Abstract
A study of \( {B}_c^{+}\to {\chi}_c{\pi}^{+} \) decays is reported using proton-proton collision data, collected with the LHCb detector at centre-of-mass energies of 7, 8, and 13 TeV, corresponding to an integrated luminosity of 9 fb−1. The decay \( {B}_c^{+}\to {\chi}_{c2}{\pi}^{+} \) is observed for the first time, with a significance exceeding seven standard deviations. The relative branching fraction with respect to the \( {B}_c^{+}\to J/\psi {\pi}^{+} \) decay is measured to be$$ \frac{{\mathcal{B}}_{B_c^{+}\to {\chi}_{c2}{\pi}^{+}}}{{\mathcal{B}}_{B_c^{+}\to J/\psi {\pi}^{+}}}=0.37\pm 0.06\pm 0.02\pm 0.01, $$
where the first uncertainty is statistical, the second is systematic, and the third is due to the knowledge of the χc2→ J/ψγ branching fraction. No significant \( {B}_c^{+}\to {\chi}_{c1}{\pi}^{+} \) signal is observed and an upper limit for the relative branching fraction for the \( {B}_c^{+}\to {\chi}_{c1}{\pi}^{+} \) and \( {B}_c^{+}\to {\chi}_{c2}{\pi}^{+} \) decays of
$$ \frac{{\mathcal{B}}_{B_c^{+}\to {\chi}_{c1}{\pi}^{+}}}{{\mathcal{B}}_{B_c^{+}\to {\chi}_{c2}{\pi}^{+}}}=<0.49 $$
is set at the 90% confidence level.
