Exclusive four pion photoproduction in ultraperipheral Pb-Pb collisions at $\sqrt{s_{\rm NN}} = 5.02$ TeV

Abstract

The intense photon fluxes from relativistic nuclei provide an opportunity to study photonuclear interactions in ultraperipheral collisions. In particular, it allows for the investigations of excited, light-flavour vector mesons. The measurement of coherently photoproduced $ \pi^+ \pi^- \pi^+ \pi^-$ final states in ultraperipheral Pb$-$Pb collisions at $\sqrt{s_{\rm NN}} = 5.02$ TeV is presented for the first time. The cross section, d$\sigma$/d$y$, times the branching ratio ($\rho\rightarrow \pi^+ \pi^+ \pi^- \pi^-$) is found to be $47.8\pm2.3~\rm{(stat.)}\pm7.7~\rm{(syst.)}$ mb in the rapidity interval $|y| < 0.5$. The invariant mass distribution is not well described with a single Breit-Wigner resonance without an interference term. Including interference with a non-resonant contribution results in the mass and width values being too far from those reported in PDG, while the production of two interfering resonances, $\rho(1450)$ and $\rho(1700)$, also provides a good description of the data. The values of the masses ($m$) and widths ($\Gamma$) of the resonances extracted from the fit assuming two interfering resonances are $m_{1}=1385\pm14~\rm{(stat.)}\pm3~\rm{(syst.)}$ MeV/$c^2$, $\Gamma_{1}=431\pm36~\rm{(stat.)}\pm82~\rm{(syst.)}$ MeV/$c^2$, $m_{2}=1663\pm13~\rm{(stat.)}\pm22~\rm{(syst.)}$ MeV/$c^2$ and $\Gamma_{2}=357~\pm31~\rm{(stat.)}\pm49~\rm{(syst.)}$ MeV/$c^2$, respectively. The measured cross sections times the branching ratios are compared to recent theoretical predictions.