Exclusive photoproduction of excited ρ mesons decaying to four pions in ultraperipheral Pb–Pb collisions at sNN=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 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, 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 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