The search was initiated based on additions to the “Standard Model,” the broad physics theory that describes fundamental particles, how they interact, and how they bind together to form composite particles. The B particle, for example, is a member of a class of composite particles known as “mesons,” which consist of one quark — the “building block” of all other particles — and one antiquark. The tau, a fundamental, non-composite particle, can be thought of as a very heavy electron.
The original Standard Model allows the B meson to decay into the tau pair, but at a rate so low as to be extremely difficult to measure. However, recent proposed extensions to the model predict that the decay should occur at a measurable rate, perhaps through the addition of unknown forces or particles. Enter the BaBaR experiment.
Located at the Stanford Linear Accelerator Center (SLAC) near Stanford University, BaBaR was built, ultimately, to determine why matter dominates the present-day universe, with very little antimatter leftover from the Big Bang. The detector is positioned around one section of a very large particle accelerator, where a beam of electrons and a beam of anti-electrons — i.e. positrons — are directed in opposite circular paths at nearly the speed of light by a large ring of very powerful magnets.
The collisions produce B and anti-B mesons, which in turn decay into a shower of other particles. These particles pass into the BABAR detector, which records their trajectories, energies, and electric charge. From this data, scientists in the BABAR collaboration (about 600 physicists from more than 70 institutions worldwide) can work backward to determine how the B mesons decay.
“Our results show, at the 90 percent confidence level, that this decay occurs so infrequently as to be undetectable by BaBar. Our result is important because of what we did not find, since this places limits of possible new physics beyond the Standard Model,” said the experiment’s spokesperson, SLAC physicist David MacFarlane, to
PhysOrg.com.
BaBar physicists analyzed the collisions by calculating the “branching ratio” for this particular decay. In essence, the size of this number tells them how often the decay occurs versus other possible decay paths for the B meson. The branching ratio for the neutral B-to-tau/anti-tau decay was determined to be — at the largest — 0.0041.
This research was published in the June 23, 2006, online edition of
Physical Review Letters.
On the Net:
http://www.slac.stanford.edu/BFROOT/
By Laura Mgrdichian, Copyright 2006 PhysOrg.com
Related stories:
A front-row seat at this summer's physics extravaganza
Nearly 20 years in the making, the largest particle accelerator in the world will start running in Switzerland this summer, offering scientists a glimpse of particles that have never been seen before.
Phantom parent molecule of important class of chemical compounds isolated for first time
A team of scientists from the University of Georgia and two European universities has, for the first time, synthesized and characterized the elusive parent molecule of an important class of chemical compounds.
Physicists: After 30 years of study, rare particle confirms prediction
High-energy physicists devoted to recreating the conditions at the beginning of the universe have for the first time observed a new way to produce those basic particles of atoms, protons and neutrons.
Using fireballs to uncover the mysteries of ball lightning
“People have been pondering ball lightning for a couple of centuries,” says James Brian Mitchell, a scientist the University of Rennes in France. Mitchell says that different theories of how it forms, and why it burns in air, have been considered, but until now there were no experimental indications of what might be happening as part of the ball lightning phenomenon.
A 'Golden Channel' for New Physics
A group of physicists has dubbed a particular particle decay, the decay of the Bs meson into a neutral kaon and neutral antikaon, as a “golden channel” for new physics, suggesting that probing and studying the decay could lead to brand-new insight into the physics laws that govern the tiniest bits of matter. The scientists discuss their ideas, and how this decay could be studied in the future, in the January 25, 2008, edition of
Physical Review Letters.
Latest Supercomputer Calculations Support the Six-Quark Theory
A new calculation, reported in the January 25, 2008 issue of
Physical Review Letters, confirms the six-quark theory of particle-anti-particle asymmetry. This is the first complete calculation of this phenomenon to employ a highly accurate description of the quarks that adds a fifth dimension beyond those of space and time.
Particle accelerator may reveal shape of alternate dimensions
When the world's most powerful particle accelerator starts up later this year, exotic new particles may offer a glimpse of the existence and shapes of extra dimensions.
New Underground Particle Detectors Proposed for Europe
Three new giant underground particle detectors have been proposed for construction in Europe that could help achieve some major milestones in physics, such as verifying the decay of a proton, which has been theorized but never observed. In turn, this could lead to a new understanding of how our universe evolved.
