We present the results of a calculation of deep inelastic electron-photon scattering at a linear collider for very high virtuality of the intermediate gauge boson up to NLO in perturbative QCD. The real photon is produced unpolarized via the Compton back s

1 TTP99-30Deep-inelastic Electron-Photon Scattering at High Charged Current Reactions

Q2: Neutral and

A. Gehrmann{De Ridder a a Institut fur Theoretische Teilchenphysik, Universitat Karlsruhe, D-76128 Karlsruhe, Germany and Deutsches Elektronen-Synchrotron DESY, D-22603 Hamburg, Germany.We present the results of a calculation of deep inelastic electron-photon scattering at a linear collider for very high virtuality of the intermediate gauge boson up to NLO in perturbative QCD. The real photon is produced unpolarized via the Compton back scattering of laser light of the incoming beam. For Q2 values close to the masses squared of the Z and W gauge bosons, the deep inelastic electron-photon scattering process receives important contributions not only from virtual photon exchange but also from the exchange of a Z-boson and a W-boson. We nd that the total cross section for center of mass energies above 500GeV is at least of O(pb) and has an important charged current contribution.

1. IntroductionProcesses induced by initial state photons provide us with an interesting testing ground for QCD. As a photon can interacts directly through a pointlike coupling with quarks or through its parton content like a hadron it has a twofold nature. Its point-like interaction gives rise to perturbatively calculable short-distance contributions 1] while its hadron-like or resolved part cannot be described with perturbative methods. It is described in terms of the parton distribution functions inside the photon. These parton distributions obey a perturbative evolution equation with a non-perturbative boundary condition usually parameterized in the form of an initial distribution at some low starting scale 0 . The pointlike and resolved processes contribute to the various real structure functions entering the cross section for e+ e? ! e?+ ! l+ X, the object of this study.

The di erential Compton cross section for the process e+ ! e+ 0, 0where the polarization of the nal state photon is not observed takes the following form 2,3], d dy2 n 1= x m2 1? y+ (1? y)? 4r(1? r) 0 e o+Pe P x0 r (1? 2r)(2? y): (1)

2. The photon spectrumThe process of Compton backscattering of laser beams o highly energetic electrons/positrons offers an e cient mechanism to transfer a large fraction of the lepton energy to the photon which structure can be investigated in deep inelastic electron-photon scattering.

The energy transferred from the electron to the backscattered photon is denoted by y . Pe; P are the helicities of the incoming lepton and laser photon with?1 Pe; P 1. In the above formula, the ratio r= y= (1? y)x0] while the fractional energy of the nal state photon y is x bounded by y 1+x . The parameter x0 is de4Ew where E and w0 are the energies ned as me of the incoming electron and photon. By tuning the energy of the incoming photon, the parameter x0 can be chosen close to 4.83, just below the threshold for production of e+ e? pairs from the collision

of laser and nal state photons. To give an order of magnitude, for an energy of the incoming lepton of 250 GeV, the laser energy is of the order of 1 eV. It is worth noting that the photon luminosity for backscattered photons is of the same order as the initial electron luminosity. It is therefore enhanced compared to the photon luminosity ob0 2 0 0