π+π and π0π0pair production in photon-photon scattering and ultraperipheral ultrarelativistic heavy-ion collisions.

We discuss ɣɣ → ππ reactions, starting from the two-pion threshold up to about √Sɣɣ = 6 GeV. Several reaction mechanisms are identified. We include the dipion continuum owing to pion exchange (for π+ π-) and ρ± exchange (for π0 π0), as well as the pronounced dipion σ(600), ƒ0(980), and ƒ2(1270) s-channel resonances. We discuss also a possible contribution of less pronounced scalar resonances ƒ0(1500) and ƒ0(1710), being glueball candidates, as well as the tensor resonances ƒ'2(1525), ƒ2(1565), and ƒ2(1950). While the contribution of ƒ0'(1525) is rather small, the contribution of ƒ0(1565) changes the spectrum around the resonance position. We find that the inclusion of the spin-four ƒ4(2050) resonant state improves the situation at √Sɣɣ ≈ 2 GeV. We estimate the relevant diphoton partial decay width to be ɣƒ4 → ɣɣ ≈ 0.7 keV. At higher energies, Brodsky-Lepage and handbag mechanisms are included in addition. We nicely describe the world data for ɣ ɣ → π π for the first time both for the total cross section and for angular distributions for ɣ ɣ → π+ π- and π0 π0 reactions simultaneously at all experimentally available energies. 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 π0 π0 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, and pion pseudorapidity are shown. The ɣ ɣ → π+ π- subprocess constitutes a background to the exclusive AA → Aρ0(→ π+ π-)A process, initiated by photon-pomeron or pomeron-photon subprocesses. We find that only a part of the dipion invariant mass spectrum associated with ɣ ɣ collisions can be potentially visible as the cross section for the AA → Aρ0A reaction is very large. [ABSTRACT FROM AUTHOR]