QCD Corrections and the Leptoquark Interpretation

arXiv:hep-ph/9703427v126Mar1997DTP/97/16ETH–TH/97–10March1997QCDCorrectionsandtheLeptoquarkInterpretationoftheHERAHigh–Q2Events1Z.KunsztaandW.J.Stirlingb,ca)InstituteofTheoreticalPhysics,ETH,Z¨urich,Switzerland.b)DepartmentofPhysics,UniversityofDurham,c)DepartmentofMathematicalSciences,UniversityofDurham,SouthRoad,DurhamDH13LE,UnitedKingdom.AbstractTheexcessofhigh–Q2eventsfoundbyH1andZEUSatHERAine+pdeep-inelasticscatteringabovetheStandardModelpredictionmotivatesustocalculatetheNLOQCDcorrectionstotheHERAscalarleptoquark(orsquark)productioncrosssec-tions.Wefindthatthecorrectionsaresignificant,oforder50%inthemassrangeofinterest.Wealsocalculatetheleptoquarkaveragetransversemomentumsquaredandfindittoberathersmall.VariousleptoquarkproductioncrosssectionsattheTevatronp¯pcolliderarealsoconsidered.WeinvestigateindetailtheleptoquarkinterpretationoftheHERAdata.FirstweassumeaminimalleptoquarkmodelwithasinglediagonalYukawacouplingtofirstfamilyleptonandquarkmasseigen-statesonly.InthiscaseconstraintsfromatomicparityviolatingexperimentsallowonlyisodoubletscalarleptoquarkproductionatHERA.ThisinterpretationcanbeconfirmedorruledoutinthenearfuturebyhighluminositydataattheTevatron.TheTevatrondataalreadyappeartoruleoutthevectorleptoquarkinterpretationoftheHERAdata.Wealsoconsideramoregeneralmodelwhichallowsforallpos-sibleleft-handed,right-handed,flavourandleptonnumberchangingcouplings.TheallowedvaluesoftheYukawacouplingsofthisgeneralmodelofferseveraldifferentinterpretationsofthedatawhichareradicallydifferentfromtheminimalmodelsolutions.HoweverthesesomewhatadhoctunedsolutionscaneasilybetestedbyfutureHERAexperiments.1E-mails:[e-mail];[e-mail].1IntroductionRecentlytheHERAcollaborationsH1[1]andZEUS[2]havepresentednewresultsonhigh–Q2deepinelastice+pscatteringbasedon14pb−1and20pb−1luminositydata,respectively.ThedataindicateanexcessofeventswithrespecttotheStandardModelexpectationpredictedbytheconventionalnext-to-leadingorderQCD–improvedpartonmodelinanewkinematicalrangeunexploredbypreviousexperiments.AthighQ2valuesQ215000GeV2H1finds7excesseventswhileZEUSfinds5events.AninterestingfeatureoftheH1eventsisthattheyareclusteredinasinglebinoftheinvariantmassofthejet–leptonsystem(Mej=√xs)around200GeVwithanestimatedexperimentalresolutionof6GeV[1].TheZEUSeventsdonotexhibitsuchaclustering,butthisisnotinconflictwiththeH1datasinceZEUShavealargerMejresolution.H1havealsopublishedresultsonchargedcurrentevents[1].Althoughtheyfindmoreeventsthanexpected,theexcessisnotstatisticallysignificant.Ifthesignaliseventuallyconfirmedastheproductionofanewheavyresonance,onewillpresumablybeabletomeasureitsspinanditsbranchingratiosbothtochargedcurrentandtoflavourand/orleptonnumberviolatingchannels.Thepossibilityofs–channelresonanceproductionatHERAhasbeensuggestedintwotheoreticalschemes.First,Buchm¨ullerandWyler[3]arguedthattheleptoquarks(LQ)necessarilyappearingingrandunifiedtheories[4,5]canalsobeaccommodatedintheorieswithconservedleptonandbaryonnumber.Theypointedoutthattheseleptoquarkscanbelight(i.e.mLQ≪MGUT)sincelowenergyexperimentsimposemuchweakerboundsonleptonandbaryonnumberconservingcouplings.Second,thesquarks(˜q)ofsupersymmetricextensionsoftheStandardModelcanhavedirect(leptonnumberviolating)couplingstoquarksandleptonsifR–parityisnotconserved.Thereforetheycanalsobecoupledtolepton–quarkinitialstatesandthusappearass–channelresonances[6].Inthepresentstudyweshallrestrictourdiscussiontotheleptoquarkinterpretationonly,althoughseveralofourresultswillremainvalidalsoforsquarkproduction.Insection2weconsiderleptoquarkproductioncrosssectionsatHERA,includingnext-to-leadingorderQCDcorrections.Fromthenumberofexcesseventsweextractthevalueoftheleptoquarkcouplings(λ)anddiscusstheconstraintsonthesecomingfromatomicparityviolatingexperiments.Wealsocommentonnon-minimalmodelswhichallowforallpossibleYukawacouplings,followingtheanalysisofref.[7].Insection3weconsidersingleandpairproductionofscalarandvectorleptoquarksinp¯pcollisionsattheTevatron.Westudyλdependentcontributions,theeffectsofQCD1corrections,signalsandbackgrounds,andexperimentallimitsontheleptoquarkmassesobtainedbyD0andCDF.Section4describesthecalculationofthenext-to-leadingorderQCDcorrectionstocolour-tripletscalarresonanceproductioninepcollisions.Thenumericalvalueofthe‘K-factor’andtheaveragetransversemomentumsquaredoftheproducedresonancearealsostudied.FulldetailsofthecalculationsaregivenintheAppendix.Ourconclusionsarecontainedinsection5.2LeptoquarkproductionatHERAandlimitsoncouplingsWefirstconsidertheminimalschemeinwhichtheleptoquarkshaveseparatebaryonandleptonnumberconservingchiralcouplingsforeachfamilyofmasseigenstates.Thisallowsustoevadeseverelowenergylimitsonleptonnumberviolatingcouplings[7,8,9].Leptoquarkswhicharecoupledtothefirstfamilyonlyarecalledfirstgenerationleptoquarks.Aleptoquarkissaidtocouplechirallyifitcoupleseithertoleft-handed(L)orright-handed(R)leptonsbutnotboth.Theallowedleptoquarkrepresentationshavebeenclassifiedinref.[8].TherearesevenBandLconservingandSU(3)×SU(2)×U(1)renormalizablecouplingsbothforscal