A precise measurement of the Bc meson mass July 27, 2007Posted by dorigo in news, physics, science.
The Bc meson is a particle composed of a b-quark and a anti-charm quark (charge conjugation applied to the state brings about the antiparticle, made by a anti-b-quark and a charm quark). It is just another meson – a bound state of two quarks – and it would not be so interesting to study, if the mass of the constituents weren’t so large. Indeed, the Bc meson is the first particle containing both b- and c- quarks, and so it does have some entertainment value for particle physicists.
In fact, there are several questions that may receive an answer by studying this particle. Is the decay rate consistent with what one would expect from the current model of these objects ? Is the mass compatible with what one would compute by modeling the state as a almost non-relativistic system composed of a charm quark jiggling around the heavier b-quark ? How about lattice QCD computations for the mass of such a unconventional meson ? Moreover, is the pattern of possible decays following what we predict ? Also, is the overall production rate in agreement with what QCD predicts (this latter question is probably the hardest to answer presently…) ?
The Bc was discovered by CDF in 1998, but its decay to a J/psi meson and a single pion had not been seen before. CDF has now performed a very simple search for this final state, by tuning the search criteria on the twin particle B-, which is much more frequently produced in proton-antiproton collisions. The B- is composed of a b-quark and a anti-u quark, and it may decay to a J/psi meson and a Kaon. The kinematics of that decay should be quite similar to the one searched for the Bc, so CDF could decide what cuts to apply to particle momenta and other observables beforehand, basing the selection on the well known B- signal, and then collect data silently, until the “signal box” was full enough to beg to be opened. By doing things this way, the search has been quite model-independent, and the results much more easy to interpret.
We now have a 8-sigma signal of Bc –> J/psi pi decays. You can see the reconstructed invariant mass of the system in the following plot:
As you see, the large dataset of 2.2 inverse femtobarns of proton-antiproton collisions allows for a significant signal to emerge on the exponentially falling background, even if this particle is not very frequently produced -indeed, there are large uncertainties in the theoretical predictions for its production rate – and even if the decay to J/psi and pion is not the most frequent one (I need to check this though… I will update later with some number). The mass of the meson is measured to be 6274.1 +- 3.2 +- 2.6 MeV, which is smaller but not inconsistent with current estimates from lattice QCD (6304 MeV), which carries some 18-mevish systematics. The ball is now in the lattice QCD field to get a better match with experimental measurements.. In the meantime, CDF will collect more of these interesting events, and improve our understanding of this intriguing particle.
More detail on the recent measurement described above can be found this preprint paper.