Double-scattering mechanism in exclusive $A A \to A A ρ^0 ρ^0$ reaction at ultrarelativistic collisions

Abstract: We calculate, for the first time, differential distributions for double $\rho^0$ meson production in exclusive ultraperipheral, ultrarelativistic collisions via a double scattering mechanism. The calculations are done in impact parameter space. The cross section for $\gamma A \to \rho^0 A$ is parametrized based on an existing calculation. Smearing of $\rho^0$ masses is taken into account. The results of our calculations are compared at the RHIC energy to the contribution of the two-photon mechanism discussed previously in the literature. The cross section for the double scattering mechanism is found to be an order of magnitude larger at $M_{\rho \rho} <$ 2 GeV and more than two orders of magnitude at $M_{\rho \rho} >$ 3 GeV, than that for the photon-photon mechanism. Compared to the two-photon mechanism the double scattering mechanism populates somewhat larger $\rho^0 \rho^0$ invariant masses and larger rapidity distances between the two $\rho^0$ mesons and gives a significant contribution to the $A A \to A A \pi^+ \pi^- \pi^+ \pi^-$ reaction. Some observables related to charged pions are presented too. We compare the results of our calculation with the STAR collaboration results on four charged pion production. While the shape in invariant mass of the four-pion system is very similar to the measured one, the predicted cross section constitutes only 20% of the measured one. We discuss a possibility of identifying the double scattering mechanism at the LHC.