Hard Exclusive Electroproduction of Pions(5)

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

prefactorstheρ+amplitudecanbeobtainedfrom(6)byreplacingpolarizedquarkdoubledistributionfunctionsbyunpolarizedones,withproperaccountoftheiroppositechargeconjugationproperties.

Inordertoobtainanumericalestimateforthepseudovectorcontribution(6)toπ+productionmodelsfortheinvolveddoubledistributions Fhavetobeconstructed.Weareguidedherebytheappropriateforwardlimitofdoubledistributions,theirsymmetryproperties,andsumruleswhichrelatethemtonucleonformfactors.Followingrefs.[20]and[16]weuse:

A: F(x,y;t)=h(x,y) q(x)f(t),

B: F(x,y;t)=h(x,y) q(x)exp t

x(1 x) .(7)

Hereh(x,y)=6y(1 x y)/(1 x)3,and q(x)denotesthecorrespondingordinarypolarizedquarkdistribution.InnumericalcalculationspresentedinthispaperwehaveusedGehrman-StirlingLOset-Aparametrizationsofpolarizedquarkdistributions[21].Furthermore,wehaveusedtheoneloopexpressionfortherunningcouplingconstantwithΛQCD=200MeV.Notethatinadditiontotheabovementionedconstraints,themodels

(7)areconsistentwithasymptoticsolutionsofevolutionequationsfordoubledistributionsandsatisfythesymmetriesofthelatter[4].Thet-dependenceofdoubledistributionsinmodelAisgovernedbytheformfactorf(t)which,likein[4],istakentobeequaltothenucleonpseudovectorformfactor[22]:

f(t)=1