**31**: 307-325

https://doi.org/10.1140/epjc/s2003-01360-8

experimental physics

## Di-jet production in photon-photon collisionsat $\sqrt{s_{\protect\bf\mathrm{ee}}}$ from 189 to 209 GeV

Di-jet production is studied in collisions of quasi-real photons at e^{ + }e^{-} centre-of-mass energies $\sqrt{s_{\protect\bf\mathrm{ee}}}$ from 189 to 209 GeV at LEP. The data were collected with the OPAL detector. Jets are reconstructed using an inclusive $k_{\perp}$-clustering algorithm for all cross-section measurements presented. A cone jet algorithm is used in addition to study the different structure of the jets resulting from either of the algorithms. The inclusive di-jet cross-section is measured as a function of the mean transverse energy $\bar{E}^{\mathrm{jet}}_{\mathrm{T}}$ of the two leading jets, and as a function of the estimated fraction of the photon momentum carried by the parton entering the hard sub-process, $x_{\gamma}$, for different regions of $\bar{E}^{\mathrm{jet}}_{\mathrm{T}}$. Angular distributions in di-jet events are measured and used to demonstrate the dominance of quark and gluon initiated processes in different regions of phase space. Furthermore the inclusive di-jet cross-section as a function of $\eta^\mathrm{jet}$ and $\Delta\eta^\mathrm{jet}$is presented, where $\eta^\mathrm{jet}_\mathrm{cntr}$is the jet pseudo-rapidity. Different regions of the $x_{\gamma}^ + $-$x_{\gamma}^-$-space are explored to study and control the influence of an underlying event. The results are compared to next-to-leading order perturbative QCD calculations and to the predictions of the leading order Monte Carlo generator PYTHIA.

*© Springer-Verlag, 2003*