Scientists using a newly designed “snowball chamber” inspired by the Academy Award winning movie Frozen are hoping to be the first to detect dark matter — a mysterious substance believed to comprise the vast bulk of the universe’s total mass.
Evidence for the existence of dark matter comes from its gravitational effect on galaxies, clusters of galaxies, and the universe at large.
But it has to date remained elusive, implying that it is composed of hard-to-detect shadow particles that pass through ordinary matter almost as though it doesn’t exist.
A number of methods have been used in the effort to detect these particles, but so far none has succeeded.
The newly proposed “snowball chamber” was described recently at a meeting of the American Physical Society, in Denver, Colorado, US.
It uses supercooled water, which, if low enough in impurities such as dust particles, and placed in a smooth enough container, can be carefully chilled to a state below its normal freezing point of zero degrees Celsius.
In that state, the liquid is “metastable”, says Matthew Szydagis, a physicist at the University of Albany, New York, meaning that it is neither unstable nor truly stable. Undisturbed, it remains liquid.
But “the water wants to freeze, so with a sufficient disturbance you can trigger the freezing process,” he explains.
That’s where Disney’s Frozen comes in, with its heroine, Elsa, who can magically cause water to instantly freeze.
In the real world, the process doesn’t occur quite that quickly, but bottles of water carefully supercooled to, for example, minus-12 degrees Celsius can be induced to freeze solid in a matter of seconds simply by banging them on a countertop.
But it doesn’t take a disturbance anywhere nearly that large to make supercooled water start nucleating into snowflake-sized crystals. Incoming subatomic particles smashing into water molecules can also shake things up enough to cause snowball-like crystals to form, Szydagis says.
In tests, the process easily detects neutrons, and is also sensitive to neutrinos. But it is unaffected by gamma radiation.
In part, this might make it useful for the development of advanced neutrino detectors. But, Szydagis says, it is more interesting for the search for dark matter.
“If dark matter happens to be around the mass of a neutron, or [otherwise] like neutrons, this would be an ideal detector for it,” he explains.
That makes it a useful addition to the arsenal of potential dark matter detectors, he adds, because most others are better geared to detect higher mass particles. And the fact it ignores gamma rays improves its sensitivity, because dark matter seekers don’t have to worry about background radiation swamping the system.
The term “snowball chamber”, Szydagis adds, was inspired by cloud chambers and bubble chambers, which detect subatomic particles from their interactions with water vapour and hot water.
“The name parallels [these two], which are technologies that for many decades were instrumental in particle physics,” he says.