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Peripheral, ultrarelativistic production of particles in heavy ion collisions

The cross sections for the production of two-pions in ultraperipheral ultrarelativistic heavy ion collisions, calculated in the impact parameter Equivalent Photon Approximation (EPA), are presented. Differential distributions in impact parameter, dipion invariant mass, single pion and dipion rapidity, pion transverse momentum and pion pseudorapidity are shown. The $γγ\to π^+ π^-$ subprocess constitutes a background to the $A A \to A ρ^0 (\to π^+ π^-) A$ process, initiated by emission of a photon by one of colliding nuclei. Only a part of the dipion invariant mass distribution associated with the $γγ$-collisions can be visible as the cross section for the $A A \to A ρ^0 A$ reaction is very large. Differential distributions for two $ρ^0$ meson production in exclusive ultraperipheral, ultrarelativistic collisions via a double scattering mechanism are presented. The cross section for $γA \to ρ^0 A$ is parametrized based on a calculation from the literature. Smearing of $ρ^0$ masses is taken into account. The results of calculations are compared to experimental data obtained at RHIC and to the contribution of the two-photon mechanism. The double scattering mechanism populates larger $ρ^0 ρ^0$ invariant masses and larger rapidity distances between the two $ρ^0$ mesons compared to the $γγ$ fusion. It gives a significant contribution to the $A A \to A A π^+ π^- π^+ π^-$ reaction. Some observables related to charged pions are presented. The results of our calculation are compared with the STAR collaboration results for 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. \end{abstract}