For decades, physicists have theorized that the current best theory
describing particle physics—the "Standard Model"—was not sufficient to
explain the way the universe works. In the search for physics beyond the
Standard Model (BSM), elusive particles called neutrinos might point the
way.
Neutrinos are sometimes called "ghost particles" because they so rarely
interact with matter that they can travel through just about anything.
However, while traveling through matter, they may be "slowed down,"
depending on the neutrino's type (or "flavor"), in what is known as a
"matter effect."
In many BSM models, neutrinos have extra interactions with matter due to new
and thus far unknown forces of nature. Different neutrino flavors might be
affected to varying extents by these interactions, and the strength of the
resulting matter effects depends on the density of matter the neutrinos are
passing through. If researchers observe matter effects that can be explained
as "nonstandard interactions" (NSI), it might point to new physics.
The IceCube Neutrino Observatory, an array of sensors embedded in the South
Pole ice, was built to detect and study neutrinos from outer space. But in
IceCube's center is a subset of more densely packed sensors called DeepCore;
this region is sensitive to lower energy neutrinos formed in Earth's
atmosphere that are potentially more strongly affected by nonstandard matter
effects. In a paper published today in Physical Review D, the IceCube
Collaboration discusses an analysis in which they examined three years of
DeepCore data to see whether atmospheric neutrinos have extra interactions
with matter. This analysis puts limits on all the parameters used to
describe NSI, an improvement upon earlier analyses that were restricted to
only the NSI regimes to which IceCube is most sensitive.
Valuable ambassadors for new physics
"Atmospheric neutrinos are a great probe for testing whether neutrinos have
NSI because they pass right through Earth, including its center, which has a
very high matter density," says Elisa Lohfink, graduate student at the
University of Mainz in Germany and a lead on this publication. Changes in
matter density directly change the neutrinos' oscillation patterns—the way
neutrinos change their flavors, or "oscillate"—and therefore which flavors
of neutrinos arrive at the South Pole detector. IceCube DeepCore is
sensitive to these matter effects because of the huge number of atmospheric
neutrinos it detects every year.
In this analysis, led by University of Mainz graduate student Thomas
Ehrhardt, the researchers looked at the oscillation patterns of neutrinos
that had arrived at DeepCore from all directions and determined whether they
matched the Standard Model expectations or showed effects from any of five
effective parameters that measure how much more a neutrino flavor interacts
than it would in the Standard Model. The researchers could then constrain
the effective NSI parameters by testing how well the oscillation pattern
matches different NSI scenarios.
First, Ehrhardt and his collaborators examined one effective parameter at a
time, yielding the results shown in the figure above. Fully free NSI were
investigated separately. Since the analysis was largely independent from any
specific underlying models, the researchers were able to constrain NSI
without relying on one model to be correct.
The researchers were able to put limits on each of the five possible NSI
parameters individually at a sensitivity that is at least comparable to the
world's combined limits, an accomplishment described by Lohfink as
"unprecedented." More importantly, the researchers say, is the finding that
IceCube can probe models in which they fit all the parameters at once. "To
our understanding, there is no other experiment in the world that can do
this from a single measurement," says Sebastian Böser, professor at the
University of Mainz. "We can put limits on an unprecedented range of models
for new physics in the neutrino sector." The result is a major improvement
on a previous IceCube analysis that looked at just one parameter.
The researchers hope that the rest of the neutrino community will pick up on
the results and incorporate them into the global fits. And Lohfink and her
collaborators are already working on a follow-up analysis using a much
larger data sample—eight years of data instead of three years—with much
better sensitivity. They hope to have an improved limit soon.
"In the long run, the IceCube Upgrade will be a real game changer for this
type of analysis," says Böser. "Not only will the Upgrade provide better
calibration and reduce the impact of systematic uncertainties, but it will
also allow us to resolve the neutrino oscillations much, much better, and
therefore let us see potential deviations from the Standard Model much more
clearly. I am truly excited about this!"
Reference:
R. Abbasi et al, All-flavor constraints on nonstandard neutrino interactions
and generalized matter potential with three years of IceCube DeepCore data,
Physical Review D (2021).
DOI: 10.1103/PhysRevD.104.072006
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