Rare loop-induced decays are sensitive to New Physics in many Standard Model
extensions. In this paper we discuss the potential of the LHCb experiment to
very rare $\mathrm{B_s} \to \mu^+ \mu^-$ decays, radiative penguin $\mathrm{b
\to s}\gamma$ decays and electroweak penguin $\mathrm{b \to s\ell\ell}$ decays.
The experimental strategies and the expected sensitivities are presented.Comment: Submitted for the SUSY07 proceedings, 4 pages, LaTeX, 3 figure

Lenzi, Michela

English

arXiv

Rare loop-induced decays are sensitive to New Physics in many Standard Model extensions. In this paper we discuss the potential of the LHCb experiment to very rare $\mathrm{B_s} \to \mu^+ \mu^-$ decays, radiative penguin $\mathrm{b \to s}\gamma$ decays and electroweak penguin $\mathrm{b \to s\ell\ell}$ decays. The experimental strategies and the expected sensitivities are presented.Rare loop-induced decays are sensitive to New Physics in many Standard Model extensions. In this paper we discuss the potential of the LHCb experiment to very rare $\mathrm{B_s} \to \mu^+ \mu^-$ decays, radiative penguin $\mathrm{b \to s}\gamma$ decays and electroweak penguin $\mathrm{b \to s\ell\ell}$ decays. The experimental strategies and the expected sensitivities are presented

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arXiv:0710.5056v1  [hep-ex]  26 Oct 2007Rare B decays atLH CbM ichela Lenzi1 aO n behalfofLHCb collaborationINFN sezione diFirenze,via G .Sansone 1,50019 Sesto Fiorentino,Firenze,ItalyAbstract. Rare loop-induced decaysare sensitive to New Physicsin m any Standard M odelexten-sions.In this paper we discuss the potentialofthe LHCb experim ent to very rare B s ! + decays,radiativepenguin b ! s decaysand electroweak penguin b ! s‘‘decays.Theexperim en-talstrategiesand the expected sensitivitiesare presented.PACS. PACS-key 14.40.Nd { PACS-key 12.60.-i1 IntroductionAlthough the Standard M odel(SM ) is successfulinexplaining alm ostallexperim entalresults ofelem en-tary particlephysics,itispossiblethatphysicsbeyondthe SM exists just above the presently reachable en-ergy scale.New Physics is expected to be accessiblefrom raredecayswhere standard m odelcontributionsaresuppressed enough to allow potentialsm alleectsto em erge.In this paper we willfocus on the avor-changing neutralcurrents(FCNCs)processes,which,in theStandard M odel,areforbidden atthetreeleveland can proceed only via loop diagram s.Ifadditionalbox and/or penguin diagram s with non-SM particlescontributetotheseprocesses,thecom plexcouplingsofnew particlesm ay resultin an enhancem entofdecayratesorin the appearanceofnon-trivialCP-violatingphases.The LHC willbe a copious source ofB m esons,with a totalbb cross-section of 500 b.LHCb is aforward spectrom eterforb physics.Itsm ain featuresarea precisevertex detector,two RICH detectorsandaversatiletriggerwith a 2 kHzoutputratedom inatedby bb events.LHCb willoperate at a lum inosity ofL = 2 1032cm 2s 1 ,correspondingto2fb 1 peryear.The reconstruction ofrare b decays at LHC is achallengeduetothesm allratesand largebackgroundsfrom varioussources.The m ost criticalis the com bi-natorialbackground from pp! bbX events,containingsecondary verticesand characterized by high chargedand neutralm ultiplicities.Thestudiesreported in thispaperhavebeen perform edusingfullysim ulated events.2 Very rare decay B0s! G iven itssim ple experim entalsignatureand the cleartheoreticalpictureforitsprediction,them easurem entaEm ail:M ichela.Lenzi@ .infn.itoftheBR ofthevery raredecay B0s ! +  isan ex-cellentprobeofNew Physicseects.ThisFCNC