A detector designed for the hunt for dark matter has made a particle physics observation that will hopefully help physicists identify important truths about our universe. No, dark matter has not been discovered, but the new result proves that these extremely sensitive detectors are valuable to scientists for a variety of reasons.
In terms of gravity, the universe behaves as if it contains far more matter than the astronomers actually identified. That's why physicists have built experiments to look for candidates for this so-called dark matter. The search for the most popular candidate for dark matter is so far empty.
But one of these dark matter experiments, called XENON1
The detector technology we use for dark matter is much more versatile, "said graduate student Christian Wittweg, PhD student at the University of Münster in Germany, Gizmodo . "We get all these cool analyzes … for free, after building an experiment that's sensitive enough to look for dark matter."
Scientists are pretty sure that the second most abundant particle in the universe (after photons, light particles) is the neutrino. However, neutrinos are very hard to detect and measure.
We know that they have mass but do not know how much. We know that they have an antiparticle, a kind of evil twin that causes both particles to be destroyed when they meet, but they do not know the nature of this antiparticle. There are a lot of neutrino secrets to solve.
The new measurement, called "two-neutrino double-electron capture," is an important step in delivering these answers.
Two-neutrino double-electron capture is an extremely rare particle interaction, first theorized in 1955. has escaped discovery for decades, "states the article published in Nature .
Two protons in the nucleus spontaneously and simultaneously absorb a pair of electrons orbiting the nucleus, releasing a neutrino pair. The experimental signature of the event is a flood of X-rays and electrons coming from other electrons orbiting the atom replacing the two electrons absorbed by the nucleus.
And when I say seldom, I seldom mean . The average time it takes for half of the xenon atoms in a sample to undergo this reaction, according to the study, is 1.8 × 1022 years. That's about a trillion times the age of the universe.
XENON1T is an experiment that is perfectly suited to measure this rare event. First, it contains a lot of xenon atoms – 3.2 tons of liquid xenon (although the xenon isotope used for this measurement represents only a small fraction of the total xenon atoms).
Second, the entire structure is buried deep in an Italian mountain shielding it from any particles that could cause a false signal.
And finally, the scientists understand just about every sound that could generate a signal in the experiment, and increase the confidence that they have indeed found something important when an anomalous signal occurs.
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 achievement. "It is the longest half-life ever directly measured," said doctoral candidate Chiara Capelli of the University of Zurich, who works at XENON, opposite Gizmodo .
. Researchers do not call their findings "discovery" because their statistics did not meet the five-standard deviation threshold that particle physicists need to use this word. Instead, they call it an "observation" because the result had a significance of 4.4 sigma.
This means that there is only a one-hundred-thousand chance that this outcome would be seen without the reaction. However, it takes a bit more observation to reach the one in 3.5 million quota demanded by physicists to announce a discovery.
Next, scientists will chase after neutrino double-electron capture without neutrino, an even rarer event where the two neutrinos collide after double neutrino electron capture and emit a gamma ray. This would show that neutrinos are their own antiparticles and would allow scientists to determine the mass of neutrinos.
This is also a complementary search for a reaction called neutrinoless double beta decay – the opposite of neutron-less double-electron trapping, where two neutrons spontaneously and simultaneously transform into protons, emitting electrons, and a pair Neutrinos destroying each other.
We do not know if these "neutrinoless" reactions would really take place, but it is an important question for particle physicists. If neutrinos really are their own antiparticle, this would help explain why neutrinos have so little mass and why there is so much more matter than antimatter in the universe.
Ultimately, scientists need more observation time. XENON will soon be upgrading to XENONnT with even more liquid xenon, allowing scientists to observe these events more frequently and observe neutrino events with even longer half-lives, explains Laura Baudis, a professor of physics at the University of Zurich. 19659002] Above all, it is evidence that these experiments are so sensitive that they can perform other important measurements besides hunting for dark matter.