Physicists at University College London have found a new upper limit on the mass of a neutrino – one of the tightest constraints yet from either particle physics or cosmology.
Neutrinos are elementary particles that travel close to the speed of light, but are very difficult to detect because they are not electrically charged. In fact, in the time it takes you to read this sentence, thousands of billions of neutrinos will have passed through your body – and you won’t have felt a thing.
According to the Standard Model of particle physics, neutrinos should be massless – just like the photons that make up light – but in reality they do have a very small mass. What the Standard Model failed to take into account is the fact that neutrinos undergo something known as oscillations, or mixing.
Neutrinos come in three “flavours”: electron neutrinos, mu neutrinos and tau neutrinos (as well as a corresponding antiparticle for each). When a neutrino is created, for example in the Sun during nuclear fusion, it has a specific flavour. Over time the neutrino can change flavour, but only if its mass is non-zero. All of the neutrinos created in the Sun are electron neutrinos. However, by the time they reach the Earth, we detect equal amounts of each flavour of neutrino; this is how we know that neutrinos must have mass.
Shaun Thomas and colleagues at UCL have now found an upper limit on that mass. It’s the tightest constraint ever placed on the neutrino mass, and comes from cosmology rather than particle physics. Thomas et al took redshift data from over 700,000 galaxies using the Sloan Digital Sky Survey and combined this with results from Wilkinson Microwave Anisotropy Probe, which measured the radiation left over from the Big Bang. They were then able to compare density distributions from the early and more recent Universe.
A Universe where neutrinos are massless would look a bit different to our own. Having massive neutrinos means that the total mass in the Universe is higher than it otherwise would be, and the evolution of the Universe is affected. Larger neutrinos suppress the growth of galaxies more than smaller ones; the more mass neutrinos have, the fewer galaxies will be able to form.
From this relationship, Thomas et al were able to provide the first limit on neutrino mass to come from a galaxy redshift survey, and the tightest constraint yet. The new limit found will complement future neutrino experiments such as KATRIN, and provides hope that the next generation of galaxy surveys may be able to put even tighter constraints on the mass.
Thomas, S., Abdalla, F., & Lahav, O. (2010). Upper Bound of 0.28 eV on Neutrino Masses from the Largest Photometric Redshift Survey Physical Review Letters, 105 (3) DOI: 10.1103/PhysRevLett.105.031301
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