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Maximum mass of lightest neutrino revealed

The mass of the lightest neutrino, an abundant ‘ghost’ particle found throughout the universe, has been calculated to be at least six million times lighter than the mass of an electron in a new UCL-led study.

Understanding neutrinos and the process through which they acquire mass could reveal secrets about astrophysics, including how the universe is held together, why it is expanding, and what dark matter is made of.

First author, Dr. Arthur Loureiro (UCL Physics & Astronomy), said, “A hundred billion neutrinos fly through your thumb from the Sun every second, even at night. These are very weakly interactive ghosts that we know little about. What we do know is that as they move, they can change between their three flavors, and this can only happen if at least two of their masses are non-zero.”

“The three flavors can be compared to ice cream where you have one scoop containing strawberry, chocolate, and vanilla. Three flavors are always present but in different ratios, and the changing ratio-and the weird behavior of the particle-can only be explained by neutrinos having a mass.”

The concept of neutrino’s mass has emerged since the discovery in 1998 earning Professor Takaaki Kajita and Professor Arthur B. McDonald the 2015 Nobel Prize in Physics. Even though the standard model used by modern physics presently can’t seem to be refreshed to assign neutrinos a mass.

Now, the study from UCL, Universidade Federal do Rio de Janeiro, Institut d’Astrophysique de Paris and Universidade de Sao Paulo set the furthest limit for the mass of the lightest neutrino for the first time. The molecule could have no mass as a lower limit is yet to be determined.

Using an innovative approach, scientists were able to calculate the mass of neutrinos. For this, they collected 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 particle accelerator experiments.

Dr. Loureiro said, “We used information from a variety of sources including space- and ground-based telescopes observing the first light of the Universe (the cosmic microwave background radiation), exploding stars, the largest 3-D map of galaxies in the Universe, particle accelerators, nuclear reactors, and more.”

“As neutrinos are abundant but tiny and elusive, we needed every piece of knowledge available to calculate their mass and our method could be applied to other big questions puzzling cosmologists and particle physicists alike.”

Scientists then use the data to create a framework to mathematically model the mass of neutrinos and used UCL’s supercomputer, Grace, to calculate the maximum possible mass of the lightest neutrino.

They found that the mass of the lightest neutrino is 0.086 eV (95% CI), which is equivalent to 1.5 x 10-37 Kg. In addition, they calculated that three neutrino flavors together have an upper bound of 0.26 eV (95% CI).

Second author, Ph.D. student Andrei Cuceu (UCL Physics & Astronomy), said: “We used more than half a million computing hours to process the data; this is equivalent to almost 60 years on a single processor. This project pushed the limits for big data analysis in cosmology.”

Professor Ofer Lahav (UCL Physics & Astronomy), co-author of the study and chair of the UK Consortiums of the Dark Energy Survey and DESI said: “It is impressive that the clustering of galaxies on huge scales can tell us about the mass of the lightest neutrino, a result of fundamental importance to physics. This new study demonstrates that we are on the path to measure the neutrino masses with the next generation of large spectroscopic galaxy surveys, such as DESI, Euclid, and others.”

The study is published in Physical Review Letters on Thursday, 22 August 2019.

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