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Study of Top Quark FCNC

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    Study of Top Quark FCNC Commun.Theor.Phys.(Beijing,China)52(2009)PP.662~64 ?ChinesePhysicalSocietyandIOPPublishingLtdVo1.52,No.4,October15,2009

    StudyofTopQuarkFCNC

    M.MohammadiNajafabadiandN.Tazik

    SchoolofParticlesandAccelerators,InstituteforResearchinFundamentalSciences(IPM),P

    .O.Box193955531,Tehran

    Iran

    2PhysicsDepartment

    ,

    SemnanUniversity,Semnan,Iran

    (ReceivedApril16,2009;RevisedJuly13,2009) AbstractWestudytheone-loopcontributionoftheeffectiveflavorchangingneutralcoupling

    s(FCNC)tcZonthe

    charmquarkelectricdipolemoment.Usingtheknownlimitsonthetopandcharmquarkselect

    ricdjpolemoments,we

    placelimitsontheseFCNCanomalouscouplings. PACSnumbers:14.65.Ha,12.15.Lk

    Keywords:topquark,flavorchangingneutralcurrent 1Introduction

    Thestandardmodel(sM)isinaverygoodagree

    mentwithpresentexperimentaldata.Nonetheless,itis believedtoleavemanyquestionsunanswered.andthisbe- liefhasresultedinnumeroustheoreticalandexperimen- talattemptstodiscoveramorefundamentalunderlying theory.Varioustypesofexperimentsmayexposetheex- istenceofphysicsbeyondtheSM.includingthesearch

    fordirectproductionofexoticparticlesathighenergy colliders.Acomplementaryapproachinhuntingfornew physicsistoexamineitsindirecteffectsinhigherorder

    processes.Sincetopquarkisfarmoremassivethanother SMfermions.itsinteractionsmaybequitesensitiveto newphysicsoriginatingathigherscale.jIfthereareany

    deviationsfromtheSMexpectationsinthepropertiesof thetopquark,theymayindirectlyleadtomodifications intheanticipatedbranchingfractions.

    IntheSM.duetotheGlashow-IliopoulosMaiani

    (GIM)mechanism,thetopquarkflavorchangingneutral current(FCNC1interactionsareabsentattreeleveland areextremelysuppressedatloopleve1.Therefore.theob- servationofanyFCNCtopquarkprocessasmokinggun fornewphysicsbeyondtheSM.

    IntheSMframework,FCNCinteractionsareinduced bytheWbosonchargedcurrentCabbibo-Kobayashi- Maskawa(CKM)transitionsinvolvingdowntypequarks

    intheloopswhicharemuchlighterthanthetopquark. InmodelsbeyondSMsuchasminimalsupersymmetric standardmodel(MSSM)orTechnicolortheory,although thetopquarkFCNCinteractionsarealsoinducedatloop leve1.theycanbegreatlyenhancedrelativetotheSM predictions.ForexampleinMSSM.inadditiontothe Wbosonloops,therearefourkindsofloopscontributing tothetopquarkFCNCinteractions.InMSSM,charged Higgs,chargino,gluino,andneutralinoloopscontribute Correspondingauthor,E-mail:mojtaba~ipm.ir tothetopquarkFCNCinteractions.Theoreticalbranch

    ingratiosofFCNCtopquarkdecaysinvariousmodels

arepresentedinTable1.23]Itisworthmentioningthat

    attheLHC,thebranchingfractionfortopFCNCde

    cay,Br(t}Zq),canbemeasuredwiththeprecisionof 6.1×100and3.1×10withtheintegratedluminosityof 100fband10fb_.,respectively.[4-5]Therearesevera1 studiesonthetopquarkFCNCwhichsomeofthemcan befoundinRefs.6121.

    Inthisarticle,ouraimistoconstraintthetopquark FCNCanomalouscouplings.intheprocessoft_+Zc.us

    ingeffectsinducedbytheelectricdipolemomentfEDM1 oftopquarkontheoneloopinducedEDMofthecharm quark.Intheanalysis,wewillusetheestimatedbounds ontheEDM'softopandcharmquarktoconstraintthe anomalouscouplings.

    Table1TheoreticalbranchingratiosofFCNCtopquark decaysinvariousmodels

    2EffectiveFCNCLagrangian

    ODetoo1thatisoftenusedtodescribetheeffectsof newphysicsatanenergyscaleofA,muchhigherthanthe electroweakscale,istheeffectiveLagrangianmethod.If theunderlyingextendedtheoryunderconsiderationonly becomesimportantatascaleA,thenitmakessenseto expandtheLagrangianinpowersofA-1:

    c=csM+?0t,(1)

    whereCsMisthestandardmodelLagrangian,Oi'sarethe operatorscontainingonlytheSMfields,72iisthedimen

    sionof0tandc4'saredimensionlessparameters.[351

    Inthetopquarksector,thelowestdimensionopera- t0rstbaltcontributet0FCNCwithtbeZverrexcanbe N0.4StudyofTopQuarkFCNC

writtenas[161

    ceff一—

    2co

    

    s

    

    Ow[Lzc+RzPRc+h.c.],(2) where9isthecouplingconstantofSU(2)L,Owisthe

    Weinbergmixingangle,andL. Rarefreeparametersde- terminingthestrengthoftheseanomalouscouplings.As sumingCPinvarianceL.

