The odd life of Bs0 mesons

The red line shows the fit to the Bs0 –> J/ψ f0 decay mode. The blue dashed line shows the fit to the background.

The Bs0 meson is a particle with the peculiar property that it can transform itself into its antiparticle. This process is described by the Standard Model of particle physics, and it was first observed by the CDF collaboration back in 2006.

Now, with much larger data samples than originally used, physicists at the Tevatron are searching for deviations from the predicted behavior of the Bs0 meson. These deviations could indicate the existence of new physics processes and might help explain the matter and antimatter asymmetry in our universe.

One of the most promising places to look for deviations from the Standard Model is the behavior of Bs0 mesons under the so-called CP transformation. This CP transformation exchanges particles and antiparticles, while flipping the spatial coordinates. Particles that are unaffected by the CP transformation are labeled “CP even” while particles that affected are labeled “CP odd.”

The Bs0 mesons observed in experiments are usually a mixture of both CP even and CP odd components. The mixture depends on how the Bs0 meson decays. Since the CP even and CP odd components each have a different lifetime, the mixture also determines the measured lifetime of the
Bs0 meson’s decay. To date, the Bs0 lifetime was only measured in decays that were a mix of CP even and odd components. Sophisticated analysis techniques are required to untangle the two lifetimes.

Recently, CDF scientists used a Bs0 decay that has only the CP odd component to directly measure its lifetime alone for the first time. The measured lifetime is 1.70 +0.12/-0.11 (statistical) ± 0.03 (systematic) trillionths of a second. Within the current experimental precision, the result is compatible with the expectation from the Standard Model. Extensions of this analysis, which investigate the behavior of Bs0 mesons in this decay mode under CP transformation, will increase the sensitivity to new physics processes.

These physicists were responsible for this analysis. From left: Thomas Kuhr, Matthias Huschle and Michael Feindt, all from Karlsruhe Institute of Technology in Germany; and Michal Kreps, University of Warwick in the United Kingdom.