The high flux of equivalent photons present in relativistic heavy ion collisions of two charges Z 1 and Z2 gives rise to the collision of two equivalent photons. The cross-sections for various processes are directly related to the corresponding 7-~ cross-sections. As compared to Y-7 physics being studied at e § e-colliders, we find that high energy states will not be so easily accessible at the existing facilities, however, the enhancement factor (Z 1 Z2) 2 in the expression for the cross section will provide very large photon fluxes for lower energies. 25.70.Np An extensive programme of 7-~ physics is going on at high energy e § e-colliders. The dominant graph is shown in Fig. 1. The charged particles e § and eemit "virtual" (sometimes also called "equivalent") photons which collide to form a neutral system X with charge parity C = + 1. There exists a vast literature on this subject, the properties of the virtual 7's are calculated in great detail and the cross sections in e § e-collisions are directly related to the corresponding 7-7 cross sections. (See [1,2,3] where many further references are contained). An early result is due to F.E. Low [4] where the measurement of the rc ~ lifetime by ~z ~ production in e e or e-e § collisions is proposed. Using a variant of the Weizs/icker-Williams method, the cross section for the process e-e + ~e-e+X is found to be related to the cross section for 7+y~X by (we use the notation of [5])
PACS:
dae--e+x(S)=q2Sdo~f(oo)dav,_~x(COs )
f (c~ [(2 + c~ ln l~-2(1--~~ (3 + o)](2c) For a collider s is given by s=4E 2 where E is the e + (e-) energy in the labsystem, m is the electron mass 1 E and 7=--. In addition to the situation V2 m 1 c2 pertaining to (1), where the final momenta of e + and e-are not measured ("untagged luminosities") one can study cases where these momenta are measured ("tagged" and "double tagged" luminosities, see e.g.[2]).It is the purpose of the present paper to discuss the 7-7 processes which occur in relativistic heavy ion (RHI) collisions. It is the additional factor (Z1 Z2) 2, where Z 1 and Z2 are the charge of the colliding heavy ions, which increases strongly the RH! cross section as compared to the e § e-case. We study the collisions of two "equivalent" photons in the system, where the two heavy ions move with opposite velocities v and -v towards each other (see Fig. 2). This is equivalent to the collision of two photons with frequency distributions nl(coz) and n2(c%) moving in opposite directions. The expressions for the frequency distributions, integrated over the impact
