Nonperturbative character of electron-positron pair production in relativistic heavy-ion collisions

Coupled-channel calculations for relativistic heavy-ion collisions are performed in order to determine the electromagnetic electron-positron pair production yeild with subsequent capture of the electron into the K shell of the target. We consider heavy projectiles impinging on ${\mathrm{Pb}}^{82+}$ and ${\mathrm{U}}^{92+}$ with small impact parameters at a bombarding energy of 1.2 and 2 GeV/nucleon, respectively. The dependence of the positron yields on the charge number of the projectile exhibits a strong nonperturbative feature. The corresponding result obtained within first-order perturbation theory is exceeded by about 2 orders of magnitude.