A detector designed to hunt for dark matter has made a particle physics observation that will hopefully help physicists establish important truths about our universe. Now, it did not spot dark matter, but the new result is that these ultra-sensitive detectors are valuable to scientists for multiple reasons.
Gravity-wise, the Universe behaved as if it contains more matter than astronomers have actually identified, so physicists have built experiments to hunt for candidates for this so-called dark matter. The hunt for the most popular dark matter candidate has so far turned up empty. But one of these dark matter experiments, called XENON1T, has now observed a process that has avoided multiple detection attempts, one that will hopefully help scientists better understand the shadowy particle called the neutrino.
use for dark matter is much more versatile, ”graduate student Christian Wittweg, Ph.D. student at the University of Münster in Germany, told Gizmodo. “We get all these cool analyzes … for free after having built an experiment sensitive enough to hunt for dark matter.”
Scientists are pretty sure that the second most abundant particle in the Universe (after photons, particles of light) is the neutrino. But neutrinos are very hard to detect and measure. We know they have mass, but don't know how much. We know that they have an antiparticle, a sort of evil twin that causes both particles to annihilate if they measure, but don't know the nature of that antiparticle. There are a lot of neutrino mysteries to solve. The new measurement, called "two-neutrino double electron capture," is an important stepping stone for providing answers.
Two-neutrino double electron capture is an extremely rare particle interaction that was first theorized in 1955 and "has escaped detection" for decades, ”according to the paper published in Nature. In the process, two protons in the atomic nucleus spontaneously and simultaneously absorb a pair of electrons orbiting the nucleus, releasing a pair of neutrinos. The experimental signature of the event is a barrage of x-rays and electrons resulting from other electrons orbiting the atom replacing the two absorbed by the nucleus. And when I say weird, I mean . The average amount of time it would take half of the xenon atoms in a sample to undergo this reaction is 1.8 × 10 years, according to the paper. That's roughly a trillion times the age of the Universe.
XENON1T is an experiment perfectly equipped to measure this weird event. Firstly, it contains a crapload of xenon atoms – 3.2 tons worth of liquid xenon (though, as a note, the xenon isotope used for this measurement makes up only a small fraction of the total xenon atoms). Secondly, the whole setup is buried deep within an Italian mountain, shielding it from pretty much any particle that could cause a false signal. And finally, scientists understand pretty much every bit of noise that could produce a signal in the experiment, increasing their confidence that they have actually found something important when an anomalous signal appears.  After 214 days of observation (177 days of usable data), the researchers' analysis revealed approximately 126 two-neutrino double electron capture events.
This is an incredible scientific accomplishment. "The longest half-life ever measured directly," Ph.D. student Chiara Capelli from the University of Zurich who works on XENON customs Gizmodo
Researchers aren't calling their results a "discovery," because their statistics didn't hit the five-standard deviation threshold particle physicists require in order to use that word. Instead, they're calling it "observation," as the result carried on significance of 4.4 sigma. That means there is only one in a few hundred thousand chance that they would see this result had not existed – but it will take a bit more observation to get to the one-in-3.5 million odds required by physicists to announce a discovery
Next, scientists will hunt for a no-neutrino, or neutrino-double electron capture, and even event in which, after the double neutrino electron capture event, the two neutrinos collide and emit in gamma ray. This would demonstrate that neutrinos are their own antiparticles, and would allow scientists to add to the neutrinos mass. It is also a complimentary search for a reaction called neutrinoless double beta decay — sort of like the opposite of neutrinoless double electron capture, where two neutrons spontaneously and simultaneously turn into protons, emitting electrons and a pair of neutrinos that annihilate one another. We don't know if these neutrinos are really going to happen, but it's an important question to physicists. If neutrinos really are their own antiparticle, it would help explain why neutrinos are so low-mass and maybe why there is so much more matter than antimatter in the Universe.
Ultimately, scientists need more observing time. XENON will soon upgrade to XENONnT with even more liquid xenon, which will allow scientists to observe these events more frequently, and to observe neutrinoless events that have even longer half-lives, says Laura Baudis, professor of physics at the University of Zurich. 19659005] But most importantly, it is proof that these experiments are sensitive enough that they can perform important measurements beyond just hunting for dark matter.