Saturation and forward jets at HERA

We analyse forward-jet production at HERA in the framework of the Golec-Biernat and Wusthoff saturation models. We obtain a good description of the forward jet cross sections measured by the H1 and ZEUS collaborations in the two-hard-scale region kT ~ Q >>

SaturationandforwardjetsatHERA

Servicedephysiqueth´eorique,CEA/Saclay,91191Gif-sur-Yvettecedex,France

C.Marquet andR.Peschanski

C.Royon§

Servicedephysiquedesparticules,CEA/Saclay,91191Gif-sur-Yvettecedex,FranceWeanalyseforward-jetproductionatHERAintheframeworkoftheGolec-BiernatandW¨ustho

saturationmodels.Weobtainagooddescriptionoftheforward-jetcross-sectionsmeasuredbytheH1andZEUScollaborationsinthetwo-hard-scaleregion(kT～Q ΛQCD)withtwodif-ferentparametrizationswitheithersigni cantorweaksaturatione ects.TheweaksaturationparametrizationgivesascalecompatiblewiththeonefoundfortheprotonstructurefunctionF2.WearguethatMueller-NaveletjetsattheTevatronandtheLHCcouldhelpdistinguishingbetweenbothoptions.

arXiv:hep-ph/0407011v2 25 Aug 2004

I.INTRODUCTION

Thesaturationregimedescribesthehigh-densityphaseofpartonsinperturbativeQCD.ItmayoccurforinstancewhentheBalitsky-Fadin-Kuraev-Lipatov(BFKL)QCDevolutionequation[1]goesbeyondsomeenergyrelatedtotheunitaritylimit[2,3,4,5,6,7].Onaphenomenologicalground,awell-knownsaturationmodel[8]byGolec-BiernatandW¨ustho (GBW)givesaparametrizationoftheprotonstructurefunctionsalreadyintheHERAenergyrange.Itprovidesasimpleandelegantformulationofthetransitiontosaturation.However,theredoesnotyetexistaclearcon rmationofsaturationsincethesamedatacanwellbeexplainedwithintheconventionalperturbativeQCDframework[9].

Infact,thestudyoftheprotonstructurefunctionsisaone-hard-scaleanalysissincetheirQCDpropertiesaredominatedbytheevolutionfromasoftscale(theprotonscale)tothehardscaleofdeepinelasticscattering.Inordertofavortheevolutionat xedtransversescale,whichisexpectedtoleadmoredirectlytosaturation,itseemsinterestingtofocusontwo-hard-scaleprocessessuchasforward-jetproductionatHERA,intheregionwherethetransversemomentumofthejetkTisoftheorderofthevirtualityofthephotonQ2andwheretherapidityintervalforsoftradiationislarge.Thiskinematicalcon gurationwhichwasalreadyproposed[10]fortestingtheBFKLevolutionisthusalsoagoodtestinggroundforsaturation.ThegoalofourpaperisthentoformulateandstudytheextensionoftheGBWmodeltoforwardjetsatHERA.

Amongthephysicsquestionsthatwewanttoraise,weanalysewhethersaturatione ectscanbesizeableinforwardjetsatHERAandwhethertheyarecompatibleornotwiththeGBWparametrizationofF2.Moregenerally,wewouldliketocomparepotentialsaturatione ectsinone-scaleandtwo-scaleprocesses.Indeed,inthetwo-scaleprocessinitiatedbyγ γ scattering,ithasbeensuggested[11]thatthesaturationscalecouldbedi erentandinfactquitelargerthantheoneofdeepinelasticscattering.However,anotherstudyofγ γ scatteringstickstothesamesaturationscaleasforγ pscattering[12].Wewilldiscussthispointwithforward-jetdatawhichhavebetterstatisticsandwiderkinematicalrangethanγ γ scattering.Ifthesaturationscaleisuniversalasassumedin[12],thenoneshouldnotexpecttoseestrongersaturatione ectsinforward-jetdatathanforF2;howeverifthesaturationscaleishigherforprocessesinitiatedbytwohardscalesasproposedin[11],saturatione ectscouldbemoreimportant.ForHERA,theexistenceoftwodi erentsaturationscalesforoneortwohard-scaleprocesseswouldbeaninterestingfeature.ThisstudyisalsoofinterestintheprospectofMueller-Naveletjets[13]attheTevatronandtheLHCwheresaturationwithtwohardscalescouldplayabiggerrole.

Theplanofthepaperisthefollowing.InSection2,weformulatetheextensionoftheGBWmodelforforwardjets.InSection3,wepresentthe ttingmethodandgivetheresultsofthe tstogetherwithseveraltestsofthetwosaturationsolutionsthatweobtain.Section4isdevotedtoadiscussionofourresultsandtopredictionsforMueller-Naveletjetsathadroncollidersthatcoulddiscriminatebetweenbothsolutions.

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