Raúl Jiménez
ICREA research professor of Cosmology and Theoretical Physics at the Institute of Cosmos Sciences at the University of Barcelona, visiting professor in the Astrophysics group at Imperial College London (United Kingdom), and researcher at the Imperial Centre for Inference and Cosmology (ICIC)
The KATRIN underground experiment reports a new upper limit on the mass of the electron neutrino (in nature there are three families of neutrinos: electron, muon and tau; the Katrin experiment is only sensitive to measuring the mass of the former) of 0.45eV. For the three families of neutrinos the total limit is < 1.35eV. This is an interesting result as it brings the limit for the total mass of neutrinos measured in the laboratory closer to the results in cosmology. In cosmology we have been measuring the total mass of the three families of neutrinos for decades using their effect on the growth of the structures of the universe. This is a different and complementary method to that of KATRIN. Recently, the DESI experiment has reported limits on the total mass of the three neutrinos of < 0.064 eV, which is a factor of 21 smaller than the limit measured by KATRIN. Even so, the KATRIN experiment explores a completely different avenue to cosmology and that is why its result is so relevant, as it brings the limit measured in the laboratory closer to that of cosmology. If one is more conservative in cosmology and wants to relax their assumptions, then the upper limit is < 0.16 eV which is still an 8-fold smaller factor than that of KATRIN.
What this tells us is that, for the moment, there are no surprises: the KATRIN limits are compatible with the most restrictive limits of cosmology and, therefore, there is no room for exotic physics that would imply that neutrinos in the cosmos behave differently than on Earth. Unfortunately, KATRIN in its final measurement will only reach the limit of < 0.9 eV and will remain far from the cosmological limits. However, there are other experiments on Earth that may very soon bring us closer to a better understanding of the nature of neutrinos and their masses. These are the NEXT experiment, which is located in the Spanish-built Canfranc Underground Laboratory, and the HyperKamionkande experiment in Japan, where the Canfranc Underground Laboratory plays a crucial role.
Underground experiments in conjunction with cosmology will bring us closer to discovering the nature of neutrinos over the next five years and thus understanding nature itself, and why there is matter in the universe and not just radiation. It is in this context that the relevance of KATRIN's results must be framed.