physicsdude

> > They've already unblinded, so it would be unethical to do anything other than report what they found.

> Absolute and utter ridiculous nonsense. Can you not just admit when you're wrong?

Wrong about what? If you think they should _not_ just report their findings, and instead try to change their analysis after already finding this event, then you're free to think that, and you're also free to say so. Personally, I don't think that that would be ethical.

> But at least 8 global, coordinated press releases? That's a choice, with consequences.

You seem to be asserting that there is something of nefarious happening. The collaboration does coordinate otherwise-unrelated releases by individual institutions. The alternative is that PR folks distributed across different institutions, few if any of whom have any training in physics or statistics, would make whatever claims they feel appropriate at whatever time they feel appropriate (relative to each other, and relative to publication of the actual paper). In what way is that preferable?

> 2.6 sigma results become amazing new discoveries.

They are not claiming a new discovery. They are claiming to have observed a single event, which is significant at the level of 2.6 sigma. A naive treatment produces a significance of more than 3 sigma, and the collaboration did what you might call "extra" work to be fair in their statistical analysis, which reduced the appropriate number to quote to be below 3 sigma. They state that clearly, and suggest interpreting the event at 2.6 sigma, the less-flashy option. To me, that shows integrity.

One of the amazing things about Cherenkov radiation is that it is not omnidirectional. There are processes (fluorescence, or scintillation, for example) which result in isotropic emission of light from materials. But Cherenkov light is emitted along a conical surface about the axis of the charged particle's direction of travel. As a sister comment said, muons tend to move linearly, so this light stacks up in a particular direction. Electrons, particularly at lower energy, scatter more, and that stacking gets washed out a bit.

That directionality is very useful. The directionality of a muon or electron produced from a neutrino interaction is correlated with the direction of the incident neutrino. That's interesting for the astrophysical kind of questions that IceCube is addressing, and it's also useful in other contexts (background rejection for fundamental physics searches, monitoring of nuclear reactors via neutrinos, ...).

Cherenkov radiation is indeed polarized, but to my knowledge that hasn't ever been made use of.

Tangentially: there's also some really cool work being done on detectors which utilize both Cherenkov and scintillation light at the same time, exploiting the directional (and other) differences to extract the most information possible out of each event.

That is a great point. I'm not sure if I fully understand your question, but I'll comment on both "testing," meaning doing the analysis, and "reviewing," meaning peer review.

The point of "blinding," as I mentioned it before, is to guard against biasing the result due to choices made by analysts while figuring out how to compute their final answer. Part of that is just psychological --- if an analyst _knows_ that the data is obscured, for lack of a better term, in a way such that they can't believe a signal that they see (for example --- that's just one way to blind), then they won't feel any emotion or excitement about things they encounter while constructing the analysis, because they know what they're looking at isn't real, and that helps them to maintain objectivity. It's also common to have multiple independent analysis teams for something like this --- they know that they'll have embarrassed themselves if they don't all get the same result both before and after unblinding, which provides pressure to maintain objectivity.

As far as review goes --- there are physicists who believe in dark matter, and there are physicists who don't believe in dark matter. But all particle physicists and nuclear physicists use the same core technical and statistical methods. For a result like this, there will be a few "core analysts" who do the analysis, typically graduate students and postdocs. Their advisors review their work, through the lens of making sure that they are doing so sanely. The collaboration will also establish an internal review committee to comb through the documentation and software which produced the result, not necessarily trying to disprove what they found, but really looking at it critically. Once it gets to a journal, they'll get reviewers who may be biased for or against the result, and who may or may not work directly on dark matter detection, but who can all assess the methodology by which the result was obtained. That's probably the best we can do with humans involved.

Thanks for clarifying, and my apologies if I came off as argumentative --- what you _are_ suggesting makes sense and is a reasonable thing to wonder about.

But I don't see anything particularly strange or coordinated is happening. From the collaboration's perspective, it's quite stressful having this event. They've already unblinded, so it would be unethical to do anything other than report what they found. If they publicize it and it's a mistake, then that's a big blow to their credibility. But if they withhold it and it's real, then they miss making the discovery and/or bias their future analyses on larger datasets without disclosing that to the community. So they are in a tough spot, and are safest to just tell the world what they saw.

