CP Violations in B Mesons: First Official Experimental Results

B mesons and anti-B mesons have the same intrinsic lifetimes, but subtle differences in the way they decay is indicative of a preference in the overall scheme of things for matter over anti-matter, at least in our sector of the universe. The explanation is believed to arise from certain basic symmetries, or lack thereof, in the way particle interactions occur. One such symmetry is called parity (P): a physical interaction ought to be the same whether viewed directly or in a (three-dimensional) mirror. A second important symmetry proposition, called charge conjugation (C), says that an interaction should be the same even if we replace all the participating particles with antiparticles.

Experiments in the 1950s and 60s showed that interactions via the weak nuclear force not only can violate the C and P propositions, but even the combined CP symmetry, and it is this fact, like a rarely expressed defective gene, which over cosmological time leads to the apparent large-scale extinction of antimatter. CP violation was first studied in the 60s with the asymmetrical decays of K mesons, which possess strange quarks (for the latest on CP violation in K's see Update 420).

Theorists believe that CP violation in B mesons (carrying a much heavier quark, the bottom quark) should be more prominent, although the Bs themselves are harder to manufacture than Ks. Two years ago Fermilab issued a rudimentary measurement of CP violation in B mesons based on rare events culled from proton-antiproton collisions (Update 405). At the B Factory at SLAC producing B and anti-B mesons is the main business. Now the scientists at the BaBar Detector at the B Factory have just released their first official results in a seminar at SLAC (contact Stewart Smith, 650-926-4775, ajsmith@slac.stanford.edu; web site), and they constitute the best evidence yet for CP violation in B mesons.

The chief parameter used to indicate CP violation is called sin (2 beta), and BaBar's measured value is 0.34, with an uncertainty of 0.20, an accuracy about twice as good as the previous value (Update 497). Results from the BELLE detector group at the KEK lab in Japan, the other premier lab dedicated to studying B mesons, are also just now available; a preprint cites a value for sin (2 beta) of 0.58, with an uncertainty of about 0.33 (http://www.lanl.gov/list/hep-ex/new). A value of zero would have implied that there were no CP violation. The uncertainties in these early measured values would therefore preclude a definite statement on the size of CP violation or on any likely agreement with theoretical estimates. Both BaBar and Belle have submitted their work to Physical Review Letters, which (pending approval) plans to publish them in the same issue.