T.F. Walsh

Two-photon processes in the parton model.

Abstract We study here some two-photon processes in colliding e ± e − beam reactions, with the aim of testing the parton model. The parton contributions should dominate in certain well defined kinematical limits, and provide an essential difference between photonic and hadronic processes.

Deep inelastic electron-proton scattering

We study deep inelastic scattering of an electron or positron on an almost real photon target in the reaction e+e− → e+e− + hadrons with C = +, where one e+ or e− is scattered at small or zero angle, yielding a virtual photon γ(k), k2 ∼ O(−me2). The other e− or e+ scatters on this virtual photon, emerging at large angle. We emphasize particularly: (i) the pointlike QCD contribution to the structure functions of the photon; (ii) the measurability of the structure functions under realistic experimental conditions; (iii) the background from the inelastic Compton process e+e− → e+e− + hadrons with C = −, which we find is controllable.

Virtual photon structure to non-leading order in QCD

Abstract We investigate the virtual photon structure functions for Λ2 ⪡ P2 ⪡ Q2, where −Q2 (−P2) is the mass squared of the probe (target) photon. We do this to next-to-leading order in QCD. The non-leading corrections significantly modify the leading log result, in particular at large x. Also, the perturbatively calculated structure function is positive at low x even for P2 ≈ 1 GeV2. (For a real photon target it becomes negative at low x.) For large P2 the QCD result approaches the box diagram (Born) structure function also when non-leading contributions are included. It is, however, important to include the large non-leading box diagram contributions in making this comparison.

GLUONS IN QUARKONIUM DECAY

We discuss what can be learned from the 3S1 quarkonium decay QQ → 3 gluons, QQ → γ + 2 gluons. The former is a way to find gluon jets and test QCD. The latter also allows us to measure gluon + gluon → hadrons, and look for pure gluonic resonances (glueballs).

Virtual photon structure

Abstract The structure functions of a real photon are calculable in QCD. The leading contribution is proportional to 1n Q 2 , with a coefficient reflecting the gluon flux in a real photon. We investigate this leading term for non-zero target photon mass. In an appropriate limit the gluon content in a virtual photon is found to vanish. The gluon radiative corrections of QCD can thus be turned off by tuning the target photon mass.

Inelastic electron-photon scattering

Abstract It is suggested that colliding e+e− beam machines may be used to study inelastic electron-photon scattering.

Quark and gluon jets in the breit frame of lepton-nucleon scattering

Gluon bremsstrahlung,q→Gq, and quark pair production from gluons,laggy, in deep inelastic reactions is investigated in the Breit frame (moving alongQ in the laboratory). These QCD effects diminish the overall forward momentum. There are also events with asingle largep⊥ forward jet. One spectacular class of events is predicted in which no forward going hadrons emerge, in the Breit frame. These effects are not mimicked by nonperturbative (limitedp⊥) parton jets at large but attainableQ2.

Hadron production in virtual photon-photon annihilation

Abstract In this article, we systematically study the amplitudes for annihilation of two virtual photons into hadron resonances and continuum states, a process measurable with colliding electron-positron beams. The asymtotic behavior of resonance form factors and their absolute scale follows from light cone or parton model considerations; definite predictions for inclusive two-photon annihilation are derived by the same methods.

Assessing QCD in deep-inelastic eγ scattering

We consider hadron production in deep inelastic scattering of electrons on photons. 1. (i)Exploiting the leading order QCD corrections due to gluon bremsstrahlung we find that the photon structure functions rapidly approach their asymptotic form which can be calculated in QCD. 2. (ii)Replacing QCD by a theory with a fixed quark-gluon coupling constant (scalar or abelian gluons) gives dramatic changes: for Bjorken x away from 0, the asymptotic form of the structure function is scale-invariant, with a shape quite unlike the QCD shape, and the structure function diverges for x = 0. 3. (iii)The pointlike component of the photon gives final state jets emerging at large angles to the axis of the real + virtual photon, in contrast to the hadronic component of the photon.

γγ production of non-strongly interacting SUSY particles at hadron colliders

Abstract Non-strongly interacting particles in supersymmetric theories — sleptons, charginos/neutralinos, and charged Higgs bosons — are difficult to detect at the Large Hadron Collider. We therefore examine the possibility of producing particles of this type in virtual γγ collisions at the LHC. Since photons can be emitted from protons which do not break up in the radiation process, very clean events can be generated, compensating to some extent for the small event numbers. Higher rates are expected, at the expense of stray hadrons, for events in which one or both protons break up.