The LHCb collaboration recently reported the first observation of the decay of the B_c⁺ meson (composed of two heavy quarks, b and c) into a J/ψ charm-anticharm quark bound state and a pair of pions, π⁺π⁰. The decay process shows a contribution from an intermediate particle, a ρ⁺ meson that forms for a brief moment and then decays into the π⁺π⁰ pair.

The B_c⁺ is the heaviest meson that can only decay through the weak interactions, via the decay of one heavy constituent quark. B_c⁺ decays into an odd number of light hadrons and a J/ψ (or other charm-anticharm quark bound states, called “charmonia”) have been studied intensively and have been found to be in remarkable agreement with the theoretical expectations. The decay of B_c⁺ into a J/ψ and a π⁺π⁰ pair is the simplest decay into charmonium and an even number of light hadrons. It has never been observed before, mainly because the precise reconstruction of the low-energy π⁰ meson through its decay into a pair of photons is very challenging in an LHC proton-proton collision environment.

A precise measurement of the B_c⁺→J/ψπ⁺π⁰ decay will allow better understanding of its possible contribution as a background source for the study of other decays of B_c mesons as well as rare decays of B⁰ mesons. From the theoretical point of view, decays of B_c into J/ψ and an even number of pions are closely related to the decays of the τ lepton into an even number of pions, and to the e⁺e^– annihilation into an even number of pions. Precise measurements of e⁺e^– annihilation into two pions in the ρ mass region (as in the B_c decay discussed here) are crucial for the interpretation of results from the Fermilab g-2 experiment measuring the anomalous magnetic dipole moment of the muon, since low-energy e⁺e^– annihilation into hadrons is an important source of the uncertainty of the g-2 measurements.

The ratio of the probability of the new decay to that of the decay of B_c⁺ into J/ψπ⁺ has been calculated by various theorists over the last 30 years. Now these predictions can finally be compared with an experimental measurement: most predictions agree with the new result obtained by LHCb (2.80±0.15±0.11±0.16).

The large number of b-quarks produced in LHC collisions and the excellent detector allows LHCb to study the production, decays and other properties of the B_c⁺ meson in detail. Since the meson’s discovery by the CDF experiment at the Tevatron collider, 18 new B_c⁺ decays have been observed (with more than five standard deviations), all of them by LHCb.

Read more in the LHCb paper.