Observation of Excess Events in the XENON1T Dark Matter Experiment
science.purdue.edu/xenon1t/?p=1474
science.purdue.eduPosted Jun 17, 2020, 2:20 PM UTC
38351
science.purdue.eduSo it looks like the observed signal is inconsistent with random noise at >3 sigma and could be explained by 1. solar axions, 2. enhanced neutrino magnetic moment, 3. excess tritium beta decays (+ axions?), or 4. something else.
54
That's is pretty exciting.
18
[removed]
-20
Resonaances "Hail Xenon Excess" has my favorite take on this so far.
"How confident should we be that it's new physics? Experience has shown again and again that anomalies in new physics searches have, with a very large confidence, a mundane origin that does not involve exotic particles or interactions. "
40
The bump is at 1.5 keV, which is twice 750 eV.
I have no sense of the significance of the third bullet point. What's special about 750 eV?
9
I think that bullet point is a joke about the 750 GeV excess and how the number is cursed now?
13
Is that really it? I assumed it was some point about thresholds for pair production or some kind of well known resonance/harmonic that everyone except me has at their fingertips.
5
At the very least, I've never heard of 750 eV being an otherwise physically significant value. And the way it's phrased without further elaboration does sound like a joke.
5
It's clear to me from the emphasis on "twice"... but maybe I've been doing too many crosswords recently.
3
That blog usually has a very satirical tone so I think the joke was definitely the intention (I chuckled to myself).
3
From the comments to the post:
750 will be a traumatic number forever ;)
1
Can’t find the actual quite, but there’s a cynical saying that a 3 sigma significance means the theory has a 50/50 shot of being true.
3
In Particle Physics I think its way way way way lower than 50%. *When it is in disagreement with the standard model
4
Since physics breakthroughs have slowed since the early 20th century, we focus on anything that may bring interesting results.
It's like the New York Islander's dynasty of the 80s. The fans still follow them relentlessly, but it's often more focused on what could be because the current output is not that great.
1†
diogenesofthemidwest is a racist that spams reddit with hate propaganda full time, and if the admins won't do something about him then the mods need to.
################################################################
1
The relevant paper. This is pretty cool!
34
An excess over known backgrounds is observed below 7 keV, rising towards lower energies and prominent between 2–3 keV. The solar axion model has a 3.5 σ significance...
Nice.
The excess can also be explained by β decays of tritium, which was initially not considered, at 3.2 σ significance with a corresponding tritium concentration in xenon of (6.2 ± 2.0) × 10−25 mol/mol.
Shit.
64
The tritium hypothesis is not preferred because rough calculations showed a cosmogenic tritium concentration of orders of magnitude lower than what is needed after getter purification. Of course that's just a rough calculation that cannot rule out tritium! I think it's just enough that it's not foolhardy to remain hopeful :)
30
Still: Some tritium contamination from whatever source vs. a new particle...
30
Of course I don't disagree, I'm just still young and overly hopeful probably 🤔
15
Hey! Get out of here you Bayesian heretic!!!
/s
9
It’s cosmogenic, so from cosmic rays scattering in detector materials as many as 10s of years before deployment.
1
[deleted]
1
3.5 Sigma is pretty exciting --- but this is still quite far from discovery. I believe the strength of the diphoton excess was around 3.8 sigma and that ended up being statistical noise. Also, I do find it interesting that their favoured axion parameter space doesn't match up with stellar cooling preferred region.
Hats off to the XENON team though, their analysis is top notch!
23
3.5 Sigma is pretty exciting --- but this is still quite far from discovery.
Yes, for sure. I certainly am not trying to be sensationalist here. Still, it's a cool result, even if not the most compelling, and we'll see how it pans out!
9
Oh yeah! I don't think you're sensationalist at all, but I've seen threads go off the rails a bit once they start getting upvoted.
With this and flavour anomalies both creating somewhat strong tension with SM results, I think particle physics has a lot to look forward to over the next few years!
7
Anomalies are always cool but this one seems even less convincing than the 750 GeV thing, and we know how that turned out. They saw a slowly rising set of counts near the lowest energies the experiment is sensitive to? That just about screams 'systematic effect'.
