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

TL;DR

This paper reports the first observation of the decay $ ext{Lambda}_b^0 ightarrow ext{psi}(2S) ext{Lambda}^0$ and measures its branching ratio relative to $ ext{Lambda}_b^0 ightarrow J/ ext{psi} ext{Lambda}^0$ using ATLAS data at 8 TeV.

Contribution

It provides the first measurement of the branching ratio of $ ext{Lambda}_b^0$ decays to $ ext{psi}(2S)$ and compares it with theoretical predictions.

Findings

Branching ratio measured as 0.501 ± 0.033 (stat) ± 0.019 (syst)

Observation of the $ ext{Lambda}_b^0 ightarrow ext{psi}(2S) ext{Lambda}^0$ decay

Result is lower than covariant quark model expectations

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.