Thursday, March 03, 2005
Bubbles and Neutralinos
I listened to Andrew Sonnenschein from Chicago give an interesting colloquia on Thursday (pdf on his website) - it was a cool mixture of cosmology, high energy and nuclear physics, and engineering. That is, they're working on a project to detect WIMPs. In particular, neutralinos (which are superpositions of the supersymmetric partners to the Higgs and W bosons (and others)), are likely candidates for dark matter. They're working on a bubble chamber method - reviving the old particle detection techinque for modern day physics. Bubble chambers were abandoned for accelerator physics in the 70s since they can't reset very quickly, but they are suitable for dark matter detection: hopefully every now and then, one of the cold dark matter particles drifting around the galaxy will collide with a nucleus via a weak interaction, and imparted kinetic energy will be sufficient to overcome surface tension effects and cause a bubble to quickly form in the superheated liquid in the test chamber.
There are many groups working in this area. Intriguingly, data from the DAMA group at Gran Sasso in Italy appears to show positive detection for WIMPs in the form of an annual modulation of detections: the Earth orbits the Sun at about 30 km/sec, and the Sun orbits the Milky Way at around 230 km/sec, and so the velocity of the Earth with respect to the galactic dark matter background oscillates yearly by about 7% (the Earth's orbital plane is inclined by 60 degrees with respect to the Sun's). And indeed DAMA has apparently found a consistent yearly oscillation with the right phase in the hit rate (although the variation is small ~ 1%). However, Richard Schnee gave a colloquia today presenting the CDMS (Cryogenic Dark Matter Search) project, and their current null detection result appears to contradict the DAMA result. They bring disks of Germanium down close to absolute zero (with lots of shielding and in the SOUDAN mine in order to eliminate as much background as possible), and then listen for phonons resulting from occasional nuclear collisions with WIMPs. So far they haven't heard any, and upgrades with much more test mass in the future will test various supersymmetric theories stringently. It will certainly be very exciting when we discover what the dark matter is made of (or at least some of the components!).
There are many groups working in this area. Intriguingly, data from the DAMA group at Gran Sasso in Italy appears to show positive detection for WIMPs in the form of an annual modulation of detections: the Earth orbits the Sun at about 30 km/sec, and the Sun orbits the Milky Way at around 230 km/sec, and so the velocity of the Earth with respect to the galactic dark matter background oscillates yearly by about 7% (the Earth's orbital plane is inclined by 60 degrees with respect to the Sun's). And indeed DAMA has apparently found a consistent yearly oscillation with the right phase in the hit rate (although the variation is small ~ 1%). However, Richard Schnee gave a colloquia today presenting the CDMS (Cryogenic Dark Matter Search) project, and their current null detection result appears to contradict the DAMA result. They bring disks of Germanium down close to absolute zero (with lots of shielding and in the SOUDAN mine in order to eliminate as much background as possible), and then listen for phonons resulting from occasional nuclear collisions with WIMPs. So far they haven't heard any, and upgrades with much more test mass in the future will test various supersymmetric theories stringently. It will certainly be very exciting when we discover what the dark matter is made of (or at least some of the components!).
// posted by Travis Garrett @ 7:30 PM 
