hint at need for
Nearly massless particles
could turn physics on its ear
By Ron Cowen
Neutrinos are the big nothings of subatomic physics. Nearly massless and with
no electric charge, these ghostly particles
interact so weakly with other matter
that more than 50 trillion of them pass
through a person’s body each second.
Yet two new experiments hint that
neutrinos may be opening a window on a
hidden world of subatomic particles and
forces. The findings could end up being
statistical flukes. But so far the results,
announced June 14 at the Neutrino 2010
conference in Athens, indicate that neutrinos and their antiparticle counterparts, antineutrinos, are not the nearly
exact mirror images of each other that
current physics supposes them to be.
If confirmed, the results would “
indicate a fundamentally new direction in
our thinking” about subatomic particles
and the origin of matter in the universe,
says theorist Rabindra Mohapatra of the
University of Maryland in College Park.
The findings may help explain how the
universe, believed to have begun with
matter and antimatter so perfectly balanced that they would have destroyed each
other upon contact, became dominated
by matter. The results “could even signal a tiny breakdown of Einstein’s theory
of special relativity,” Mohapatra says.
Current theories of particle physics
assume that known forces arise from
interactions with neighboring particles
and obey special relativity, which holds
that the speed of light and the laws of
physics are always the same regardless of a particle’s speed or rotation. For
that to hold true, particles and antiparticles—including neutrinos and their
“if the masses are different ... then the most sacred symmetry
of quantum field theory, CPT, is broken.” — TOM WeILeR
Data from the MINOS experiment (detector at the Soudan Underground Laboratory in
Minnesota shown) hint that neutrinos and antineutrinos might have different masses.
antipartners — must have the same mass,
Mohapatra says. But new data from the
MINOS (for Main Injector Neutrino
Oscillation Search) experiment seem to
contradict that notion.
The three known types of neutrinos
(electron, muon and tau) transform from
one type into another as they travel.
During a 735-kilometer journey from
Fermilab in Batavia, Ill., to the Soudan
Underground Laboratory in Minnesota,
about 37 percent of muon antineutrinos
disappeared — presumably morphing into
another neutrino type — compared with
just 19 percent of muon neutrinos, reports
MINOS spokesman Robert Plunkett of
Fermilab. That difference suggests a difference in mass between antineutrinos
and neutrinos — although more data will
be needed to confirm the observation.
Data collected so far leave a 5 percent
chance that the particles weigh the same.
“If the masses are different for neutrinos and antineutrinos, then the most
sacred symmetry of quantum field theory,
CPT [for charge, parity and time], is broken
in the neutrino sector,” says Tom Weiler
of Vanderbilt University in Nashville.
If particle interactions are thought of
as a movie, CPT symmetry requires that
whatever physics occurs during the show
must be the same when the film is run for-
ward or backward (time), viewed through
a mirror (parity) and when replacing each
particle by its antiparticle (charge).