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A detector designed to search for dark matter has been able to observe particle physics and help physicists to establish important truths about our universe. No, dark matter has not been spotted, but the new result proves that these ultra-sensitive detectors are valuable to scientists for many reasons.
From the point of view of gravity, the universe behaves as it contained much more material than what astronomers actually identified. Physicists have therefore built experiments to search for candidates for this so-called dark matter. The search for the most popular candidate for dark matter has been futile until now.
But one of these experiments on dark matter, called XENON1T, has now observed a process that has avoided many attempts to detect, a process that will hopefully help scientists better understand the dark particle called neutrino.
"This proves that the XENON detector technology we use for dark matter is much more versatile," said Christian Wittweg, a PhD graduate student at the University of Münster, Germany. Gizmodo. "We get all these interesting analyzes … for free after building a sensitive enough experience to drive out dark matter."
Scientists are pretty 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 a mass, but we do not know how much. We know they have an antiparticle, a kind of evil twin that annihilates the two particles if they meet, but we do not know the nature of this antiparticle. There are a ton of neutrino mysteries to solve.
The new measurement, called "double neutrino electron capture", is an important springboard for providing these answers.
Dual neutrino electron capture is an extremely rare particle interaction that was first theorized in 1955 and "eludes detection for decades," according to the paper published in Nature.
In the process, two protons of 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 dam of X-rays and electrons resulting from other electrons orbiting the atom, which replaces the two absorbed by the nucleus.
And when I say rare, I mean rare. According to the document, the average time required for the reaction of half of the xenon atoms of a sample is 1.8 × 1022 years. It's about a trillion times the age of the universe.
XENON1T is a perfectly equipped experience to measure this rare event. First, it contains a significant number of xenon atoms – 3.2 tons of liquid xenon (although, let us note, the isotope of xenon used for this measure is only a small fraction total xenon atoms).
Secondly, the entire configuration is buried deep within an Italian mountain, which protects it from any particle that could generate a false signal.
Finally, scientists understand just about every noises that can generate a signal during the experiment, which reassures them that they have found something important when an abnormal signal appears.
After 214 days of observation (177 days of usable data), the researchers' analysis revealed about 126 double electron capture events by two neutrinos.
It's an amazing scientific achievement. "It's the longest half-life that has been measured directly," said Chiara Capelli, a PhD student at the University of Zurich, who works on XENON. Gizmodo.
Researchers do not qualify their results as "discovery" because their statistics have not reached the threshold of five standard deviations required by particle physicists to use this word. Instead, they call this an "observation" because the result had a significance of 4.4 sigma.
This means that there is only one chance over a few hundred thousand to see this result if the reaction does not exist – but it will take a little more observer to get to the 3.5 million odds required by physicists to announce a Discovery.
Next, scientists will look for a neutrino-free double electron capture, an even rarer event in which, after the double neutrino capture event, the two neutrinos collide and emit a gamma ray. This would demonstrate that neutrinos are their own antiparticles and allow scientists to quantify the mass of neutrinos.
It is also a complementary research to a reaction called double neutrino-free decay – a bit like the opposite of neutrin-free neutron capture, where two neutrons spontaneously and simultaneously transform into protons, emitting electrons and a pair of neutrinos which cancel each other out.
We do not know if these "neutrin-free" reactions would actually occur, but it is an important issue for particle physicists. If neutrinos are really their own antiparticle, that would help explain why neutrinos have such a low mass and perhaps why there is so much more matter than antimatter in the universe.
In the end, scientists need more time to observe. XENON will soon move to XENONnT with even more liquid xenon, which will allow scientists to observe these events more frequently and observe events without neutrality with an even longer half-life, explained Laura Baudis, Professor of Physics at the University of Zurich.
But above all, it is proved that these experiments are sensitive enough to be able to carry out other important measurements, beyond the search of the dark matter.
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