Biggest dark matter detector spots a single weird particle
science.org270 points by randycupertino 16 hours ago
270 points by randycupertino 16 hours ago
I read their preprint[1] and they did a thorough job. They investigated a number of the things I'd suspect if I were looking for mis-reconstructed events or weird backgrounds.
So it's certainly interesting!
That said, particle physics history is full of 3 sigma particle "discoveries" that disappeared with more data. They're collecting more, so hopefully we'll learn more in a few more years.
[1] https://lz.lbl.gov/wp-content/uploads/sites/6/2026/08/LZ_Pre...
This sort of thing is really useful for helping people to understand what the purpose of scientific publication is. It's not about presenting finished products to society, it's more like a Discord where you communicate new findings with other authors. Often the reason for a communication is because you found something weird, and you want other people to know about it so they can help you confirm or rule it out as bad data. People shouldn't feel gunshy about this. (My field even has a conference for failed results, CFAIL.) I like to highlight examples of this stuff, because I see so many angry online comments when a paper turns out to be "wrong" or doesn't replicate.
Yep. Publishing like this gives a heads up to those operating similar observatories to keep an eye out for similar events. And it gives a nudge to theorists that might help them start looking in a more fruitful direction, appropriately caveated that it may be a statistical fluke.
You have accurately described a email mailing list. Where in the value-add here?
> You have accurately described a email mailing list. Where in the value-add here?
Pre-prints are basically a mailinglist where you post your paper prior to peer review.
The value over a simple mailinglist is:
1. Stable URL and citation to enable other work and discussions to cite and reference it.
2. Versioning of the paper, allowing updates to be made without having mail out the paper, while allowing everyone to find all prior versions
3. Host for a PDF and data that might be quite large
4. Centralized searchable long term archive of scientific papers
5. Scalability, arxiv gets 30,000 submissions a day, no one wants to receive 30,000 PDFs in their inbox everyday
You think that people's findings should be communicated by email? that their email chains are what should go into the permanent record and be cited and printed out and included in journals and such?
would you include all the quoted text in the reply-alls, or is that too much?
That's how Linux is built.
Science has too many threads to do it successfully though
> would you include all the quoted text in the reply-alls, or is that too much?
Only quote the relevant part and reply to it, just like this very comment.
And Linux has a large mailing archive of various lists and threads that are searchable and available to everyone and get this: free access
The linux developers' mailing lists are not producing anything like scientific papers...
Kind of arrogant no? Linux kernel development mailing lists are producing something immensely valuable with a much clearer impact on economic indicators than your average scientific paper. Comparing them is hard, but it's patently absurd to say there's nothing being produced compared to scientific papers.
Dunno who you're arguing with, I didn't say they didn't produce anything of value. I said they aren't producing scientific papers. Conversations are not like papers. The scientists have conversations (sometimes on mailing lists!) as well. The analog to scientific papers in the Linux world are... scientific papers. And the occasional essay on the mailing list, which---get this---would be more valuable to humanity if it was subsequently reproduced as a paper with references and explanations and the like.
(Notwithstanding the absurdity of academic publishing, of course.)
No so wrong. The oldest journals started as smailing list :)
https://www.scientificamerican.com/blog/information-culture/...
If I had the email address of every researcher in my field, I would never send a mass email to them describing my latest goofy idea. I would, however, send my latest goofy idea to a conference with those same reviewers (if I felt it was technically appropriate and correct.)
I mean we use to have newsgroups which basically implemented this.
Which of course is the point: it is in fact quite similar to a mailing list, just with some extra protocol surounding it to make it manageable.
So the attempted snark about it up thread is stupid.
Instead of reading everybody's spam or having to have a centralized body decide who gets to send messages to the mailing list, journal editors filter which things are worthy of publication and in which journal such that readers don't have to wade through garbage or uninteresting results.
They were - in the past. I imagine that right now, Discord is their Discord.
Or this[2] 2007 Science paper on ultra high energy cosmic ray source candidates ("anisotropy") that we had to retract because significance started dropping almost the day the paper was approved.
It was a fascinating experience as a junior member to follow the collaboration internal conversation and investigation on this, because a lot of extremely principled scientists were clearly deeply worried about losing their hard earned reputation. In the end, I am convinced that we were simply unlucky.
Reminds me of the FTL neutrinos too, where the scientist where pretty much "hey, something is wrong, can you help us figure it out?" and the general public were the ones screaming "OMG! Physics is dead!"
Then when it comes out as measurement error, the public is all "Damn these scientists are all hype machine clowns..."
