Astronomers Present A New Mass Estimate For The Lightest Neutrino

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At any given second, roughly 600 trillion neutrinos are moving through your body. We don’t feel them, and we can barely detect them because they have no electric charge and are incredibly light. However, we do not know exactly how light they are.

We do not have a precise value for the mass of the neutrino, but we have estimated its upper limit using a variety of sources. Some information comes from particle physics and some comes from what we know about the cosmos. Now researchers have combined much of this information using a new estimation technique and employed a supercomputer to churn out an answer.

As reported in Physical Review Letters, the team estimates the maximum possible mass of the lightest neutrino to be 1.5 x 10-37 kilograms. That’s over 10 million times smaller than the mass of the electron. Like comparing a pea to an elephant. A truly tiny value for the particle.  

“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,” lead author Dr Arthur Loureiro, from University College London (UCL), said in a statement.

“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.”

Electrons and neutrinos are leptons. Leptons in the universe come in three types or flavors. There is the electron, the muon, and the tau particle. Neutrinos too come in three flavors, electron neutrinos, muon neutrinos, and tau neutrinos, with the electron neutrino being the lightest. But neutrinos have a peculiar ability; they can change their flavor.

Researchers have described them as "flavor-queer", and they oscillate from one type to the next. This property is relevant to the question of the mass of neutrinos. The oscillation is only allowed by the laws of physics if the neutrinos have different masses, so their masses cannot all be zero, for example.  

“The three flavours can be compared to ice cream where you have one scoop containing strawberry, chocolate and vanilla. 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,” explained Dr Loureiro. 

The three flavors of neutrinos put together have a maximum total mass of less than 4.6310-37 kilograms. These calculations were performed using the UCL supercomputer Grace.

“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,” second author Andrei Cuceu, a PhD student at UCL, added.

In the upcoming years, astronomical observation projects such as the Dark Energy Survey and the European Space Agency’s Euclid will perform more detailed cosmological measurements that could also help to refine the neutrino mass estimate.

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