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

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 $\sqrt{s_{\mathrm{N}\mathrm{N}}}=5.02$ TeV is presented for the first time. The cross section, d$\sigma$/d$y$, times the branching ratio ($\rho \to {\pi }^{+}{\pi }^{+}{\pi }^{-}{\pi }^{-}$) is found to be $47.8\pm 2.3(\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{t}\mathrm{.})\pm 7.7(\mathrm{s}\mathrm{y}\mathrm{s}\mathrm{t}\mathrm{.})$ mb in the rapidity interval $\mathrm{\mid }y\mathrm{\mid }<0.5$. 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 ($\mathrm{\Gamma }$) of the resonances extracted from the fit assuming two interfering resonances are $m_1=1385\pm 14(\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{t}\mathrm{.})\pm 3(\mathrm{s}\mathrm{y}\mathrm{s}\mathrm{t}\mathrm{.})$ MeV/$c^2$, ${\mathrm{\Gamma }}_1=431\pm 36(\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{t}\mathrm{.})\pm 82(\mathrm{s}\mathrm{y}\mathrm{s}\mathrm{t}\mathrm{.})$ MeV/$c^2$, $m_2=1663\pm 13(\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{t}\mathrm{.})\pm 22(\mathrm{s}\mathrm{y}\mathrm{s}\mathrm{t}\mathrm{.})$ MeV/$c^2$ and ${\mathrm{\Gamma }}_2=357\pm 31(\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{t}\mathrm{.})\pm 49(\mathrm{s}\mathrm{y}\mathrm{s}\mathrm{t}\mathrm{.})$ MeV/$c^2$, respectively. The measured cross sections times the branching ratios are compared to recent theoretical predictions.