Ultraperipheral Collisions at RHIC and LHC

A brief introduction to the physics of ultraperipheral collisions at collider energies is given. Photon-hadron (proton/ nucleus) and photon-photon interactions can be studied in a hitherto unexplored energy regime. 1 Some features of Ultraperipheral Collisions (UPC) Photon-photon and photon-hadron interactions can also be studied in hadron-hadron collisions [1]. This may be surprising since in general such collisions are dominated by strong interactions between the hadrons. However, by choosing collisions with large impact parameter b (or, equivalently, small momentum transfer) one can suppress these strong interactions. The time-dependent electromagnetic field of a fast moving charged particle can be thought of as a spectrum of (quasireal, or equivalent) photons [2], see Figure 1. The determination of the equivalent (or WeizsäckerWilliams) photon spectrum corresponding to a fast particle moving past an observer on a straight line path with impact parameter b is a textbook example [3]. The probability P (b) of a specific photonhadron reaction to occur in a collision with an impact parameter b is given by P (b) = N(ω, b)σγh(ω), where σγh is the corresponding photoproduction cross section. The equivalent photon spectrum can be calculated analytically, a useful approximation for qualitative considerations is