The BaBar experiment

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The primary goal of BaBar is to study the nature of the weak interaction using B mesons. The most exciting result from BaBar so far is the first observation that the Charge Conjugation - Parity (CP) symmetry - which relates matter to antimatter - is violated in the decay of B mesons.  In particular, BaBar has measured sin 2 beta to be 0.710 +/- 0.039, where beta is one of the three angles of the Unitarity Triangle, a representation of the CKM matrix that characterizes the weak interaction.   Vub, an element of the CKM matrix that determines the length of one of the sides of the Triangle is also being measured with increasing precision in both inclusive and exclusive B decays. Overall, BaBar's physics program includes a broad set of measurements to determine all sides and angles of the Unitarity Triangle with sufficient precision to overconstrain the underlying parameters of the Standard Model. It also includes a wide variety of searches for new physics, including rare or forbidden decays of the B and tau, and unexpected CP violation.This program has already produced the first intriguing hint of physics beyond the Standard Model. Measurements of sin 2 beta using decay modes involving higher order loops with heavy virtual particles - so called "penguin" decays - produce a value that differs by 2.6 standard deviations from the value measured in charmonium modes. Heavy, as yet unknown, particles would be expected to cause such a difference. These penguin modes are all rare, and establishing whether this observed difference is a statistical fluctuation or a sign of new physics will require a substantial increase in data.  In the summer and fall of 2006, the PEP-II e+e- collider that provides BaBar's beams will undergo upgrades to achieve the needed higher luminosity. BaBar is simultaneously upgrading its muon system to ensure that we can fully utilize the additional data. We expect to increase the size of the BaBar dataset by a factor of 2.5 from now until the completion of data taking in September 2008.