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So after everybody assuming that the Large Hadron Collider shutdown last month meant no new physics discoveries until spring at least, we now out of fucking nowhere have two (2!) completely unrelated non-LHC scientific experiments finding almost exactly the sort of “something not right” that nobody in particle physics has managed to score in ten years– within two days of each other.

Short version:

• PAMELA is a European Space Agency satellite that counts the different kinds of particles passing through space near earth. They’ve found more positrons passing through space than astronomers would have predicted.

• CDF is one of the two detectors at Tevatron– the LHC’s immediate predecessor to the “most powerful particle accelerator in the world” title. CDF finally got enough data to show that they’re finding more muons than physicists would have predicted, and the muons are showing up in a really weird place.

This is interesting because both of these seem to be explainable by the existence of a new particle type or types, which would be sort of the holy grail of experimental physics right now– people have spent the last 30 years desperately trying to find any holes in the “Standard Model” of particle physics and basically failing. Even better, both of these results (with varying degrees of plausibility) have some reason to believe that the new particle they point to is in fact the particle responsible for “dark matter”.

To go into a little more detail: These days when you go out looking for particles, it’s usually not possible to detect the particles directly (generally because most particles that interest physicists these days only exist for fractions of an instant before decomposing), so instead what you have to do is look for things that whatever particle you’re actually interested in eventually decays into– interesting particle A decays into less interesting particles B and C, which decay into recognizable sprays of boring but long-lived particles D through F. Modern particle physics is tested by building detectors to count particles D through F and then checking whether the theoretical models come up with approximately the same counts that the detectors did.

This is basically what both the CDF and PAMELA results above are about– with the difference being that with PAMELA they’re counting particles coming from deep space, whereas with CDF they’re counting particles coming from collisions inside Tevatron. In both cases, they’re seeing an excess of some simple particle where they don’t expect to. In both cases, this could very possibly just be experimental noise– both the things found in excess here have difficult aspects to accurately detecting them, and these results are going to need to be closely checked before they’re taken too seriously. (The obvious question with the CDF result is whether the same results can be seen by D0, which is the other detector sitting next to CDF on the Tevatron ring.) But either result could mean something is happening to produce those particles that physicists don’t know about. In the case of PAMELA, there’s a good candidate for where the extra particles come from; CDF is a bit more puzzling.

In the case of PAMELA, most people who are taking bets on what the extra positrons mean are hoping that what we’re actually seeing is the faint glow of dying dark matter. Dark matter research, these days, has basically come to the consensus that dark matter is some kind of special “WIMP” particle that’s immune to the electromagnetic or strong forces– so that it passes right through normal matter, and light passes right through it– but which still interacts with normal matter via gravity. This conclusion was partly reached because several leading alternate explanations for dark matter were shown to be insufficient to explain all the dark matter that exists, and partially because of smoking gun data like the bullet cluster observation, where astronomers caught a photo where two galaxies collided and you could actually see the dark matter (mapped via gravitational lensing) separating out from the galaxies they’d previously been clumped inside of. We’ve never seen a particle that behaves the way the “WIMP” would have to, but maybe it’s just really heavy, too heavy for any of the particle accelerators we’ve built to have found it.

There’s maybe a way to detect dark matter without a particle accelerator, though: dark matter doesn’t interact with normal matter, but maybe it interacts with itself. Maybe it’s possible for dark matter particles to collide with each other, and maybe when that happens they annihilate and release energy. And if this “dark matter annihilation” is occurring, then it would look very much like the positrons-from-nowhere that PAMELA is seeing– the positron excess is consistent with the idea of some kind of floating particle scattered everywhere throughout space destroying itself and leaving positron debris, and the mass of the particle that this debris would  probably have come from is right in the range where WIMP models tend to put their new particle.

In fact, several people who’ve made dark matter particle models have already come forward to show that the PAMELA data neatly meets their pet suggestion for what the dark matter particle is. And by “come forward” I mean “published papers, before the PAMELA results even came out.” There’s a weird little subplot here where the PAMELA data actually “leaked” several months ago– and by “leaked” I mean “the PAMELA people presented their data at a conference”. After the conference everyone who’d seen the PAMELA talk promptly went home, looked over the pictures of the PAMELA powerpoint slides that they’d taken on their cell phone cameras, and posted papers on arxiv.org about the consequences of the PAMELA results. Then the PAMELA people got all pissy that people had, you know, done things with the information they gave them, rather than waiting around for the PAMELA people to finish getting their paper into print. Whatever. Anyway, this is where things get tricky.

The PAMELA result I’m talking about here was published last thursday; then the CDF results came out on friday. Everybody knew the PAMELA results were coming. The CDF results on the other hand were a complete surprise– although CDF had been working on a version of the results since at least July and it’s probable a good number of people inside CDF knew this was coming. I’m still not sure why the CDF people decided to publish the day after PAMELA did, but the two results have basically nothing to do with each other. When the guy who writes resonaances was asked about this, he said:

…from the experimental point of view there is only one connection between PAMELA and CDF: both see an excess in a leptonic final state. Otherwise, the two signal are completely different, in particular, the energy scale is different. It is very likely that two anomalies are not linked in a simple manner, but models that explain both appear more sexy to theorists.

So, here’s the punchline: As mentioned, after the early PAMELA reports more than one group with a dark matter theory wrote a paper about how whatever it was they were already working on totally fit the PAMELA results. One of these groups published a paper a few months ago about the “SuperUnified Theory of Dark Matter”, which fit the PAMELA data but not very cleanly; it predicted something about “lepton jets” that would happen as a result of the existence of the dark matter particle, and suggested that the LHC look for these lepton jets. I think normally clutter like this would be seen as a flaw; except, it turns out that these “lepton jets” would show up something very like the same weird muon excess that CDF now turns out to have been seeing already. In fact, this makes the ’superunified’ dark matter theory the only paper currently in the wild with an explanation of muon sprays of the particular weird type that CDF saw.

So: Two potentially groundbreaking unrelated results announced in physics over the course of two days, and one published theory turns out to have predicted both of them as being a consequence of dark matter. That could just be a coincidence, but if so it would be a really weird one. Actually more likely than it being a coincidence is that one of the ’superunified’ authors just heard some kind of rumor from somebody in CDF that they might publish something about a muon excess– these big group experiments leak like sieves– and tweaked their paper to fit. One of the authors is already publicly claiming he had no idea CDF had been seeing this; the other hasn’t said anything either way yet.

More on this stuff if you care:

Papers:

CDF’s new findings
PAMELA’s new findings

The “superunified” theory, which explains PAMELA and maybe CDF
The “minimal dark matter” theory, which is an example of something that explains PAMELA but not CDF

Blog posts:

Tomasso Dorigo, who works for CDF, on the muon thing

The Resonaances guy, who works for the LHC’s parent organization, on the CDF muon thing

Resonaances guy on the PAMELA thing

Peter Woit’s roundup on these subjects, including an elaborate soap-opera bit in which Woit discovers that a draft of CDF’s as-far-as-anyone-knows-totally-surprising results have apparently been sitting by accident in a google-indexed public web directory since July