This is getting media attention because it would be a big deal to the general public if this ends up being a real. Someone in the field wouldn't claim that it's real, but the possibility is catnip to folks looking for a sensational headline. That the press releases are synchronized in time is because the result was just released right now and they're all doing their commentaries right away --- for something like this, each institution independently negotiates a release with the local researchers who are involved. They all promise to wait until the result is officially released, out of respect for the scientific process, like you say, but the different institutions aren't coordinating with each other. They're just all respecting their own researchers.

Is a 2 or 3 sigma fluctuation worth a lot of press? Personally, I don't think so. But we don't know if it's a fluctuation yet, and no institution is going to pass on having made it clear, if this does turn out to be real, that they were involved.

All that being said: I would very much like these folks to continue to receive funding. They are professional and do excellent work, as demonstrated here.

Particle physics is not a particularly large community. There is a hand-countable number of experiments like this, and the folks working on each of them know the folks working on each of the others. The collaborations executing each experiment are comprised of scientists employed by multiple independent institutions, both public and private, typically across national borders. Internally, the collaborations have a democratic structure with individual researchers acting as institutional representatives serving in what is essentially a parliamentary structure to make decisions. The software to determine results is always public within the collaboration and reviewed well in advance of making any truly public disclosure like this.

Culturally, an attempt to intentionally distort or misrepresent data to suggest a result like this would not be tolerated. You can imagine a bad actor writing a single-author paper with fakery, but a collaboration at this level is inherently critical of itself and everyone holds each other accountable.

But mistakes do happen. Exciting results or hints of exciting results can appear due to well-intentioned researchers making convenient mistakes which get glossed over for psychological reasons and then add up to appear as something significant. You can read about "blinding" (which is mentioned in the paper) to get a feel for the techniques researchers employ to not only guard against fabricating results intentionally, but even unintentionally.

This raises what is (I think) an interesting question. CERN is a collider, so they are _trying_ to produce lots of stuff, and they do (lots and lots of stuff). They can't write it all to disk, and most of it isn't interesting enough to try.

The work being done here falls into the category of "low background physics" --- they aren't trying to produce anything, and actually put quite a bit of effort into doing the opposite, by removing all sources of particles (e.g. sourcing materials free of radioactive contaminants, physically cleaning all surfaces and purifying all fluids involved, etc).

So the detector, if built properly, is fairly quiet, and you try to write as much data to disk as you can (e.g., if something even fairly-potentially interesting happens, you save it). Then when you analyze the data like this, you ignore the majority of what you've got --- only a teeny fraction makes it into an analysis of this caliber.

At some level, it could be (and that would be an great discovery as well!). It's a question of probabilities: it's unlikely to be any of the things that we already know about, but that doesn't mean that it's something new. Unlikely things happen --- infrequently. As stressed in the article and elsewhere in comments, more data should elucidate what is going on. That's the difficulty of these kinds of searches: there is one event, and we can't make clear, confident statements about one event.

The detector from which data is taken to do this analysis contains 7 tons of liquid xenon. It is inside of a larger detector, which contains hundreds of tons of water and more than 10 tons of a scintillator. One of the functions of that outer detector is to absorb neutrons and other infiltrates coming from the rock.

When doing this kind of thing, the analysts will plot the rate of events as a function of "distance from the outer surface" and confirm that it decreases, and in this case "is 0" inside of the detector used for analysis.

But keep in mind that all statements are made statistically, so it's not that the event _can't_ be an external neutron, but that it is _very_ unlikely to be.

The data-driven approach is referring to a more general technique --- it's a less fancy concept than renormalization, which only comes up when doing full-blown relativistic quantum field theory. Specifically, it refers to the use of a couple of well-grounded mathematical tricks to sidestep doing a from-first-principles calculation by, effectively, translating the results of different measurements into a prediction for something new (in this case, a quantity that contributes to the calculation of the magnetic moment).

