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Muons offer clue
to why universe
isn’t just space
By Ron Cowen
Less than a trillionth of a second after
the Big Bang, another tumultuous event
unfolded. In a cosmos born with equal
parts matter and antimatter— which
should have annihilated each other — matter somehow began to dominate.
Physicists now have uncovered a new
clue about what caused this fortunate
imbalance, which led to the existence of
galaxies, planets and people.
The new result is based on seven years
of studying trillions of short-lived particles called B mesons produced at the
Fermi National Accelerator Laboratory’s Tevatron particle collider in Batavia,
Ill. Scientists on the Tevatron’s DZero
experiment have found hints that when B
mesons disintegrate, they produce about
1 percent more pairs of muons, a heavy
version of the electron, than pairs of the
muon’s antiparticle, the antimuon. That
imbalance, a signature of a phenomenon
known as CP violation, may bode well for
eventually understanding how matter
outstripped antimatter in the universe.
The finding, reported at a Fermilab
seminar May 14 and posted online at
arXiv.org May 18, also improves the odds
that the Large Hadron Collider, the European accelerator that recently superseded
the Tevatron as the world’s most powerful
atom smasher, will discover new elementary particles or other novel physics.
Although small, the 1 percent surplus
is 50 times larger than the asymmetry
between matter and antimatter predicted
for B meson decays by the standard model
of particle physics, notes DZero spokes-person Stefan Söldner-Rembold of the
University of Manchester in England.
the dZero detector (shown) at fermilab’s tevatron accelerator has recorded an
excess of matter over antimatter in the decay of particles called B mesons. that
excess could help physicists figure out why matter exists in the universe today.
“It was a goose bump situation,” says
Söldner-Rembold of the moment in early
May when he and his 500 DZero collabo-
rators realized what they had discovered.
“We were very excited because it means
there’s some new physics beyond the
standard model that has to be within our
reach for the asymmetry to be so large.”
Although there’s less than a 0.1 percent
chance that the DZero results are a fluke,
by the standards of particle physics the
results should be regarded as hints that
still must be confirmed,
Grossman of Cornell
University. Söldner-
Rembold notes that
the DZero findings are
similar to an asymmetry
in matter-antimatter
production discovered in
2008 by another Tevatron experiment,
called CDF, but the new results have
much less uncertainty.
Theories that might account for the
DZero result include supersymmetry,
which assumes that each elementary par-
ticle in the standard model has an as-yet-
undiscovered heavier superpartner, notes
theorist Marcela Carena of Fermilab,
who is not a member of the discovery
team. Other possible theories, she notes,
include a model in which gravity and
other forces operate in extra, hidden
dimensions, and the notion of a fourth
family of quarks beyond the three known
generations (up and down, strange and
charm, and top and bottom).
“Some new
physics beyond
the standard
model ... has to be
within our reach.”
