Year 2019. In an underground laboratory, a kilometer and a half under the Massif of Gran Sasso in Italy, a dark matter detector witnessed something extraordinary: the radioactive disintegration of an atom of Xenon-124. It is the slowest (and therefore rare) process ever registered.
They touched the cosmic lottery. The Xenon-124 has a semi-width of 1.8 × 10²² years. That is 18 followed by 21 zeros: 18,000 trillions of years. To put it in perspective, the universe has “barely” about 13.8 billion years, so that the process that Italian scientists could observe in 2019 is a billion times more durable than the age of the universe itself, as described by the researchers in the journal Nature.
A little context. The “semi -experience” is a statistical measure similar to half -life, but specifically defines the semi -dear period of a radioactive substance. Uranium-238, for example, has a semi-width of 4.5 billion years. In the case at hand, the semi-experience tells us how long it has to pass so that half of a very large group of Xenon-124 atoms disintegrate and become another element, the teluro-124.
For an individual atom, its disintegration is a purely random event. A concrete atom could disintegrate in the next second or be stable for a much greater time than its semi -experience. For a group of atoms, the semi -experience is a very reliable prediction of its collective behavior. If you had a container with a large number of Xenon-124 atoms, you would have to wait 18,000 trillions of years for half of the atoms to transform.
How did they do it? With a very large container, which contained 3.2 tons of ultra -overthopuro liquid xenon. We refer to the Xenon1T experiment of the National Laboratory of Gran Sasso, in downtown Italy. A dark matter detector designed for the direct search for the hypothetical massive particles of weak interaction (WIMP).
The detector was designed with extreme sensitivity and built under a mountain to isolate it from cosmic radiation. But what he captured was not dark matter, but the whisper of an atom of Xenón-124 decomposing; transforming into Teluro-124. The weirdest event ever witnessed.
It is not a hyperbole. It really was a milestone of experimental physics that we should not have seen even in a billion lives of the universe. But although the probability that an atom of Xenon-124 disintegrate in a year is practically nil, the detector contained almost 10,000 billion xenon atoms in the two tons of volume that were analyzed.
With such an overwhelming amount of “lottery tickets”, the probability that at least one disintegrate during the observation period increased dramatically. During the 177 days of data collection, the team observed not one, but a total of 126 events that could later confirm how the decay of the Xenon-124, a type of radioactive disintegration allowed by the standard model of particle physics, but practically undetectable.
What did they see. An atom of Xenón-124 disintegrates when its nucleus simultaneously captures two electrons of the innermost layers. This causes two protons to become neutrons, transforming the atom into Teluro-124. But the energy released is carried by two neutrinos, which escape without being detected.
What the Xenon1T photomultipliers detected up to 126 times was the X-ray waterfall and omer electrons that occur when the electrons of the upper layers of the Xenon-124 fall to fill the gaps that have left the two captured electrons. This is the energy firm, the “flash” that betrays the weird event of the universe.
Has it served for something? For more than it seems. Although there was no luck with dark matter, the detection showed that Xenon1T can capture an incredibly weak and rare signals, validating its design. But the measurement also provided experimental data to test and improve the theoretical models that describe the structure and stability of atomic nuclei.
This observation is a general trial for an even more ambitious goal: the search for double electron catches without neutrinos. If this hypothetical process was detected, it would demonstrate that neutrinos are their own antiparticles (what is known as Majorana particles). This would explain why the universe is made of matter and not of antimatter.
Imagen | LNGS
In WorldOfSoftware | When no result is a good result: Xenon’s story and the search for dark matter
