Untitled

Abstract

Some of the most important physics processes which can be studied at high-energy electron-proton colliders such as HERA involve the hard scattering of quasi-real photons emitted from the incoming electron. Several of these processes have been calculated to next-to-leading order in QCD perturbation theory and can therefore, at least in principle, be used for precision Standard Model tests. A common feature of such analyses is the use of the equivalent photon approximation, which simplifies the calculations significantly. We examine the validity of this approximation in the context of a typical hard scattering processthe production of large transverse momentum photons. By comparing the exact and approximate cross sections we show that, with an appropriate choice of parameter in the electron structure function, the approximation is generally very good in the relevant kinematic regions, and any residual errors are typically less than other theoretical uncertainties in the calculation.

Single-inclusive production of hadrons and jets in lepton-nucleon scattering at NLO

“…In processes where the scattered lepton is observed, such as N → hX, one can in fact select such contributions by requiring the scattered lepton to have a low scattering angle. The incoming lepton then effectively acts merely as a source of quasi-real photons, and the process may be very accurately described in terms of a (perturbative) distribution function for photons in leptons known as the Weizsäcker-Williams (WW) distribution [11,[44][45][46][47]. This approach has been widely used with much success in the HERA physics program [11].…”

Aspects of two-photon physics at lineare + e − colliders

“…Measurements of deep inelastic scattering have the advantage that higher order corections ("k-factors") are expected to be smaller than in case of real γγ scattering. The ultimate F γ 2 measurement might come [74] from eγ colliders, which are a hybrid of the e + e − and γγ colliders discussed in this paper. More experimental information about the details of the spectator jets from resolved photons, as well as of the behaviour of total cross sections for hadronic processes involving real photons in the initial state, is also needed.…”

Isolated prompt photon production at DESY HERA

“…According to the results of Ref. [16] the error introduced by using the approximation (6) a t these small angles should not be greater than a few percent. Note that (6), in contrast with the full Weizsacker-Williams spectrum used in [8], should also more closely match the experimental situation, where some sort of tagging or antitagging will almost certainly be necessary.…”