For decades, physicists have suspected an interloper. A reclusive,
hypothetical subatomic particle might be creeping into studies of neutrinos,
nearly massless particles with no electric charge. A new study casts doubt
on that idea, but leaves unsolved the mystery of what caused peculiar
results in certain neutrino experiments.
“We still don’t have the answer,” says physicist Kate Scholberg of Duke
University, who was not involved with the new result. “It’s simultaneously
satisfying and unsatisfying.”
Neutrinos, which come in three known varieties, have shown up in greater
numbers than expected in some experiments. That strange behavior raised the
tantalizing prospect that a stealthier fourth type of neutrino, called a
sterile neutrino, might be awaiting discovery. But new data from the Micro
Booster Neutrino Experiment, or MicroBooNE, favor the canonical neutrino
trio.
An earlier experiment called MiniBooNE, located at Fermilab in Batavia,
Ill., had for years found more neutrinos than expected at low energies, a
hint strengthened with more data in 2018. An even earlier neutrino
experiment, performed in the 1990s, had also seen a similar signal.
With MiniBooNE, scientists studied a phenomenon called neutrino oscillation.
The three known varieties of neutrinos — electron neutrinos, muon neutrinos
and tau neutrinos — can transform, or oscillate, from one type to another as
they travel. MiniBooNE looked for electron neutrinos produced when muon
neutrinos oscillated.The apparent glut of electron neutrinos seen by
MiniBooNE could indicate that the switch seemed to happen more often than
expected, potentially due to sterile neutrinos muddling up the oscillations.
But there was a catch. Particle detectors can’t directly spot neutrinos,
instead identifying them by observing other particles spit out when
neutrinos interact within a detector. And MiniBooNE tended to confuse
electrons — a signature of electron neutrinos — with photons, particles of
light that could indicate another particle. That left scientists unsure
whether the excess events were really electron neutrinos. The sterile
neutrino remained a question mark.
Enter MicroBooNE. Also at Fermilab, the experiment uses an advanced type of
detector that can tell electrons from photons. So scientists set out to
investigate the excess events — aiming to find out whether they involved
electrons or photons. But MicroBooNE, confusingly, found no excess at all.
In an October 1 seminar and a paper posted at arXiv.org, MicroBooNE had
mostly eliminated the possibility of extra events involving photons. The new
result, reported October 27 during a virtual seminar, rules out many of the
possible types of extra events involving electrons, making the sterile
neutrino idea less plausible.
It’s not clear why one experiment saw an excess while the other didn’t. The
difference between the two measurements might come down to the different
materials used in the detectors, Scholberg says — carbon in the case of
MiniBooNE, argon for MicroBooNE.
Other possible explanations for the excess events that MiniBooNE found
remain to be investigated, some of which might go beyond standard physics.
The detections, for example, might involve electrons paired with their
antimatter partners, positrons. That pair could point the finger at
different hypothetical subatomic stuff, in particular, what’s called an
axionlike particle.
The researchers “have eliminated a lot of possibilities of what this excess
could be, so I found the results pretty compelling,” says physicist Mayly
Sanchez of Iowa State University in Ames, who was not involved with the
study. “You’re giving fewer and fewer places to hide to these sterile
neutrinos.”
But all hope for sterile neutrinos is not lost: A more complicated scenario
involving a sterile neutrino combined with other theorized new phenomena
could still explain the excess events.
“There’s still a mystery afoot,” says physicist Bonnie Fleming of Yale
University, a cospokesperson of the MicroBooNE experiment. “We have more
work to do. There’s no doubt about that.”
References:
M. Toups et al.
First search for an excess of electron neutrinos in MicroBooNE with
multiple final-state topologies. Virtual seminar, October 27, 2021.
P. Abratenko et al.
Search for an excess of electron neutrino interactions in MicroBooNE
using multiple final state topologies. microboone.fnal.gov. Posted October 27, 2021.
MicroBooNE collaboration. Search for Neutrino-induced neutral current ∆
radiative decay in MicroBooNE and a first test of the MiniBooNE low energy
excess under a single-photon hypothesis.
arXiv:2110.00409. Submitted October 1, 2021.
Mark Ross-Lonergan.
Search for anomalous single-photon production in MicroBooNE as a first
test of the MiniBooNE low-energy excess. Virtual seminar, October 1, 2021.
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