Can Sapphire Crystals Capture Exotic Dark Matter?

By Ryan F. Mandelbaum on at

Physicists often build experiments looking for a specific something. Maybe that something consists of dark matter, new kinds of particles, or new ways that particles might interact with one another. Other physicists are trying to use these experiments’ old data in new ways, to look for something other than that specific something.

Confused? Here’s an example: One researcher is hunting for a less popular idea to explain dark matter, the elusive source of most of the Universe’s gravity. He’s using data from an experiment designed to observe a less exotic thing. This is physics, so he hasn’t found anything yet. But that doesn’t stop the results from being important.

“We didn’t see [the new particles], but I wasn’t surprised,” Jonathan Davis from King’s College, London told Gizmodo. “This is more of a proof of concept showing that you can use new ideas to probe all sorts of energies.”

You might be familiar with an important cosmic conundrum. Regular matter only makes up four per cent of the Universe’s energy, while some sort of strange, invisible “dark matter” makes up around 25 per cent (a mysterious thing called dark energy makes up the rest). One especially promising candidate is “weakly interacting massive particles,” or WIMPs, that consist of a hypothetical particle that causes such a tiny jostle on atoms that scientists haven’t seen it yet. Davis is instead looking for a hypothetical particle stronger than the proposed WIMPs but still too weak for regular experiments to detect.

It’s a SIMP, for “strongly interacting massive particle.” The interaction between a SIMP and a nucleus might be as strong as the interactions between the particles within the nucleus.

WIMP-hunting experiments exist and are buried deep underground to filter out more strongly-interacting particles. But researchers haven’t found any WIMPs yet. The thick rock above these experiments would stop the SIMPs, though. So Davis asked the physicists at the CRESST collaboration in Germany if he could look at the data from their v-cleus experiment. This sapphire crystal-containing detector sits on the Earth’s surface and is cooled. In theory, the temperature of the detector should increase by a tiny amount should a particle pass by; SIMPs would leave a specific signature in the data.

Davis didn’t find anything with the preliminary, proof-of-concept run, according to the paper published recently in Physical Review Letters, but he was able to help physicists understand what a SIMP can’t look like. That’s of crucial importance to particle physics so other experimenters don’t have to look for the same kinds of particles.

The experiment didn’t rule out as many possible particles as some others have and overlapped with some existing experiments, pointed out one physicist, James Beacham from CERN and the Ohio State University, in an email to Gizmodo. Still, he says, “I love results like these, and they’re a large part of the bread-and-butter of theorists and phenomenologists, since we experimentalists rely upon them to scrutinise our results after they’re released to point out where new physics could still be hiding and how we could have missed it in our data.”

Another CERN physicist, Steven Lowette, was excited about the results, since he’s similarly hunting for SIMPs at the LHC, and other particles that might have slipped past other physicists. “The SIMP is one that would give a special signature the LHC was not built for.”

Davis’ is another interesting example of physicists using existing experiments in different ways than they were intended in order to find new particles or phenomena others might not be looking for. Beacham himself has tried to coordinate searches like these. Other physicists are trying to take advantage of vacant physical space near other experiments, like walls or steam tunnels near the high-energy particle collisions of the LHC. Here they might make far cheaper (sometimes plastic) additions, like MoEDAL and MilliQan to spot particles the original experiment wasn’t meant to find.

But Davis himself wants to see more directed searches for these SIMPs and other exotic particles.

“Hopefully it will encourage people to build experiments to look for other sorts of weird dark matter.” [PRL]


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