That was a fiendish thing to debug; if I recall correctly it was a slightly and intermittently defective connector.
I’m fine with that. Put it at the feet of pop science blogging.
I’m less fine with the time and resources spent on mouse models. They already know you’d get the same utility from a magic 8 ball, but they do it anyway.
Mouse models are useful to discard very bad ideas. There was a recent experiment to use bacteria to kill cancer https://news.ycombinator.com/item?id=46306894 They tried like 40 bacterias in vitro, then like 9 in mice, and only 1 was useful in mice and they will continue only with that, perhaps in humans. Anyway, as you suggest, there is a high chance it will fail.
Also, you can do nasty stuff to mice that would never be allowed with humans. In that experiment they injected cancer cells in mice with a bad inmune system, so they could get like 90 mice with cancer and run the experiment in a short time. No ethical committee would approve that in humans.
Wow haha, that is a lot of authors! Never seen this before!
Most horrifying is a 2014 Science paper on Ebola with 58 authors, 5 of whom died of Ebola before publication. https://www.science.org/content/article/ebolas-heavy-toll-st...
The ATLAS-CMS joint Higgs boson mass measurement paper has close to 6000 authors: https://doi.org/10.1103/PhysRevLett.114.191803
I think the final COVID consortium report has something like 30k authors.
> we had to retract because significance started dropping almost the day the paper was approved.
It's stories like this that raise my p(we are in a simulation).
Are there any other candidate particles besides WIMPs that the observation could be from, assuming it’s a real signal?
In one sense anything that passed all their background rejection is a WIMP. To interact with a nucleus through so much matter, it's not interacting electromagnetically. The main candidate for a strong force interaction would be a neutron, and they did a lot of work to model that and eliminate it as a background. So definitionally it would be a WIMP.
They were pretty model agnostic in what they were looking for. They modeled and simulated a number of different ways a WIMP could interact with normal matter. If this is a discovery, more data will be needed to figure out the nature of that interaction and how it fits into particle physics.
But there's always a chance it's something completely new, or some extremely rare manifestation of things we already know about, but have never seen before. And even if it is WIMP, it may not be the right type of WIMP (wrong mass, or wrong interaction strength) to explain cosmological dark matter.
If neutrons are on the list, how are they ruled out from a random decay event emitting particles, from some mineral in the surrounding rock?
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 experiment is set up to make any already understood interactions some combination of easy to identify or extremely improbable.
They have a lot of shielding for the detector, and also if there were a significant source of neutrons they'd expect to see other evidence, both in the detector and in the separate "veto" detector that surrounds the main detector.
That's not to say it can't be a neutron, but it would be surprising if it were.
The mainstream TV news report that I saw about this ended with a comment about how we should continue to fund this detector. Made me wonder if the nature of this release involved forces other than purely scientific ones. Apparently funding has already been cut for the successor to the LUX-ZEPLIN detector.
> it’s far too early to claim a discovery, physicists warn...“How do you even make sense of one event?” muses Tom Shutt, a particle astrophysicist at SLAC National Accelerator Laboratory and co-founder of the LZ project. “We just decided we should publish and think really, really, really hard about what that event could be.”
Very hard to manage jumping the gun by reporters. Sounds like they saw some new data. No idea what it is.
Looking forward to the follow up.
> The detector lurks 1480 meters deep in the Sanford Underground Research Facility, in a former gold mine in South Dakota.
Glad to see such things getting re-purposed instead of just sealed off and abandoned.
How many years until we've discovered "everything"?
Probably never. We've only mapped like 29% of the seafloor, actually explored like %5 of it, and we've barely pierced the earth's crust.
I think it's cool that there's still unconfirmed hypothesizes, and still unexplained phenomena in the science that's investigating these hypothesizes.
I hope this turns into a real discovery about something; but even if it's an equipment malfunction, hopefully it's a lesson that can be turned into improving the detector.
The moment there are no more unconfirmed hypotheses you can assume something is wrong with sciences. All provable models (theories, explanations) that we have, or could have, are by definition wrong or incomplete.
Astronomy/Physics is overflowing with unexplained physics phenomena these days.
Especially after JWT started looking deeper into the early universe.
So you have 7 tons of Xenon as the detector, hoping that some dark matter will bump into a nucleus. How do you exclude other effects?
To start with, they do a lot of work to eliminate radioactive backgrounds in the materials they're using, and they put the detector deep underground to shield from cosmic rays.
Additionally, when a particle interacts with the nucleus, the ratio of how much energy ends up as scintillation light versus ionization is different than when a particle interacts with an electron, which is most of the background processes.