pro-cessisalsohelicitysuppressed,resultingin apredictionfortheSM branchingratioof(3:55 0:33) 10 9 .Thisbranchingratioisknown toincreaseasthesixth poweroftheratiooftheHiggsvacuum in M SSM expectationvalue,tan [1].Any im provem enton thelim iton thisBR isthereforeparticularly im portantto probelarge-tan m odels.The anom alusm uon m agnetic m om entm easured atBNL disagreeswith the SM expectationby2.7 In thecontextofCM SSM atlargetan ( 50),this indicates that the gaugino m ass is in the range400-650 G eV/c2,that corresponds to a B0s ! + branching ratio of10 7   10 9 .The presentlim itonthe BR provided by Tevatron is< 7:5 10 8 at90%CL,that is expected to im prove up to < 2:0 10 8by the end ofthe Tevatron run.This is stillabout6tim es higher than the SM expected value.G iven theextrem ely low branching ratio ofthesignal,a detailedunderstandingofthebackgroundiscrucialin thisanal-ysis.Severalsources ofbackground were considered:com binatorialbackground (two realm uonsthatcom -binetoform asignalcandidate);m isidentied hadronsand exclusivedecayswith very sm allbranching ratios,thatcould sim ulate the signal.A good invariantm ass resolution is crucialto re-ducethecom binatorialbackground,butalsotoreducethe contam ination ofm isidentied two-body decays.G ood m ass resolution also allows a clear separationbetween Bd and Bs decays.G ood m uon identicationand good vertex resolution arealsocritical.TheLHCbtwo-track vertex resolution is 110m in the direc-tion,while the average precision ofthe track im pactparam eter is  40m .A G aussian t to the recon-structed invariantm ass distribution for signaleventsgivesaresolution of18M eV/c2.Theeciencytoiden-tify realm uons from B decays is  95% ,while theprobability to m isidentify a hadron either due to theoccupancy in m uon cham bersorbecause itdecaysinight is below 1% for hadrons with m om enta largerFlavor Physics Contributed Talk11010 20 0.1 0.2 0.3 0.4 0.5#$%& # '"		(&+)& !'##2/;.0 $# "# 'A-&## /&# $%L;#)#&## /:=.>0123456789100 1 2 3 4 5 6 7 8 9 10#)	"'*		(&+)& !'##2/;.0<σ7σ&## /:=.>0Fig.1.Left:BR( 10 9 )exclusion at90% condencelevelasa function ofthe integrated lum inosity (fb 1 ),undertheassum ption thatonly background ispresent.Right:BR( 10 9 )observation (3)ordiscovery (5)asa function oftheintegrated lum inosity (fb 1).than 10 G eV/c.The com bined trigger eciency,forsignalevents passing the selection used in the analy-sis,isgreaterthan 90% .The analysis is based on a very ecient soft pre-selection that rem oves a good fraction of the back-ground whilekeeping m ostofthesignalfalling withintheLHCb acceptance[2].Each eventisthen weightedby its likelihood ratio on the relevant distributions.Three likelihoods were dened: a geom etricallikeli-hood thattakesinto accountvariablesrelated to thevertex,pointing and isolation;them uon identicationlikelihood;and the invariantm asslikelihood.In Fig.1 (left),the 90% CL exclusion region forthebranching ratio isshown asa function oftheinte-grated lum inosity,undertheassum ption thatonly theexpected background is observed.The two lines cor-respond to two background hypothesis (i.e.,nom inaland shifted values),and theregion between them rep-resents the uncertainty com ing from the lim ited M Cstatistics.LHCb has the potentialto exclude the in-teresting region between 10 8 and the SM predictionwith very little lum inosity ( 0:5fb 1 ).In Fig.1 (right),theLHCb potentialto discoverasignalis shown as a function ofthe integrated lum i-nosity.LHCb hasthe potentialfora 3 (5)observa-tion (discovery) ofthe SM prediction with  2 fb 1( 6 fb 1)ofdata.3 Radiative