Double-tag events in two-photon collisions at LEP

Abstract

Double-tag events in two-photon collisions are studied using the L3 detector at LEP centre-of-mass energies from σγ∗γ∗ GeV to 209 GeV. The cross sections of the e⁺e ⁻ →e⁺e ⁻ hadrons and σγ∗γ∗ processes are measured as a function of the product of the photon virtualities, σγ∗γ∗, of the two-photon mass, W_γγ, and of the variable Y=ln(W²_γγ/Q²). The average photon virtuality is ≶Q²₁≶=≶Q²₂≶=16 GeV². The results are in agreement with next-to-leading order calculations for the process √s = 189 in the interval 2≰Y≰5. An excess is observed in the interval 5<Y≰7, corresponding to W_γγ greater than 40 GeV. This may be interpreted as a contribution of resolved photon QCD processes or the onset of BFKL phenomena.