Dark Matter Detector Records One of the Rarest Events Known to Science | Hacker News Reader

Dark Matter Detector Records One of the Rarest Events Known to Science

sciencealert.com

130 pointselorant38 comments

Arstechnica has a much better summary:

https://arstechnica.com/science/2019/04/dark-matter-detector...

But the best source is the original paper, which is quite readable even for a non-expert:

https://arxiv.org/pdf/1904.11002.pdf

This is, I am sure, a dumb question, but I read this https://en.wikipedia.org/wiki/XENON and I just don't know enough to completely understand the concept, but my dumb question is how do they know the event they saw was a spontaneous decay and not caused by a WIMP or whatever Dark Matter is? If it is such a rare event, how do we know that it wasn't a dark matter particle that caused it but the reaction was different than anticipated ? Can anyone take on a ELI5 here ?

It's not quite as rare as the article makes it out to be. It's not like they by chance happened to detect a single event that only happens every few trillion years. They had a TON (actually three tons) of xenon. Decay events happened every few days, and they collected over 100 of them. Those events have a particular "signature" that allows them to be distinguished from other kinds of events. Even then, they needed to apply statistical methods to compute the odds that the data they were seeing was real and not from random chance, and the result was that the odds were not quite high enough to qualify as a discovery by the usual standards.

Details are in the original paper, which is pretty accessible:

https://arxiv.org/pdf/1904.11002.pdf

I'm always amazed at how much good science happens based on inferring things from what's known or guessed with high confidence.

Why is that amazing? How would you expect it to work?

The most powerful phrase in statistics is "suppose that it is..."

Wow, 2800 pounds of Xenon, at $120/gram, is about $150M. Hope they don’t have leaks. I thought these detectors used dry cleaning fluid.

SNO has some $300M worth of heavy water in its neutrino detector. They manage to get it on loan for one dollar[1].

I know DEAP uses argon[2], for example. IIRC the move to these more exotic fluids are due to the need to suppress the background noise.

[1]: Slide 10, https://indico.cern.ch/event/661324/attachments/1511564/2373...

[2]: http://deap3600.ca/

I worked on a different xenon-based experiment[1] for my PhD. We were looking for similar rare decays to learn about neutrinos. Our observation of double beta decay of xenon 136 used to be the record for rarest decay.

Xenon has some very nice properties as a particle detector for rare events:

1. For one thing, it's a noble element, and so it can be purified very well to reduce background decays of other elements.

2. It's also a very heavy atom, and so it's self-shielding from external radiation. The core of the detector is shielded very well from radiation coming from the outside, so any signal you see there is most likely from decays of xenon or from things like dark matter WIMPs that don't interact much with matter.

3. It is a natural scintillator[2]. It gives off light when an interaction or decay ionizes the xenon atoms. That lets you actually detect the event, and by collecting the scintillation light, and the electrons from the ionization, you can get a decent measurement of the energy of the event.

4. It's recyclable. The XENON1T experiment follows the XENON100 experiment. The 100 kg from XENON100 were reused in XENON1T, and the tonne from XENON1T will be reused in future experiments. So the cost gets amortized.

$120/g sounds on the expensive side. The price is always changing based on supply and demand. One manufacturer deciding to use xenon in some process, or finding a way to replace xenon with argon, can swing the price by an order of magnitude.

As for leaks, I can say on our experiment we took the possibility very seriously. The entire xenon gas system was made of ultra-high-vacuum plumbing, and we helium leak checked every connection. When the xenon was outside the experiment in bottles, we had sniffers around the bottles to make sure they weren't leaking. We also had emergency systems in place if we needed to recover the xenon, including a "balloon of last resort" that would've captured the xenon in the event of a catastrophic failure.

[1] https://www-project.slac.stanford.edu/exo/

[2] https://en.wikipedia.org/wiki/Scintillator

Also, you don't want to breathe it; xenon is an anesthetic gas and will put you to sleep.

Can you talk more about the "baloon of last resort" and the process to recapture the gas from it?

