What is the neutrino experiment

GERDA: Will the neutrino change the standard model?

GERDA investigates the question of whether neutrinos are their own antiparticles. The answer places the highest demands on the experiment, with which researchers retreated 1,400 meters underground. If the nature of the neutrinos contradicts the standard model of particle physics, a value for the mass of the particles can also emerge in the end.

Researchers in the GERDA detector

For us experimenters, the volatile neutrinos have never made it easy: Since they extremely rarely interact with matter and so camouflage themselves so well, they only got into the experimental clutches 26 years after their theoretical prediction. And for decades there were far fewer particles than expected, because the three different types of neutrinos mutate into one another and thus escaped detection. As a result, the particles cannot be massless, as physicists had assumed since the 1960s in the so-called Standard Model of particle physics.

But despite its low interaction, the neutrino is a very important particle. Because after the photons (light particles) it occurs most frequently in the universe; it also plays an important role in supernova explosions and in the evolution of the universe.

Numerous working groups around the world are currently trying to elicit another riddle from neutrinos: Are they their own antiparticles? This question has important implications for the theoretical description of the particles. If neutrinos were their own antiparticles, the neutrino would come into conflict a second time with the standard model of particle physics. No particle has ever achieved this.

The question is over seventy years old and was asked when the existence of the particles had not yet been confirmed experimentally. At that time, the Italian Ettore Majorana developed an alternative to the theory of the British Paul Dirac, in which he predicted the existence of antiparticles. According to Dirac, neutrinos and antineutrinos are two different things, according to Majorana one and the same thing. What is now true can only be answered with a reaction called "neutrinoless double beta decay".

The neutrino-free double beta decay

Double beta decay

The history of neutrinos began around eighty years ago with thesimpleBeta decay is just beginning. This is a radioactive process in atomic nuclei in which a neutron becomes a proton and an electron is emitted. When the physicist Wolfgang Pauli tried to understand this process in 1930, he only succeeded when he demanded the additional participation of a new particle; a light (or even massless) neutral particle that was later named "neutrino".

At thedouble Beta decay now converts two neutrons into two protons at the same time. In addition to the two electrons, two antineutrinos are usually also formed. But if the neutrino corresponded to its antiparticle, the two particles inside the nucleus could cancel each other out ([+1] + [-1] = 0). If such a neutrino-free double beta decay had been detected, it would be clear that neutrinos are their own antiparticles.

The reaction is revealed by the fact that the two electrons produced carry all of the energy that is released during the reaction because they do not have to share it with the neutrinos (\ (E = 2039 \) kilo-electron volts (keV) for germanium-76 ). In order to be able to prove this, one has to look out for corresponding pairs of electrons and precisely measure their energies.

For experiments on neutrino-free double beta decay, those atomic nuclei are best suited for which simple beta decay is not possible for energy reasons. This is the case, for example, with germanium-76, molybdenum-100, tellurium-130 and neodymium-150. However, the reaction occurs extremely rarely. The half-lives are \ (10 ​​^ {25} \) years (a 1 with 25 zeros) and more. For a germanium-76 crystal weighing two kilograms, this corresponds to a single decay per year.

In one experiment, however, thousands of muons from the cosmos also fly through the crystal and the neighboring mounts every second. Their signals have to be filtered out in a complex process. In addition, the naturally occurring radioactivity of the environment, especially radon, contributes to disturbances, so that the experiments take place in laboratories that are deep underground protected from cosmic radiation and that can only be operated in the purest environment.

Different approaches

The Gran Sasso underground laboratory

A competition has now broken out between different groups as to which method can best be used to reduce the interference signals and to produce a result with the greatest significance: NEMO has chosen molybdenum-100 foils and set up their detector in France, CUORE in Italy is trying Tellurium-130 at cryogenic temperatures and SNO + in Canada uses neodymium-150 in a large scintillator tank. MAJORANA in the USA and our GERDA group in Italy rely on the well-known and well-developed germanium technology.

MAJORANA and GERDA work together on the computer simulation of their experiments and regularly exchange experiences with each other, although they use different concepts to filter out the interference from cosmic rays. MAJORANA will shield the detectors with lead and copper in a mine in the USA. The concept of shielding by GERDA is based on the use of water and liquid argon, so that the detectors are surrounded by the purest possible material with a low mass number (= number of protons and neutrons in the core of the atoms).


Development of GERDA

The abbreviation GERDA stands for GERmanium Detector Array (germanium detector array). The experiment is located in the Italian Laboratori Nazionali del Gran Sasso (LNGS), 1400 meters below the Abruzzo. Around ninety people from 14 institutes in six European countries work in the GERDA experiment.

At GERDA we are now building the detectors for measuring the electron energy from a germanium-76 sample, so that the sample and detector are identical. This means that the energy of the two electrons emitted in germanium-76 can be measured elegantly and very precisely. Germanium detectors are usually packed individually in jugs, because the semiconductor germanium has to be cooled down considerably by liquid gas in order to achieve the desired excellent energy resolution. At GERDA, however, several “bare” germanium detectors will hang in a tank of liquid argon at a temperature of minus 196 degrees Celsius; this eliminates a lot of interfering material in the vicinity of the sensitive detectors. The argon tank is surrounded by ultra-pure water, which is used to detect Cherenkov light, which is emitted by the few muons of the remaining cosmic radiation. A special film on the wall makes the blue Cherenkov light more visible to the light detectors mounted in the water.

Germanium detectors from GERDA

In order to keep interference as low as possible, all materials - including the steel plates from which the water tank is constructed - must be examined and selected for possible radioactive contributions before installation. Special measures also had to be taken with the delivery of the 37 kilograms of 86 percent enriched germanium-76. The semiconductor was transported to Germany from the manufacturing site in Siberia in a steel safe weighing several tons on a low-loader in order not to expose the germanium to cosmic rays during a flight. This prevents new isotopes from being generated in the sample. With the amount of germanium present, half-lives of a few \ (10 ​​^ {26} \) years can be observed.

The inner argon tank has been filled since December 2009 and the outer water tank since April 2010. The last tests are currently underway. The first signals from three detectors were recorded in the argon tank in June 2010, and the first measurements with the enriched detectors began in summer 2010. The first part of the data acquisition is designed for a year of measurement; after that, another twenty kilograms of new detectors will be added in the second section. The first phase aims for an exposure of 15 kilogram years, the second 100 kilogram years.

If we can prove the neutrino-free double beta decay, we expect to be able to determine the effective mass of the neutrinos up to about one hundred milli-electron volts, the exact value still depending on which theoretical model is used. No experiment has yet provided this proof.

At the moment GERDA has a good lead over MAJORANA, which should also be used. That is why our efforts continue to focus on optimizing the new detectors and reducing the subsurface.