Knocking on Heaven's Door - Lisa Randall [184]
The most riveting talk of the day occurred the morning I arrived. Harry Nelson, who is a professor at the University of California Santa Barbara, talked about year-old CDMS results. You might wonder why a talk about old results should receive so much attention. The reason was that everyone at the conference knew that only three days later the experiment would release new data. And rumors were flying that scientists at the CDMS experiment had actually seen compelling evidence of a discovery, so everyone wanted to understand the experiment better. For years theorists had listened to talks about dark matter detection but had listened primarily to their results and had paid only superficial attention to the details. But with imminent dark matter detection conceivable, theorists were eager to learn more. Later in the week, the results were released and disappointed the audience’s greatly exaggerated expectations. But at the time of the talk, everyone was absorbed. Harry steadfastly managed to give his talk despite the many probing questions about the soon-to-be-released results.
Because it was a two-hour informal presentation, those of us in attendance could interrupt whenever necessary to understand as much as possible. The talk nicely addressed questions that the audience, which consisted mostly of particle physicists, would find confusing. Harry, who was trained as a particle physicist—not as an astronomer—spoke the same language we did.
With these extraordinarily difficult dark matter experiments, the devil is in the details. Harry made that abundantly clear. The CDMS experiment is based on advanced low-energy physics technology—the kind more conventionally associated with so-called condensed matter or solid state physicists. Harry told us how before joining the collaboration he would never have believed such delicate detections could possibly work, joking that his experimental colleagues should be grateful he wasn’t a referee on the original proposal.
CDMS works very differently from scintillating xenon and sodium iodide detection experiments. It has hockey-puck-size pieces of germanium or silicon topped by a delicate recording device, which is a phonon sensor. The detector operates at very low temperature—low enough to be just at the border between superconducting and non-superconducting. If even a small amount of energy from phonons, the sound units that carry the energy through the germanium or silicon, much like photons are the units of light—hit the detector, it can be enough to make the device lose superconductivity and register a potential dark matter event through a device called a superconducting quantum interference device (SQUID). These devices are extraordinarily sensitive and measure the energy deposition extremely well.
But recording an event isn’t the end of the story. The experimenters need to establish that the detector is recording dark matter—not just background radiation. The problem is that everything radiates. We radiate. The computer I’m typing on radiates. The book (or electronic device) you’re reading radiates. The sweat from a single experimentalist’s finger is enough to swamp any dark matter signal. And that doesn’t even take into account all the primordial and man-made radioactive substances. The environment and the air as well as the detector itself carry radiation. Cosmic rays can hit the detector. Low-energy neutrons in the rock can mimic dark matter. Cosmic ray muons can hit rock and create a splash of material, including neutrons that can mimic dark matter too. There are about 1,000 times as many background electromagnetic events as predicted signal events, even with reasonably optimistic assumptions about the mass and interaction strength of the dark matter particles.
So