Patiently waiting for a new particle: the Majorana neutrino

The road from Rome to the Adriatic in Italy would probably be much steeper and more winding if it were not for the A24 motorway. Part of this motorway goes through a tunnel under the Apennines. This engineering work makes life easier not only for drivers, but also for academics. Particle physicists have built the Gran Sasso laboratory in a branch of the tunnel. That\'s where they study neutrinos.

Although neutrinos (together with photons) are the most common particles in the Universe, we still do not know much about them. Neutrinos interact very poorly with matter. So poorly that they are often referred to as "ghost particles". To be able to observe neutrinos, scientists need a really quiet place.

"There is a lot of noise on the surface of the earth that would disrupt our measurements. Particles are coming from every direction, also from outer space. For example, about a million muons hit a square meter of the surface in one hour" - told PAP a participant in research in Gran Sasso, Prof. Marcin Wójcik from the Jagiellonian University. But much less happens in a laboratory inside a mountain, protected by a 1300 m thick layer of rock: only 1 muon per hour passes through 1 square meter. This makes it much easier to distinguish between what is happening in the detector and the signals from outside the experiment.

In the GERDA (GERmanium Detector Array) experiment - although it was featured in the prestigious journal "Nature" (http://nature.com/articles/doi:10.1038/nature21717) - not much really happens. Scientists wait for a signal that happens very, very rarely.

It is all about the decay in the germanium crystal, which consists mainly of Ge-76. According to theory, two protons in this isotope periodically change into a set of other particles: two neutrons, two electrons and two neutrinos. This phenomenon can occur for a single Ge-76 once in a sextillion years (one with 21 zeros) - and that is several billion times longer than the age of the Universe... But waiting for such a unique event actually does make sense. After all, there are quite a lot of germanium atoms in the detector - over 35 kilograms for now, and the detector can capture each such decay. Although the total operating time of the device is only two years, GERDA has already detected thousands of such events.

But that\'s not all. Scientists believe that they will find traces of an even more unique process. They hope to observe how two protons in such a Ge-76 nucleus decay into two neutrons and two electrons. And there will be no trace of two neutrinos, which should form. In simple terms, one can imagine that one of the neutrinos has turned into an antineutrino and annihilated with the other.

It would mean that one neutrino was a particle of matter, and the other of antimatter. They would belong to two worlds: matter and antimatter. In other words, neutrinos would be their own antiparticles.

Italian physicist Ettore Majorana proposed this theory early in the 20th century, and the neutrino, which would be its own antiparticle, was named Majorana neutrino in his honour.

"The discovery would be as significant as the discovery of the Higgs particle or the discovery of neutrino oscillation" - smiled Wójcik. He added that such an observation would shed light on why there is more matter than antimatter in the Universe. Furthermore, the discovery would allow to better estimate of the mass of neutrino. And this has a great significance for physicists.

For now it is known that this Holy Grail of particle physics - so called neutrino-less double beta decay of Ge-76 - is very rare: "If it occurred once every 50 quadrillion (one with 24 zeros - PAP) years for a single Ge-76, we would have seen it already" - says the scientist. He added that it was a record-breaking accuracy. "And we would like to observe at least a dozen such events. Then we would probably recognize it as a discovery" - the scientist sighed.

The physicist described the GERDA experiment in the Gran Sasso laboratory. Germanium crystals are placed in a large, 65-cubic-meter copper-lined barrel made of steel. During the experiment, this tank is filled with a very cold liquid - argon, which further damps the interaction of particles from the outside. Inside the enriched germanium crystals suspended in these conditions, or in their vicinity, isotopes occasionally decay into other elements. During this decay, characteristic electrical signals are emitted. By analysing these signals, scientists are able to deduce what has happened in the tank.

For now, the detector contains over 35 kg of Ge-76. Recent results indicate that it is still not enough, because the target decay may occur less often than we thought. But because with the current amount of material the observations go too slowly, physicists want to expand their experiment. The next step would be to join forces with the US team and develop a detector in which initially 200 kg, and ultimately a ton of germanium 76 would be under close observation. The LEGEND team established for this purpose, which also includes scientists from Jagiellonian University, has already begun to work. If in this isotope two neutrinos sometimes indeed "annihilate", this rare phenomenon would certainly be observed much faster with a thousand kilograms of germanium.

Such experiments, however, are quite challenging, also a financial one, as germanium 76 is a very expensive isotope. If, however, research will help determine why there is more matter than antimatter - it will be money well spent.

In addition to the scientists from the Institute of Physics of the Jagiellonian University, researchers from Italy, Switzerland, Russia, Germany and Belgium also participate in the GERDA experiment. The next meeting of all participants will take place at the Institute of Physics of the Jagiellonian University in June. New results of measurements will be published during the meeting. And who knows, perhaps this time the result will not be zero...

PAP - Science and Scholarship in Poland, Ludwika Tomala

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