Two distinct heavy quarks living together

The proper-decay-length (top) and the invariant-mass (bottom) distributions of the Bc candidates, along with the fit projections.

The Bc meson is the only meson that consists of two distinct heavy quarks, the bottom quark and the charm quark. It was first discovered by CDF in 1998 from about 20 Bc decay events with a neutrino in the final state. The doubly heavy composition of the Bc meson means that its lifetime is shorter than that of the other B mesons, since both the bottom quark and the charm quark can contribute to the decay of the Bc.

Measuring the lifetime of the Bc improves our understanding of quark properties such as the quark masses and also contributes to our understanding of how quarks bind to each other.

Previously, all measurements of the lifetime of the Bc were made in decays with a neutrino in the final state. The neutrino escapes undetected and corrections are needed to model the momentum of the Bc meson in order to measure its lifetime. Now CDF researchers have measured the Bc lifetime for the first time in an all-hadronic decay mode, BcJ/ψ π. We measure all the decay particles, including the negative pion and two oppositely charged muons, which arise from the decay of the J/ψ particle.

The lifetime of the Bc meson is determined by a simultaneous fit to the lifetime and the mass of the Bc candidate events.

Using data from about 270 Bc decay events collected at the CDF II detector, CDF researchers measure the Bc lifetime to be 0.452 ± 0.048 (stat) ± 0.027 (syst) picoseconds, nearly four times shorter than that of the Bu meson. This measurement has a comparable precision with the world’s best measurement and confirms the earlier experimental work and the theoretical predictions of the Bc lifetime. This work is also the topic of the Ph.D. dissertation of Hao Song at the University of Pittsburgh.

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edited by Andy Beretvas

These CDF physicists contributed to this data analysis. Top row from left: Paul Shepard and Hao Song, both from the University of Pittsburgh. Bottom row from left: Fermilab physicists William Wester, Pat Lukens, Jeff Appel and Ron Moore.