Exclusive photoproduction of excited ρ mesons decaying to four pions in ultraperipheral Pb–Pb collisions at sqrt(s_NN) = 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 π+π−π+π− final states in ultraperipheral Pb–Pb collisions at sNN=5.02 TeV is presented for the first time. The cross section, d σ /d y , 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 m1=1385±14(stat.)±3(syst.) MeV/ c 2, Γ1=431±36(stat.)±82(syst.) MeV/ c 2, m2=1663±13(stat.)±22(syst.) MeV/ c 2 and Γ2=357±31(stat.)±49(syst.) MeV/ c 2, respectively. The measured cross sections times the branching ratios are compared to recent theoretical predictions.