Hadronic photon-photon scattering at LEP

Hadronic interactions of two quasi-real photons have been studied extensively both during the LEP1 and the LEP2 data taking periods. The higher energies available at LEP2 in particular opened regions of phase space where hadronic processes can be predicted reliably by perturbative QCD calculations, usually available to next-to-leading order in the strong coupling constant for the process concerned. Over a wide range of observables and phase space good agreement is observed between measurements and theory, however, there are a few exceptions. The L3 collaboration has found large discrepancies for high momentum particle and jet production between theory and experiment, and measurements of open b-quark production by DELPHI, L3, and OPAL are consistently in excess of the theoretical values. Three new measurements have now become available and will be discussed in this paper: on jet and hadron production by OPAL and open beauty production by ALEPH. 1. Cross section for open b-quark production Until recently the only measurement of open b-quark production in two-photon collisions had been published by the L3 collaboration [1]. In this paper the cross section was measured to be about three times the prediction of NLO QCD. Similar results have been reported by DELPHI [2] and OPAL [3] at conferences. All these results have in common that the cross section is obtained from a fit to the transverse momentum spectrum of leptons with respect to jets. This quantity is expected to be enhanced for leptons from the leptonic decays of b-hadrons in comparison to leptons from processes involving lighter quarks. The standard method of identifying b-decays by tagging the displaced secondary vertex due to the longer lifetime of b-hadrons had not been used in two-photon processes at LEP, as the lower average momentum and hence smaller displacement of the secondary vertex made such techniques difficult to apply. ALEPH [4] as now published a measurement from 698 pb (〈see〉 = 196 GeV) using for the first time lifetime information to tag b-quarks in two-photon interaction at LEP. The tagging algorithm relies on the impact parameter (the distance of closest approach to the main vertex) of tracks to identify decay products from long lived particles. A fit to the distribution of the ratio of the impact parameters and their estimated measurement uncertainties is used to derive the probability that the track originated at the main vertex. These probabilities are then combined to probabilities that the whole event, the first jet, and the second jet contain no decay products from long lived particles, and used as selection criteria in the event selection. The jets are ordered such that the mass of the first jet is closest to the b quark mass of 5 GeV/c, the mass of the second jet is the second closest, etc. The selection is performed using Iterative Discriminant Analysis. A total of three iterations is used, with the new discriminant function generated for the remaining events after the cut on the previous function has been applied. Figure 1 (left) shows the discriminant 1 to appear in the proceedings of HEP’07, Manchester, U.K., 19.-25. July 2007 3 D -2 -1 0 1 2 E v e n ts /0 .5