Neutrinos weighing in lighter than thought

Cosmologists from University College London have come up with an upper mass limit on the neutrino.

The neutrino, as I seem to have mentioned a lot in recent weeks, is a type of particle with very low mass that travels at extremely high velocities and interacts very weakly with ordinary matter – so weakly that a neutrino could happilly pass through the entire bulk of the Earth without noticing a thing. However, on even larger scales, such as galaxies, there’s a lot more matter to pass through and so a lot more interactions happen.

Lots of neutrinos are produced in places like stars where nuclear reactions are going on. They accompany the creation or destruction of a type of particle called leptons, of which the electron is the most commonly heard. With neutrinos being produced by the many stars about and with their occasional interactions happening at very high velocities, the transfer of momentum from the neutrinos to the matter it strikes must have an effect. The effect would be to start to smooth over space a little, since they’d be rushing around in all kinds of directions, catching the galaxies every now and again.

Shaun Thomas, supervised by Professor Ofer Lahav and Dr. Filipe Abdalla, produced work in part of his PhD thesis on this and continued it afterwards, culminating in a measure of how smooth the universe is. From this came an upper limit to the momentum of the neutrinos that would be doing the smoothing and from that a limit to the mass. This limit is 0.28 electron volts, or less than one billionth the mass of a hydrogen atom. Previous results put the upper mass limit at 1.8 eV and future work with the Dark Energy Survey will try to drive down the limit even further.

The work has received coverage on the BBC, through a UCL press release and on Jon Butterworth’s blog. Unlike him, I don’t have a picture of Shaun smiling by a poster, but I do have an earlier one of him doing something even more typical of a UCL astrophysicist…

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