Doubt cast on ‘sterile’ particles leaves a neutrino mystery unsolved

For many years, physicists have suspected an interloper. A reclusive, hypothetical subatomic particle could possibly be creeping into studies of neutrinos, practically massless particles with no electrical demand. A new examine casts question on that idea, but leaves unsolved the thriller of what induced peculiar success in specific neutrino experiments.

“We still really don’t have the solution,” says physicist Kate Scholberg of Duke University, who was not involved with the new consequence. “It’s concurrently satisfying and unsatisfying.”

Neutrinos, which come in three identified versions, have revealed up in better figures than expected in some experiments. That strange actions elevated the tantalizing prospect that a stealthier fourth type of neutrino, identified as a sterile neutrino, may well be awaiting discovery. But new knowledge from the Micro Booster Neutrino Experiment, or MicroBooNE, favor the canonical neutrino trio.

An before experiment known as MiniBooNE, positioned at Fermilab in Batavia, Sick., experienced for many years located more neutrinos than envisioned at small energies, a trace strengthened with a lot more information in 2018 (SN: 6/1/18). An even previously neutrino experiment, performed in the 1990s, had also witnessed a identical signal.

With MiniBooNE, experts examined a phenomenon named neutrino oscillation. The three recognised varieties of neutrinos — electron neutrinos, muon neutrinos and tau neutrinos — can transform, or oscillate, from a single sort to yet another as they journey. MiniBooNE seemed for electron neutrinos made when muon neutrinos oscillated.The obvious glut of electron neutrinos found by MiniBooNE could suggest that the switch appeared to materialize far more normally than expected, perhaps because of to sterile neutrinos muddling up the oscillations.

But there was a catch. Particle detectors just cannot instantly location neutrinos, alternatively identifying them by observing other particles spit out when neutrinos interact within just a detector. And MiniBooNE tended to confuse electrons — a signature of electron neutrinos — with photons, particles of gentle that could indicate a further particle. That still left researchers uncertain regardless of whether the surplus activities were being genuinely electron neutrinos. The sterile neutrino remained a question mark.

Enter MicroBooNE. Also at Fermilab, the experiment utilizes an innovative type of detector that can tell electrons from photons. So scientists set out to investigate the excessive events — aiming to find out irrespective of whether they associated electrons or photons. But MicroBooNE, confusingly, uncovered no excessive at all. In an Oct 1 seminar and a paper posted at, MicroBooNE experienced typically eliminated the chance of further situations involving photons. The new outcome, claimed October 27 throughout a digital seminar, rules out lots of of the attainable kinds of more gatherings involving electrons, producing the sterile neutrino idea significantly less plausible.

It’s not apparent why 1 experiment observed an extra even though the other didn’t. The big difference between the two measurements could possibly come down to the different products applied in the detectors, Scholberg claims — carbon in the scenario of MiniBooNE, argon for MicroBooNE.

Other doable explanations for the surplus situations that MiniBooNE located continue to be to be investigated, some of which could possibly go outside of conventional physics. The detections, for illustration, might contain electrons paired with their antimatter companions, positrons. That pair could position the finger at unique hypothetical subatomic stuff, in unique, what is referred to as an axionlike particle.

The researchers “have eliminated a great deal of choices of what this surplus could be, so I found the final results very powerful,” suggests physicist Mayly Sanchez of Iowa Condition College in Ames, who was not involved with the study. “You’re providing much less and much less locations to cover to these sterile neutrinos.”

But all hope for sterile neutrinos is not lost: A additional difficult state of affairs involving a sterile neutrino mixed with other theorized new phenomena could however explain the excessive occasions.

“There’s nevertheless a mystery afoot,” states physicist Bonnie Fleming of Yale University, a cospokesperson of the MicroBooNE experiment. “We have a lot more function to do. There’s no doubt about that.”