Seminar: First LUX-ZEPLIN dark matter search results

[mfb]

TL;DR Summary

LUX-ZEPLIN is one of the largest dark matter experiments, using liquid xenon. First results will be shown in a seminar.

Today 16:00 UTC, in ~7.5 hours.
Announcement, Link to Zoom meeting (why is this just an image on the website, not a link?)

LUX-ZEPLIN (LZ) uses several tonnes of xenon to search for signals of dark matter interacting with it. It started taking data last year and it should easily set better exclusion limits than previous experiments - or find a dark matter signal.

It should also tell us more about the low-energy excess seen by XENON1T. If they have achieved the background rates predicted here (figure 5) and the efficiency isn't too different from XENON1T, we should expect a background of a few events/(tonne*year) in the 2-3 keV bin and less than 1/(tonne*year) in the 3-4 keV bin. Even if they are off a bit a signal of ~30 should be easily visible.
If LZ doesn't see anything and rules out the XENON1T result then we might never figure out what caused it - the experiment was disassembled to make space for the larger XENONnT experiment, which also started taking data last year and might publish first results soon.

 

[mfb]

No dark matter seen, the exclusion limits are a significant improvement over previous experiments (slide 66). First exclusion limits going below 10^-47 cm².

~2 effective months worth of data from 3 months of running (they have large veto regions). Backgrounds are as low as required.
No excess at low energy seen (slide 62) but the detection efficiency at 2-3 keV is almost zero (slide 48). They expect to say more with more data.

XENONnT and LZ want to collaborate to build an even larger detector together.

A recording will be made available on YouTube and the paper should appear on arXiv tomorrow.

 

[Vanadium 50]

Here are their limits:

Paper draft is here: https://lz.lbl.gov/wp-content/uploads/sites/6/2022/07/LZ_SR1_Paper_7July2022.pdf

 

[strangerep]

No dark matter seen,

Why am I not surprised?

XENONnT and LZ want to collaborate to build an even larger detector together.

This is even less surprising.

(Too cynical?)

 

[mfb]

You never find something until the detector is sensitive enough or the accelerator is powerful enough to find it, and then you do.

See neutrinos, see gravitational waves, charm, bottom and top quark, Higgs boson, ...

 

[Vanadium 50]

(Too cynical?)

No, but maybe too uninformed.

(1) The LZ expected sensitivity is just beyond that of the previous experiments. This was likely to be deliberate - with less data the question is "why didn't you wait?" and with more it's "why did you wait". A consequence of this is that it is extremely unlikely to be in a situation where the previous generation saw absolutely nothing and the next generation sees an unambiguous signal. (This happens when one has a downward fluctuation and one an upward fluctuation)

(2) Sure, they may be looking in the wrong place. All but one class of experiment is looking in the wrong place. Problem is, we won't know which ones until someone has a discovery.

(3) It was very unlikely that both LZ and XENON would both be funded. There was intense pressure from the agencies to merge even then. It should have happened, and would have happened except for the sheer stubbornness of one of the participants. That moment has passed, and sanity is being restored. Of course these experiments will merged, just as LUZ and ZEPELIN merged to form LZ.

It's just geometry. You win as the volume, and lose as the surface area, so one big experiment is better than N little ones.

 

[ohwilleke]

It should also tell us more about the low-energy excess seen by XENON1T. . . . If LZ doesn't see anything and rules out the XENON1T result then we might never figure out what caused it - the experiment was disassembled to make space for the larger XENONnT experiment, which also started taking data last year and might publish first results soon.

Saying that the excess was "seen" by XENON1T is in my humble opinion an overstatement, because, in part due to the disassembly of the apparatus before the analysis was fully vetted, there are very plausible sources of non-dark matter backgrounds that were not ruled out by it, leaving any affirmative signal it claims to have seen hopelessly, and irremediably suspect.

Any exclusions from XENON1T are reliable, because a missing background can't enhance an exclusion derived from non-detection of any signal, but any affirmative low energy excess can't be said to be a credible finding. Put another way, the known sources of potential systemic error swamp the XENON1T signal.