Has a "baloon of last resort" ever been needed in any similar experiments?

We never had to use it, thankfully.

Our experiment (like the XENON experiments) used liquid xenon. The boiling point is roughly -110 °C, so it requires cooling to stay liquid. If that cooling had failed, the xenon would have started to boil and turn to gas. Gaseous xenon takes up something like 300x more volume than liquid.

So we had a few things to deal with this. We had some pretty giant UPSes to provide backup power. Imagine a shipping container filled with lead acid batteries. That was enough to keep the cooling running for about a day. During that time, we would start recovering the xenon. That would involve running compressors to stuff it back into bottles before power ran out. We also had a limited ability to cryopump the xenon out. Cryopumping involves cooling a gas cylinder (usually with liquid nitrogen) so the gas condenses inside. But that was always limited by the amount of LN we had on hand, which wasn't much.

But suppose we couldn't get all the xenon back into the cylinders before power ran out, or if our compressors failed. First, our detector would have failed. It was made of thin copper to reduce radioactivity. The xenon would have mixed with the HFE (Novec) fluid we were using for coiling. After that, as the xenon continued to boil, it would have burst some burst disks built into the system. And those were connected to the balloon.

The "balloon" was some plastic material, about 10x10x20 meters that we had stashed in an alcove off to the side of our experiment. It would have hopefully contained the xenon.

Since we never had to use it, I'm not completely sure what the process would have been. We would have shipped it off to some industrial gas facility, and they have the equipment to distill it out. And then we'd probably have to spend more time purifying it ourselves.

I doubt such a thing has ever been used. Most experiments, even underground ones, weren't dealing with the constraints we were. The mine we were in wasn't dedicated to science, and a dedicated facility would have had better support. For example, we couldn't run generators to deal with a power failure because there were limits to how much diesel equipment could be running underground with only natural ventilation, and the ventilation fans didn't have backups. Likewise, a dedicated facility would have had a better supply of liquid nitrogen. We only had one portable dewar, rather than a large tank.

I think the volume is even more than that. “XENON1T uses 3.2 t of ultra-pure liquid xenon (LXe), of which 2t are within the sensitive volume of the time projection chamber (TPC)”

From the paper linked above. https://arxiv.org/pdf/1904.11002.pdf

Thanks to the XENON1T dark matter detector lodged under the Gran Sasso mountains of Italy, scientists have recorded one of the rarest events to ever be detected: a special type of radioactive decay in xenon-124.

It's an amazing feat, because the decay of this isotope is extremely, extremely slow. In fact, xenon-124 has a half-life of 1.8 x 10 to the power 22 years – roughly one trillion times longer than the age of the Universe.

Does that qualify as impossible odds?

Dredging up memories from high school chem: not actually impossible odds. Half-life means the time it takes for half of the atoms out of a given quantity to decay. There are 6.02E23 atoms per mole, and about 8 moles per kg for xenon. So with a large enough quality of xenon-124, some decay event should happen within a reasonable timeframe. Of course it’s still a tremendous technical feat to actually setup and detect such an event.

I think the article just did a poor job of portraying the rarity of the event. This article links to a Gizmodo article[1] which does a better job of explaining that they had 3.2 tonnes xenon-124 and many occurrences:

> After 214 days of observing (177 days of usable data), the researchers’ analysis revealed approximately 126 two-neutrino double electron capture events.

[1] https://gizmodo.com/dark-matter-detector-makes-incredible-ne...

Indeed, larger samples will observe higher rarity events/outcomes.

It's probability, not rule I think.

(I'll repost one of my comments in a similar thread, with a few minor changes.)

If you have only one xenon atom in a box, to have a 50% chance that to see that it decayed you should wait a "1.8 x 10^22 years—or about a trillion times the age of the Universe".

If you have two atoms in a box, you must wait approximately half of the time to have a 50% chance to see the decay. (It's not exactly one half, there are some technical details here, but one half is a good approximation.)

