More than expected

This plot shows the invariant-mass distribution of the Bc+ → J/ψμ+ candidate events using the full CDF data sample with a Monte Carlo-simulated signal sample. The calculated backgrounds are superimposed.

This plot shows the invariant-mass distribution of the Bc+ → J/ψμ+ candidate events using the full CDF data sample with a Monte Carlo-simulated signal sample. The calculated backgrounds are superimposed.

In 1998 CDF was the first to observe the Bc+ meson, which consists of two quarks: an antibottom quark and a charmed quark. The discovery consisted of a measurement involving approximately 20 decays in which the decay products were a J/ψ, a charged lepton (muon or electron) and an unobserved neutrino.

Using the full Tevatron Run II data set, we now observe approximately 740 events in the muon decay mode. CDF looked for a signature of three muons, the mass of two oppositely charged muons being consistent with that of the J/ψ particle. This larger data set allows us to make the first measurement of the production cross section of the Bc+ meson.

One of the principal challenges in the analysis was the determination of the backgrounds, which are shown in the above figure. In the largest background, the J/ψ is correctly identified, but the third muon is misidentified as a pion, kaon or proton. Of the 1,370 Bc+ candidates, 630 are identified as being background.

In order to minimize the error, we compared our measurement to that of a decay that is already well measured (B+ → J/ψ + K+). The cross section for B+ is 2.78 ± 0.24 microbarns for conditions very similar to our measurement of the Bc+. Using well-known properties of the B+ decay, we find the final cross section for Bc+ production to be 29 ± 4 nanobarns.

Turgun Nigmanov (left) and Paul Shepard, both from the University of Pittsburgh, are the primary analysts for this result.

Turgun Nigmanov (left) and Paul Shepard, both from the University of Pittsburgh, are the primary analysts for this result.

Our result is higher than the theory expectation (by two standard deviations), but the theory calculation was done 10 years ago (kT factorization). Measurements at the LHC collider, where the cross sections should be many times larger, could resolve this problem in our understanding of a meson that is both beautiful and charmed.

CDF scientists performed a job well done in determining the background, a difficult, interesting challenge.

This is my last Frontier Science Result for CDF. I’d like to thank my CDF colleagues for writing so many interesting and important physics papers that were the subject of this column. Finally, Leah Hesla deserves special praise for her wonderful job of editing.

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