decays b ! sTheradiativepenguin b ! s decay isanotherexam -pleofrem arkableinterest.Itstotalbranching fractionis very sensitive to physicsbeyond the SM asit m aybeaected by thepresenceofcharged HiggsorSUSYparticles in the loop.Presently,the world average isin good agreem ent with the theoretical SM predic-tion.Nevertheless,in the SM the em itted photon inradiativedecaysisexpected to be predom inantly left-handed;thisSM prediction isstilluntested,and right-handed com ponents arise in a variety ofnew physicsm odels.No clarifyingresultshavebeen obtained up tonow due to lim ited statistics.In principle,a testofthe Standard M odelcan bem adebym easuringthedirectCP violationthatresultsin adierenceofthedecayratesB ! X and B ! X.In SM the direct CP asim m etry is reliably predictedto be less than 1% ;however in som e SM extensionsthe contribution from new particlesin the loop couldincreaseitup to10%   40% [3].Unfortunatly,inclusivedecaysarewelldescribed theoretically butarediculttoaccessexperim entally;whileexclusivecasesarethe-oretically m uch m oredicultto calculate.A m ore sensitive test ofthe SM can be m ade bym easuring the CP asym m etriesfrom the interferenceofm ixing and decay am plitudesin radiativeB neutraldecayswhenB0s andB0s arerequiredtohavetransitionsto thesam enalstateX 0.Ifthephoton ispolarized,as predicted in the SM ,the CP asym m etry from them ixing should vanish [4].The current world averagefor this CP asym m etry is consistent with 0,but theerrorsarestilllarge.In the LHCb experim ent,radiative b ! s decayscan be reconstructed in the m odes B0 ! K 0 andB0s !  [5].The m ain source ofbackground is as-sum ed to be bb  inclusive events where at least oneb-hadron is em itted in the LHCb acceptance region.The reconstruction algorithm s and the oine selec-tion criteria forthe decaysB0 ! K 0,K0 ! K +  and B0s ! , ! K+ K   aresim ilar.Charged trackshaveto beconsistentwith therequested particleiden-tication and inconsistent with originating from thereconstructed prim ary vertex.Selected K 0 or can-didatesarecom bined with photon candidatesoftrans-verse energy largerthan 2.8 G eV,in orderto rem oveM ichela Lenzi Rare B decays atLHCbTable 1. Annualyields and background-to-signalratiosfor selective decays at LHCb.B =S ratios are lim ited byM C statistics.M ode Yield B =SB m eson decaysB0! K0 68000 < 0:6B0s !  11500 < 0:55B baryon decaysb !  750 < 42b ! (1520) 4200 < 10b ! (1670) 2500 < 18b ! (1690) 2200 < 18Electroweak decaysB0! K0s 18774  3B0! eeK0s 9240  5low energy  and 0.The reconstructed B candidateisrequired to be com patible with com ing from a pri-m aryvertex,which isaverypowerfulcutagainstcom -binatorialbackground.Finally,to suppressthe corre-lated background from the decays B0 ! K00 andB0s ! 0,a cuton theK 0 and  decay (helicity)an-glewith respecttotheB direction isapplied.Them assdistributionsofB candidatesafterthetriggerand o-line selection are expected to be (69:9 2:2)M eV=c2and (70:9 2:1)M eV=c2 respectively.Theexpected an-nual(2 fb 1 )yieldsand background oversignalratios(B =S)aregiven in Tab.1.4 b !  