In non-relativistic quantum mechanics, there is the concept of the "wavefunction," from which you can predict the results of measurements of a particle/system. But in most real-world scenarios, you can't actually compute the wavefunction, and so naively you can't make any predictions. But there is something called the "optical theorem," which relates a single evaluation of the wavefunction to a scattering cross-section, which is something that can be measured. If you're familiar with complex analysis, there is also the "residue theorem" which allows you relate individual function values with integrals of the function in the complex plane. Basically, you can combine those two relations to translate kind one integral (which you need to compute) into a different integral (which can be measured).

This is what was done here --- just instead of a simple QM wavefunction, the relevant concept is called a "vacuum polarization function."

Axions can seem a bit goofy. One thing to know is that when talking about particle physics, we like to talk about particles, but "particles" are really a concept from classical physics. When doing quantum physics, one is fundamentally concerned with waves. If you've read any about quantum mechanics, you've heard of "wave/particle duality," which is something of a connection between the two pictures. Another thing to know is that axions should interact electromagnetically.

The bottom line is that because axions would necessarily be very "light" (that is, very much not-massive), it is misleading to picture them as "particles" and better to picture them as "waves." So while it's true that axions would feel the electromagnetic field of an atom's nucleus, that's really just because it's an electromagnetic field. So to make an experiment which is sensitive by modern standards, you say "forget individual atoms, I'm just going to make a cavity and crank it up to large electromagnetic field." And that's exactly what is done in practice.

Reprioritization of direct experimental searches for dark matter is already happening. WIMPs are by no means being abandoned, but because we are closing in on the neutrino fog background (which is mentioned in another comment), it's been recognized that to myopically cling to the same kind of experiment which dominated the 2000s and 2010s is not a strategic move (both from the "we expect to see something" and the "responsible use of tax dollars" perspectives).

For example: axions, an alternative DM candidate mentioned in another comment, have seen a significant growth in attention in recent years, and the usual detector technology for axion searches is currently being refined and scaled up, from benchtop-scale, dedicated experiments to lab-scale, wide searches.

At the same time, different groups which have developed past WIMP detectors are merging to collaborate on the larger, next-generation detectors. And there is R&D and prototyping happening to create detectors which, although looking for WIMPs, are sensitive in entirely different mass ranges than those of yesteryear.

This is absolutely correct, as there are no solar neutrinos with energy above ~20 MeV (but below that, the solars dominate). Thanks for the clarification.

Primordial black holes do not require new fundamental physics, but for them to constitute the primary component of dark matter would require a revision, at some level, of our narrative of the history of the universe. This gets a bit outside of my core knowledge, but as it stands there aren't any super solid mechanisms for generating black holes of a plausible mass distribution, early in the universe, such that we would see what we see today.

But, IMO, this is worthy of more study both theoretically and experimentally. An update to the evolution of the universe would be awesome!

The "floor" is due to solar neutrinos - the sun is a continuous source of neutrinos and is located much closer to us than virtually every past supernova was. It is also a soft floor, as there are analysis tricks that can be played to distinguish between interactions from neutrinos and WIMPs at the statistical level. For this reason, the current fashion is to refer to a "neutrino fog," which can be entered into to some depth, as opposed to a "neutrino floor."

Particle physicist here. I've worked on direct detection DM experiments in the past, and personally know some folks who work on the LZ experiment. That direct detection experiments, such as LZ, have not detected a signal does not contradict any predictions.

Indeed, relevant to what an experiment like LZ might see, there really isn't much in the way of "predictions" which can be "contradicted." What we have at this point are mechanisms to calculate the interaction rate _given at least one free parameter_. If we were to detect a non-zero rate, then we would "know" the free parameter of a single-parameter theory underlying that calculation. If we were to continue to detect a non-zero rate, then we would try to do so using different materials, and look at the time dependence of the rate (or, really, the dependence of the rate on the Earth's direction of travel in our local galaxy). That would help us choose between different theories, pin down the free parameters, and confirm that what we're seeing is consistent with "heavy stuff just sitting out in the universe."

But, from a particle physics perspective, right now there are no predictions to contradict - just an opportunity to detect something.