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LHCb Observes B_c Decays to Psi(2S) pi

March 12, 2013

Admittedly no neutrinos are involved in this particular physics process, but since the conference hasn’t started yet and I have some free time, I thought I would mention anyway, due to its interest, the paper appeared yesterday on the Arxiv, where LHCb reports of observing (with a significance exceeding five standard deviations, as required) about 20 decays (20+-5, to be exact) of the B_c meson into a Psi(2S) and a pion.

The Psi(2S) is reconstructed of course in its decay to muon pairs, which provide for triggering data collection. A clean reference signal of B_c decays to J/Psi pi also provides a very straightforward calibration and reference, such that LHCb can measure the branching fraction of the decay as a ratio. The result says that the B_c decays exactly four times less frequently to Psi(2S) pi than it does to J/Psi pi: the ratio is 0.250 +- 0.068 +- 0.014 +- 0.006, where the first uncertainty is statistical, the second is systematic, and the third is related to the knowledge of the ratio of branching fractions to muon pairs of the two Psi states.

An obligatory figure follows. In it you can see the two twin signals in the mass distribution of the B_c candidates. On the left the decay to J/Psi pi, on the right the decay to Psi(2S) pi. I should mention that LHCb probably uses the rooFit standard of drawing error bars for Poisson counts, which is fine (the bars encompass the 68% coverage) except that for zero-event bins these should be extended from 0 to 1.8 events, while these stop at 1.25, with a smaller coverage than the others. A detail, admittedly… But if anybody in LHCb is listening: fix that – in CMS we do it right, why shouldn’t you ?

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