polarization m easurem entsRadiativedecaysofpolarized b baryonsto b ! representanattractivepossibilitytom easurethehelic-ity ofthephoton em itted in theb ! squarktransition[6].Thephoton polarization can betested by m easur-ing the angular distribution ofthe photon in the bdecay oreven through the angulardistribution oftheproton com ing from the ! p   decay.The study ofthis channelis challenging becausethe long lifetim e of baryon m eans that it willtyp-ically traverse a large fraction ofthe tracking systembeforedecaying.A possiblesolution isto considerde-caysto heavier resonances;thesubsequentdecay toK   allows to trace back the decay ofthe b.Theevent selection is sim ilar to the one presented aboveforB m esons[7].The expected eventyieldsand B =Sratiosfor2 fb 1 are given in Tab.1.Itcan be notedthattheheavier m odesarehavea higherstatisticalpower,butsince the distribution ofthe proton polar-ization is expected to be at (i.e.the proton asym -m etry isuniform p = 0 due to parity conservation),the intrinsic sensitivity islower.The m ain conclusionis that,assum ing a b polarization ofat least 20% ,LHCb can m easuretheright-handed com ponentofthephoton polarization from b ! (1115) decaysdownto 15% at 3 signicance after ve yearsofrunning.Theadditionalcontribution from the(X)resonancesto the m easurablerangehasbeen estim ated to be2%at m ost.The dependence ofthe photon polarizationsensitivity on theinitialb polarization (in the rangeP b = 20  100% )hasbeen found to be ofthe orderofa few percent.5 A F B m easurem entElectroweak b ! s‘‘ decay is a FCNC processwhichproceedsvia a b! s transition through a penguin di-agram .New Physics processes can therefore enter atthe sam elevelasSM processes.In particularthe branching ratio asa function ofthesquared invariantm assofthe dilepton system can beaected in m ostNew Physicsscenarios.However,the experim entally accessible exclusive de-caysare aected theoretically by hadronic uncertain-ties.A possiblesolutionistostudyratioswherehadronicuncertaintiesaresignicantly reduced.Theforward{backward asym m etry A F B isdened forthetransition b ! s‘‘sby theangle‘ between the‘+and theb hadron ightdirectionsin thedi-lepton restfram e.The shape ofthe asym m etry A F B as a func-tion ofthe lepton-lepton eective m ass m 2‘‘and es-pecially the position ofthe zero crossing (i.e.the m 2‘‘valuecorresponding to A F B = 0)arealm ostunaectedby hadronic form factoruncertainties,thusprovidinga good basisforsearching fordeviationsfrom the SMpredictions[8].Thanks to its very clean experim entalsignature,theexclusive decay Bd ! K has been chosen to ex-tractA F B .Theselection isbased on theidenticationoftwo m uons with opposite charge and ofthe rele-vanthadronic nalstate [9].Very strictrequirem entson the vertex quality are applied to reduce the back-groundsfrom cascadesem ileptonic b ! c,c ! sand from twosem ileptonicb ! cdecays.Thesepro-cesseshavetobewellundercontrol,astheycan inducea biason A F B .Thebackground from ccresonancesisrem oved by vetoing theJ= and  (2S)m asswindowsin the di-m uon eective m assdistribution.LHCb expects a 15 M eV/c2 resolution on the Bm assand 10M eV/c2 on thedi-m uon m ass.Theresolu-tion forF B is4m rad.Theexpected yield foronenom -inalyearofrunning atLHCb (2 fb 1 ) is about7200events with a background to signalratio B =S ’ 0:5.The overalltriggerand reconstruction eciency ises-tim ated to be around 1% .Using theresultsobtained from thefullsim ulationofthe B0 ! K0 channel,LHCb hasestim ated thesensitivity to the forward-backward asym m etry in a"toy"M C study.ThetypicalbehaviourofA F B versusm2,after one year ofrunning at the nom inallum i-nosity (2fb 1 )isshown in Fig.2.W ith 2 fb 1 ofdatathe precision on the pointofzero-crossing (A F B = 0)isexpected to be 0.46 G eV 2=c4;while at10 fb 1 theprecision im proves to 0.27 G eV 2=c4.No bias in them ean ofthem easured zero-crossing pointisobserved.Flavor Physics Contributed Talk)4/c2 (GeV2q0 2 4 6 8FBA-0.6-0.4-0.200.20.40.6_:p`	ﬁvjlAh¶·¸x;>=ÔµÁÆ¾ÃËÁÄ½§åÃÂÁÄÉyÀßÑËÂÉy×gÓ¦Àº¸÷ÌFig.2.TheA F B distribution from an exam ple2 