Muoproduction of exotic charmonia at COMPASS

TL;DR

This paper reports the first observation of the exotic charmonium-like state X(3872) produced via muoproduction at CERN, providing new insights into its properties and contributing to the understanding of exotic hadrons.

Contribution

It presents the first evidence of X(3872) production in muon-induced reactions, with detailed cross section measurements and analysis of decay distributions, advancing the study of exotic charmonia.

Findings

Observation of X(3872) production with 5 sigma significance

Disagreement in the $\pi^+\pi^-$ mass distribution shape compared to previous data

Estimated cross section times branching ratio as 71±28(stat)±39(syst) pb

Abstract

Exotic charmonium-like states have been targeted by various experiments in the last 15 years, but their nature still is unknown. Photo-(muo)production is a new promising instrument to study them. COMPASS, a fixed target experiment at CERN, analyzed the full set of the data collected with a muon beam between 2002 and 2011, covering the range from 7 GeV to 19 GeV in the centre-of-mass energy of the (virtual)photon-nucleon system. Production of the X(3872) state in the reaction $\mu^+~N \rightarrow \mu^+(J/\psi\pi^+\pi^- )\pi^{\pm} N'$ has been observed with a statistical significance of around 5 $\sigma$. The shape of the $\pi^+\pi^-$ mass distribution from the decay $X(3872)\rightarrow J/\psi\pi^+\pi^-$ shows disagreement with previous observations. The product of the cross section and the branching fraction of the $X(3872)$ decay into $J/\psi\pi\pi$ is estimated as 71$\pm$28(stat)$\pm$39(syst) pb. The results obtained for the production of the $Z_c^{\pm}(3900)$ are also reported as well as future perspectives.