Although direct detection of new particles will be the main focus of the LHC,
indirect New Physics searches are expected to provide useful complementary
information. In particular, precision measurements of rare processes occurring
in flavour physics are of utmost importance in constraining the structure of
the New physics low energy effective Lagrangian. In this paper, few key LHCb
studies, including $B_s-\bar{B}_s$ mixing and rare decays through the quark
level $b\to s$ loop transition, are presented to illustrate New Physics effects
at low energy.Comment: Proceeding of Moriond QCD 2008, 5 page

Hicheur, A.

English

arXiv

Although direct detection of new particles will be the main focus of the LHC, indirect New Physics searches are expected to provide useful complementary information. In particular, precision measurements of rare processes occurring in flavour physics are of utmost importance in constraining the structure of the New physics low energy effective Lagrangian. In this paper, few key LHCb studies, including $B_s-\bar{B}_s$ mixing and rare decays through the quark level $b\to s$ loop transition, are presented to illustrate New Physics effects at low energy.Although direct detection of new particles will be the main focus of the LHC, indirect New Physics searches are expected to provide useful complementary information. In particular, precision measurements of rare processes occurring in flavour physics are of utmost importance in constraining the structure of the New physics low energy effective Lagrangian. In this paper, few key LHCb studies, including $B_s-\bar{B}_s$ mixing and rare decays through the quark level $b\to s$ loop transition, are presented to illustrate New Physics effects at low energy

