Weighing up the options for neutrino mass

By Tushna Commissariat

As I am sure all of you know, the 2015 Nobel Prize for Physics was awarded yesterday to Arthur McDonald and Takaaki Kajita “for the discovery of neutrino oscillations, which shows that neutrinos have mass”. Following on from yesterday’s neutrino-flavoured excitement, here’s an explanation of why it’s so important that we better understand neutrino mass.

Our current observations and theories of neutrino oscillations suggest that at least two of the currently known three flavours of neutrinos have non-zero mass. While we know the mass differences between the different neutrino flavours accurately, their actual masses have not been measured. It’s not for lack of trying, it has simply proven very difficult to make the measurements.

Most attempts to directly measure the neutrino mass involve studying the decay of certain particles into a neutrino and some other charged particles. If we know the mass of the parent particle and the masses and momentums of the charged decay products, we can, in principle, determine the mass of the neutrino. The neutrino mass will undoubtedly be very small, and therefore a successful experiment must be exceedingly precise and well understood in order to differentiate between a zero-mass and non-zero-mass result.

Also, the mechanism by which neutrinos acquire mass remains a mystery. All fundamental particles acquire mass as they collide with the Higgs boson – photons are massless as they do not do this. But we also know that collisions with the Higgs boson change the handedness of a particle – left-handed particles become right-handed and vice versa.

So far we have only seen left-handed neutrinos. Indeed, the Standard Model of particle physics does not include right-handed neutrinos and therefore predicts that neutrinos have no mass.  As a result, the SM must be extended to explain massive neutrinos.

One such extension allows for both left- and right-handed neutrinos. The left-handed “Dirac neutrinos” acquire mass via the Higgs mechanism and three flavours (electron, muon and tau) have already been detected. The right-handed “sterile neutrino”, however, interacts much more weakly and has yet to be found. This hypothetical and much-debated fourth type of neutrino would contribute mass but would only interact with the other three “active neutrinos” – making sterile neutrinos much more difficult to detect. Watch the video above to learn more about this elusive particle from Physics World’s features editor Louise Mayor.

Another extension of the Standard Model suggests that extremely heavy right-handed neutrinos briefly exist before they collide with the Higgs boson to produce light left-handed “Majorana” particles. In this scenario, the neutrino and its antimatter counterpart – the antineutrino – are identical. Left-handed neutrinos would couple directly with right-handed antineutrinos, giving neutrinos mass. The Enriched Xenon Observatory-200 (EXO-200) is searching for this type of neutrino.

Despite the fact that neutrino research has now won its fourth Nobel prize, the field is far from exhausted – what with other possible flavours (that enticingly may even be dark-matter candidates), extremely high-energy cosmic neutrinos that experiments like IceCube are studying and primordial neutrinos (that came into being mere seconds after the Big Bang and were recently detected by the Planck telescope), nuclear-monitoring using antineutrinos and a whole host of new neutrino observatories springing into action, there is, undoubtedly, lots more in store.