Measurement of the branching ratio \(\Gamma(\Lambda_b^0 \rightarrow \psi(2S)\Lambda^0)/\Gamma(\Lambda_b^0 \rightarrow J/\psi\Lambda^0)\) with the ATLAS detector

Abstract

An observation of the \(\Lambda_b^0 \rightarrow \psi(2S) \Lambda^0\) decay and a comparison of its branching fraction with that of the \(\Lambda_b^0 \rightarrow J/\psi \Lambda^0\) decay has been made with the ATLAS detector in proton--proton collisions at \(\sqrt{s}=8\,\)TeV at the LHC using an integrated luminosity of \(20.6\,\)fb\(^{-1}\). The \(J/\psi\) and \(\psi(2S)\) mesons are reconstructed in their decays to a muon pair, while the \(\Lambda^0\rightarrow p\pi^-\) decay is exploited for the \(\Lambda^0\) baryon reconstruction. The \(\Lambda_b^0\) baryons are reconstructed with transverse momentum \(p_{\rm T}>10\,\)GeV and pseudorapidity \(|\eta|<2.1\). The measured branching ratio of the \(\Lambda_b^0 \rightarrow \psi(2S) \Lambda^0\) and \(\Lambda_b^0 \rightarrow J/\psi \Lambda^0\) decays is \(\Gamma(\Lambda_b^0 \rightarrow \psi(2S)\Lambda^0)/\Gamma(\Lambda_b^0 \rightarrow J/\psi\Lambda^0) = 0.501\pm 0.033 ({\rm stat})\pm 0.019({\rm syst})\), lower than the expectation from the covariant quark model.