fb 1toyexperim ent.The solid line showsthe tresult.Recenttheoreticalwork [10]hashighlighted otherinterestingasim m etriestobestudied,asthelongitudi-nalpolarizationfraction oftheK 0(FL )and thesecondofthetwo polarization am plitudeasym m etries(A(2)T).The param etersare allpredicted with high precisionfrom theory in the SM and m any extensions beyondthe SM .Theavailablestatisticsarelim ited by the re-quirem entto restricttheregion ofthedi-m uon m assesfrom 1 to 6 G eV 2,which isfavoured by sm alltheoreti-calerrors.In thisregion,theLHCb expected resolutionwith an integrated lum inosity of2 fb 1 is0.016 in FLand 0.42 in A(2)T[11].6 R K m easurem ents atLH CbFinally,the ratio ofb ! s and b ! ees decaysinany exclusive m ode is also a clean probe ofthe SM .Lepton universalitypredictsthisratiotobeunity witha theoreticalerrorbelow 1% [12].In theSM ,theratioofb ! sand b ! eesdecaysisexpected to beverycloseto unity,nam ely R K = 1:000 0:001.Deviationsoftheorderof10% can occurwith neutralHiggsbosonexchange in m odels that distinguish between leptonavours(foristance,them inim alSUSY m odelatlargetan).In LHCb thereconstructionofthetwodecaym odesB+ ! K +  and B + ! K + ee allowsan extraction oftheratioR K ofthetwobranchingfractions,integratedover a given di-lepton m ass range [13].The two de-cays are reconstructed with the sam e procedure andrequirem ents described above,except that a properbrem s-strahlungcorrectionisessentialin theB+ ! K + eechannel.Thedi-lepton m assrangeischosen to be1<m2ll< 6 G eV2=c4 in orderto avoid ccresonances.Theeventyields are extracted from a two-dim ensionaltto theK ‘‘and ‘‘m assesin orderto takeinto accountthe backgrounds from b ! J= s and B0 ! K 0‘‘.Theexpected yieldsaregiven in Tab.1.W ith 10 fb 1we expect a relative error on R K between 4% and6% depending on the levelofbackground and the ef-ciency ofthe trigger.The study ofthe m ost likelysourcesofsystem atic errorsshowsthatthiserrorwillbe statistics-dom inated.7 ConclusionsThe LHCb experim ent has a prom ising potentialforthe study ofrare loop-induced decays,which are sen-sitive to new physicsin m any Standard M odelexten-sions.In particular,forthe very raredecay B0s ! ,present experim ents will detect a signal only whenthe BR is strongly enhanced by New Physics.W itha sensitivity exceeding the BR expected in the SM ,LHCb willbeableto discoverboth enhancem entsandsuppression.In addition,LHCb hasgood potentialform easuring the helicity ofthe photon em itted in theb ! s decay,the forward-backward asym m etry AF Bfor the transition b ! llsand the ratio ofb ! sand b ! eesdecaysin a num berofexclusivem odes.The experim entalstrategies,the expected annualsignalevent yields and the estim ates on backgroundto signalratioshavebeen presented.References1. A.J.Buras,Phys.Lett.B 566,(2003)115G . Buchalla and A.J. Buras, Nucl.Phys.B 400,(1993)225M .M isiak and J.Urban,Phys.Lett.B 451,(1999)1612. D .M artinez,LHCb-2007-0333. K .K iers,A.Soni,G .-H.W u,PhysRev.D 62 (2000)1160044. D .Atwood,M .G ronau and A.Soni,Phys.Rev.Lett.79,(1997)1855. L. Shchutska, A. G olutvin and I. Belyaev, LHCb-2007-0306. T.M anneland S.Recksiegel,Nucl.Part.Phys.24,(1998)9797. F.Leggerand T.Schietinger,LHCb-2006-0138. M . G ronau, Rencontres de M oriond 2003,hep-ph/03042409. J.D ickens,V.G ibson,C.Lazzeroni,M .Patel,LHCb-2007-038J.D ickens,V.G ibson,C.Lazzeroni,M .Patel,LHCb-2007-03910. F.K ruger and J.M atias, Phys.Rev.D 71 (2005)09400911. U.Egede,LHCb-2007-5712. A.Ali,ICHEP 2002,hep-ph/021018313. P.K oppenburg,LHCb-2007-034

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