Minnesota's oldest and deepest iron mining site.
Astronomical and cosmological observations lead to the conclusion that our galaxy is embedded in a halo of Cold Dark Matter (CDM). The composition of the CDM is unknown, and its existence shows that the Standard Model of particle physics is incomplete. Several theories of particle physics, such as Supersymmetry, predict the existence of Weakly Interacting Massive Particles (WIMPs) with properties suitable for explaining the CDM halo: Stable, neutral, with weak-interaction-scale cross-sections needed to explain the observed density, and masses similar to those of atomic nuclei. The goal of the Cyrogenic Dark Matter Search (CDMS) experiment is to directly detect WIMPs in our halo by measuring nuclear recoils from WIMP-nucleus interactions.
The expected rate of WIMP interactions is already constrained to be very small (less than one event per kg-year) and the expected nuclear recoil energy is very low (100 KeV or less) so background rejection is crucial. One requirement is the ability to distinguish nuclear recoils from electron recoils arising either from beta decays or compton scattering of gamma rays. The CDMS strategy is to measure both the phonon energy and charge energy of events in Germanium and Silicon crystals. A dilution refrigerator cools the detectors to 50 mK to allow a precise measurement of the phonon energy down to a 10 KeV threshold. The phonon channel measures the total energy of an event, while the charge channel provides discrimination between nuclear and electron recoils: The dense energy deposition from a slow-moving nuclear recoil produces only 1/3 of the ionization of a relativistic electron recoil with the same energy. Furthermore, the CDMS detectors measure the arrival time of athermal ballistic phonons from the recoil, allowing us to identify and reject backgrounds produced at the surfaces of our detectors.
Another requirement is to eliminate neutron backgrounds, since neutrons produce nuclear recoils identical to those from WIMP interactions. To eliminate the fast neutron flux induced by cosmic rays, the experiment is located deep underground. CDMS is currently located in the Soudan mine in Northern Minnesota, where it is neighbors with the MINOS neutrino experiment. An active scintillator shield vetoes events in coincidence with the remaining cosmic-ray flux. Further layers of lead and polyethylene absorb ambient gamma and neutron radioactivity. Our goal is to meet the increasingly stringent demands of remaining background-free as our exposure in kg-years increases.
CDMS is currently taking data with about 5 kg of active detector mass. Meanwhile, we are starting construction of the next phase of our experiment, SuperCDMS, which will use larger detectors for a total of 25 kg of active mass, and will be located in a deeper site at Snolab in Canada. The sensitivity of SuperCDMS will allow us to cover a very interesting range of WIMP predictions, including the "focus-point" region of minimal-Supersymmetry.
