Neutrino mass gets an upper limit

Neutrinos in the universe in terms of percentage of mass.

Neutrinos, the mysterious and elu sive chargeless particles with extremely tiny masses that pervade the entire universe, come in three flavours that result from a mix of three neutrino masses. While the differences between their masses were known, little information was available about the mass of the lightest species until now. The processes through which neutrinos obtain their mass could reveal secrets about astrophysics, including how the universe is held together, why it is expanding and what dark matter is made of.

“A hundred billion neutrinos fly through your thumb from the sun every second. What we do know is that as they move, they can change between their three flavours, and this can only happen if at least two of their masses are non-zero,” explained Arthur Loureiro of University College, London (UCL), the lead author of the study. “The three flavours can be compared to ice cream. Three flavours are always present but in different ratios, and the changing ratio, and the weird behaviour of the particle, can only be explained by neutrinos having a mass,” he added.

The concept that neutrinos have mass is a relatively new one, with the 2015 Nobel-prize winning discovery that was made in 1998. However, the Standard Model, which is so successful in describing the universe to a very large extent, is yet to reconcile with a massive neutrino.

The team used an innovative approach to calculate neutrino masses by using data collected by both cosmologists and particle physicists. This included data from 1.1 million galaxies from the Baryon Oscillation Spectroscopic Survey (BOSS) to measure the rate of expansion of the universe, and constraints from accelerator experiments. The researchers used the information to prepare a framework for mathematically modelling the mass of neutrinos and with UCL’s supercomputer, Grace, calculated the maximum possible mass of the lightest neutrino to be 0.086 eV, which is equivalent to 1.5 x 10-37 kg. They calculated that the three neutrino flavours together have an upper limit of 0.26 eV. “This new study demonstrates that we are on the path to actually measuring the neutrino masses with the next generation of large spectroscopic galaxy surveys, such as DESI, Euclid and others,” said Ofer Lahav of UCL and co-author of the study.