Physicists cast doubt on neutrino theory
Subatomic particles are no-shows in experiments, increasing doubts about
their existence

Date:
August 11, 2020
Source:
University of Cincinnati
Summary:
Physicists are raising doubts about the existence of an exotic
subatomic particle that failed to show up in twin experiments.



FULL STORY ========================================================================== University of Cincinnati physicists, as part of an international research
team, are raising doubts about the existence of an exotic subatomic
particle that failed to show up in twin experiments.


==========================================================================
UC College of Arts and Sciences associate professor Alexandre Sousa
and assistant professor Adam Aurisano took part in an experiment at the
Fermi National Accelerator Laboratory in search of sterile neutrinos,
a suspected fourth "flavor" of neutrino that would join the ranks of
muon, tau and electron neutrinos as elementary particles that make up
the known universe.

Finding a fourth type of neutrino would be huge, Sousa said. It would
redefine our understanding of elementary particles and their interactions
in what's known as the Standard Model.

Researchers in two experiments called Daya Bay and MINOS+ collaborated
on complementary projects in an intense effort to find sterile neutrinos
using some of the world's most advanced and precise tools.

"We apparently don't see any evidence for them," Aurisano said.

The study was published in the journal Physical Review Letters and was
featured in Physics Magazine, published by the American Physical Society.



========================================================================== "It's an important result for particle physics," Sousa said. "It provides
an almost definitive answer to a question that has been lingering for
over 20 years." The research builds on previous studies that offered tantalizing possibilities for finding sterile neutrinos. But the new
results suggest sterile neutrinos might not have been responsible for
the anomalies researchers previously observed, Aurisano said.

"Our results are incompatible with the sterile neutrino interpretation
of the anomalies," he said. "So these experiments remove a possibility
-- the leading possibility -- that oscillations into sterile neutrinos
solely explain these anomalies." Neutrinos are tiny, so tiny they can't
be broken down into something smaller.

They are so small that they pass through virtually everything --
mountains, lead vaults, you -- by the trillions every second at virtually
the speed of light. They are generated by the nuclear fusion reactions
powering the sun, radioactive decays in nuclear reactors or in the
Earth's crust, and in particle accelerator labs, among other sources.

And as they travel, they often transition from one type (tau, electron,
muon) to another or back.



==========================================================================
But theorists have suggested there might be a fourth neutrino that
interacts only with gravity, making them far harder to detect than the
other three that also interact with matter through the weak nuclear force.

The experiment Daya Bay is composed of eight detectors arrayed around six nuclear reactors outside Hong Kong. MINOS+ uses a particle accelerator
in Illinois to shoot a beam of neutrinos 456 miles through the curvature
of the Earth to detectors waiting in Minnesota.

"We would all have been absolutely thrilled to find evidence for sterile neutrinos, but the data we have collected so far do not support any kind
of sterile neutrino oscillation," said Pedro Ochoa-Ricoux, associate
professor at the University of California, Irvine.

Researchers expected to see muon neutrinos seemingly vanish into thin
air when they transitioned into sterile neutrinos. But that's not what happened.

"We expected to see muon neutrinos oscillating to sterile neutrinos and disappear," Aurisano said.

Despite the findings, Aurisano said he thinks sterile neutrinos do exist,
at least in some form.

"I think sterile neutrinos are more likely than not to exist at high
energies.

At the very beginning of the universe, you'd expect there would be
sterile neutrinos," he said. "Without them, it's hard to explain aspects
of neutrino mass." But Aurisano is skeptical about finding light sterile neutrinos that many theorists expected them to find in the experiments.

"Our experiment disfavors light or lower-mass sterile neutrinos," he said.

Sousa said some of his research was truncated somewhat by the global
COVID-19 pandemic when Fermilab shut down accelerator operations
months earlier than expected. But researchers continued to use massive supercomputers to examine data from the experiments, even while working
from home during the quarantine.

"It's one of the blessings of high energy physics," Aurisano
said. "Fermilab has all the data online and the computing infrastructure
is spread out around the world. So as long as you have the internet
you can access all the data and all the computational facilities to do
the analyses." Still, Aurisano said it takes some adjusting to work
from home.

"It was easier when I had dedicated hours at the office. It's a challenge sometimes to work from home," he said.


========================================================================== Story Source: Materials provided by University_of_Cincinnati. Original
written by Michael Miller. Note: Content may be edited for style and
length.


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Link to news story: https://www.sciencedaily.com/releases/2020/08/200811125036.htm

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