CERN Physicists Observes New Decay Mode of Charmed Beauty Meson

Physicists from the LHCb Collaboration at CERN’s Large Hadron Collider (LHC) have reported the first observation of the decay of the Bc+ 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. This new 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 Bc+ is the heaviest meson that can only decay through the weak interactions, via the decay of one heavy constituent quark.

It 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 Bc+ 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 Bc+→J/ψπ+π0 decay will allow better understanding of its possible contribution as a background source for the study of other decays of Bc mesons as well as rare decays of B0 mesons,” the LHCb physicists said.

From the theoretical point of view, decays of Bc 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 Bc 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 Bc+ into J/ψπ+ has been calculated by various theorists over the last three decades.

Now these predictions can finally be compared with an experimental measurement: 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 detector allows the LHCb researchers to study the production, decays and other properties of the Bc+ meson in detail.

“Since the meson’s discovery by the CDF experiment at the Tevatron collider, 18 new Bc+ decays have been observed (with more than five standard deviations), all of them by LHCb,” they said.

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LHCb Collaboration. 2024. Observation of the B+c→J/ψπ+π0 decay. arXiv: 2402.05523