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 ${\pi }^{+}{\pi }^{-}{\pi }^{+}{\pi }^{-}$ 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$\sigma$/d$y$, times the branching ratio ($\rho \to {\pi }^{+}{\pi }^{+}{\pi }^{-}{\pi }^{-}$) 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, $\rho (1450)$ 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 $m_1=1385\pm 14(stat.)\pm 3(syst.)$ MeV/$c^2$, ${\Gamma }_1=431\pm 36(stat.)\pm 82(syst.)$ MeV/$c^2$, $m_2=1663\pm 13(stat.)\pm 22(syst.)$ MeV/$c^2$ and ${\Gamma }_2=357\pm 31(stat.)\pm 49(syst.)$ 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

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