Exclusive four pion photoproduction in ultraperipheral Pb-Pb collisions at $\sqrt{s_{\mathrm{NN}}}=5.02$ 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 $\ddot{I}{€}^{+}\ddot{I}{€}^{-}\ddot{I}{€}^{+}\ddot{I}{€}^{-}$ final states in ultraperipheral Pb$-$Pb collisions at $\sqrt{s_{\mathrm{NN}}}=5.02$ TeV is presented for the first time. The cross section, d$\ddot{I}ƒ$/d$y$, times the branching ratio ($\ddot{I}\to \ddot{I}{€}^{+}\ddot{I}{€}^{+}\ddot{I}{€}^{-}\ddot{I}{€}^{-}$) is found to be $47.8\pm 2.3(stat.)\pm 7.7(syst.)$ 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, $\ddot{I}(1450)$ and $\ddot{I}(1700)$, also provides a good description of the data. The values of the masses ($m$) and widths ($\hat{I}“$) of the resonances extracted from the fit assuming two interfering resonances are $m_1=1385\pm 14(stat.)\pm 3(syst.)$ MeV/$c^2$, $\hat{I}{“}_1=431\pm 36(stat.)\pm 82(syst.)$ MeV/$c^2$, $m_2=1663\pm 13(stat.)\pm 22(syst.)$ MeV/$c^2$ and $\hat{I}{“}_2=357\pm 31(stat.)\pm 49(syst.)$ MeV/$c^2$, respectively. The measured cross sections times the branching ratios are compared to recent theoretical predictions.

24 pages, 3 captioned figures, 3 tables, authors from page 18, figures at http://alice-publications.web.cern.ch/node/10862