A neutrino has a mass, physics says, but it is elusive to figure out and extremely hard to measure – a neutrino is capable of passing through a light year (about six trillion miles) of lead without hitting a single atom.

How do you measure a ghost particle?   We have a giant particle detector already, say cosmologists - space.   A new survey of galaxies in the universe has University College London researchers putting neutrino mass at no larger than 0.28 electron volts – less than a billionth of the mass of a single hydrogen atom. They say this is one of the most accurate measurements of the mass of a neutrino to date and their research is due to be published in an upcoming issue of Physical Review Letters.  The work resulted from the PhD thesis of Shaun Thomas and will be presented at the Weizmann:UK conference at UCL today.

The work is based on the principle that the huge abundance of neutrinos (there are trillions passing through you right now) has a large cumulative effect on the matter of the cosmos, which naturally forms into "clumps" of groups and clusters of galaxies. As neutrinos are extremely light they move across the universe at great speeds which has the effect of smoothing this natural "clumpiness" of matter. By analyzing the distribution of galaxies across the universe (i.e. the extent of this "smoothing-out" of galaxies) scientists are able to work out the upper limits of neutrino mass.

Central to this new calculation is the existence of the largest ever 3D map of galaxies, called Mega Z, which covers over 700,000 galaxies recorded by the Sloan Digital Sky Survey and allows measurements over vast stretches of the known universe.

The cosmologists at UCL were able to estimate distances to galaxies using a new method that measures the colour of each of the galaxies. By combining this enormous galaxy map with information from the temperature fluctuations in the after-glow of the Big Bang, called the Cosmic Microwave Background radiation, they were able to put one of the smallest upper limits on the size of the neutrino particle to date. 

Dr. Shaun Thomas commented, "Although neutrinos make up less than 1% of all matter they form an important part of the cosmological model. It's fascinating that the most elusive and tiny particles can have such an effect on the Universe."

Dr. Filipe Abadlla added, "This is one of the most effective techniques available for measuring the neutrino masses. This puts great hopes to finally obtain a measurement of the mass of the neutrino in years to come."

The authors are confident that a larger survey of the Universe, such as the one they are working on called the international Dark Energy Survey, in which UCL is heavily involved, will yield an even more accurate weight for the neutrino, potentially at an upper limit of just 0.1 electron volts.