r_b and new physics a comprehensive analysis

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1、hep-ph/9602438McGill-96/04UdeM-GPP-TH-96-34WIS-96/09/Feb-PHRandNewPhysics:bAComprehensiveAnalysisP.Bamerta,C.P.Burgessa,J.M.Clinea,D.Londona,bandE.NardicaPhysicsDepartment,McGillUniversity3600UniversitySt.,Montr´eal,Qu´ebec,Canada,H3A2T8bLaboratoiredePhy

2、siqueNucl´eaire,Universit´edeMontr´ealC.P.6128,succ.centre-ville,Montr´eal,Qu´ebec,Canada,H3C3J7cDepartmentofParticlePhysicsWeizmannInstituteofScience,Rehovot76100,IsraelAbstractarXiv:hep-ph/9602438v312Aug1996Wesurveytheimplicationsfornewphysicsofthedisc

3、repancybetweentheLEPmeasure-mentofRbanditsStandardModelprediction.Twobroadclassesofmodelsareconsidered:(i)thoseinwhichnewZbbcouplingsariseattreelevel,throughZorb-quarkmixingwithnewparticles,and(ii)thoseinwhichnewscalarsandfermionsaltertheZbbvertexatonelo

4、op.WekeepouranalysisasgeneralaspossibleinordertosystematicallydeterminewhatkindsoffeaturescanproducecorrectionstoRboftherightsignandmagnitude.Weareabletoidentifyseveralsuccessfulmechanisms,whichincludemostofthosewhichhavebeenrecentlybeenproposedinthelite

5、rature,aswellassomeearlierproposals(e.g.supersymmetricmodels).Byseeinghowsuchmodelsappearasspecialcasesofourgeneraltreatmentweareabletoshedlightonthereasonfor,andtherobustnessof,theirabilitytoexplainRb.11.IntroductionTheStandardModel(SM)ofelectroweakinte

6、ractionshasbeentestedandconfirmedwithunprecedentedprecisionoverthepastfewyearsusingmeasurementsofe+e−scat-teringattheZresonanceatLEP[1]andSLC[2].AparticularlystrikingexampleoftheimpressiveSMsynthesisofthedatacamewiththediscovery,atCDFandD0[3],ofthetopquar

7、kwithamasswhichisinexcellentagreementwiththevalueimpliedbythemeasurementsatLEP.Thebiggest—andonlystatisticallyimportant—flytobefoundsofarintheprover-bialSMointmentistheexperimentalsurplusofbottomquarksproducedinZdecays,relativetotheSMprediction.Withtheana

8、lysisofthe1994dataasdescribedatlastsummer’sconferences[1][2],thisdiscrepancyhasbecomealmosta4σdeviationbetweenexperimentandSMtheory.Thenumbersare:Rb≡Γb/Γhad=0.2219±0.0017,whileRb(SM)=0.2156.(1)TheSMpredictionassumesatopmas