Then, whatever is left, they try to model using known processes. After all that, there's one event that they can't account for. And that's what the news is about.
I agree. Moreover, I'm not sure if it's the same team, but in a similar experiment while removing all the other effects, they discovered that Xenon 124 is radioactive, but the half life is super long and no one had seen it before. https://xenonexperiment.org/observing-the-rarest-decay-proce...
That's a pretty cool discovery in its own right.
Wow! That's in https://en.wikipedia.org/wiki/Xenon now.
Xe-124, half-life 1.1 * 10^22 years. That's crazy.
Makes me wonder if all atoms with 2+ nucleus elements (protons and neutrons) are radioactive but the halflife is so far out as to make something we'll never detect.
Probably not, they have a pretty good handle on why atoms decay, to the point they can predict some to be radioactive before it's actually observed to be (like Bismuth-209).
Also even if something is REALLY REALLY long lasting, you can still check for the halflife by observing enough of it, they've been able to rule out proton halflives under 10^34 years (the universe is on the order of 10^10 years old) but by observing enough protons (like say 50,000 tons of water) you would expect at least some to decay.
> they have a pretty good handle on why atoms decay
Oh, they actually don't. Radioactive decay, AFAIK, is still an open physics mystery. We know it happens, we don't know why, what causes it, or if there even is a cause. We can predict factors that make it more likely.
> to the point they can predict some to be radioactive before it's actually observed to be (like Bismuth-209).
Right, but Xenon 124 wasn't predicted to be radioactive which is what makes it fascinating. It shows holes in what we can predict as being radioactive which is what makes me wonder about everything being radioactive but the timetable is too far out.
Where is the mystery? Any system can spontaneously transform into a new state with a probability greater than zero unless some conservation law prevents it. In a sense it's just quantum tunneling.
There's just no way deuterium is radioactive, unless hydrogen is radioactive too.
Some theories predict that protons decay, but the half life is like 1E31 or 1E35 years (compare to the Xe124 that has a half life of only 1E24 years). All experiments so far to measure the proton decay have failed, anyway. https://en.wikipedia.org/wiki/Proton_decay
I don't remember anything specific about deuterium, and the method that Xe124 uses is not available, and I can't imagine a razonable alternative method, so my guess is that deuterium is as stable as protons.
Those double beta decays are also interesting because they can probe whether the neutrino is a Majorana particle.
Yeah, it's funny, for experiments like this you spend 90% of your time modeling and subtracting noise, and 10% analyzing the signal that results. Had the same experience in X-ray astronomy. 3 years building a detailed model of all the sources of noise, then subtracting it out and finally starting on the science.
I worked a little on the Virgo interferometer, I would say about 99.9% of the work on those types of detectors is limiting and subtracting noise. ( ≧ᗜ≦)
I always wondered if it would happen in my lifetime. Hope it turns out to be something interesting (AKA) dark matter.
Not well informed on the topic- but the title made me think of the recently launched Roman Space Telescope.
The difference: LUX-ZEPLIN, which is underground, is waiting to detect a dark matter particle itself. On the other hand, NGRST seeks to observe the effects of dark matter.
> Such behavior could require dark matter to be more complicated than just a single new type of particle. For example, the dark matter particle might have some internal structure, like an ordinary atom, so it would only interact if hit hard enough to excite it to a higher energy internal state
I get a little shiver imagining that the dark matter might be something like ordinary atoms. Imagine that other 85% of the universe could have its own parallel atomic table, chemistry, even some kind of life utterly alien to us?
I've heard this referred to as a "dark sector", or "hidden sector":
https://en.wikipedia.org/wiki/Hidden_sector
The scary thought is that, were it true that there is voluminous richly-interacting dark matter, we would be the actual dark sector.
"Imagine a universe, like ours, overlapping ours, except where some of its matter spontaneously rips itself apart, and other matter can be mashed together if you squeeze it hard enough." "That's nightmare fuel! Fortunately it's probably impossible, so far as we can tell."
I can imagine other scary thoughts, though. Ever read any Warhammer lore?
It's possible that there could be an entirely different charge mechanism that works exactly like ours but they don't interact at all; but the whole premise of dark matter is that it doesn't seem to have any self-interaction outside of gravity.
The premise of dark matter is that it is something with gravity that does not otherwise interact with any of our detectors (e.g. "normal" matter); or that any such interactions are weak enough that it is plausible we have not noticed.
By itself, that does not exclude the possibility of dark matter having other interactions which do not interfere with our detectors.
Our observations are much more consistent with a form of dark matter which does not self-interact, or which does so incredibly weakly.
