Two major experiments have found no evidence for sterile neutrinos – hypothetical particles that could help explain some puzzling observations in particle physics. The KATRIN experiment searched for sterile neutrinos that could be produced during the radioactive decay of tritium; whereas the MicroBooNE experiment looked for the effect of sterile neutrinos on the transformation of muon neutrinos into electron neutrinos.
Neutrinos are low-mass subatomic particles with zero electric charge that interact with matter only via the weak nuclear force and gravity. This makes neutrinos difficult to detect, despite the fact that the particles are produced in copious numbers by the Sun, nuclear reactors and collisions in particle accelerators.
Neutrinos were first proposed in 1930 to explain the apparent missing momentum, spin and energy in the radioactive beta decay of nuclei. The they were first observed in 1956 and by 1975 physicists were confident that three types (flavours) of neutrino existed – electron, muon and tau – along with their respective antiparticles. At the same time, however, it was becoming apparent that something was amiss with the Standard Model description of neutrinos because the observed neutrino flux from sources like the Sun did not tally with theoretical predictions.
Gaping holes
Then in the late 1990s experiments in Canada and Japan revealed that neutrinos of one flavour transform into other flavours as then propagate through space. This quantum phenomenon is called neutrino oscillation and requires that neutrinos have both flavour and mass. Takaaki Kajita and Art McDonald shared the 2015 Nobel Prize for Physics for this discovery – but that is not the end of the story.
One gaping hole in our knowledge is that physicists do not know the neutrino masses – having only measured upper limits for the three flavours. Furthermore, there is some experimental evidence that the current Standard-Model description of neutrino oscillation is not quite right. This includes lower-than-expected neutrino fluxes from some beta-decaying nuclei and some anomalous oscillations in neutrino beams.
One possible explanation for these oscillation anomalies is the existence of a fourth type of neutrino. Because we have yet to detect this particle, the assumption is that it does not interact via the weak interaction – which is why these hypothetical particles are called sterile neutrinos.
Electron energy curve
Now, two very different neutrino experiments have both reported no evidence of sterile neutrinos. One is KATRIN, which is located at the Karlsruhe Institute of Technology (KIT) in Germany. It has the prime mission of making a very precise measurement of the mass of the electron antineutrino. The idea is to measure the energy spectrum of electrons emitted in the beta decay of tritium and infer an upper limit on the mass of the electron antineutrino from the shape of the curve.
If sterile neutrinos exist, then they could sometimes be emitted in place of electron antineutrinos during beta decay. This would change the electron energy spectrum – but this was not observed at KATRIN.
“In the measurement campaigns underlying this analysis, we recorded over 36 million electrons and compared the measured spectrum with theoretical models. We found no indication of sterile neutrinos,” says Kathrin Valerius of the Institute for Astroparticle Physics at KIT and co-spokesperson of the KATRIN collaboration.
Meanwhile, physicists on the MicroBooNE experiment at Fermilab in the US have looked for evidence for sterile neutrinos in how muon neutrinos oscillate into electron neutrinos. Beams of muon neutrinos are created by firing a proton beam at a solid target. The neutrinos at Fermilab then travel several hundred metres (in part through solid ground) to MicroBooNE’s liquid-argon time projection chamber. This detects electron neutrinos with high spatial and energy resolution, allowing detailed studies of neutrino oscillations.
Sterile neutrinos are a no-show (again)
If sterile neutrinos exist, they would be involved in the oscillation process and would therefore affect the number of electron neutrinos detected by MicroBooNE. Neutrino beams from two different sources were used in the experiments, but no evidence for sterile neutrinos was found.
Together, these two experiments rule out sterile neutrinos as an explanation for some – but not all – previously observed oscillation anomalies. So more work is needed to fully understand neutrino physics. Indeed, current and future neutrino experiments are well placed to discover physics beyond the Standard Model, which could lead to solutions to some of the greatest mysteries of physics.
“Any time you rule out one place where physics beyond the Standard Model could be, that makes you look in other places,” says Justin Evans at the UK’s University of Manchester, who is co-spokesperson for MicroBooNE. “This is a result that is going to really spur a creative push in the neutrino physics community to come up with yet more exciting ways of looking for new physics.”
Both groups report their results in papers in Nature: Katrin paper; MicroBooNE paper.
