Narrow Results

It is advocated that geometry of the interaction region of two heavy nuclei colliding at large impact parameters is important for the relative role of light-by-light scattering and QCD-initiated processes. Exclusive production of resonances is possible by dense electromagnetic fields in the interior space between the nuclei. The cross-section of the two-photon processes is evaluated and some examples are considered. It is speculated that the exclusive production of ${\rho }^0$-mesons by “two-photon” processes forbidden by the Landau–Yang rule may be allowed within strong magnetic fields due to odd number of photons becoming involved.

Threshold behavior of the cross-sections of ultraperipheral nuclear interactions is studied. Production of $e^{+}{e}^{-}$ and ${\mu }^{+}{\mu }^{-}$ pairs as well as ${\pi }^0$ and parapositronium is treated. The values of corresponding energy thresholds are presented and the total cross-sections of these processes at the newly constructed NICA and FAIR facilities are estimated.

Perturbative and nonperturbative terms of the cross-sections of ultraperipheral production of lepton pairs in ion collisions are taken into account. It is shown that production of low-mass $e^{+}{e}^{-}$ pairs is strongly enhanced (compared to perturbative estimates) due to the nonperturbative Sommerfeld–Gamow–Sakharov (SGS) factor. Coulomb attraction of the nonrelativistic components of those pairs leads to the finite value of their mass distribution at lowest masses. Their annihilation can result in an increased intensity of 511 keV photons. It can be recorded at the NICA collider and is especially crucial in astrophysical implications regarding the 511 keV line emitted from the Galactic center. The analogous effect can be observed in lepton pairs production at LHC. Energy spectra of lepton pairs created in ultraperipheral nuclear collisions and their transverse momenta are calculated.