Busy Times for BaBar
It's been another busy and successful year for BaBar. The B-factory presented an impressive 48 new results at the Lepton-Photon conference in Daegu, South Korea, including some using the most recent Run-6 data. The new results spanned the BaBar physics spectrum and included new measurements of Charge Parity (CP) violation in B-decays, decay rates of τ-leptons, and the search for new charm and charmonium states.
Undoubtedly the most exciting development over the past year has been the observation of mixing in the neutral D-meson (symbolized by D0), first announced by BaBar in March and confirmed by Belle soon after (see a recent SLAC Today article by JoAnne Hewett for an interesting discussion on the implications of this discovery). The discovery of D0 mixing, thought unobtainable when the B-Factories began, was first achieved in the decays of D-mesons to K- and π-mesons. Since the spring, these first measurements have been confirmed by measurements of mixing in other channels including the three-body decay modes and in the lifetime measurements of decays into two K-mesons and two π-mesons, all of which were presented in Daegu. The mixing results have generated significant theoretical interest as well with a recent search on SLAC-SPIRES yielding a large number of citations to the original BaBar paper.
BaBar has also continued work on measuring the Unitarity Triangle of the Cabibbo-Kobayashi-Maskawa (CKM) matrix, an important test of the Standard Model. At the Lepton-Photon conference, new measurements on all of the sides and angles were presented. In particular, for the first time BaBar has measured the CP-asymmetries in B0 decays to two neutral rho mesons (denoted ρ0), a key ingredient in extracting the Unitarity Triangle angle α. The angle α is one of the three critical quantities that allow the B-factory experiments to probe the complex phase of the CKM matrix. The B0 meson decays to ρ0ρ0 only one time in a million, making for an extremely challenging analysis. This decay was first observed by BaBar in the summer of 2006. Utilizing all of the available data, including part of the Run-6 dataset, BaBar was able to follow up this observation with a time-dependent CP analysis. In order to extract α, measurements from all possible charge configurations of B-meson decays to ρρ are combined. Without the information from the ρ0ρ0 measurement, the precision on α was limited to about 20 degrees. With the additional information provided by the ρ0ρ0 decays, the precision on α will continue to increase as the BaBar dataset increases.
All in all, BaBar continues to increase the scope and precision of its probes of the Standard Model and is uncovering surprises at the luminosity frontier. Even as BaBar's 300th publication has been submitted, the physics world still looks toward the B-factories for exciting discoveries to come. We at BaBar are looking forward to collecting and analyzing the impressive data delivered by PEP-II, with the expectation that new surprises are lurking somewhere.
Mathew Graham, SLAC Today, August 23, 2007