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

Abstract

The observation of a wide peaking structure in the $B_{c}^{+} γ$ 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.

Figures (1)

• Figure 1: Distribution of the mass difference $M(B_{c}^{+}\gamma)-M(B_{c}^{+})$, where the $B_{c}^{+}$ candidate mass lies within the range $[6240, 6300]~\mathrm{Me V\!/}c^2$. The inset displays the mass distribution of the $B_{c}^{+}$ candidates reconstructed from the $B_{c}^{+} \rightarrow\xspace J/\psi \pi^+$ decay. The fit results are overlaid.