In addition, the article is deeply confused about p-values.
In statistical terms, the solar axion hypothesis has a significance of 3.5 sigma, meaning that there is about a 2/10,000 chance that the observed excess is due to a random fluctuation rather than a signal.
That's the textbook way to not do it.
11
There was a question about that being a systematic too. This is unlikely to be something that simple because no such trend is shown in data from rn220, in fig. 6 from the paper.
What is your problem with the interpretation of sigmas in the presser? Those are frequentist sigmas, so it seems to make sense to me that 3.5 sigma means the data is inconsistent with background, with a 2/10,000 chance of being explained by a statistical fluctuation of the background. There was also a question made during the webinar about the look-elsewhere effect but I think what was said is that that's expected to be small because this is not a parameter space sweep.
I guess if you want to be very particular about wording, one could say that it is really that the modelled background has a 2/10,000 chance of fluctuating that high.
Note that the wording specifically says 'a signal', not 'the signal'; it does not preclude signal being some other instrumental artifact not included in the background model.
7
It's the classic mistake. "A 0.02% chance of being background" is not even close to the same thing as "a 0.02% chance of getting something at least this extreme if it were background". It's not even like one is a slightly more technically correct version of the other -- the second is right by definition, and the first is just completely wrong.
11
I had to re-read the wording a few times to spot it, but you're right. They mixed up P(X|Y) with P(Y|X)
7
Yeah, that sounds right; it does sound like the presser is not worded correctly. I don't think your criticism about the actual analysis holds water though, as the rn220 calibration is in the same energy range but there is no such a shape; reading into the arrangement of a couple error bars sounds like waay over-reading of tea leaves to me especially at only 3.5 sigmas, especially when interleaved calibration runs do not show the same odd arrangement.
2
That just about screams 'systematic effect'.
It certainly does. Also, the way just one bin in the histogram seems reeeeally big makes me think there's a random error there too.
3
If you watch the talk I think it clarifies this a bit more. First of all, the models they are looking for (solar neutrinos and axions) only have signals at this particular energy, so there is no look elsewhere effect and they were targeting this region from the start. Also, the data is actually peaking above the flat background where the efficiency is flat, and it is hard to image how the efficiency during the turn-on could ever be higher than the plateau. Additionally in their calibration data they are able to model the turn on quite well.
So if its not BSM (likely) I think its way more likely to be the tritium than an efficiency turn on systematic.
1
ELI5?
7
Basically, extensive amounts of work goes into predicting and modeling the signals in the detector. Despite their best efforts to eliminate possible sources of error, the scientists on the collaboration have concluded they have seen interactions in quantities that cannot be adequately explained. These results are of course treated with healthy skepticism by everyone, including the collaboration presenting the results.
To give an analogy:
Imagine you have deep and extensive knowledge about how fast a gun can fire. You know from times you've gone to the gun range that you can send 300 rounds per minute down range. The next time you go to the range, you notice 320 rounds per minute are ending up downrange. Has your gun started firing faster on it's own? Or is there someone next to you also firing a gun at 20 rounds per minute?
You could describe this question as a mysterious "rate excess", which is what the XENON folks observed--an interaction rate in the detector that was higher than what they expected.
7
I second this.
2
Personally, I can't wait to see what theorists would make of this! Also, given LZ and XENONnT, we _should _ (fingers crossed) determine definitively in ~1.5 years if this is tritium or not.
4
I worked on this project in my undergrad! Very exciting to hear any news about it.
3
If anyone is interested, the seminar unveiling the result is recorded here. I found it quite informative.
2
How is this not getting more coverage?
1
I'm working on the NEXT project searching for neutrinoless double beta decay, 0nbb, in 136Xe. For a second I thought this was a 0nbb announcement. Phew! Maybe my dissertation is safe for another couple years.
It’ll be the time of 0nbb astronomy if it’s found! Your PhD will be intact ;)
how will that astronomy be like, can you tell more?
It’s not really that’s what they call it for dark matter and i just spun the phrase stupidly in an attempt to keep a positive attitude. It’ll just be a rare event search for a terrestrial based phenomena (as opposed to potentially WIMPs from outer space)
[deleted]