 

[ohwilleke]

No dark matter seen, the exclusion limits are a significant improvement over previous experiments (slide 66). First exclusion limits going below 10^-47 cm².

Another press conference that could have been an email.

 

[Orodruin]

Another press conference that could have been an email.

That’s not how large collaborations work …

 

[Vanadium 50]

Another press conference

Nonsense.

No press conference. It was a scientific seminar. You are surely not suggesting that there be some sort of credential check before signing in, are you?

 

[ohwilleke]

Nonsense.

No press conference. It was a scientific seminar. You are surely not suggesting that there be some sort of credential check before signing in, are you?

Of course not. The line between an open participation scientific seminar and a press conference, however, can be pretty thin.

I am echoing the humorous phrase "a meeting that could have been an email" in the context of a press conference/seminar that, given the fact that the results reached were unexceptional, did not warrant the fanfare. A mere release of the draft paper and a written press release, without a live presentation or Q&A, just like hundreds of other new scientific paper released every day, would have been more than adequate.

Often, press conferences or scientific seminars are the forum in which new results are announced because they are particularly notable, newsworthy, or hard to explain, and when someone uses this method to announce a result, it sets up a certain expectation for something like that, while in this case, the ultimate results announced were utterly pedestrian and predictable: A null result, and methods that have already been widely vetted that worked as they were supposed to work. There is an ever so slight of a sense of "crying wolf" following the hype leading up to the announcement.

I'm not saying that the results aren't worth sharing or that they don't add to the accumulation of knowledge or that they were inappropriate to discuss in a scientific seminar. Instead, I'm just making the ultimately minor point that they "could have been an email" because it doesn't appear that there was much that called for discussion beyond what was in the paper itself, and this was also not anything ground breaking, so embargoing it to release it with a heavily hyped scientific seminar was unnecessary and a bit of a tease.

It's like applying to CalTech as a long shot, and getting a response to your application in a thick envelope, only to find a single page rejection letter and a lot of packing peanuts when you open it.

 

[Vanadium 50]

A mere release of the draft paper and a written press release, without a live presentation or Q&A, just like hundreds of other new scientific paper released every day, would have been more than adequate.

When you get in a hole, stop digging.

It is normal and proper for there to be a seminar coincident with the release of a new result.

 

[fresh_42]

Everybody who wants to seriously discuss MOND is invited to create an appropriate thread, i.e. on the basis of a scientific source, in
https://www.physicsforums.com/forums/beyond-the-standard-model.66/
or
https://www.physicsforums.com/forums/cosmology.69/

This thread is not about MOND.

And nowhere is a place to exchange cynism and sarcasm compared to the other position.

 

[Maarten Havinga]

Then I'll just post about this venture: forget it! The standard model tells the reality on elementary particles.

And nowhere is a place to exchange cynism and sarcasm compared to the other position

It was a metaphor, and a realistic one too. I was merely giving a reason for other people's cynism.

 

[mfb]

Most oft the seminar was introducing the detector, how it takes data, how it deals with backgrounds and so on. That's very interesting and a seminar is a nicer presentation than a performance paper (which will take more time to write anyway).

Saying that the excess was "seen" by XENON1T is in my humble opinion an overstatement, because, in part due to the disassembly of the apparatus before the analysis was fully vetted, there are very plausible sources of non-dark matter backgrounds that were not ruled out by it, leaving any affirmative signal it claims to have seen hopelessly, and irremediably suspect.

Data points were above the expected level, that's called an excess, it's obviously present. Its source is unclear - but that's a different question.

 

[mfb]

It should also tell us more about the low-energy excess seen by XENON1T.

XENONnT did that now: Progress of the XENONnT experiment

Slide 26 compares the XENON1T excess with the new XENONnT result, and it's clear it was not a real signal. Probably some tritium contamination, maybe just a statistical fluctuation, but certainly not new physics.