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Biggest dark matter detector spots a single weird particle

science.org

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Re: Biggest dark matter detector spots a single weird particle

#31

Post by advisedwang »

> 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?

Re: Biggest dark matter detector spots a single weird particle

#32

Post by wuliwong »

martinpw wrote: post #24
SaberTail wrote: post #9

Earlier quoted context omitted.

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. Th…

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≦)

Re: Biggest dark matter detector spots a single weird particle

#33

Post by physicsdude »

imglorp wrote: post #29
SaberTail wrote: post #15

Earlier quoted context omitted.

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…

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.

Re: Biggest dark matter detector spots a single weird particle

#34

Post by SaberTail »

imglorp wrote: post #29
SaberTail wrote: post #15

Earlier quoted context omitted.

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…

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?

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.

Re: Biggest dark matter detector spots a single weird particle

#35

Post by physicsdude »

scotty79 wrote: post #19

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.

Re: Biggest dark matter detector spots a single weird particle

#36

Post by procflora »

scotty79 wrote: post #19

Why couldn't it be just a weirdly energetic neutrino originating from the neighborhood of some black hole?

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!

Re: Biggest dark matter detector spots a single weird particle

#37

Post by WarmWash »

smueller1234 wrote: post #10
SaberTail wrote: post #5

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-…

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 h…

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..."

Re: Biggest dark matter detector spots a single weird particle

#38

Post by physicsdude »

IsTom wrote: post #14
parineum wrote: post #11

> 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?

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.

Re: Biggest dark matter detector spots a single weird particle

#39

Post by lofaszvanitt »

SaberTail wrote: post #17
parineum wrote: post #11

> 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.…

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.

Re: Biggest dark matter detector spots a single weird particle

#40

Post by hyperhello »

advisedwang wrote: post #31

> 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 un…

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.
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