Does a fourth neutrino exist?

A new study claims it is time to seriously discuss that claim.

(PhysOrg.com) -- Physicists know that neutrinos (and antineutrinos) come in three flavors: electron, muon, and tau. In several experiments, researchers have detected each of the neutrino flavors and even watched them “oscillate” back and forth between flavors. But starting in the early ‘90s, some experiments have also revealed a nagging anomaly: muon antineutrinos oscillate into electron antineutrinos at a 3% higher rate than predicted. Physicists can reconcile this discrepancy by adding a fourth neutrino with a specific mass, although such a move would require modifying the Standard Model, the theory of subatomic particles that has taken decades to build. In a new study, a team of physicists thinks it’s time to put the question of the fourth neutrino’s existence to the test.


In their study published in a recent issue of Physical Review Letters, Michel Cribier, et al., have proposed an experiment that would reveal whether a fourth flavor of neutrino really exists. If it does, then it would have huge implications not only for neutrino science, but also for understanding the building blocks of matter overall.
The first hints that something was amiss came in the early ‘90s from the Liquid Scintillator Neutrino Detector (LSND) experiment at Los Alamos National Laboratory. In the experiment, an antimuon beam bombarded a target, revealing a greater number of antielectron neutrino oscillations than predicted. Or in other words, antineutrino oscillations seemed to be occurring at a faster-than-expected rate.

But Cribier and his coauthors’ main motivation for carrying out a test of a fourth neutrino rests on the results of a more recent finding, which is now known as the Reactor Antineutrino Anomaly. In a recent study, physicists (including some from the recent paper) at the French Atomic Energy Commission (CEA) in Saclay recalculated the rate of antineutrino production in nuclear reactors that was first calculated in the 1980s. Using improved techniques, the scientists estimated that the rate of antineutrino production is about 3% more than previously predicted. Even after rechecking the new estimates, the 3% antineutrino surplus remains. As a consequence, the same physicists reanalyzed more than 20 previous reactor neutrino experiment results, finding more discrepancies.

The simplest physics explanation for this anomaly is the existence of a fourth neutrino. Keep on reading...