Non-perturbative chiral approach to Kp --gamma Y reactions(2)

A recently developed nonperturbative chiral approach to describe the S=-1 meson-baryon reactions has been extended to investigate the near threshold $K^- p \to \gamma \Lambda, \gamma \Sigma^0$ reactions. With the parameters governed by chiral SU(3) symmetr

I.INTRODUCTION

ThenearthresholdK p→γYreactionwithY=Λ,Σ0haslongattractedalotofinterest,mainlybecauseofthepossibilityofusingthisreactiontoresolvethedebates[1–7]overthestructureoftheΛ(1405)resonance.Mostoftheearliertheoreticalinvestigations

[8]neglectedtheinitialstrongK pinteractions.Itwas rstdemonstratedbySiegelandSaghai[9]thattheinitialK pinteractionscandrasticallychangethepredictedcaptureratesandthuscansigni cantlyaltertheinterpretationofthedata.Withthephenomenologicalseparablepotentials,they,however,neededanabout30 50%deviationofthecouplingconstantsfromtheSU(3)valuestoobtainanaccuratedescriptionofthedata.

Obviously,theprogresscanbemadeonlywhentheinitialstronginteractionsandthephotoproductionamplitudesareconsistentlydescribedwithinthesametheoreticalframe-work.Forinvestigatingthelowenergyprocessesinvolvingstrangehadrons,suchastheprocessesconsideredinthiswork,itisnowgenerallybelievedthatthebeststartingpointisthee ectivechirallagrangianwithSU(3)symmetry.ThemostdevelopedapproachinthisdirectionisChiralPerturbationTheory[10–14].Suchanapproach,however,becomespowerlessinfacingtheS= 1meson-baryoninteractionsbecauseoftheformationofares-onancejustbelowtheK pthreshold.Analternativenonperturbativechiralapproachwas rstdevelopedbyKaiser,SiegelandWeise[15].Theessentialideaistode nethemeson-baryonpotentialsbytheSU(3)chirallagrangianandsumasubseriesofthechiralexpansionbysolvingacoupled-channelLippmann-Schwingerequation.Theyconsiderthelowestorderandnexttolowestorderchirallagrangians,andimplementedsuitableformfactorstomaketheresultingmeson-baryonpotentialsof niterangesuchthattheloopsbecomeconver-gent.Therangeparametersofformfactorsandsomeofthesecondorderchirallagrangiansare ttedtothedata.Thisunitarycoupled-channelapproachgeneratesdynamicallytheΛ(1405)resonanceasaquasiboundmesonbaryonstate,andreproducesthelowenergydataforK pelasticandinelasticchannels.ThegenerationoftheΛ(1405)resonanceisactuallynotameritofthechirallagrangianssinceitcanbeobtainedinasuitablecoupledchannelK