Reacción a "A new measurement of the upper limit of the neutrino mass"

The neutrino is an extremely light elementary particle with no electrical charge that interacts very weakly with ordinary matter, which makes it extremely difficult to detect. Despite this, it plays a crucial role in astrophysical and cosmological phenomena and its study offers a lot of information about physics beyond the Standard Model, the most complete experimentally confirmed theory of particle physics that we currently have. The theoretical prediction that neutrinos could oscillate between different types or ‘flavours’ (electronic, muonic or tauonic neutrino) — a phenomenon that could only be explained if they had mass — was confirmed experimentally by the Super-Kamiokande and SNO collaborations, which were recognised with the Nobel Prize in Physics in 2015 for Takaaki Kajita and Arthur B. McDonald.

Although neutrino oscillation experiments allow the differences between the squared masses of the different neutrino flavours to be measured, they do not allow a direct study of the absolute scale of their mass. In this sense, only the combination of various cosmological observations allows an upper limit to be set on the sum of the masses of all neutrinos within the framework of standard cosmology. Motivated by this limitation, since the 1990s, attempts have also been made to directly measure the neutrino mass through experiments on Earth that study the disintegration of atomic nuclei, especially tritium. This is the case of the pioneering experiments in Mainz, Troitsk and, more recently, KATRIN.

In this article, KATRIN presents its latest results in the measurement of the effective mass of the electron antineutrino (the antiparticle of the electron neutrino, which will have the same mass) through the integral reconstruction of the beta decay of tritium, one of the lowest energy decays.

In this decay, a neutron transforms into a proton, emitting an electron and an antineutrino whose combined energy is 18.6 keV (kiloelectronvolts). By measuring the electron's energy with great precision, KATRIN infers, thanks to the principle of conservation, the amount of energy carried by the antineutrino. The mass of the neutrinos would modify the energy distribution of these particles. Specifically, the maximum energy that electrons can reach (the so-called tail of the decay spectrum) would be slightly reduced with respect to what they would reach if neutrinos had no mass. This effect is extremely subtle and, in fact, has not been directly observed, which prevents obtaining an exact measurement of the mass of neutrinos and only allows upper limits to be established.

After 259 days of measurements of more than 36 million electrons, KATRIN combines the results of five data collection periods to establish an upper limit on neutrino mass of 0.45 eV (less than one millionth of the mass of the electron), reducing its own previously published limit by a factor of 2. The collaboration aims to improve this result even further by completing the planned 1,000 days of data collection. The article explains in detail the technical characteristics of the experiment, as well as the meticulous handling of statistical and systematic uncertainties in order to derive the limit.

In conclusion, although it does not introduce radically new ideas with respect to previous publications, KATRIN refines the precision of the method and, by increasing the measurement time, manages to improve its results and continues on the path to determining a fundamental parameter in particle physics such as the mass of neutrinos.