Science Today: New Measurement of the Unitarity Triangle
Using information obtained from detailed studies of B meson decays to K*0rho+ and rho+rho- can teach us about the tiny difference between matter and anti-matter. Our understanding of this phenomenon is encoded in three angles of a special triangle, called the unitarity triangle. These angles are called beta, alpha, and gamma. BaBar physicists have been able to constrain the second angle of the unitarity triangle to within seven degrees. This result is the culmination of over five years of work by BaBar in the pursuit of the angle alpha.
SLAC's B-factory was built to understand nature's preference of matter over anti-matter. The first landmark discovery in 2001 from the SLAC B-factory was the measurement of beta, which established that an asymmetry exists in the decay of B mesons versus the decay anti-B mesons. Particle and anti-particle decay at different rates. The angle beta is now measured to one degree by the BaBar and Belle experiments. Three years ago, BaBar released a result that laid down a proof of principle for measuring the second angle alpha using a previously neglected method, the decay of a B meson to a rho+rho- final state.
The elusive signal occurs one in forty thousand times. As every interesting signal event is swamped under a further 50 background events, this is a very challenging measurement to embark upon. BaBar has recently released a new measurement of this decay. Starting from a sample of almost four hundred million B meson pairs BaBar physicists have isolated 730 signal events that tell us something about the matter anti-matter difference. Performing this analysis is like searching for a needle in a haystack, and then trying to thread the needle once it has been found. If that is not challenging enough, there is a further complication to measuring alpha. This complication comes from troublesome loop contributions. These are additional ways to go from the initial B meson to the rho+rho- final state that quantum mechanically interfere with the interesting part of our signal.
The loop contributions are very rare, in this case occurring about nine times in every million B decays. That makes them hard for experimentalists to see. It is also hard for theorists to accurately calculate what we should expect to happen. The contamination to our precise measurement of alpha from these loop contributions is important, and it has to be well understood. Prompted by recent measurements of these rare loop processes, theorists have outlined a new method to constrain the loop contributions to rho+rho-. The crucial point realized was to be able to relate the pure loop process of a B decaying into K*0 rho+, to the loop contribution of the decay of a B meson into rho+rho-. This gives a clean constraint on the pesky loop contribution to our signal. Doing this has enabled BaBar physicists to measure the second angle of the unitarity triangle with an uncertainty of seven degrees. Physicists now have enough information to define the unitarity triangle using this new measurement of alpha, and the previous measurements of beta.
The best is yet to come as BaBar continues to profit from the efforts of the many talented people working on the PEP-II accelerator. The B-factory will continue to accumulate data until the fall of 2008. In doing so the BaBar Collaboration will have doubled the available data to use in studying this bizarre phenomenon of matter anti-matter asymmetry.
—Adrian Bevan, SLAC Today, June 21, 2007
Above image: The unitarity triangle. (Image courtesy of CKMfitter collaboration.)