Why is there matter in the universe? Where did the antimatter go? We exist because the matter “defeated” antimatter, but we currently don’t know how this process occurred.
The study of neutrinos, the most mysterious particles within the Standard Model, might help us understand what happened right after the Big Bang and how the matter came to “survive” the annihilation process with the antimatter.
The Big Bang theory, which gave birth to our universe, holds that in the beginning, when the universe was born, there were both matter and antimatter: every type of particle has a corresponding antiparticle.
Then it would be expected that they annihilate and ultimately give rise to a Universe made entirely of energy – photons, for example. However, today, 13 billion years after the Big Bang, we have a universe full of matter – no trace of antimatter. Where did the antimatter go? What happened to the antimatter?
For decades, scientists attempt to answer these questions – basically since antimatter was discovered. The existence of antimatter was predicted by physicist Paul Dirac, in 1928, through the famous equation that bears his name (at first, it wasn’t clear that this equation predicted the existence of antimatter; Dirac’s purpose was to describe the behavior of electrons).
Positrons, antiparticles of electrons, were discovered in 1932 by Carl Anderson. Since the Big Bang theory was introduced in physics, the disappearance of antimatter has become an absolute mystery – one of the most fascinating mysteries of modern physics.
Currently, it is believed that the solution may be related to the fact that in the world of antimatter there are laws that are different in comparison with laws in the world of matter (eg: antiparticles behave slightly differently); therefore, an asymmetry between matter and antimatter, which was measured inclusively, at particle accelerators, for example in the study of neutral kaons and antikaons.
But the measured asymmetry does not justify the complete disappearance of antimatter. This is why scientists are looking for other processes that might help us better understand what happened.
The study of neutrinos could give us a hand. Neutrinos are particles with no electric charge; they have a tiny mass, as well as other interesting quantum properties. But what is still unknown is whether neutrinos are the same particles as antineutrinos or they are two different classes of particles.
The answer to this question is extremely important. If the neutrino and antineutrino are in fact the same particle (as the Italian physicist Ettore Majorana held in 1937), a process called “violation of lepton number conservation” would take place in the Universe.
If the lepton number wouldn’t be conserved, the antimatter can be converted into matter, which means that we could have a natural explanation of the disappearance of antimatter from the Universe.
Leptons are spin-1⁄2 particles; a group of particles that do not “feel” the strong nuclear force. There are six types of leptons, classified into three generations. The first generation comprises the electron and electron neutrino; the second generation comprises muon and muon neutrino; the third generation comprises the tau and the tau neutrino.
In particle physics, the lepton number is a conserved quantum number, representing the number of leptons minus the number of antileptons in an elementary particle reaction.
Currently, all the processes that have been studied showed the conservation of this lepton number.
To measure an eventual anomaly, scientists are studying the so-called double beta decay without neutron emission. Beta decay is a process during which an unstable nucleus becomes a (more) stable nucleus, emitting an electron and an antineutrino. In a double beta decay, two electrons and two electron antineutrinos are emitted. But if the neutrino and antineutrino are the same type of particle, the two antineutrinos can annihilate and just two electrons are emitted.
Such processes are currently studied underground laboratories; however, so far, no double beta decay process (where just two electrons are emitted) has been observed. Some experiments, such as CUORE, conducted in Gran Sasso underground laboratory in Italy, have been improved and, in the near future, scientists will continue to study these interesting processes.
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