Although it makes up everything we see and touch around us, ordinary matter only accounts for about 15 percent of the mass of the universe. The other 85 percent is believed to be dark matter, a theoretical substance that doesn't interact with ordinary matter and has so far eluded our best efforts to directly detect it. Now, physicists from Brown University have proposed a new way to find dark matter, using a huge tub of helium in a superfluid state.
Dark matter doesn't interact with the electromagnetic force, meaning it doesn't absorb, reflect or emit light. So if we can't see, hear, smell or otherwise detect dark matter, it's only natural to ask: how do we know it's there at all? Its presence is felt through its gravitational effects, and the extra mass it adds helps plug some holes in our understanding of the movements of stars and galaxies, gravitational lensing, and how the universe evolved.
Since dark matter can't be directly detected, experiments are being conducted to learn about it indirectly instead. It might be produced in particle collisions at the Large Hadron Collider, and while the facility wouldn't be able to contain it, any dark matter created would carry off energy and momentum, allowing scientists to ID the stuff by mapping this missing energy and momentum. Researchers at the University of Nevada are attempting to track waves of dark matter sweeping over the Earth by tracking desynchronization between atomic clocks in orbit and on the ground.
One of the most rigorous tests, the Large Underground Xenon (LUX) experiment, concluded last year. LUX contained a huge tub of xenon gas built in a facility 1 mile (1.6 km) underground to reduce interference from natural radiation. The idea goes that on the rare occasion when a dark matter particle bumps into one of the xenon particles, the tiny ripples from the collision should be detectable. Unfortunately, the experiment ran its 20-month course without a single such event.
Not detecting any dark matter doesn't mean the test was a bust: instead, it helps narrow down the range of masses that dark matter might have. Although dark matter "particles" could have a mass as huge as a dwarf planet, LUX was specifically searching for masses more than five times that of a proton. Coming up empty-handed might indicate that its mass is smaller than the instrument can detect – and that's where the new Brown system comes in.
"Most of the large-scale dark matter searches so far have been looking for particles with a mass somewhere between 10 and 10,000 times the mass of a proton," says Derek Stein, co-author of a paper describing the new detector. "Below 10 proton masses, these experiments start to lose their sensitivity. What we want to do is extend sensitivity down in mass by three or four orders of magnitude and explore the possibility of dark matter particles that are much lighter."
To do so, the new system would use a tank of superfluid helium instead of xenon. The thinking goes that if the nucleus of the atoms in the tank is bigger than the incoming dark matter particles, then the dark matter would just bounce off without disturbing the detector particle. Since the nucleus of xenon is about 100 proton masses, that limits how light an incoming particle can be and still be detected. Helium, however, has a nuclear mass of just four proton masses, extending its sensitivity to lighter particles.
In fact, the Brown researchers say their design will be able to detect particles between 1,000 and 10,000 times lighter than previous experiments could pick up. Using helium is only part of the improvement, too: the instrument has been cleverly designed to amplify a signal from just a single atom.
If a dark matter particle bumps into a helium atom, it creates sound wave-like excitations called phonons and rotons. These would travel through the superfluid unimpeded until they reach the surface, releasing helium atoms into the vacuum above the fluid.
There an array of small, positively-charged metal pins is lying in wait, and when the helium comes close to one of them, it turns that atom into a positively-charged helium ion. The ingenious part is that because both are now positively charged, the ion will be repelled away, zipping off with enough energy to be easily detected by a calorimeter.
"If we put 10,000 volts on those little pins, then that ion going is going to fly away with 10,000 volts on it," says Humphrey Maris, co-author of the study. "So it's this ionization feature that gives us a new way to detect just the single helium atom that could be associated with a dark matter interaction."
The system is still just a proposal at this stage, and the researchers say that before it could be built and run, they first need to experiment with the dynamics of superfluid helium and how their pin ionization idea might work.
The research was published in the journal Physical Review Letters.
Source: Brown University