In the global race to discover the particles that compose dark matter, scientists of the Cryogenic Dark Matter Search experiments have retaken the lead.
Researchers from UCSB, along with scientists from 16 other institutions, are currently collaborating on CDMS -located in a half-mile deep mine in Soudan, Minnesota – to detect dark matter’s mysterious particles. Approximately 85 percent of the entire mass of the universe consists of dark matter, and the remaining 15 percent is matter made up of electrons, neutrons and protons.
Due to dark matter’s peculiar properties, UCSB physics professor emeritus David Caldwell said scientists have not yet found a way to easily detect it.
“It is called dark matter because there is no electromagnetic radiation from whatever [dark matter] is so there’s no light, no radio waves, nothing that you can easily detect,” Caldwell said. “That is why this is a difficult experiment where you’re looking for a dark matter particle to come along and hit an atom in your detector.”
Dark matter is thought to consist of Weakly Interacting Massive Particles, UCSB physics professor and one of the CDMS experiment principal investigators Harry Nelson said. As these particles rarely interact with any other form of matter, their presence is highly difficult to detect, he said.
“Weakly Interacting Massive Particles feel two out of the four known forces of nature, but they don’t feel the other two,” Nelson said. “The two they feel are gravity and the weak interaction. They don’t feel the electromagnetic interaction that holds atoms together, and they don’t feel the strong interaction that holds atomic nuclei together.”
The CDMS experiments use cryogenic detectors – radiation sensors that find, track and identify high-energy particles that operate at very low temperatures, Caldwell said. The experiments consist of germanium crystals – a hard silver-white metal similar to tin – shielded from all possible sources of interference, including cosmic rays and almost all forms of radiation. The crystals operate at 40 millikelvin, a temperature very close to absolute zero where all molecular motion ceases, Caldwell said. He said these conditions are necessary for picking up traces of any WIMPs.
“The idea there is that you have to get the germanium detector that cold in order that you can measure a signal in two ways from such a particle as it strikes an atom of a detector,” Caldwell said. “It can … produce heat, and in order to measure this very small amount of heat, you have to be at an extremely low temperature.”
The probability of a WIMP interacting with a germanium crystal, however, proves yet another great challenge, UCSB researcher and CDMS scientist Rupak Mahapatra said.
“With the model detector material that we have, we expect less than 100 hits per day,” Mahapatra said. “You would have to wait 100 days with our detectors to get one WIMP hit, so it’s not so bad because we have been taking data for many years, but we haven’t seen any yet and the hope is that you keep searching and eventually find it.”
Mahapatra said probability has to be on their side in order to make the CDMS experiments a success.
“To make sure that it is on our side, we try to take away any background,” he said. “Anything that can mimic a WIMP we want to avoid and that is why CDMS, our experiment, is the best experiment in the world. It is the only experiment that is able to get down to zero background.”