Articles & Interviews

Neutrinos are truly elusive particles: electrically neutral, very light and weakly interacting. That is why we know less about them than about other particles of the Standard Model of particle physics. The quantum-mechanical phenomena of “neutrino oscillations” have allowed us to understand much better the properties of these particles, but still there are some discrepancies between the experimental results and the commom framework in which the neutrino oscillations are studied. Could these disagreements be understood if we introduce a new, “sterile”, neutrino in our formalism? This is the question studied in this paper of the month.

Neutrinos are the most elusive particles of the Standard Model (SM) of particle physics. They can be produced in a number of reactions, such as natural radioactivity in the earth (geo-neutrinos), nuclear fission in reactors (reactor neutrinos), supernova explosions (supernova neutrinos), and fusion processes in the Sun (solar neutrinos).

A galaxy rotation curve is a plot of the orbital velocities of stars or gas in the observed galaxy versus the radial distance from the galaxy's center. What turns out is that galaxies in our universe seem to achieve too high velocities, such that the gravity generated by the observable matter (stars, gas) could not hold them together. This has led scientists to believe that there is some extra matter, not visible, that generates the extra gravity that galaxies need to stay intact. This extra matter is what we call dark matter, one of the most fascinating problems in physics nowadays. Unlike normal matter, dark matter does not interact with the electromagnetic force; therefore it does not absorb, emit or reflect light and because of this it is very hard to detect. It is possible that it interacts through gravity and weak force (WIMPs-weakly interacting massive particles), but it is also possible that it interacts only through gravity, which would make it even harder to spot.

The Cosmic Neutrino Background (CNB) is a solid prediction of the Standard Models of Particle Physics and Cosmology. In the early universe, neutrinos were formed as part of the thermal bath, a hot plasma filling the universe (thermal production). As the universe expanded and cooled down to MeV temperatures, the neutrinos decoupled from the thermal bath, traveling freely through space ever since. Neutrinos are very weakly interacting particles, which makes the detection of this neutrino background very challenging and so far experimentally inaccessible. However, on the other hand, this same property implies that once we succeed in measuring the cosmic neutrino background, we can not only learn something about the properties of neutrinos but also the CNB is a window to the very early universe, back to the times of the formation of light elements in Big Bang Nucleosynthesis (BBN), when the neutrinos decoupled from the thermal bath at about two minutes after the “Big Bang”. For comparison, the cousin of the CNB, the better-known Cosmic Microwave Background (CMB), consisting of background photons instead of neutrinos, had lead to major breakthroughs in modern cosmology. This window however only leads back to temperature around 1 eV, nearly 400,000 years later than the CNB window. This paper discusses how physics beyond the standard model, in particular non-thermally generated right-handed neutrinos, can modify the CNB predictions.