Particle physics tries to give answers to fundamental questions such as “what are we made of?” or “what is the origin of matter in the Universe?”. As far as we can tell the answer to the first question is that all known visible matter is made from only 4 fundamental particles: two types of quarks dubbed “up” and “down” which combine into the protons and neutrons with which all atomic nuclei are formed; the electrons that orbit the nuclei inside the atoms and the mysterious neutrinos, which have only very weak interactions and extremely small masses and are hence very hard to produce or detect.
These 4 species are replicated 3 times in Nature in what we call the 3 families or generations of particles. The particles of each generation are identical to their counterparts in all the interactions they have, the only difference is that each of them is more massive than the others. In this way, the second generation contains a heavier version of the electron, that also has the same negative electric charge, but that is about 200 times heavier and we call the muon m. And in the third generation the tau t, which is about 3500 times heavier than the electron.
The neutrinos only have very weak interactions in which they are produced or detected together with electrons, muons or taus. For this reason, the 3 neutrinos are called electron neutrino, muon neutrino or tau neutrino depending on the other particle they interact with.
The neutrino sector already provided a big and very surprising discovery that was awarded with the Physics Nobel prize in 2015 for the discovery of the neutrino oscillation phenomenon, in which the three different types of neutrinos have been observed to convert into one another and the probability of this conversion to take place oscillates with the distance travelled by the neutrino. This phenomenon is an indirect evidence of neutrino masses. Indeed, in the Standard Model of particle physics neutrinos are massless and so far we have not managed to detect directly the neutrino masses, we only know that they must be at least a million times lighter than the electron, which is the next-to-lightest known particle.
Our best description of the fundamental particles that constitute the building blocks of everything we see is through quantum physics, which states that at this microscopic level particles also behave as waves. And, just like waves in a pond, particles are able to superpose or interfere with each other. Neutrino oscillations are a consequence of this quantum nature and neutrinos being massive.
We can envision the lightest neutrino n1, characterized by a wave (figure 1):
And a heavier neutrino n2, characterized by a different wave (Figure 2):
Because of their quantum nature these two neutrinos do not need to coincide exactly with the “electron neutrino” or the “muon neutrino” which are not characterized by their mass but by which particle they interact with. An electron neutrino can instead correspond to the superposition of the two waves that are n1 and n2, just like two waves can combine into a more complex pattern when they meet in a pond (figure 3):
The green wave of the electron neutrino thus corresponds to the sum of the yellow and blue waves of n1 and n2. When both of them are in a crest the green has a higher crest and when one is in a crest and one in a valley the green wave is in-between.
The muon neutrino would then correspond to the difference between the waves of n1 and n2 (Figure 4):
The blue wave now starts going down when the yellow goes up producing a very distinct red shape from the electron neutrino one.
If the yellow and blue wave -that is the n1 and n2– had the same mass, they would propagate at the same speed. Thus, if they originally combined to an electron neutrino, the shape would remain the same in the propagation and it always correspond to an electron neutrino:
If, on the other hand, n1 is lighter than n2, it will propagate faster, producing a mismatch between the two which, at some point, will be large enough so that the shape the two waves combines to correspond to a muon neutrino instead. When the propagation continues at some point n1 “overtakes” n2 and the shape corresponds again to an electron neutrino. Thus, a neutrino that was originally associated to the electron will keep oscillating between an electron and a muon neutrino as it propagates.
Therefore, the discovery of neutrino oscillations implies the existence of neutrino masses, which in turn is a source of new physics beyond the Standard Model, since a new mechanism is needed to explain the origin of masses.
But neutrino oscillations could still hold more secrets, for instance they could be related to the second of the big fundamental questions: “what is the origin of matter in the Universe?”. In the Standard Model, for each matter particle there is an antimatter particle or “antiparticle” which has opposite charge but the same mass. Particles and antiparticles are created together and also annihilate each other and therefore it is a mystery why the Universe is made of matter with no hint of antimatter. However, neutrino oscillations might favour matter over antimatter (or viceversa) and could then provide a hint to the solution of this fundamental mystery.
The ESSnuSB would produce an unprecedented high intensity beam of muon neutrinos whose oscillation to electron neutrinos would be studied around 540 km away with a huge water detector located underground in the Garpenberg mine after two oscillations have taken place. The experiment would then be repeated with antineutrinos. If a difference between the two oscillations is discovered, this would provide evidence that neutrino oscillations, unlike most other processes in the Standard Model, differentiates between particles and antiparticles. This discovery would hint that neutrinos might be responsible for the excess of matter over antimatter observed in the Universe, that prevents matter to be completely annihilated by antimatter and therefore provides the seeds to galaxies, stars, planets and life.