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arXiv:0805.2474v1  [hep-ex]  16 May 2008N EW P H Y SIC S SEA R C H AT LH C BA.HICHEURaOn behalfofthe LHCb collaborationEcole Polytechnique Federale de Lausanne -LPHE,Lausanne 1015,Switzerland.A bstractAlthough directdetection ofnew particleswillbe the m ain focus ofthe LHC,indirectNew Physics searches are expected to provide usefulcom plem entary inform ation. In par-ticular,precision m easurem entsofrare processesoccurring in avourphysicsare ofutm ostim portancein constrainingthestructureoftheNew physicslow energy eectiveLagrangian.In thispaper,few key LHCb studies,including B s   B s m ixing and raredecaysthrough thequark levelb ! s loop transition,are presented to illustrate New Physics eects at lowenergy.1 IntroductionW eak decays of hadrons are generically described by a low energy eective ham iltonianexpressed asan expansion oflocaloperatorsO i1:H eff =XiCiO i (1)TheW ilson coecientsC iincludetheshortdistanceeectsand arecom puted perturbativelyat the electroweak scale and then derived at the  m b scale through the renorm alizationgroup equations. The m atrix elem entsofthe O i operatorsrepresentthe long range eectsrelated to hadronization and arederived non-perturbatively,using varioustechniques(Q CDsum rules,Lattice,etc...).NotethattheO i alsom ix underrenorm alization,theconsequencebeing thata given Ci coecientm ay receivecontributionsfrom otherC j coecients:in thiscase,wetalk abouteectivecoecients,Ceffi associated to O i.In thisfram ework,theintervention ofvirtualnew heavyparticlesin loop dom inated processeswillaecttheC i coecients.W earethereforein search forany observablesensitiveto thesecoecientsand forwhich the theoreticaluncertaintiesarerelatively sm all.2 B s m ixingTheB s   B s m eson oscillation isdescribed by the B = 2 box diagram sshown in gure 1.sB sBbssb,W H,W Ht tsB sBbssb,W H,W HttFigure 1:Box diagram sforB s m ixing.In Standard M odel,the loop ism ediated by a W boson.Forillustration,the case where a charged Higgsisinvolved isdepicted.In the Standard m odel,the box diagram scarry the weak phase(VtbVts)2.The bestwayto probenew contributionsin thebox isto com pareB s and B s decaysto com m on C P nalstatesasafunction oftheB s propertim e.Sim ilarly totheB d case,thepreferred nalstatesfC P are the onesinduced by the b! ccs quark tree transition,leading to the golden m odeB s ! J=	 and otherm odeslessfavored experim entally such asB s ! J=	(0),B s ! caAdlene.Hicheur@ cern.chand B s ! D+s D s .Allthese m odescarry the weak phaseVcbVcs.The am plitude ofthe m ixing-induced asym m etry,A C P (t) = ( B s(t)! fC P )  (B s(t)! fC P ) ( B s(t)! fC P )+  (B s(t)! fC P )isproportionalto sin(2s),where 2s = 2arg( VtsVtbVcsVcb)=   0:04 to a precision of5% in theStandard M odel.Thissm allvalueim pliesthatthem easurem entofa sizeableam plitudewillpointdirectly to the intervention ofNew Physics.Table 1 shows results ofsensitivity studies perform ed for various decay channels with theLHCb sim ulated events. The num bershave been obtained fora statisticscorresponding tooneyearofdata taking atnom inallum inosity L = 2 1034cm  2 :s 1 .Sam ple Expected yield/2 fb  1 (2s)B s ! J=	 (+   )()3 8.5k 0.109B s ! J=	 (+   )(0)3 3k 0.142B s ! J=	 (+   )0()4 2.2k 0.154B s ! J=	 (+   )0()5 4.2k 0.080B s ! c(4h)(K+ K   )3 3k 0.108B s ! D+sD  s3 4k 0.133AllpureC P eigenstates - 0.046B s ! J=	 (+   )(K + K   )3;6130k 0.023Allm odes - 0.021Table 1:Expected yieldsand sensitivitieson 2s. Reconstructed subm odesare indicated between brackets. Forc reconstruction,4h m eansa com bination offourcharged kaonsorpions.Fora shortterm scenario with an integrated lum inosity of0:1 fb 1 thesensitivity to 2sisstillbetterthan 0:1,which m eansthata deviation of 0:3 would be detected with veryearly data.New physics contribution to the m ixing is usually param eterized in a m odel-independentway2:< B sjHtotaleff jB s >< B sjHSMeffjB s >= CB se2iB s (2)Itfollowsfrom thisparam eterization thatsin(2s)becom essin(2(s   B s)).FitsforCB sand B s param etershave been perform ed using available experim entaldata7. The recentTevatron resultson the m ixing phase and width dierence   s8,  1:20 < 2s < 0:06 and0:06 <   s < 0:30 ps 1 at 90% condence level,triggered a statisticalanalysis9 whichallowed to constrain the CB s   B s param eters space, suggesting a hint for beyond SMcontributions. No doubtthatthe com ing im provem entsin Tevatron resultsand above all,therstLHCb results,willdenitely clarify thepictureand help usquantify m oreaccuratelythe sizeofany New Physicscontribution.3 R adiative b! sb ! s is one ofthe benchm ark New Physics probe in b physics. It is m ediated by theelectrom agneticdipoleoperator,O 7 = s(m bR + m sL)bF,whereR = 1+ 5;L = 1  5.Theam plitude isthereforedriven by the eectiveW ilson coecientCeff7.G iven thatm s < < m b,one photon polarization issuppressed by m s=m b:the photon ism ostly right-handed in b decaysand left-handed in bdecays.However,enhancem entofthesuppressed polarization could com efrom New Physicscontributions.Furtherm ore,itcan beshown thatthe m ixing-induced asym m etry in B 0 ! V 0 decayshasthe sam e suppressionfactorasthe photon polarity.Severalradiativedecaysstudieshavebeen perform edin LHCb,am ongthem :B d ! K0(K +   ),and B s ! (K+ K   ) 10. These m odeshave been jointly analyzed and com m on selectioncutshavebeen applied,when possible.Forthe photon,a ECAL clusternotassociated to atrack isrequired,along with a transverseenergy cutto suppress0 background.To recon-structK 0 and ,im pactparam etersand particle ID cutsare applied to pionsand kaons,as wellas vertex quality requirem ents for K  and K K . B ight is used to reject prom ptbackground from prim ary production vertex.Thestudieshaveshown thatyieldsof68k and 11k signaleventsareexpected with 2fb 1forB d ! K0 and Bs ! ,respectively.W ith thisstatistics,a 1% sensitivity isexpectedfortheC P asym m etry.A dedicated photon polarization study wasperform ed fortheB s ! channel11 and has shown a sensitivity better than 0.2 for the suppressed polarizationfraction.4 Electrow eak b! sl+ l This transition is governed m ostly by electroweak and electrom agnetic penguin operators,O 7, O 9 and O 10. The rate is dom inated by jCeff9j2, jC10j2 and the sign of Ceff7. Aninteresting observable isthe leptonsforward-backward asym m etry which ishighly sensitiveto therelativesign ofCeff7and C10.In theleptonspairrestfram e,weconsidertheanglellofthe leptonswith respectto the B m eson m om entum .Theasym m etry isthen dened as:A F B (^s=m 2llm 2b)=R10dcoslld2 (B ! X sl+l )dcosllds^ R0 1dcoslld2 (B ! X sl+l )dcosllds^R10dcoslld2 (B ! X sl+ l  )dcosllds^+R0 1dcoslld2 (B ! X sl+ l  )dcosllds^(3)Thepoint s^0 wherethisquantity cancelsto zero hasa particulartheoreticalinterestsinceitisknown with a reasonableaccuracy12.Studieshavebeen perform edforthedecayB d ! K0+   13.Particularcarehasbeen takento apply selection cutsthatdon’tbiasthe dim uon invariantm assdistribution. 7.2k signalevents are expected for an integrated lum inosity of2 fb 1 . Figure 2 shows the resultingexpected asym m etry distribution,along with theoreticalpredictions on the shape (takenfrom reference14).Therem arkablefeatureofthepredictionsliesin thefactthat,besidethes = mll2 (GeV2)AFBFigure2:D im uon forward-backward asym m etry asa function ofthesquared dim uon invariantm ass.Theleftplotshows the experim entalresult while the rightplot shows theoreticalpredictions for severalworking hypotheses.Rightplot: the solid line isfor the SM -based prediction and the dotted and dashed lines representresultsfromdi erentSUSY-based scenarioswhere the signsofC10 and Ceff7W ilson coe cientsare  ipped.resultthatthe zero point ofthe asym m etry is known to a precision of 0:6 G eV 2 in theStandard M odel,m odels where Ceff7> 0 predictthatthe asym m etry does notcancel. Inthesam estudies,othervariableswith m inim altheoreticaluncertainties,such asthefractionoflongitudinalpolarization ofthe K 0,have been considered asinteresting probesand areexpected to providefurthersensitivity to New Physics.The analysisofB + ! K + l+ l  m odes(with l= e; )hasalso been considered15. Ithasbeen shown16 thatthefollowing ratio:R X =Rsm axsm indsd (B ! X + )dsRsm axsm indsd (B ! X e+ e  )ds(4),can be predicted with a very good precision in the Standard M odel. In particular forX = K ,thisratio isequalto one atthe 10 4 level. Substantialdeviationsfrom thisvaluecould occurfrom scalar sRblland pseudo-scalar sRbl5loperatorscontributing to theeective ham iltonian.The corresponding W ilson coecientsinclude the lepton m assesandarethereforeresponsiblefora possibledierence between electron and m uon m odes.Experim entally,selections have been optim ized to reject backgroundsoftype X l+ l  withbadly reconstructed X and ithasbeen shown thatthesensitivity to R K reachesfew percentwith an integrated lum inosity of10 fb 1 .5 B s ! +  Thisrarem odeism ediated by second orderannihilation diagram ssuch astheoneshown ingure 3.sBbstµ+µ−W,Z γ0 0 0, ,A h HFigure 3: Annihilation diagram ofthe decay B s ! + . In M SSM or D ouble Higgs M odel, Z boson or m ediation can be replaced by neutralHiggsbosons,asdepicted.It is suppressed by a factor  m 2=m2B in the Standard M odel,leading to a branchingratioof 3 10 9 ,17.A possibleenhancem entcould occurthrough neutralHiggsm ediationin constrained M inim alSuperSym m etry orTwo HiggsM odelswith large tan 17. In thatcase,phenom enology predicts (B s ! +   )/m2bm2tan6M 4A 0.O n the experim entalside18,the decay is easy to reconstruct but is em bedded in a hugebackground com ing from leptonic bdecays.Sensitivity studieshavebeen perform ed to testthediscovery powerasa function ofstatistics.Figure4 showstheresults.Early observationwith 2 fb 1 ispossible forSM -like rateswhile discovery can be envisaged with even lowerstatisticsifNew Physicsenhancesthe branching ratio.C onclusionThe key studies reviewed in this paper reectthe New Physicssensitivity tim eline fortheLHCb experim ent.Firstdata (integrated lum inosity <  0:5fb 1 )willgiveusrstanswerson any substantialenhancem entofStandard M odelsuppressed observables,such astheweakm ixing phase s in the B s oscillations or the rate ofB s ! +   . The 2 fb 1 m ilestonewillthen consolidatetherstobservations.Finaldata sam pleofwhatonecould qualify asarst"phase", 10fb 1 ,isexpected to giveusm oreinsighton theavourstructureofNewPhysicsthrough precisem easurem entsofratesand C P asym m etriesand willalsoallow m oresignicantdeterm ination ofdierentialrateasym m etriesand polarizationsin rareprocesses.R eferences1. G .Buchalla,A.J.Burasand M .Lautenbacher,Rev.M od.Phys68,1125 (1996).2. G .Buchalla etal.,arXiv:0801.1833 [hep-ph],and referencestherein.123456789100 1 2 3 4 5 6 7 8 9 10#)	"'*		(&+)& !'##2/;.0<σ7σ&## /:=.>0Figure 4: O bservation (3)and D iscovery (5)lim its for B s ! + as a function ofintegrated lum inosity incase where both signaland background are observed.3. CERN-THESIS-2006-042.4. CERN-THESIS-2007-051.5. CERN-LHCb-2007-027.6. CERN-LHCb-2007-101.7. UTtCollaboration,arXiv:0707.0636.8. See,e.g,D0 Collaboration,arXiv:0802.2255.9. UTtCollaboration,arXiv:0803.0659.10. CERN-LHCb-PHYS-2007-030.11. CERN-LHCb-PHYS-2007-147.12. A.Alietal.,Phys. Rev.D55,4105 (1997).13. CERN-LHCb-2007-038,CERN-LHCb-2007-039,CERN-LHCb-2007-057.14. A.Alietal.,Phys. Rev.D61:074024 (2000).15. CERN-LHCb-2007-034.16. G .Hiller,F.K ruger,Phys. Rev.D69:074020 (2004).17. K .S.Babu,C.F.K olda,Phys. Rev. Lett.84:228-231 (2000). G .Isidori,A.Retico,JHEP 0111:001 (2001).A.J.Burasetal.,Phys. Lett.B546:96-107 (2002).18. CERN-LHCb-2007-033.

New Physics Search at LHCB

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