Painting neutrinos into a corner

As elementary particles go, neutrinos—’little neutral ones’—are not newcomers: they were proposed by physicist Wolfgang Pauli in 1930 and revealed by experiment in 1956.  The ‘Standard Model’ of particle physics says they are massless. However, experiments and observations have shown otherwise. This makes the neutrino the only confirmed particle that doesn’t fit the Standard Model (which is otherwise extremely successful).

The neutrino mass isn’t well constrained, either. Pinning it down would help us to better understand how neutrinos fit into the particle-physics zoo.

But even talking of ‘the’ neutrino mass is misleading. Neutrinos come in at least three kinds, or ‘flavours’, which can transform into each other. Experiments show that there are mass differences between at least two of the kinds. Current particle-physics experiments are not sensitive enough to measure the individual masses, so the mass limits we can obtain are limits on the sum of the masses of the three kinds.

Observations of neutrinos from the Sun, the atmosphere, and nuclear reactors have put the lower limit for the combined mass at 0.05 eV—an amount equivalent to a tenth of a millionth of the (already tiny) mass of the electron.

The strongest upper limits come from cosmology. Until recently, the best figure was 0.23 eV. That was obtained from the measurements of the cosmic microwave background by the Planck spacecraft, coupled with information about the patterns in which galaxies are clustered from four galaxy redshift surveys. Two of those surveys were done in Australia: the 6dF Galaxy Survey of more than 125,000 galaxies, made with the 1.2-m UK Schmidt Telescope, and the WiggleZ survey of almost 240,000 galaxies, carried out with the 4-m Anglo-Australian Telescope.

Now Signe Riemer-Sørensen (University of Oslo) and her colleagues David Parkinson and Tamara Davis (both University of Queensland) have painted neutrinos even further into a corner. By adding extra data from the WiggleZ survey to the dataset used by the Planck team, they have pushed the upper mass limit down to 0.18 eV—an improvement of over 25%.