Finding That There's Nothing to Find

In 1967, a team of scientists hauled a big pile of gear-- electronics, particle detectors, a giant slab of iron-- into the burial chamber at the base of one of the pyramids at Giza. This sounds like a scene from a science fiction or fantasy novel-- throw in the fact that their first attempt was interrupted by the Six Day War and you've got an element of a Tim Powers secret history story-- but the goal wasn't the opening of an interdimensional portal or the raising of the dead. Instead, they were using astrophysics to do archaeology: their detectors measured the number of cosmic ray particles coming from all different directions, and used that to "x-ray" the pyramid looking for hidden chambers.

The idea behind the experiment is that high-energy particles blasting in from outer space will sometimes collide with atoms in the upper atmosphere and create subatomic particles called muons, heavier versions of electrons. These muons are created with a good deal of energy, and can penetrate a fair distance through solid rock, reaching down to the burial chamber under the pyramid. They aren't completely unaffected by the rock, though-- it does screen some of them out-- so an empty space somewhere inside the pyramid would lead to a higher than expected number of muons coming from that direction.

Their experiment worked beautifully, clearly showing the contours of the pyramid. In one test, they even showed that they could see the other pyramids by the decrease in muons along a line that passed through two of them. Despite one false alarm-- they had all their detectors set up at one and of the burial chamber, but had forgotten to account for the empty part of the room when they calculated the expected number from that direction-- they didn't find anything.

The physicist in charge of the experiment, Nobel laureate Luis Alvarez (who I wrote to back in 1981), put a different spin on it. They didn't fail to find a chamber, he said, they showed that there are no chambers to find.

I was reminded of this story yesterday, because the big physics story of the day was the release of the first results from the LUX dark matter search, which failed to find anything. This led to a New York Times piece by the great Dennis Overbye with the odd headline Dark Matter Experiment Has Detected Nothing, Researchers Say Proudly. It's kind of an odd formulation, but not an inaccurate one. LUX is a really nice experiment, and their data are beautifully clean, something to be very proud of. They just don't see any evidence in their data for the existence of dark matter particles with the properties that might've been hinted at by some earlier experiments.

This wasn't the only such experiment released this week, though. With much less fanfare (I only saw it via Matt Strassler's blog post), the ACME collaboration released a preprint proudly announcing another lack of detection, this time of an electric dipole moment (edm) of the electron. They beat the previous sensitivity limit by a factor of 12, but their results are still consistent with the eelectron edm being exactly zero. I've written about this a bunch of times before-- I wrote up the previous record by Ed Hinds' group back in 2011, and did a feature on edm searches for Physics World back in 2009. I'm a bit of an edm groupie, really-- I would never have the patience to do these experiments myself, but I think they're incredibly cool, and love hearing the latest.

Both of these, and a host of other searches for new physics (arxiv version) are placing limits on physics beyond the Standard Model. In the case of LUX, they ruled out some popular candidates for dark matter, and the ACME result does something similar. In most of the obvious extensions of the Standard Model of particle physics, the new particles that get added have properties that allow a much larger edm than you get with known physics, and you can infer something about the properties of those particles from the size of the edm. With fairly conservative assumptions, the ACME result can be seen as putting tight limits on the possibility of any such particles with masses up to about the same energy scale probed by the Large Hadron Collider. This takes a big bite out of the possible models for physics beyond the Standard Model, though I have confidence in the nearly infinite ingenuity of theorists confronted with inconvenient experiments to find a way to push things just a little beyond the current sensitivity.

This sort of work, as you might imagine, requires a certain personality type. You need to be able to go into an experiment expecting to detect nothing but hoping to see something, and still get excited by null results. It also demands a level of obsessive attention to statistical and experimental detail that's rare even among physicists.

But the people working for both of these collaborations have every reason to be proud of their detectors not finding anything. They haven't failed to find exotic new particles, they've shown that there are no exotic particles to find.

------

(A version of the "no chambers to find" line is quoted in this PDF about Alvarez's "detective" work, and also in different words in his autobiography. "Featured image" from Wikimedia.)

(This post was prompted by an exchange of tweets with Ben Lillie, in which I quoted the Alvarez line, which led to Ben discovering Alvarez's fascinating career, to his great delight. I count that as my good deed for Wednesday.)