Don’t ask me how or why, but this is essentially the universal (pun intended) consensus amongst cosmologists.
That doesn't rule out "dark matter having other interactions which do not interfere with our detectors" Dark sector theories which include other dark particles or new particle interactions are not controversial amongst cosmologists in this space. For example there's a whole area of study around "dark photons" which would mix with our photons and interact with dark matter.
I guess so. If you could sort of measure the dark matter distribution, there might be a structure under it, but strictly speaking isn’t it limited to saying it’s not at least likely to emit light but does have mass.
It's very hard to explain the gravitational halo around the galaxies if your dark matter can interact with itself. If it interacted like normal matter, it would have a distribution similar to the gases, and not spread way into intergalactic space.
But it does bound it: if dark matter has self-interactions then the apparent distribution would be different. Regular matter forms stars and galaxies and all the structures we see because it can self-interact.
Since where we see dark matter mass shadows we don't see structure formation, what self interaction it may have must be very, very limited.
Why couldn't it be just a weirdly energetic neutrino originating from the neighborhood of some black hole?
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.
I actually saw one of the authors present the data yesterday, one of the audience comments was indeed that this could be explained by an astrophysical neutrino striking the detector (all other neutrino sources have energy thresholds much lower than the detected data), but the data on astrophysical neutrinos is sparse (in part why detectors like IceCube exist), so I believe the argument is that the chances that an astrophysical neutrino would strike the detector is negligible.
Thank you very much for this first hand info. One might argue that chances of theoretical particle existing and showing up are at least as negligible.
Based on my very amateurish skim of this and a related paper, maybe so? They simulated the neutrino background as solar and cosmic ray atmospheric sources, so any source with a different energy distribution is perhaps a possible explanation for the event, I think.
But probably more likely is for this to have been a particularly energetic event in the tail of one of the known sources of neutron recoil detection they did model. More events needed!
> If the new result is real, more signals should emerge soon. LZ researchers have already collected three times as much data as they used in the paper.
It sounds like this implies they've seen 3x more events but it seems like they would have said that if it were the case. Have they just gathered more data about the single event or is this 4 separate events they're talking about?
Detectors like this work on exposure. They're always on (except for calibration and maintenance), waiting for events to happen. This paper was written with 2.8 tonne-years of data. That is, 4.7 tonnes of liquid xenon for a little mmore than half a year. The detector has 7 tonnes, and the 4.7 number reflects cuts they made on parts of the detector that either they don't understand as well, or have higher backgrounds.
As they better understand the detector, they can use more of that mass. They have data from it, but they just didn't use it. And they're always collecting more data, too, as time passes.
So the 3x is saying they have something like 8.5 tonne-years of data.
Hm, they have 7 tonnes of Xenon. Events detected all around in the matter, but the PMTs can localise where the event happened. So they can virtually segment parts of the detector where they are sure all the outside effects are understood and taken care of.
If this anything like CERN detectors, they get amounts of data so vast that they have to discard almost all of it to be even able to record it. Depending on heurestics you use to discard data you might be discarding what you are looking for and after adjustment will get some new interesting events, but still actually processing the candidates might take a long time.
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.
They collected x3 more hay, and they still have to processes it and try to find any needle mixed with it.
Hopefully it the new data may have 3 additional events, or perhaps 2 or perhaps 4 or perhaps 10 or perhaps... Or the reported event may be false event caused by a lucky coincidence, and they may find 0 additional events.
This is 1σ. It’s fun and interesting, but means nothing.
> LZ physicists estimate there’s about a one in 200 chance the event is a statistical fluke
That's more than 3σ.
Such detectors will never see dark matter because it is little black holes.
Depending on the size of the black holes these detectors can see them.
i have zero knowledge of physics, but I just can't believe something like dark matter exists. My intuition is that some math just isn't correct and falls apart at the scales of the observable universe. No idea what equations are used for getting to the reslut of having 85% unexplained matter in the universe, but I really assume there's just a constant missing or our math in general just inaccurate or not considering specific effects.
I'm not a physicist either, but the amount of evidence pointing in the direction of dark matter is extremely significant, it's a little silly to just dismiss all that based on your intuition.
> No idea what equations are used for getting to the result
"Basic" gravitational equations applied to observed behaviour: clumping and rotation of galaxies, lensing (light bending), etc.
The behaviour implies "something" is exerting force in a mass like way - but there's a shortfall of visible mass.
The observational hole left by the apparent behaviour of "unseen matter" attracts a lot of theory.
