A neutrino experiment reveals (again) that something is missing in our universe

A large electron counting machine has indirectly raised a measure of the most slippery particle known in physics – and added to the evidence of dark matter.

This measurement is the first result of an international effort to measure the mass of neutrinos – particles that fill our universe and determine its structure, but that we are barely able to detect. Neutrinos, according to the KATRIN experiment (Karlsruhe Tritium Neutrino, based in Germany), have no more than 0.0002% of the mass of an electron. This number is so low that even if we counted all the neutrinos in the universe, they could not explain the missing mass. And this fact adds to the pile of evidence of the existence of dark matter.

KATRIN is basically a very large machine to count the very high energy electrons that spring from a sample of tritium – a form of radioactive hydrogen. with a proton and two neutrons in each atom. Tritium is unstable and its neutrons disintegrate into electron-neutrino pairs. KATRIN looks for electrons and not neutrinos because they are too weak to be accurately measured. And the machine uses tritium gas, according to Hamish Robertson, KATRIN scientist and professor emeritus at the University of Washington, because it is the only source of electron-neutrinos simple enough to get a good measure of mass.

Neutrinos are more or less impossible to measure accurately by themselves because they have such a low mass and tend to do without detectors without interacting with them. So, to determine the mass of neutrinos, Robertson told Live Science, KATRIN counts the most energetic electrons and works from that number to derive the mass of the neutrino. The first results of KATRIN have been announced and researchers have reached a rapid conclusion: neutrinos have a mass not exceeding 1.1 electronvolt (eV).

Electronic volts are the mass and energy units used by physicists when they talk about the smallest things in the universe. (At the basic particle scale, energy and mass are measured using the same units, and the neutrino-electron pairs must have combined energy levels equivalent to their original neutron.) The Higgs boson, which confers its mass to other particles, has mass of 125 billion EV. The protons, the particles in the center of the atoms, have a mass of about 938 million eV. The electrons are just 510,000 eV. This experience confirms that neutrinos are incredibly tiny.

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KATRIN is a very big machine, but his methods are simple, Robertson said. The first chamber of the device is full of gaseous tritium, whose neutrons naturally decompose into electrons and neutrinos. Physicists already know how much energy is needed when a neutron decays. Part of the energy is converted to mass of the neutrino and mass of the electron. And the rest pours into these newly created particles, dictating their speed very roughly. Usually, this extra energy is distributed roughly evenly between the electron and the neutrino. But sometimes, most or all of the remaining energy is spilled into one particle or another.

In this case, all the remaining energy after the formation of the neutrino and the electron is poured into the electronic partner, forming a very high energy electron, said Robertson. This means that the mass of the neutrino can be calculated: it is the energy involved in the decay of the neutrons minus the mass of the electron and the maximum energy level of the electrons in the experiment.

The physicists who designed the experiment did not try to measure neutrinos; these are allowed to escape the machine without being touched. Instead, the experiment channels the electrons into a giant vacuum chamber, called a spectrometer. An electric current creates a very powerful magnetic field that only the highest energy electrons can cross. At the other end of the chamber is a device that counts the number of electrons crossing the field. As KATRIN slowly increases the intensity of the magnetic field, said Robertson, the number of electrons going through is decreasing, which almost amounts to fading. But at the very end of this spectrum of electronic energy levels, something is happening.

"The spectrum dies suddenly before reaching the end point [where the electron would have all the energy released in the tritium decay]because the mass of the neutrino can not be stolen by the electron. It must still be left for neutrinos, "said Robertson," the neutrino mass must be less than this tiny amount of missing energy at the end of the spectrum. "And after several weeks of operation, the experimenters have reduced that number to about half the number physicists knew before.

The idea that neutrinos have a mass is revolutionary. The Standard Model, the basic theory of physics that describes the subatomic world, had once insisted that neutrinos had no mass, Robertson said. By the 1980s, Russian and American researchers had tried to measure neutrino masses, but their results were problematic and imprecise. At one point, Russian researchers fixed the neutrino mass at exactly 30 eV – an interesting figure that would have revealed that neutrinos were the missing link that would have explained the great gravitational structure of the universe, filling the entire mass missing – except one that turned out to be wrong.

Robertson and his colleagues began working with tritium gas at the time, after realizing that the low-level radioactive substance was the most accurate source of neutron decay available to science.

"It has been a long search," Robertson said. "The [incorrect] The Russian measure of 30 eV was very exciting because it would have closed the universe by gravitation. And it's always exciting for that reason. Neutrinos play a big role in cosmology and have probably shaped the large-scale structure of the universe. "

All these faint particles fly around everything else with their gravity, and take and lend energy to all other matters. Although the number of masses decreases, adds Robertson, the precise role of these small particles is complicated.

The researcher stated that the number of 1,1 eV is interesting because it is the first neutrino mass experimentally derived number that is not high enough to explain by itself the structure of the rest of the world. # 39; universe.

"There is a matter we do not know yet, there is this dark matter" and there can not be neutrinos that we know about, he said.

Thus, this small number coming from a large vacuum chamber in Germany adds at the very least to the pile of evidence that the universe contains elements that physics still does not understand.

Originally published on Science live.