Physicists from LHCb collaboration at CERN’s Large Hadron Collider (LHC) have made the first observation of the collapse of the Bc+ meson. This results in a J/ψ charm-anticharm quark bound state (consisting of two heavy quarks, b and c) and a pair of pions π+π0. This new decay process shows a contribution from an intermediate particle, the ρ+ meson, which forms for a short time and then decays into π+π0 pairs.

The Bc+ is the heaviest meson and decays only through weak interactions due to the decay of one heavy constituent quark.

It decays into an odd number of optical hadrons, and J/ψ (or another attractive and anti-attractive quark-bound state called Charmonia) has been intensively studied and found to be in remarkable agreement with theoretical predictions.

The decay of Bc+ to J/ψ and π+π0 pairs is the simplest decay to charmonium and even-numbered optical hadrons.

This has never been observed before. The main reason for this is that in the LHC proton-proton collision environment, it is very difficult to accurately reconstruct low-energy π0 mesons through their decay into a pair of photons.

“Accurate measurements of the Bc+→J/ψπ+π0 decay will allow us to better understand its possible contribution as a background source for the study of other decays of Bc mesons and rare decays of B0 mesons,” said the LHCb physicist.

From a theoretical point of view, J/ψ and the decay of Bc into an even number of pions are closely related to the decay of the τ lepton into an even number of pions and the e+e- annihilation into an even number of pions.

Accurate measurements of e+e- annihilation into two pions in the ρ mass region (like the Bc decay discussed here) are possible using the Fermilab G-2 experiment, which measures the anomalous magnetic dipole moment of the muon and is important for interpreting the results. The annihilation of low-energy e+e- into hadrons is an important source of uncertainty in g-2 measurements.

The ratio of the probability of a new decay to the probability of a decay from Bc+ to J/ψπ+ has been calculated by various theorists over the past 30 years.

Now these predictions can finally be compared with experimental measurements. Most predictions agree with the new result 2.80±0.15±0.11±0.16.

The large number of b quarks produced in LHC collisions and the excellent detectors allow LHCb researchers to study the formation, decay, and other properties of Bc+ mesons in detail.

“Since the discovery of the meson by the Tevatron Collider’s CDF experiment, 18 new Bc+ decays (with more than 5 standard deviations) have been observed, all from the LHCb,” the researchers said.

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LHCb collaboration. 2024. Observation of B+c→J/ψπ+π0 collapse. arXiv: 2402.05523