Limiting factors for the elusive sterile neutrino

By Tushna Commissariat

More data are definitely needed in the quest for the sought-after sterile neutrino. That much was clear as more than 10 different global neutrino detectors announced at the Neutrino 2016 conference in London that they have found no evidence for the slippery particle’s existence. The sterile neutrino is a hypothetical and much-debated fourth type of neutrino that would contribute mass, but only interact with the other three “active neutrinos”, making it that much more difficult to detect. In the video above, Physics World features editor Louise Mayor explains why researchers are so keen to nail down this particle, should it exist, as it may single-handedly explain some of the biggest mysteries in physics today, including dark matter.

Indeed, almost every neutrino experiment presenting results – including T2K, Daya Bay, NOvA, RENO and MINOS+ – mentioned the sterile neutrino, but not one of them has come any closer to actually detecting the particle. The closest they have come is to set better and more stringent limits on which energies it is *not* at – you can see the excluded area in the graph below, presented by MINO’s Justin Evans from the University of Manchester. In case you are left wondering what experimental evidence physicists do have for sterile neutrinos, it lies mostly in discrepancies in their oscillation data that deviate from theoretical predictions.

Latest limits: MINOS slide on sterile neutrino limits. ( Courtesy: MINOS collaboration/University of Manchester)

Maury Goodman, a particle physicist at the Argonne High-Energy-Physics neutrino group, is a long-time “non-believer” of the sterile neutrino, although he can see why it is so tempting for experimentalists to go after the Yeti of particles. (Take a look at a previous blog “Are ‘sterile neutrinos’ dark-matter particles after all?” where Goodman explains his reservations.) When I caught up with him at the conference, Goodman told me that instead, he would like some of the main unanswered questions of the standard three-flavour neutrino model answered first – such as resolving the mass hierarchy and determining the actual masses of the three known neutrinos, figuring out how exactly neutrinos acquire their mass as well as determining whether neutrinos are Dirac or Majorana particles.