The B_c meson is a particle made up by a heavy bottom quark and a charm quark. Such a combination used to be a relatively rare occurrence in a hadron collision when the center of mass energy of the collisions is in the same ballpark of its mass, but after its discovery at the Tevatron a few years ago in semi-exclusive decay modes, its fully reconstructed decay to a J/Psi meson and a pion has become available for detailed studies at the LHC. Other decay modes are less straightforward to evidence, but in a sense this makes things more fun! Note that due to the simultaneous presence of two heavy quarks in the meson, which both want to decay by weak interaction into lighter states, the phenomenology of B_c decay is quite varied and interesting to study.
So now 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. The 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 ?