They have 2 tons of xenon. I'm not sure if 1 ton = 1000kg or 907 kg, but it doesn't change the result too much. I'm also not sure about the isotope of Xenon they are using, so I'll use 131 as the atom mass, but it doesn't change the result too much. With 2 tons you have 2 x 907 * 1000 / 131 x 6.02x10^23 = 8E28 xenon atoms, so you expect to have a few decays per hour (if I did the calculations correctly).

I'm not sure if they can detect the 100% of the decays and the noise level, so it probably takes a longer time to detect a good signal.

The decay in question is from xenon-124, so they're probably using a natural isotope mix which will also contain those quasi-stable ones.

Wikipedia says that the natural abundance of Xenon-124 is 0.095% = 1/1000. Adding that correction I get a few decays per day that is more similar to the data reported in other commit that is 126 events in 177 days.

Edit: I think I'm missing a ln_e(2) in the calculation, but it's only a 30% more or 30% less. It's not a huge change in the result.

article says “1,300 kilograms (2,866 pounds) of xenon isotope”

not sure if that was intentional..

The other article didn't have the exact number, so I used the number of Wikipedia that says something like 3.6 tons in total and 2tons inside the measurement part (or something like that).

>If you have two atoms in a box, you must wait approximately half of the time to have a 50% chance to see the decay. (It's not exactly one half, there are some technical details here, but one half is a good approximation.)

Really? As far as I know radioactive decay is mostly believed to be a Poisson process (or rather the time it takes for a single atom is believed to be exponentially distributed), in which case the time it will take is exactly one half, on average.

Are the two of you using the same answer to the question "Does a X% chance of 2 events (in given interval) contribute X% or 2X% to the chance of seeing one event?"? That is, do you mean [time for a 50% chance of seeing ≥1 decay] or [time to see, on average, 0.5 decays].

In my case I'm talking about the (average) time until the first decay.

Poisson distribution is the limit of having infinite particles around.

It is, although the only difference is that the rate of this particular process changes after the first decay. Which makes it more of a continuous time Markov process, but the general idea is similar.

If you put two atoms A and B in an box and wait 1/2 of the half life, then the probability that the atom A survives is 1/sqrt(2). The probability that the atom B survives is also 1/sqrt(2). So (assuming independence) the probability that both survive is exactly 1/2. So you are right!

(I'd prefer to say that it's a property of the exponentially distribution, not the Poisson process.)

For something this exceedingly, impossibly rare I’d expect an in-depth discussion of how this is NOT an instrument mistake or a mis-interpretation of the measurements. There appears to be none of that.

If you read the paper I’m sure you’ll find the information you’re looking for. These types of experiments spend decades understanding all potential background events and trying to limit them.

That’s also one of the harder things to summarize or bring to a lay audience.

It's a 4.4 sigma detection, so not a discovery by particle physics standards. https://www.nature.com/articles/s41586-019-1124-4

I've heard suggestions that the result is more of a curiosity than of fundamental importance, and that this is why they get to publish a uncertain result in nature.

4.4 sigma ... uncertain result. You partical physicists are a funny bunch!

Needs to be 5 sigma

The famous 750 GeV bump of 2016 reached a maximum significance of 4.4 before dropping down as more data was added.

Particle physics has a lot of events, so locally improbable things get likely when considering the totality of the data sets.

The Pierre Auger Observatory, which can be described as doing astronomy using particle physics, published in Science about an exciting anisotropy at very high energies at about 2.5 sigma. (It was a prescription, so it would have been published even as a negative result but likely not in Science.) Alas, it turned out to be a statistical fluke as well. We had to publish a note to the effect later.

The particle physicists in the collaboration were by and large against calling it a discovery. The astronomers (by and large) shared your sentiment about the particle physicists being curious folk. :)

https://arxiv.org/abs/0712.2843

Here's the arXiv link: https://arxiv.org/pdf/1904.11002.pdf

Look for the discussion of backgrounds.

Reply on news.ycombinator.com