UC Berkeley

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 π + π − π + π − final states in ultraperipheral Pb–Pb collisions at s NN = 5.02 TeV is presented for the first time. The cross section, dσ/dy, times the branching ratio ( ρ → π + π + π − π − ) is found to be 47.8 ± 2.3 (stat.) ± 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, ρ(1450) and ρ(1700), also provides a good description of the data. The values of the masses (m) and widths (Γ) of the resonances extracted from the fit assuming two interfering resonances are m 1 = 1385 ± 14 ( stat . ) ± 3 ( syst . ) MeV/c 2, Γ 1 = 431 ± 36 ( stat . ) ± 82 ( syst . ) MeV/c 2, m 2 = 1663 ± 13 ( stat . ) ± 22 ( syst . ) MeV/c 2 and Γ 2 = 357 ± 31 ( stat . ) ± 49 ( syst . ) MeV/c 2, respectively. The measured cross sections times the branching ratios are compared to recent theoretical predictions.