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?

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SaberTail 7 hours ago | parent [-]

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.

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gus_massa 7 hours ago | parent | next [-]

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...

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pfdietz 3 hours ago | parent | next [-]

Those double beta decays are also interesting because they can probe whether the neutrino is a Majorana particle.

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vintermann 7 hours ago | parent | prev | next [-]

That's a pretty cool discovery in its own right.

	▲	jakzurr 6 hours ago \| parent [-]
		Wow! That's in https://en.wikipedia.org/wiki/Xenon now. Xe-124, half-life 1.1 * 10^22 years. That's crazy.

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cogman10 4 hours ago | parent | prev [-]

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.

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cwmma 3 hours ago | parent | next [-]

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.

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cogman10 3 hours ago | parent [-]

> 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.

	▲	mr_mitm 2 hours ago \| parent [-]
		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.

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marcosdumay 3 hours ago | parent | prev [-]

There's just no way deuterium is radioactive, unless hydrogen is radioactive too.

	▲	gus_massa 3 hours ago \| parent [-]
		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.

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martinpw 7 hours ago | parent | prev [-]

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.

	▲	wuliwong 5 hours ago \| parent [-]
		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≦)