Double-Tag Events in Two-Photon Collisions at LEP

TL;DR

This study measures double-tag events in two-photon collisions at LEP, comparing results with QCD predictions and observing potential signs of complex photon interactions at high energies.

Contribution

It provides the first detailed measurement of cross sections for gamma*gamma* -> hadrons at high energies and explores possible indications of BFKL phenomena.

Findings

Results agree with NLO calculations for certain Y ranges.

An excess observed at high W_gammagamma suggests new QCD effects.

Potential evidence of resolved photon processes or BFKL dynamics.

Abstract

Double-tag events in two-photon collisions are studied using the L3 detector at LEP centre-of-mass energies from root(s)=189 GeV to 209 GeV. The cross sections of the e+e- -> e+e- hadrons and gamma*gamma* -> hadrons processes are measured as a function of the photon virtualities, Q1^2 and Q2^2, of the two-photon mass, W_gammagamma, and of the variable Y=ln(W_gammagamma^2/(Q1 Q2)), for an average photon virtuality <Q2> = 16 GeV2. The results are in agreement with next-to-leading order calculations for the process gamma*gamma* -> q qbar in the interval 2 <= Y <= 5. An excess is observed in the interval 5 < Y <= 7, corresponding to W_gammagamma greater than 40 GeV . This may be interpreted as a sign of resolved photon QCD processes or the onset of BFKL phenomena.