collaboration is hunting for signs of new physics—perhaps supersymmetry, or
perhaps the Higgs particle—in ‘penguin modes,’
rare channels of decay from B mesons to other particles.
They hope to catch their quarry with their physics
net. This is a set of data so huge that the current 15-month run will
double the amount of data collected by the BABAR experiment during all
previous runs from 1999 to 2004.
“There have been intriguing hints for new physics
in penguin modes, which makes this a hot topic,” said BABAR Spokesman
The search for new physics among penguin modes and
other rare decay channels is one of many research goals for the
international collaboration during the current run. It will take the
vast amount of new data that the PEP-II accelerator is providing, and
BABAR is recording, to be sensitive to tiny effects involved as well as
to attain the statistical significance needed to possibly resolve
whether new physics effects are taking place.
The current run started mid-April 2005, paused
briefly from mid-October to mid-November to re-certify accelerator
safety systems and upgrade PEP-II vacuum chambers, and will continue
through July 31, 2006.
BABAR also took advantage of the down time to install new drift chamber
electronics that will allow full exploitation of the increasing
luminosity delivered by PEP-II. In addition, the experiment is
benefiting from upgrades to one-third of the muon identification
The PEP-II accelerator produces B mesons and their
anti-particle, B-bar mesons. Both decay into other particles through the
weak force, but the matter and antimatter particles decay at different
rates because of an asymmetry in the behavior of the weak force (it
violates charge-parity symmetry).
According to the Standard Model of known physics,
this asymmetry in matter and antimatter decays should remain the same
whether decays occur in a particular set of penguin channels or in the
more common ‘charmonium’ modes. The data to date, however, indicates
that in some of those penguin decays, there seems to be far less
asymmetry than seen in the charmonium modes of decay. If this holds up,
it means some kind of new physics is showing up in these rare
decays—thanks to quantum loops.
A quantum loop means particles of any mass can
instantly materialize, and then disappear, during the decay process.
Quantum loops dominate penguin modes, but not charmonium modes. The
brief-lived virtual particles are usually W bosons, which mediate the
weak force. But other particles with similar behavior could enter the
loop, including a charged Higgs particle, which physicists still
ardently seek, or a neutralino, one of the proposed supersymmetry
particles. This makes decays involving loops very sensitive to new
physics at high masses. If the new particles participate in penguin
decays, they would alter the expected asymmetry between matter and
“Each individual penguin mode might have a
different contribution from new physics,” said Riccardo Faccini, BABAR
Physics Analysis coordinator.
Indeed, recent papers on two new types of penguin
decays showed that one follows standard model physics and the other
suggests that non-standard physics might be influencing the decays.
B-factory physicists are also looking for new physics in other rare
decays with loops, such as tau and charm particle decays.
BABAR experimenters hope to have a much better
understanding by the end of this run. The current average of all penguin
channels shows about three-sigma effect for new physics, still below the
five-sigma signal usually needed to claim a discovery. Even if these
hints aren’t confirmed, the measurements madeby BABAR and by Belle, the B-factory in Japan, will
constrain where new physics could be found. The constraints will
ultimately help experiments at the Large Hadron Collider currently being
built at CERN interpret any discoveries made by directly producing new
Penguin decays are elusive prey. Out of 400 million
events, BABAR has measured only several hundred of them. Fortunately,
the hunters are getting smarter.
“We have developed quite refined tools,” Faccini
said. “The doubling of the data set is really important because of the
much improved sensitivity. We have a very good silicon vertex tracker to
measure these different modes and in recent years we’ve been learning
how to squeeze every last bit out of it. We also have refined our
The PEP-II team is also on their side, delivering a
record luminosity of just over 1x1034/cm2/s in the
last few days before the mid-run downtime in October. The accelerator
team has plans for a further 20 percent improvement in luminosity before
next summer. They also expect an almost doubling of luminosity after
they install major improvements in the fall of 2006.