Neutrinos might explain matter in universe

Deep under the northern Italian mountains, in Gran Sasso, lies a facility where exciting research operations are taking place. A coalition of more than a hundred scientists mostly from Asia, Europe and Northern America are looking for the answer to a long-standing question: why is there more matter than anti-matter in the universe?

To obey the laws of conservation of mass, charge and energy, there should be as much matter as anti-matter in the universe. However, this would mean that matter and antimatter would cancel each other out, and the universe wouldn’t exist. Somehow, there is more matter in the universe, something happened to unbalance the symmetry of matter and antimatter. The Standard Model of physics might be wrong or incomplete and this would call for a new theory of physics or potentially an approach to a Grand Unification Theory.

The way scientists at Gran Sasso are pursuing to respond to this issue is by looking at neutrinos. These particles are the second most abundant ones in the universe, after photons. Because they are very difficult to detect, due to their tiny mass and neutral charge, very little is known about them. However, it is known that they can travel great distances, through planets and stars without interacting with matter at all, and that they can have three different masses due to the existence of their three different quantum flavours, which is for now not explained by the Standard Model. To add to their peculiarity, their flavour can change over time when they’re travelling.

Neutrinos’ mass can be measured by looking at a rare and yet unobserved event, neutrinoless double beta decay. When beta decay occurs, an atom becomes more stable by changing a neutron into a proton and releases an electron so its charge remains the same. This phenomenon also releases a neutrino for the conservation of mass, energy and lepton number. In the end, by losing a neutron and gaining a proton, the atom turns into its neighbouring element on the periodic table.

Now when double beta decay happens, an atom skips an element on the periodic table by turning two neutrons into two protons and two electrons. A first beta decay event happens, releasing a neutrino that is then captured by another neutron successively decaying to change into a proton and emit an electron, without the ejection of a neutrino. This process is still hypothetical and has never been spotted. Its observation would be a way to measure the exact neutrino’s mass and a proof that neutrinos are Majorana particles. Unlike Dirac particles like electrons and positrons (the positive electron’s counterpart) that annihilate each other, Majorana particles have their own kind for opposite, i.e. the opposite of a neutrino would not be an antineutrino, but another neutrino.

The neutrinoless double beta decay project at Gran Sasso named CUORE, which stands for Cryogenic Underground Observatory for Rare Events and is also Italian for heart, is to take place at 10mK, an ultra-low temperature close to absolute zero and to stay there for five years. Because neutrinoless double beta decay releases a very low amount of energy, keeping the experiment at a low temperature eliminates error measurements; the colder the environment, the higher the precision can be. The chamber in which the radioactive elements, Tellurium Dioxide crystals, are laid to undergo radioactive decay needs to be insulated from background radioactivity, consequently the chamber has been covered with ancient Roman Lead, which presents very low levels of radioactivity since it has been crafted hundreds of years ago. As for cosmic rays that keep bombarding Earth, having the experimental site buried under a mountain acts as a barrier keeping cosmic particles at bay.

Several other experiments are taking place around the world to uncover neutrinos properties. CUORE is the first one to use a setup which will remain close to absolute zero temperature for several years. Of all the particles existing in the universe, neutrinos are one of the most mysterious. It would be a surprise to find out that a quiet particle like the neutrino holds the answers to the existence of matter in the universe and subsequently to our own existence.

To find out more, visit the webpage of the Istituto Nazionale di Fisica Nucleare:


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