Anthony Villano is a post-doc working on the Cryogenic Dark Matter Search (CDMS) and Super CDMS experiments. CDMS is looking for weakly-interacting massive particles (WIMPs) that are a possible dark matter candidate. Physicists have not convincingly isolated a WIMP signal, but their new, more-advanced WIMP detector, SuperCDMS-Soudan, has the best sensitivity of any detector they have ever made.
Villano has been working on a simulation program for cosmogenic neutrons, particles that interfere with the detector’s ability to find a WIMP signal. Cosmogenic neutrons are created from muon interactions in rock. Muons rain down from the sky, and come through the ½ mile of shielding rock to the CDMS detector in the Soudan Underground Laboratory. While muons are easily distinguished from WIMP signal, cosmogentic neutrons are not. The cosmogenic neutrons are uncharged particles which deposit energy discretely, or, not continuously along their paths as muons would. The discrete energy depositions are common to WIMPs as well. Both these traits are common with WIMPs as well. Luckily for physicists, neutrons can interact twice, but WIMPs probably never will, which allows physicists to eliminate signals that have two distinct energy depositions. If a muon enters the cavern, doesn’t hit the detector, but creates a neutron that interacts once in the detector, that signal is indistinguishable from a WIMP. You need a simulation to estimate the number of times this will happen during the experimental run so that the expected level of neutron background is known. Villano says that you design the experiment so that cosmogenic neutrons are as rare as possible so you can increase the sensitivity to WIMP signals.
The WIMP has no charge and interacts via the weak force, which means it has a very low probability of interacting with the everyday matter in the CDMS detectors. Villano uses the analogy of billiard balls which are all the same mass. When you smack two together you get a very efficient transfer of energy that is easy to detect. Physicists call this transfer of energy, a nuclear recoil. They have tuned their detectors to create the biggest recoils possible by making target particles that are as near to the predicted mass of WIMPs as possible. "The tricky part," Villano says, "is if the models are off a bit and the WIMP was actually much lighter than we thought, that brings the expected energy from 10s of KEV to two or less keV."
SuperCDMS will probably run for two years at Soudan before a new one is built at SNOlab, an underground laboratory in Canada that is almost three times as deep as Soudan. "SNOlab should have factor of 200-300 less neutrons, but we have to understand that in detail," Villano says. The SNOlab detector will be larger as well. They have 15 discs at Soudan and SuperCDMS will have around 72 discs, total.