What can’t it be?

I’ve been trying to think more seriously about the entity that set off the LZ experiment. I was dismissive of the possibility of it being dark matter, perhaps unfairly so. On further reflection, I feel only fairly dismissive.

First, the formal significance that is claimed is 2.6σ. That seems to be a fair and fairly conservative statement. So it seems something did happen, the question is what. The odds of a 2.6σ event are about 1 in 200, so a fluke seems unlikely.

I can think of three generic possibilities:

1. It’s a fluke.
2. It’s dark matter (huzzah!)
3. It’s some strange new particle-entity that is not dark matter.

I remain of the opinion that the most likely possibility is that is is a fluke. I know I just said that seems unlikely, but “seems” is doing a lot of work here. I think it is fair to say that known explanations are excluded at this confidence, but that leaves a lot of unknowns. Most unknowns turn out to be boring.

We’ve been down this road many times. The LHC has been running for many years now. It detected the Higgs particle, which was the last missing piece of the Standard Model. That’s a great success. It was hoped that the LHC would also reveal new physics beyond the Standard Model. That it has not done. But there have been many hints during its operation. These appear as glitches (deviations from expectation); there have been multiple glitches that appeared to be significant at the 2.something σ level. Each time such a glitch was reported, it resulted in a flurry of theoretical activity. Within days, the arxiv preprint server would be inundated with hundreds (literally hundreds) of preprints staking out a claim as to what the latest glitch might be. Each time more data were acquired, the glitch went away. So this seems like another such occasion, making (1), a fluke, seem like the least improbable outcome.

This is why the [arbitrary] threshold for discovery is set at 5σ (1 in 3.5 million). Flukes at the 2.something σ level happen too easily, too often. This phenomenon is not restricted to particle physics. The same thing happens in astronomy all the time. All the gorram time. It’s almost as if nature places more probability in the tail of the distribution for “weird” events than the Gaussian statistics we use say there should be.

Given this experience, finding a new particle outside the bounds of the Standard Model of Particle Physics seems a lot less likely than 1 in 200. Seems is again doing a lot of work here. How likely 1 in 200 hundred seems depends on your priors. If you’ve been working on this for a long time in the hopes of a positive result, these odds seem pretty good:

“I have been waiting for a positive result for, oh, my god, 40 years. So you can bet I’m very, very excited by the LZ results.”

Katy Freese

If, like me, you’ve already convinced yourself that dark matter cannot explain the kinematic data for galaxies, then it is just another opportunity for the eyes to roll up inside the skull. Obviously we can’t both be right, which is why we do these experiments.

So what about the other possibilities, (2) and (3)? If the event is real, it is tempting to believe that it is dark matter. That’s what the experiment was designed to detect, after all, so that’s a natural first assumption. But just because you’re looking for something doesn’t guarantee that when you find something you find what you’ve been looking for.

As I’ve reviewed before, we’re way past the regime of what we expected to find. So it’d be great to find something, but there’s little reason at this point to expect that something to be what we set out to find. So it might be dark matter (2) or it might be something completely different (3). I’ve no way to weigh the odds of those possibilities because they both seem very unlikely.

For WIMP-like dark matter we at least have some expectations, and this event does not conform to those expectations. It is too high energy. For every event like this one, the detector should have already recorded lots and lots of lower energy events.

That doesn’t mean it isn’t dark matter (2), just that it isn’t what we expected. But we already missed out on what we were expecting, so maybe? Or maybe it is something unrelated (3), or nothing at all (1). One event does not help override whatever assumption bias we might have had about these possibilities to start.

To be convincing…

It occurs to me that I am likely to hold the putative detection of dark matter to a higher standard than other scientists. So what would I find convincing?

• First, the detection of a putative new particle has to cross the traditional 5σ threshold.
• Second, the same result must be obtained by independent experiments.
• Third, the new particle has to be demonstrated to have the right properties to be dark matter.
• Fourth, the new particle has to exist with the required mass density to be the dark matter.
• (Fifth, it should naturally explain the observed MONDian phenomenology.)

I put the fifth criterion in parentheses because it is a very high bar, and I can conceive of being persuaded to soften it. It is also more advanced tuition than most physicists have contemplated, so let’s start with the more basic requirements.

At present, none of the above criteria have been met by the new LZ result. There is an old claim to a significant detection by DAMA that satisfies the first criterion but was never independently reproduced, so fails the second criterion. Nobody outside the original experiment takes it seriously now for that reason. It is sobering to realize that while new and exciting, LZ has not yet progressed as far down this list as DAMA.

Supposing both the first and second requirements are met, then we have a new particle. That is certainly interesting, but by itself would not mean we’ve found dark matter. When we’re in the realm of imagining new particles, there is a vast landscape of possibilities. Only some features in that landscape could qualify as dark matter; there are other possibilities that could exist but have nothing whatever to do with dark matter. So the properties of the particle have to be right, and there is a long list to satisfy.

From a generic astronomical perspective, the fourth criterion is the most important. We have a pretty good idea of the mass density that dark matter needs to have in the solar neighborhood to explain Galactic kinematics. That’s what these experiments are trying to detect: the dark matter particles in our Galaxy’s dark matter halo that happen to be passing through just now. So for whatever we detect to qualify as the dark matter, there needs to be the right number of particles with the right mass to satisfy astronomical requirements.

This fourth criterion is a pretty high bar. It does not suffice merely to find a new particle, it’s gotta be the right kind of particle and these particles have to exist with the right density. That can be measured, but it requires lots of detections: enough to work out the properties of the particles and how many are whizzing by. This is part of the disconnect I had in the last post with people complaining about statistical significance: they were focused on the significance of this one event; I was worried about having enough detections to estimate the mass density, or at least see if it is anywhere near the right ballpark. I’m more concerned with the astrophysical relevance of a new particle than with the new particle itself.

This is a generic problem in the field of dark matter, being as it is at the interface of two different fields. Astronomers need dark matter; we don’t care so much what it is if it performs as required. Particle physicists heard we astronomers needed dark matter, so are more than happy to suggest possibilities, but are more interested in the particle itself than what it does for galaxies. I don’t say what it does for the universe because that’s a lower bar where any dynamically cold, non-baryonic mass will do. The cosmic density needs to be right, but there’s no way to measure the cosmic average, only the local density of the Galactic dark matter halo.

The highest bar is the fifth criterion, which I think warrants its own post.