Equivalent g beams at e+e-, ep colliders

• EM field of relativistic charged particle = flux of “equivalent” photons.

• Weizsacker-Williams (EPA) formula for an e+- beam:

• Characteristics of g-induced collisions at e+e- (LEP), ep (HERA) colliders:

  • Peaked at lower c.m. energies than parent beams: 0.1√s <~ Wgg,gp<~ 0.5√s
  • Quasi-real (Q2~0) photons: parent e± scattered very close to beam, produced particles have low pT.

• Schematic kinematics for gg and gp: