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Photon Structure and the Production of Jets, Hadrons, and Prompt Photons
We give a pedagogical introduction to hard photoproduction processes at HERA, including the production of jets, hadrons, and prompt photons. Recent theoretical developments in the three areas are reviewed.
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aANL-HEP-CP-99-73PhotonStructureandtheProductionofJets,Hadrons,andPromptPhotonsMichaelKlasenHEPTheoryGroup,ArgonneNationalLaboratory,Argonne,IL60439,,includingtheproductionofjets,hadrons,-protonscatteringatHERAproceedsdominantlythroughtheex-changeofasinglephotonwithsmallvirtualityQ?-ductionlimitandinthepresenceofahardfactorizationscaleMγ,p,theelectron-posedinto11dσep=??dyFγ/e(y,Q2max)dxγFa/γ(xγ,Mγ2))dσ(n),b??0??0??1dxpFb/p(xp,Mp20(1)TheWeizs¨acker-WilliamsspectrumofphotonsintheelectronFα1γ/e(y,Q2max)=yln Q2maxQ2max?
q)pairs,whichin
turnevolveintoavectormeson-,analmostreal
photonradiatedfromtheelectroncanthusinteracteitherdirectlywiththe
partonsintheproton(ponent)oractasahadronicsourceof
partonswhichcollidewiththepartonsintheproton(ponent).
2MichaelKlasen
Inthelattercase,onedoesnottesttheprotonstructurealonebutalsothe
,boththephotonandtheprotonstructure
cannotbecalculatedtheoretically,buthavetobedeterminedexperimentally.
BeforeHERAstartedtakingdata,informationonthepartondensitiesin
thephoto/?γscatteringat
e e?
thisprocess,itisdi?culttoobtaininformationonthegluondensity,whichis
suppressedrelativetothequarkdensitiesbythes