Coherent photoproduction of ρ⁰, ω and excited vector mesons in ultraperipheral PbPb collisions

The invariant-mass distribution for the coherent photoproduction of dipions in ultraperipheral PbPb collisions is measured using data, corresponding to an integrated luminosity of 224.6 ± 9.6μb⁻¹, collected by the LHCb experiment in 2018 at a nucleon-nucleon centre-of-mass energy sNN=5.02 TeV. In the mass range from 400 to 1200 MeV, the results are consistent with previous experiments, with the spectrum dominated by the ρ⁰ meson, which interferes with a nonresonant component, together with a smaller ω meson contribution. In an extended mass range up to 2300 MeV, models previously used do not fit the data and a consistent description requires the introduction of two resonances at masses of 1350 ± 20 MeV and 1790 ± 20 MeV with widths of about 300 MeV. The cross-section for each meson is measured differentially in twelve bins of rapidity from 2.05 to 4.90. The ρ⁰ cross-section increases with rapidity from about 400 to 600 mb and is measured with a typical precision of 8%, while the cross-section times branching fraction for the ω, ρ′ and ρ′′, with the statistical precision of the data, do not have a pronounced rapidity dependence and are between 0.5 and 1.5mb, with uncertainties up to 30%. A large nuclear suppression is observed for the ρ⁰ meson compared to expectations based on photoproduction on the proton that use the impulse approximation. Significant suppression is also observed compared to that predicted by elastic scattering described in the Glauber approach, or with the addition of inelastic scattering in a Gribov-Glauber model.