Researchers managed to capture the "last elementary particle"

The tau neutrino left a trail in the lab; Although two types of neutrino particles were found years ago, scientists have not been able to discover the tau neutrino until now.

by Tamara Traubman

At the end of the week, physicists announced that they succeeded in discovering the tau neutrino particle - the only particle that has not yet been discovered among the elementary particles that make up the matter in the universe. The neutrino particles are created in places where there is a lot of energy like the sun, and float freely. The tau neutrino is the most mysterious particle among the three types of neutrons, which pass through any material - like ghosts - without leaving a mark and cannot be seen or felt. In fact, the tau neutrino is so elusive that some physicists doubted whether they would ever be able to observe it directly.

The research team - consisting of 54 physicists from the USA, Japan, South Korea and Greece - detected the tau neutrino with the help of the most powerful particle accelerator in the world, the "Tevatron", located at the Fermi National Laboratory in Chicago, and using scanning devices that scanned tons of material to discover the The tiny traces of the rare particle.

"Every experiment related to neutrinos is not an easy adventure," says Prof. Shmuel Elitzur, from the Far Institute of Physics at the Hebrew University, in response to the discovery. According to him, "the very discovery is an impressive operation".

Neutrino particles have almost no mass and no electric charge. He can pass through anything without interruption. Physicist Prof. Haim Harari, president of the Weizmann Institute of Science, says that "nothing can stop him, everything is as if transparent to him. Right now, billions of neutrino particles are passing through our bodies, without us noticing."

Another thing that made the discovery difficult is that, although the two brothers of the tao neutrino - the electron neutrino and the muon neutrino - are very common, it itself is only rarely created, because its creation requires an extremely large energy. Due to its rarity, the chance of it damaging the material and leaving traces on it is almost zero. However, based on mathematical calculations and results of previous experiments, physicists were convinced that the tau neutrino must exist. However, the particle managed to elude them for 25 years.

In the experiment they conducted, the researchers launched a beam of about 100 trillion neutrino particles, including a few tau neutrino particles. The beam hit a sandwich of iron plates and plates smeared with emulsion, the material also found in photographic films. When a tau neutrino hits an iron atom, the collision between them produces a tau particle. When the tao hits the emulsion board it leaves a mark on it. The scientists developed the panels in the lab, and a scanner and computer software located these sections. It took the team three years to analyze the results of the experiment. They discovered four tau neutrinos, which created small but clear traces in the emulsion. "This discovery provides the last stone in the list of the building blocks of the material," says Prof. Harari.

All the elementary particles that make up the building blocks of matter are included in what is known as the "standard model" - a theoretical model that describes their properties. According to the model, there are two types of building blocks: the quarks, which make up all the atomic nuclei of every substance in the universe - from humans to gold - and the leptons, which include the electron and the muon and tau particles (similar to it in everything except their mass) and the neutrino particles.

According to the standard model it would appear that neutrinos have no mass. But about a year ago, an experiment in Japan revealed evidence that there was probably a small mass, which the scientists could not determine what it was. "The thing that everyone is waiting for with bated breath is information about the mass of the tau neutrino," says Prof. Harari.

Knowing the mass of the tau neutrino may solve some mysteries, such as how much the neutrino particles contribute to the total mass in the universe. According to one hypothesis, the neutrino is part of the "dark matter" - an invisible substance, which, based on indirect observations, is thought to make up more than 80% of the matter in the universe.

Prof. Harari says that although the tau neutrino appears to be a good contender, most dark matter does not appear to be made of it. In an experiment that began five years ago at the CERN particle accelerator in Geneva, they tried to estimate the mass of the particle. Harari, who participated in its planning, said that the experiment was done in such a way that only a mass greater than one electron volt (a unit for measuring mass) could be detected. According to him, since so far no results have been received showing that it has mass, it is likely that if it is part of the dark matter, then it is only a small part of it.

Several other experiments in the world are trying to estimate the mass of the neutrino with more sensitive measuring devices. "I will not be surprised if in the near future we will hear about the results of new measurements of the mass," said Prof. Harari.

Its existence was proposed in the 30s

Physicist Wolfgang Pauli was the first to propose the existence of neutrino particles, in the XNUMXs. This, after physicists who preceded him noticed the process of radioactive decay, in which an electron was ejected from the atom, and the atom turned into another atom. However, further experiments showed that the electron emitted from the nucleus of the atom has a different energy each time and it is always smaller than expected according to the theoretical calculation. These measurements contradicted the law of conservation of energy.

Pauli came up with the idea that in the decay another particle is emitted, elusive and invisible, massless, on which almost no force acts, therefore it easily passes through any material. This particle - given the name neutrino - simply "steals" some of the energy. Almost thirty years have passed since then until a physicist named Fred Raines discovered the first neutrino particle, in an experiment that earned him the Nobel Prize in '95.
{Appeared in Haaretz newspaper, 23/7/2000}
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