Two-photon exchange in p⊆)p→ℓ +ℓ −X and a comparison with QCD

Abstract

A thorough study of lepton-pair production from two-photon annihilation in p⊆)p collisions is presented. The differential cross section is calculated over a large range of energies (27⩽√ s⩽800 GeV as a function of the dilepton mass M as well as the dilepton transverse momentum Q T and the Feynman variable x F. No kinematical approximations (such as the equivalent photon approximation) are made. For Q T ≈ 0 the two-photon mechanism represents an important fraction of the pp→e +e −X cross section already at ISR energies, whereas at ISABELLE energies it dramatically dominates in the interval 0⩽ Q T⪅1 GeV. At ISR energies these conclusions follow from a direct comparison of the two-photon contribution with pp→e +e −X data. For the ISABELLE energy range the expected O( α s ) QCD contribution to pp→ℓ +ℓ −X, corrected for soft gluon radiation to all orders (in leading bilogarithmic approximation), was taken as a reference. At larger Q T and ISR energies the γγ contribution is negligible, whereas at √ s = 800 GeV γγ/QCD≅ 10–20% almost everywhere. Furthermore, two-photon candidate events from the ISR are shown to be in reasonable agreement with theory. A decomposition of the γγ cross section into contributions from both proton vertices being elastic, inelastic and of mixed configuration is given. The results provide important clues for a future isolation of the two-photon mechanism.