If you hung around the Wilson Hall café at Friday lunch in recent years, you might have noticed a conspicuous group of men, well-dressed in dark suits and ties, and sometimes wearing dark shades. They’re not prohibition-era gangsters—no, these are the young scientists of the Chicagoland Observatory for Underground Particle Physics, or COUPP, strutting their stuff.
But you haven’t seen them lately, because they have literally gone underground—more than a mile underground, in a laboratory, deep in a Canadian nickel mine. They have fled there not to escape the long arm of the law, but to take shelter from the relentless cosmic rays that rain down on the surface of the earth from outer space and to start up a new particle detector.
Our Pierre Auger Observatory scientists built detectors in the Argentinian pampa and are eager to catch and study as many cosmic-ray particles as they can at the highest energies they can find. So why do our COUPP scientists flee to the depths of the earth to get away from these naturally occurring particles?
They are there, a mile underground at SNOLAB, to seek the most exotic cosmic particles of all: the constituents of the famous dark matter that swarms throughout the space of our galaxy and throughout the universe. Unlike cosmic rays, these particles hardly interact with matter at all, and go right through the earth. Very rarely, perhaps once a year in a handful of material, one of the dark-matter particles will hit an atomic nucleus and make it gently recoil. That’s the quest of the COUPP posse: they want to find a kind of particle that’s never been detected before.
The COUPP experiment looks like a bootlegger’s still, but it is actually a modern version of a type of particle detector that Fermilab used with accelerator experiments for many years: a bubble chamber. An atom recoiling from a dark-matter collision makes a little bubble in a vessel that looks like a giant test tube, filled with 60 kilograms of superheated liquid. The device records the sound this collision creates and takes pictures of the bubble as it grows. (In the ’70s and ’80s, experiments used giant movie cameras to record the tiny bubbles that formed the tracks of charged particles. The biggest bubble chamber ever used at Fermilab is on proud display outside our Silicon Detector Lab, 15 feet wide and complete with its 70-mm cameras.)
In the last few months, the COUPP team members have shed their cool black suits for safety helmets and white bunny suits and spent months in a deep underground cleanroom assembling their apparatus. Last week, they installed the last major pieces of their machine. Now they are ready to start filling it to see what kind of new particle bubbles up. Stay tuned!
Craig Hogan is the head of the Center for Particle Astrophysics.