Hard Exclusive Electroproduction of Pions(2)

We investigate the exclusive electroproduction of pi^+ mesons from nucleons. To leading twist, leading order alpha_s accuracy the corresponding production amplitude can be decomposed into pseudovector and pseudoscalar parts. Both can be expressed in terms

1Introduction

Hardexclusiveelectroproductionofmesonsfromnucleonshasbecomea eldofgrowinginterest.TherecentprogressintheQCDanalysisofsuchprocessesisbasedonafactoriza-tiontheorem[1](seealso[2,3,4,5])validinthecaseoflongitudinallypolarizedphotons:atlargephotonvirtualities,Q2 Λ2QCD,theunderlyingphoton-partonsub-processesaredominatedbyshortdistancesand,hence,canbecalculatedperturbatively.Ontheotherhand,allinformationonthelongdistancedynamicsofquarksandgluonscanbecollectedinthedistributionamplitudeoftheproducedmesonandingeneralized(skewed[2,6]ordouble[2])partondistributionfunctionsofthenucleon.Asaconsequence,experimentalinvestigationsofmesonproductionprocessesallowtoprobedetailsofthequark-gluondynamicsinnucleonsbeyondourcurrentknowledgeobtainedmainlyfrominclusivehigh-energyprocesses.OngoingandplannedmeasurementsatDESY(HERA[7],HERMES

[8]),CERN(COMPASS[9]),andJe ersonLab[10]arethereforeofgreatinterest.Inourpreviouswork[4,11]wehavederivedtheleadingorderQCDproductionam-plitudesforneutralpseudoscalarmesonsandvectormesons.Thecorrespondingcrosssectionshavebeendiscussedwithinaspeci cmodel[12]fordoublepartondistributions.Inthispaperweconsiderhardelectroproductionofchargedpions.Interestinthispro-cessarisesinparticularfromthefactthatitreceivessigni cantcontributionsfromtwoproductionmechanisms:either(i)thephotoninteractswithaquarkfromthenucleonwhich,afterhardgluonexchange,combineswithasecondquarkofthetargettothe nalpion,or(ii)thepionisproducedfromthemesoncloudofthenucleon.Thelattermech-anismisoftenreferredtoasthepionpolecontribution.Inmeson-cloudmodelsofthenucleonthesecondmechanismdirectlyinvolvestheelectromagneticformfactorFπ(Q2)ofthepion(seee.g.[13]andreferencestherein).PionelectroproductionhasthereforeoftenbeenconsideredasagenericprocesstodetermineFπ(Q2)(seee.g.[14]).However,asdiscussedinref.[15],theremaybesigni cantuncertaintiesduetotype(i)contribu-tions.Thelatterhavebeenmodeledinref.[15]intermsofhardgluonexchangediagramswithnonperturbativefactorsestimatedbyanoverlapoflight-conewavefunctionswithChernyak-Zhitnitskytypedistributionamplitudes[17].

Inthisworkweaddressquestionssimilartothoseraisedin[15].ThedistinctivefeatureofourapproachisasystematicuseofperturbativeQCD(PQCD)factorization

[1,2].WithinaconsistentPQCDanalysisallnecessaryinformationaboutthenucleonstructureiscontainedinquarkdoubledistributions.Thelatterareconstrainedthroughthebehaviorofordinaryquarkdistributionfunctions,aswellasthroughsumrulesandsymmetryproperties(foradiscussionsee[4,16]).Inparticularweshowthatactuallybothproductionmechanisms,(i)and(ii),canbeformulatedintermsofquarkdoubledistributionfunctionsofthenucleon,i.e.theexistenceofthepionpolecontribution tsintothePQCDfactorizationframework.

Aftermodelingtheinvolveddoubledistributionfunctionswecalculatethedi erentialπ+productioncrosssectionintheregionofsmallmomentumtransfers, t<0.5GeV2.We ndthatbothmechanisms,(i)and(ii),contributesigni cantly.Whileonepion