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BABAR
Tests Matter-Antimatter Theory in New Ways
By Kate Metropolis
A physicist in the twenty-third century telephones her automobile
mechanic. “Hello,” she says, “I’ve been having trouble with my new car.
It’s a red Alpha Romeo. Any chance you can fix it? Oh, by the way, it’s
made out of antimatter.”
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Graphic by Alan Chou |
The only repair manual the mechanic has
is for the matter model. “No problem,” he thinks. “I’ll just follow the
manual and substitute antimatter parts,” so he tells his customer to
bring in her car. Using a special tool kit to handle the antimatter, the
mechanic works on the car, following precisely the instructions in the
manual.
A few days later, the customer is back in his shop. She’s not happy.
“The car just doesn’t drive the way it should,” she complains.
“Ah,” says the mechanic, “there’s a subtle difference between the way
matter and antimatter behave. The rules for the car just don’t carry
over perfectly to the anticar.”
In fact, the universe would be vastly different if the laws of physics
were precisely the same for all particles and their antimatter
counterparts. The Big Bang, the explosion in which the
universe was created, produced exactly equal amounts of matter and
antimatter, and whenever they get close enough, they annihilate each
other. If the annihilation had been complete, all matter, as we know it,
would have been converted into light.
This dominance of matter in the universe, which most of us never wonder
at, is bizarre. It is as extraordinary as measuring how long it takes a
marble to roll down a board and discovering that your result depends on
the time of day you do the measurement.
The laws that govern particle behavior might have been as blind to the
difference between matter and antimatter as the law of gravity is to the
difference between morning and afternoon. But the fact that they do
distinguish matter from antimatter, a phenomenon particle physicists
call CP violation, means that there’s “a one in a billion imbalance in
favor of ordinary matter,” says
BABAR
physicist Christophe Yeche, from Saclay.
The current theory of particle physics, called the standard model,
predicts that all the differences in the behavior of matter and
antimatter can be expressed in terms of three angles—called alpha, beta,
and gamma—that, like the angles of a triangle, add up to 180 degrees.
(This is a strange-sounding prediction; it would take an article at
least this long to explain it, so I hope you’re willing to take it on
faith.)
“Measuring each of these angles,” says
BABAR
physics analysis coordinator Jeff Richman, “is a holy
grail of this field.”
The Quest for Alpha
B mesons and anti-B mesons provide the largest window into the
phenomenon of CP violation. Their behavioral difference lies in the
probability of their decays into particular particles: the B mesons
decay into given states a different percentage of the time than the
anti-B mesons.
Both
BABAR
and Belle, a collaboration in Japan, use B mesons to explore CP
violation. The initial plan of both groups was to measure the angle
alpha by looking at the very rare events in which a B meson (or an
anti-B meson) decays into a pair of charged pions. However, while the
experiments were being built, the CLEO collaboration at Cornell
performed measurements suggesting that the pion approach was fatally
flawed, at least for the amount of data that
BABAR
and Belle will acquire, because the decay process was so
complicated to interpret. “Nature,” says Yuval Grossman, a theorist at
the Technion, the Israel Institute of Technology, “was unkind.”
The hope of measuring alpha still burns bright in
BABAR,
thanks to kindness on the part of nature and hard work by members of the
collaboration from LBNL, Liverpool, and Saclay.
The researchers discovered that the decay of a B meson to the particle
pair rho+ rho- is more likely than the decay to charged pions and the
decay was well suited to measuring CP violation.
However, measuring this decay was challenging, according to Andrei
Gritsan, leader of the rho-rho team at LBNL. “We had to distinguish very
rare decays, occurring only three times in a hundred thousand, and
measure their parameters in the presence of a large background.”
BABAR‘s
first results characterizing the rho+ rho– decay were published in
Physical Review, and the collaboration will present even better results
this summer, after analyzing additional data.
Then the physicists measured alpha by comparing the time it takes a B
meson to decay into rho+ rho- with the time it takes its antiparticle,
an anti-B meson, to decay into the same set of particles.
“Our direct measurement is a first,” says
BABAR
physicist Christos Touramanis, head of the Liverpool group.
These results have been submitted to Physical Review. The precision so
far is limited, but the results are consistent with standard model
predictions.
Or, as Richman puts it, “This is one more banana peel the standard model
could have slipped on.”
Grossman, who is not a member of
BABAR,
characterizes the results as “very important. A year ago, we didn’t know
you could use rho+ rho- to get a really fundamental parameter.”
“A Physics Program, Not a Single Measurement”
BABAR
has also obtained new results on another of the
holy-grail angles: beta.
Four years ago, the first major result announced by both
BABAR
and Belle was the measurement of beta in a certain class
of B decays, called the “golden modes” because the experimental
measurement and its theoretical interpretation are so straightforward.
The standard model predicts that measurements of beta in B decays will
yield the identical value; yet many physicists expect some types of B
decays to disagree with the golden-mode value.
“The goal is to try to conclude whether the data show signs of physics
beyond our current understanding,” says Zoltan Ligeti, a theorist at
LBNL. “Evidence that the standard model can’t account for data would do
that.”
Both collaborations have been pushing hard to analyze different classes
of B decays to obtain measurements of beta.
BABAR
results so far are consistent with the standard model.
Belle interprets one of its results as a possibly inconsistent with the
standard model.
“It’s constructive to think about what Belle’s result might mean if it
becomes statistically significant,” Grossman says, “but right now it is
not statistically significant.”
BABAR
has found additional B decays that can be used to search for
contradictions to the standard model, but initial measurements still
agree with standard model predictions. “The measurement of the angle
beta is not just a single measurement,” Richman emphasized at the
BABAR
collaboration meeting in April. “It’s a physics program.”
Stay Tuned
BABAR
and Belle have both started the marathon of preparing for the
International Conference of High Energy Physics, to be held this August
in Beijing. New data are pouring in (the PEP-II accelerator is now
delivering B mesons at triple the rate it was designed to), and more
data promise more accurate measurements.
“We’ve demonstrated that we can do these measurements,” Richman says.
“Now PEP-II has given us a lot more data. We could discover that
everything still fits together, or that the new results can’t fit into
the simple standard model picture. A lot of us will be on the edge of
our seats when the new results come out.”
“Understanding the laws of nature is really what we are after,” Grossman
says.
“People sometimes think that the most important discovery is a new
particle. That is particularly exciting when it teaches us something new
about the fundamental laws. But experimental results like the
measurements of alpha and beta, which could prove our theories right or
wrong, are just as valuable.” |
