Standard Model Tests at very high Q 2 at HERA

StandardModelTestsatveryhighQ2atHERAG.EckerlinonbehalfoftheH1andZEUSCollaborationsDESY,HamburgNotkestr.85,22607Hamburg,GermanyAbstract.Withthee-pcolliderHERAatDESYtestsoftheStandardModelathighestavailableQ2havebeenperformed.Recentresultsonthecrosssectionfordeepinelastice-pscatteringuptoQ2=40000GeV2arepresented.Ananalysisofeventswithhighmissingtransversemomentumptandanisolatedleptonisdiscussed.Limitsontheproductionofleptoquarks,excitedfermionsandonContactInteractionsaregiven.INTRODUCTIONWiththee-pcolliderHERAatDESYelectronprotonscatteringprocesses1atamaximumcenterofmassenergyps=300GeVoerthepossibilitytostudydeepinelasticscattering(DIS)inchargedandneutralcurrentprocessesuptoamomentumtransferofQ2=90000GeV2.Withthelargedatasamplefrom1997bothcollaborationsH1andZEUShaverenedtheiranalysesandmeasuredcrosssectionsuptoQ2=40000GeV2.Thisextendspreviousmeasurementsofxedtargetexperimentsby2ordersofmagni-tudeinQ2.AnewdomaininDIShasbeenexploredandsearchesforpossibledeviationsfromtheStandardModel(likeLeptoquarksorContactInteractions)havebeenperformed.WithincreasedluminositynewlimitsonexcitedfermionshavebeenderivedextendingbeyondtheLEPmassrange.AdetaileddescriptionoftheH1andZEUSexperimentscanbefoundin[1]and[2]respectively.DISAtHERAdeepinelasticscatteringof27:5GeVpositronson820GeVprotonsisstudiedandcrosssectionsaremeasuredovermorethan4ordersofmagnitudeboth1)AlthoughHERAhasrunwithpositronsinsteadofelectronsfrom1994until1997throughoutthepaperthenameelectronisusedfore+aswell.inQ2andx.ThemomentumtransferQ2=q2=(kk0)2isderivedfromkandk0thefourvectorsoftheinitialandnalleptonrespectively.InadditionfurtherLorentz-invariantscanbedened:theBjorkenscalingvariable:x=Q22Pq(1)andthefractionalenergytransfertotheprotoninitsrestframe:y=PqPk;(2)wherePisthe4-momentumoftheincomingproton.InthequarkpartonmodelthemassintheelectronquarksystemisgivenbyM=psx,withsbeingtheepinvariantmasssquared.AtagivenstwooftheLorentz-invariantsareneededtofullycharacterizethekinematicsoftheepscattering.Inthedatacollectedduring1994-1996bothH1andZEUSreportedanexcessofDISeventsathighestQ2andhighy[3,4].Withthedatarecordedin1997thestatisticsavailableismorethandoubledandanextensiveanalysisofthecrosssectionswereperformed.NeutralCurrentCrossSectionsNeutralcurrentDISmediatedbytheneutralbosonsorZshowascatteredelectronandahadronicnalstate.Theeventsareexpectedtobebalancedintransversemomentum.Thekinematicscanbederivedfrom4measurablequanti-ties,theenergyEeandthepolarangleeofthescatteredelectron,aswellastheenergyEhandtheanglehofthehadronicsystem[5].TheH1analysis[3]usestheelectronquantitiestoderivethekinematics(electronmethod),whereasZEUS[4]usesthetwomeasuredangles(doubleanglemethod).Bothexperimentsusedthealternativemethodasacrosscheck.Thedierentialcrosssectionisgivenby:d2dxdQ2=22xQ4[Y+F2(x;Q2)y2FL(x;Q2)YxF3(x;Q2)](3)withY=1(1y2).Fig.1showsthesingledierentialcrosssectiond=dQ2measuredbytheZEUScollaborationusingthedatafrom1994to1997.AllpointsagreewiththeStandardModel(SM)predictionswithinerror,withtheexceptionofthehighestbin(Q235000GeV2)whichcontainstwoeventswhere0.29areexpected.TheH1collaborationmeasuredthedoubledierentialcrosssection.Theresultsarepresentedintheformofthereducedcrosssection(e+p)=xQ4221Y+d2dxdQ2(4)ingure2for6dierentxrangescomparedwiththestandardmodelexpectationbasedontheMRSHevolutionequations[6].AlsoshownarethelowQ2datafromSLAC[7],BCDMS[8]andNMC[9].ThelowQ2datajoinwellthehighQ2crosssection.BoththestandardmodelexpectationbasedonMRSHpartondistributionsaswellasanexttoleadingorderH1tdescribetheobservedcrosssectionover4ordersofmagnitudeforallrangesinx.Onlyforx=0:45andQ215000GeV2thedatapointsareabovetheexpectation,howeverwithlesssignicancethanforthe1994-1996dataalone.Table1givesthenumberofobservedeventsseenbyH1aboveQ2=15000GeV2for0:1y0:9,comparingtheresultsfromthepre1997datawiththefullstatisticsupto1997.10-810-710-610-510-410-310-210-11103104ZEUSPreliminary1994-97Q2(GeV2)dsNC/dQ2(pb/GeV2)e+pDataCTEQ4DPDFband{}stat.stat.¯syst.Q2(GeV2)dsNC/dQ2/ds/dQ2(CTEQ4)1101031041101031040.80.911.11.21031040.80.911.11.2103104FIGURE1.Thee+pneutralcurrentcrosssectionddQ2measuredbyZEUS(uppergure)andtheratiodata/SM(lowergure)isshown.TheStandardModelexpectationshownisbasedontheCTEQ4Dparametrisationofthepartondensityfunction.SLACBCDMSNMCH1preliminaryx=0.07(x9000)x=0.13(x3000)x=0.18(x800)x=0.28(x100)x=0.45(x12)x=0.65(x2)MRSHNLOQCDFitQ2/GeV2s10-210-111010210310410510-1110102103104FIGURE2.Thee+pneutralcurrentreducedcrosssection(seetext)measuredbyH1isshowntogetherwithlowQ2datafromSLAC,BCDMSandNMC.TheMRSHbasedStandardModelexpectationandanexttoleadingorderQCDFitfromH1areshownascurves.TABLE1.NumberofeventsaboveQ215000GeV2mea-suredbyH1andtheStandardModelexpectationincom-parisonforthedataupto1996andthefulldatasampleNCEventswithQ21994-1996data1994-1997data15000GeV2(14pb1)(37pb1)Nobserved1222Nexpected4.710.7614.82.13Probability61035.9102LimitsonLeptoquarksH1analysedtheneutralcurrentdatasampleintermsofLeptoquarksbycom-paringtheeventrateatvariousMbinswiththeStanda