π+ π- and π⁰ π⁰ pair production in photon-photon and in ultraperipheral ultrarelativistic heavy ion collisions

Original · EN

We discuss γγ→ ππ reactions, starting from the two-pion threshold up to about √sᵧᵧ = 6 GeV. We include the dipion continuum due to pion exchange (for π+π-) and ρ± exchange (for π⁰π⁰) as well as the dipion σ(600), f₀(980) and f₂(1270) s-channel resonances. We discuss also a possible contribution of less pronounced scalar resonances f₀(1500) and f₀(1710) as well as the tensor resonances f'₂(1525), f₂(1565) and f₂(1950). We find that the inclusion of the spin-four f₄(2050) resonant state improves the situation at √sᵧᵧ ≈ 2 GeV. We estimate the relevant diphoton partial decay width to be Γf₄ → ᵧᵧ ≈ 0.7 keV. At higher energies, Brodsky-Lepage and hand-bag mechanisms are included in addition. We nicely describe the world data for γγ→ ππ for the first time both for the total cross section and angular distributions. The cross section for the production of two-pions in ultraperipheral ultrarelativistic heavy ion collisions is calculated in the impact parameter space Equivalent Photon Approximation (EPA). We obtain the nuclear cross section of 46.7 mb for π+π- and 8.7 mb for π⁰π⁰ at the LHC energy √sNN = 3.5 TeV and minimal cut on pion transverse momentum pₜ> 0.2 GeV. Differential distributions in impact parameter, dipion invariant mass, single pion and dipion rapidity, pion transverse momentum as well as pion pseudorapidity are shown. The γγ→ π+ π- subprocess constitutes a background to A A → A ρ⁰ (→ π+ π-) A process.