Phys. Rev. Lett. 109, 191801 (2012)

The BaBar detector collected data at SLAC's PEP-II electron–positron collider from 1999 until 2008. Analysis of that data continues, and J. P. Lees et al. now present some precise measurements involving bottom (b) quarks that have implications more widely in particle physics.

BaBar picked up the decay products from the pair of B mesons (which contain b quarks) created in the specially tuned PEP-II collisions. By carefully selecting those interactions that produced a photon — when a b quark decays radiatively to a strange quark — Lees et al. have extracted the 'branching fraction' for B → Xsγ (where Xs may be anything that contains a strange quark). They've also measured the direct CP-asymmetry for decays involving down as well as strange quarks.

These quantities are determined by Feynman diagrams that contain loops — and those loops could involve new physics. BaBar's results, however, are consistent with standard-model predictions and hence constrain new physics, including certain models for families of Higgs bosons.