Observation of orbitally excited $B_{c}^{+}$ states

TL;DR

This paper reports the first observation of orbitally excited $B_c^+$ meson states through a wide peaking structure in the $B_c^+ \, \gamma$ mass spectrum, confirming theoretical predictions and advancing understanding of heavy-quark hadron dynamics.

Contribution

It provides the first experimental evidence of orbitally excited $B_c^+$ states, matching theoretical predictions and enhancing knowledge of heavy-quark meson structures.

Findings

Significant peaking structure observed with over seven sigma significance.

Two narrow peaks identified at specific masses consistent with P-wave $B_c^+$ states.

Results align with theoretical models of excited heavy-quark mesons.

Abstract

The observation of a wide peaking structure in the $B_{c}^{+} \gamma$ mass spectrum is reported using proton-proton collision data collected by the LHCb detector at center-of-mass energies of $7$, $8$, and $13~\text{TeV}$, corresponding to a total integrated luminosity of $9~\text{fb}^{-1}$. The statistical significance over the background-only hypothesis exceeds seven standard deviations. The width of the observed structure is larger than the expectation from a single-peak hypothesis, and is well described by an effective minimal model consisting of two narrow peaks located at $6704.8 \pm 5.5 \pm 2.8 \pm 0.3~\mathrm{Me\kern -0.1em V\!/}c^2$ and $6752.4 \pm 9.5 \pm 3.1 \pm 0.3~\mathrm{Me\kern -0.1em V\!/}c^2$. The uncertainty terms are statistical, systematic, and associated to the knowledge of the $B_{c}^{+}$ mass, respectively. The measured peak locations are in line with theoretical predictions for lowest excited $P$-wave $B_{c}^{+}$ states, marking the first observation of orbitally excited beauty-charm mesons and providing important insights into the internal dynamics of hadrons containing two heavy quarks.