Dark matter detector makes incredible neutrino observation


A detector designed to search for dark matter made an observation of particle physics that we hope will help physicists establish important truths about our universe. No, it did not locate dark matter, but the new result proves that these ultra-sensitive detectors are valuable to scientists for several reasons.

Gravitationally, the universe behaves as if it contained much more matter than astronomers actually identified, so physicists have built experiments to hunt for candidates for this so-called dark matter. The search for the most popular candidate for dark matter has so far been empty.

But one of these dark matter experiments, called XENON1T, has now observed a process that has avoided several detection attempts, which we hope will help scientists better understand the somber particle called the neutrino.

"This proves that this XENON detector technology we use for dark matter is much more versatile," said Christian Wittweg, a doctoral student at the University of Münster, Germany. Gizmodo. "We got all these legal analyzes … for free after having built an experiment sensitive enough to hunt for dark matter."

Scientists are sure that the second most abundant particle in the universe (after photons, light particles) is the neutrino. But neutrinos are very difficult to detect and measure.

We know they have mass, but they do not know how much. We know they have an antiparticle, a kind of evil twin that causes both particles to annihilate if they meet but do not know the nature of this antiparticle. There are a ton of neutrino mysteries to solve.

The new measurement, called "double-electron capture of two neutrinos," is an important step in providing these responses.

The capture of two electrons with two neutrinos is an extremely rare particle interaction that was theorized for the first time in 1955 and "escaped detection for decades," according to the article published in Nature.

In the process, two protons in the atomic nucleus spontaneously and simultaneously absorb a pair of electrons that orbit the nucleus, releasing a pair of neutrinos. The experimental signature of the event is a dam of X-rays and electrons resulting from other electrons that orbit the atom, replacing the two absorbed by the nucleus.

And when I say rare, I mean rare. The average amount of time it would take half of the xenon atoms in a sample to undergo this reaction is 1.8 × 1022 years, according to the article. That's about a trillion times the age of the universe.

XENON1T is a perfectly equipped experiment to measure this rare event. First, it contains a charge of xenon atoms – 3.2 tons of liquid xenon (although, as a note, the xenon isotope used for this measurement makes up only a small fraction of the total xenon atoms).

Secondly, the whole configuration is buried inside an Italian mountain, protecting it from almost any particle that could cause a false signal.

And finally, scientists understand virtually every noise that can produce a signal in the experiment, increasing the confidence that they actually found something important when an anomalous signal appears.

After 214 days of observation (177 days of usable data), researchers' analysis revealed approximately 126 events of two electron capture with two neutrinos.

This is an incredible scientific achievement. "It's the longest half-life ever measured directly," said PhD student Chiara Capelli of the University of Zurich who works at XENON. Gizmodo.

Researchers are not calling their results "discovery" because their statistics have not reached the limit of five standard deviations that physicists require to use that word. Instead, they call it "observation," because the result carried a significance of 4.4 sigma.

That means there is only a chance in a few hundred thousand that they would see this result if the reaction did not exist – but it will take a little more notice to get to the one-in-a-million chances required by physicists to announce a breakthrough .

Then, scientists will hunt for a double electron capture without neutrinos, or neutrinos, an even rarer event in which, after the double electron capture event of neutrinos, the two neutrinos collide and emit a gamma ray. This would demonstrate that neutrinos are their own antiparticles and would allow scientists to put a number on the neutrino mass.

It is also a complementary research to a reaction called double beta decay without neutrinos – as the opposite of the capture of double electrons without neutrinos, in which two neutrons transform spontaneously and simultaneously in protons, emitting electrons and a pair of neutrinos that annihilate.

We do not know if these "no substitution effect" reactions would actually happen, but it is an important issue for particle physicists. If neutrinos are really their own antiparticle, this would help explain why neutrinos have such a low mass and perhaps why there is much more matter than antimatter in the universe.

Ultimately, scientists need more time to observe. XENON will soon upgrade to XENONnT with even more liquid xenon, which will allow scientists to observe these events more often and observe events without neutrinos that have an even longer half-life, explained Laura Baudis, a professor of physics at the University of Zurich.

But more importantly, it is proof that these experiments are sensitive enough that they can perform other important measurements beyond the simple search for dark matter.


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