B Physics in the Snowy Alps
Among the pyramidal Alps in the sleepy town of La Thuile, Italy, members of the BaBar collaboration are currently presenting spectacular results at the premier winter conference, Rencontres de Moriond. Last week's presentations focused on electroweak interactions and unified theories, and this week discusses quantum chromodynamics and hadronic interactions.
The collaboration announced evidence for charm mixing, where a D meson changes into its own antiparticle. This kind of matter to antimatter transformation has been found in other kinds of mesons before, but it took the power of the PEP-II accelerator's luminosity and the BaBar detector's sensitivity to find this rare "mixing" in D mesons. The analysis also depended on an innovative use of a parallel computing system at SLAC that sped up crucial calculations.
"It will be one of the most important results in particle physics this year," predicted BaBar Physics Analysis Coordinator Jim Olsen. "But that's not the only result. We have many new measurements coming out, including some of BaBar's flagship analyses."
In all, BaBar is presenting more than thirty new results at winter conferences in La Thuile, the Canadian Rockies at Lake Louise, and Aspen, Colorado. "Producing physics measurements in a timely way depends on the acquisition and processing of our immense dataset by our international team of physicists and computing professionals," said BaBar Computing Coordinator and SLAC physicist Gregory Dubois-Felsmann. "It's great to see that the team's dedicated work yields exciting results like these."
Researchers are presenting a very precise measurement of a quantity called sin2β, where β (beta) is an angle in a triangle whose area describes the amount of difference between matter and antimatter. The small asymmetry in the behavior of matter compared to antimatter is called CP violation. In 2001, BaBar's measurement of sin2β was the first observation of CP violation in the B-meson system. With the improved measurement, this quantity is now known to a precision of a few percent.
BaBar physicists have also gone after other ways to measure sin2β, and are comparing them against the "golden" mode of measurement. So far, these more difficult modes agree, leading the experimenters on to still more challenging analyses in the search for hints of new kinds of physics. One example of these modes is the decay of a B0 meson or an anti-B0 meson to D+D-. Belle found an asymmetry larger than theory predicts, while BaBar measured a small asymmetry consistent with Standard Model predictions. Both experiments are collecting more data to resolve this difference.
Another key result is the first observation of electron-positron annihilations that decay into two oppositely charged rho mesons. Physicists still don't know the intermediate steps in the process, which might involve virtual photons or neutral rho mesons.
—Heather Rock Woods, March 23, 2007
Above image: La Thuile, Italy