    R8xerea1.Intheaboverelation, .Rarethelefthandedandrighthandedprojectionop- erators.ThetopFCNCanomalousinteractionleadsto

    thefollowingbranchingfractionforthet_-?Zcfinthe limitofzeromassofb,cquarks): Br(t一兰

    

    (K+)(m;m)(m;+2m)

    2(m}m).(m}+2m)

    0.5(4+).

    RecentupperboundfromCDF fractionoftZq(q=u,c),is Therefore,onecanconclude

    (3)

    experimentforbranching 3.7%(with95%C.L.).1

    +<0.074.

ablerangefortheFCNCparameters(,n~R)l0

    i

    Ceff=dqq'Y5grUuqF,(4)

    -

    whereFistheelectromagneticfieldtensoranddis thetopquarkelectricdipolemoment(EDM)whichisa realnumberbyhermiticity.Oneshouldnotethatthisis aCPviolatingterm.Therefore,anonvanishingvaluefor

    EDMofafermionisofspecialinterestasitsignifiesthe presenceofCPviolatingjnteractions.

    IntheSM,topquarkcannothaveanEDMatleast

    tothreeloops.TheSMpredictionforthetopEDMis 1031_1032e-cmwhichistoosmalltobeobservable.In contrast,inextensionsofSMsuchasMSSM,thissitu

    ationchangessharplyandthetopquarkEDMcanarise attheoneloop.Inbeyondstandardmodeltheoriesthe typicaltopEDMisoforderof10-18100e-amwhichis

    largerthantheSMpredictionbymorethan10ordersof magnitude.[192o]

    ThecontributionoftopquarkFCNCintroducedby Eq.(2)totheonshellc57couplingisgiventhroughthe 3EstimationofConstraintsdiagramshownintheleftsideofFig.1.Usingtheef-

    Weconsidertheeffectiveinteractionofthequarksfectiveinteractionsandaftersomealgebra,

    therespective

    withonshellphotonstomakepredictionoftheacceptoneloopvertexcanbewrittenas xatx{dakf+(2)4(mt(+)一矾)4t~LnR(k-(Pl+P2)Pl?P2+mrn))q](km

    )((p一尼).m2)(2)

    Oneshouldnotethattherealrecontributionstobothmagneticandelectricdipolemomentsoft

    hecharmquark

    whichwehavekeptonlythetermscontributingtoEDM.Afterevaluatingtheintegralover:

where

    d.:47rsin2COS2Ow×dt×【(+),(t,.)+LRg(t)]

    c

    )=2×—

    1+3x

    

    t(xt-4/3)-2(2xt-

    1)log(xt)

    wherext=mt2fm2zandzc:m2cr1H2

    z.Inobtainingthe

    aboverelation,wehaveignoredofthetermsproportional to(mc2/2z),whichinfactisanegligiblequantity. InI21l,theauthorshaveestimatedtheupperbound of7x100onthetopand1x10onthecharmquark

    electricdipolemomentsusingtheexperimentalboundon neutronelectricdipolemoment.Thecombinationofthese boundsandEq.(6)lcadstotheexclusioncontourshown intherightsideofFig.1.However.inthisexclusion contouranotherparameterizationsfortopFCNCanoma- lOUScouplinghasbeenusedwhicharerelatedtothepa- rametersofEq.(2)by:R,LgY.Ifwecombine

    ourresultwiththatobtainedbyCDFexperiment,which mentionedbefore,theboundsonR,

    Lareestimatedas:

    <3×10,R<0.27.Thesevaluesarecompati

    =

    (6)

    blewiththeonesestimatedintheotherstudies.Inf161 theanomalouscouplingisconstrainedbyexperimen

    talupperboundsontheinducedflavorchangingneutral

couplingsofthelightfermions.Inderivingtheupper

    boundon,severalflavorchangingprocessessuchas

    KL_+』上一,BoB0mixingandB_l+2havebeen

    used.Finally,theobtainedupperboundonLin[16

    is5x10.Hence,theallowedregionforLfromthe

    presentworkiSoneorderofmagnitudesinallerthanthe

    oneobtainedinf161.Finally,wenotethat,fromTable

    1,MssMcangiveBr(t__+zc1aslargeas106orTech-

    nicolorcangiveBr(t-_+Zc1aslargeas10_...Therefore

    thenewboundson,Rcanbeusedtoconstrainthe

    parametersoftheorieswhicharesensitivetoFCNCsuch

    aSMSSMandTechnicolor.

    M.MohammadiNajafabadiandN.TazikVo1.52

    gA

    Fig.1(a)TheFCNCone-loopcontributiontothevertex;(b)TheexclusioncontourontopFCN

    Canomalous

    couplings.

    4Conclusion

    Inthisarticle,withintheframeworkoftheeffectiveLagrangianapproach,weperformedacalc

    ulationofthe

    radiativecorrectionsinducedonthecharmquarkelectricdipolemomentbytheeffectiveFCN

    CvertextcZ.Usingthe

    presentupperboundsonthetopandcharmquarkEDM'S,thenewconstraintsonthetopFCNCanomalouscouplings

    wereestimated:L<3×10

    3,R<0.27.Theseconstraintsareregularandcomparablewiththeonesobtainedin thepaststudiesandtheestimatedlimitonLisslightlybetter. References

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