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A graphic of a neutrino event recorded by the MiniBooNE experiment. The ring of light, registered by some of the more than 1,000 light sensors inside the detector, indicates the collision of a muon neutrino with an atomic nuclei. Graphic: Fermilab
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Neutrinos, elementary particles
generated by nuclear reactions in the sun, suffer from an identity crisis as
they cross the universe, morphing between three different “flavors.” Their
antimatter counterparts do the same thing.
A team of physicists, including some from
MIT, has found surprising differences between the flavor-switching behavior of
neutrinos and antineutrinos. If confirmed, the finding could help explain why
matter, and not antimatter, dominates our universe.
“People are very excited about it
because it suggests that there are differences between neutrinos and
antineutrinos,” says Georgia Karagiorgi, an MIT graduate student and one of the
leaders of the analysis of experimental data produced by the Booster Neutrino
Experiment (MiniBooNE) at the Fermi National Accelerator Laboratory.
The new results, announced in June and
submitted to the journal Physical Review
Letters, appears to be one of the first observed violations of CP
symmetry: the theory that matter and antimatter should behave in the same way.
CP symmetry violation has been seen before in quarks, but never in neutrinos or
electrons.
The finding could also force physicists
to revise their Standard Model, which catalogs all of the known particles that
make up matter. The model now posits only three flavors of neutrino, but a
fourth (or fifth or sixth) may be necessary to explain the new results.
“If this should be proven to be correct,
it would have major implications for particle physics,” says John Learned,
professor of physics at the University
of Hawaii, who is not
part of the MiniBooNE team.
So far, the researchers have enough data
to present their results with a confidence level of just below 99.7% (also
called 3 sigma), which is not high enough to claim a new discovery. To reach
that level, 5-sigma confidence (99.99994%) is required. “People are going to
rightfully demand a really clean, 5-sigma result,” says Learned.
Unexpected oscillations
Since the 1960s, physicists have been gathering evidence that neutrinos can
switch, or oscillate, between three different flavors—muon, electron, and tau,
each of which has a different mass. However, they have not yet been able to
rule out the possibility that more types of neutrino might exist.
In an effort to help nail down the
number of neutrinos, MiniBooNE physicists send beams of neutrinos or
antineutrinos down a 500-meter tunnel, at the end of which sits a
250,000-gallon tank of mineral oil.
When neutrinos or antineutrinos collide
with a carbon atom in the mineral oil, the energy traces left behind allow
physicists to identify what flavor of neutrino took part in the collision.
Neutrinos, which have no charge, rarely interact with other matter, so such
collisions are rare.
MiniBooNE was set up in 2002 to confirm
or refute a controversial finding from an experiment at the Liquid Scintillator
Neutrino Detector (LSND) at Los Alamos National Laboratory. In 1990, the LSND
reported that a higher-than-expected number of antineutrinos appeared to be
oscillating over relatively short distances, which suggests the existence of a
fourth type of neutrino, known as a “sterile” neutrino.
In 2007, MiniBooNE researchers announced
that their neutrino experiments did not produce oscillations similar to those
seen at LSND. At the time, they assumed the same would hold true for
antineutrinos. “In 2007, I would have told you that you can pretty much rule
out LSND,” says MIT physics professor Janet Conrad, a member of the MiniBooNE
collaboration and an author of the new paper.
MiniBooNE then switched to antineutrino
mode and collected data for the next three years. The research team didn’t look
at all of the data until earlier this year, when they were shocked to find more
oscillations than would be expected from only three neutrino flavors—the same
result as LSND.
Already, theoretical physicists are
posting papers online with theories to account for the new results. However,
“there’s no clear and immediate explanation,” says Karsten Heeger, a neutrino
physicist at the University
of Wisconsin. “To nail it
down, we need more data from MiniBooNE, and then we need to experimentally test
it in a different way.”
The MiniBooNE team plans to collect
antineutrino data for another 18 months. Conrad also hopes to launch a new
experiment that would use a cyclotron, a type of particle accelerator in which
particles travel in a circle instead of a straight line, to help confirm or
refute the MiniBooNE results.
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