* Physics "allows for" various types of particles with various types of properties - these may or may not all exist, some do. eg: Neutrino's from the sun barely interact with anything, consistently capturing them is a challenge. Maybe Dark Matter is a new hard to observe gravity particle.
* Physics has equations formed by "human scale" observation and sometimes tweaked for scales beyond direct human experience. eg: relativistic tweaks related to speeds approaching that of light. Maybe Dark Matter is a warping of observation at galaxy scale.
The opening paragraphs of, say, https://en.wikipedia.org/wiki/Dark_matter cover the ground of speculation.
For those who are interested in learning why physicists think dark matter exists, there's an excellent and accessible talk[1] freely available over at PIRSA where an astrophysicist goes through exactly what we know about the universe and how many different kinds of evidence all seem to point in a similar direction: dark matter exists.
“constant missing in our math” is roughly how inventing new particles in physics works.
When your equations are missing a number to work, you announce a new particle.
> I just can't believe something like dark matter exists.
That's a lot like a blind person saying they can't believe light exists.
It takes only a small amount of knowledge of physics to understand why and how dark matter might be possible, even likely. So small, in fact, that I can describe it in this comment.
The tl;dr is that all the senses you experience - sight, touch, smell, taste, and hearing - depend on the electromagnetic (EM) interaction. Touch, smell, taste, and hearing all depend on how electrons interact, and sight depends on how photons interact with electrons. But EM is only one of four fundamental interactions that we know of. We humans are essentially blind to all the others, without using devices to detect their presence.
But using devices we've invented, we can detect all sorts of things that we can't detect directly with our senses. X-rays and gamma rays, for example. But those are still just high-energy photons, not a different kind of particle altogether. They just help illustrate how limited our senses are.
A better example is the neutrino. They're pretty close to being "dark matter," because they don't interact via electromagnetism. As a result, they can pass right through your body, because there's nothing much to stop them. You're just as invisible to a neutrino as a neutrino is to you. It's estimated that about 100 trillion neutrinos pass through your body each second.
But neutrinos aren't perfectly "dark" - although they don't interact with electromagnetism, they do interact via the weak nuclear interaction, which is mostly something that happens inside the nuclei of atoms. (They also interact via gravity, but they have very small mass, so that doesn't help us detect them.)
We can detect neutrinos by building huge tanks full of very pure substances like water or argon, and burying them deep underground, to shield them from other interference. We can then look for the tell-tale signs that occur when a neutrino just happens to have a direct hit on an atomic nucleus, something that doesn't happen very often because nuclei are very small. That's why we need large tanks - to increase the odds of a hit.
The IceCube neutrino detector in Antarctica (https://icecube.wisc.edu/science/icecube/) extends to 2.5 km underground, and Super-Kamiokande in Japan (https://www-sk.icrr.u-tokyo.ac.jp/en/sk/) is buried 1 km below a mountain. They're able to detect neutrinos with high confidence, because aside from the tell-tale sing we can often even relate the neutrinos they detect to astronomical sources such as supernovae and supermassive black holes.
Neutrinos show that it's possible to have matter that doesn't interact via electromagnetism, which is all but invisible to us. And not just invisible - it can pass right through us. In the case of neutrinos, we're just "lucky" that they participate in the weak nuclear interaction, so we can detect them if we try hard enough. But what if a particle didn't do that? Then you'd have real dark matter - particles that we can't detect at all, except via the energy they carry, which participates in the gravitational interaction. But it's very difficult to detect tiny particles using gravity - which is why the first place we think we've detected dark matter is at large scales, in the motion of galaxies, where the collective mass of dark matter is large enough to be detected.
With all this in mind, a question dark matter skeptics would need to answer is, why wouldn't we expect dark matter to exist? We've identified quite a large zoo of particles, and what distinguishes each of them is that they each participate differently in the different interactions that we know about. Here's a summary of the fundamental particles and their participation in the fundamental interactions - which are gravity, electromagnetism (EM), weak nuclear, and strong nuclear (the latter two are different types of interactions that happen to have very generic names):
Electron: gravity, EM, weak
Neutrino: gravity, weak
Photon: gravity, EM
Quark: gravity, EM, weak, strong
Gluon: gravity, strong
Dark matter: gravity, ?
There's no physical reason we wouldn't expect a particle like dark matter to exist. It doesn't even have to have no interaction other than gravity - it could just have a very small degree of interaction that we can't easily detect, which is what the OP experiment is counting on.
If you accept the existence of X-rays and neutrinos, then it's not very consistent to draw the line at dark matter, once you have some understanding of the physics involved.