We discuss the latest studies on rare semileptonic decays of $B$-mesons and $\Lambda_b$ at the ATLAS detector

Crosetti, G

Policicchio, Antonio

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Rare Semileptonic Beauty Decays in ATLAS∗A. PolicicchioandG. CrosettiUniversity of Calabria and INFN Cosenza,Via P.Bucci, 87036 Arcavacata di Rende, Rende, ItalyWe discuss the latest studies on rare semileptonic decays of B-mesonsand Λb at the ATLAS detector.PACS numbers: 81. IntroductionRare B-decays, produced by b → s, d quark FCNC transitions, are for-bidden at the tree level in the Standard Model (SM). These decays occurat the lowest order only through one-loop “penguin” and “box” diagrams.The branching ratios of these decays are very small: from 4× 10−5 for rareradiative decay B0d → K∗γ to 10−15 for rare Cabibbo suppressed leptonicdecay B0d → e+e−.Rare B-decays careful investigation is mandatory for testing ground ofthe Standard Model and offer a complementary strategy in the search ofnew physics by probing the indirect effects of new interactions in higherorder processes. The probing of loop-induced couplings provide a means oftesting the detailed structure of the SM at the level of radiative corrections.In particular, FCNC involving b → s, d transitions, provide an excellentprobe of new indirect effects by yielding informations on the masses andcoupling of the virtual particles running in the loops and this explain theattention they have received in recent years. The study of such decays givealso other opportunities: to find the values of |Vts| and |Vtd| CKM matrixelements and to provide new informations on long-distance QCD effects inmatrix elements of the tensor current.∗ Presented at Physics at LHC conference, Krakow, Poland, 3-8 July 2006on behalf of ATLAS B-Physics group(1)ATL-PHYS-CONF-2006-01125 September 20062 policicchio printed on September 22, 2006In the last years the B-factories BaBar and Belle presented the firstresults for B → (K∗,K)l+l− branching ratios and forward-backward asym-metry (AFB) in these rare semileptonic decays ([8],[6],[7]). The experi-mental branching ratio values are consistent with SM predictions. Moreprecise measurements are instead needed for differential dimuon invariantmass and forward-backward asymmetry distributions, in order to discrimi-nate between SM and new physics predictions.In this report we will pay our attention to the semileptonic decays withµ+µ− pair in final state. We discuss the latest simulation results at ATLASdetector. The branching ratios in SM of these decays can be found in Table1.Decay channel Br. ratio Ref. Form Factors Wilson Coeff.B+ → K+µ+µ− 3.5 × 10−7 [1] QCD LCSR NNLOΛb → Λµ+µ− 2.0 × 10−6 [2],[3] HQET NLOB0d → K0∗µ+µ− 1.3 × 10−6 [4],[5] Quark Model NLOB0s → φ µ+µ− ∼ 10−6 [4],[5] Quark Model NLOTable 1. Branching ratios in SM of rare B-decays with µ+µ− pair in final states.2. Theoretical DescriptionFrom the theoretical point of view, the b → q (q = s, d) transitions aredescribed using the effective HamiltonianHeff = −4GF√2VtbV∗tq10∑i=1Ci(µ)Oi(µ) (1)in the form of Wilson expansion (see e.g. [9]). The set of Wilson coef-ficients Ci(µ) depends on the current model and contains the lowest ordermodel contributions and perturbative QCD corrections. The scale param-eter µ is approximately equal to the mass of b-quark (∼ 5GeV ). Thisparameter separates the perturbative and nonperturbative contributions ofthe string interactions. The nonperturbative contribution is contained inthe matrix elements of basis operators Oi(µ) between the initial and finalhadronic states. For the calculation of these matrix elements it is necessaryto use different decay-specific nonperturbative methods: QCD Sum Rules,Heavy Quarks Effective Theory, Quark Models and Lattice calculations.The accuracy in nonperturbative calculations depends on a method, but itis not less than 15%. The accuracy of the Wilson coefficient for with NLOand NNLO QCD corrections ([10]) is not greater than 15% if µ parameterranges in [mb/2, 2mb].policicchio printed on September 22, 2006 33. ATLAS Muon Trigger StrategyAt the start up, the target peak luminosity for the LHC is 2×1033cm−2s−1,rising to the full design luminosity of 1034cm−2s−1 after a few years of run-ning. There will be an average of 4.6 and 23 interactions per bunch crossingfor initial and design luminosity respectively. About 1% of collisions willproduce a bb¯ pair. The challenge for the trigger is to select those eventsof most interest for the ATLAS B-Physics program ([11]), within the lim-ited trigger and storage resources available. The ATLAS B-Physics trigger([12],[13]) is based on a lepton signature at the first level, which can be ac-companied by additional, lower transverse energy signature of leptons andjets. These signatures are refined in the higher trigger levels where specificdecays are reconstructed.For semileptonic dimuon decays, the trigger will be limited to a dimuontrigger. This is based on the detection of two muons found in the first triggerlevel with a transverse momentum (pT ) threshold of 6GeV and 4GeV respec-tively. At the second level, the muon pT measurements will be confirmed inthe muon precision chambers, the tile calorimeter and then extrapolated tothe Inner Detector in order to reject muons from pi/K decays. At the EventFilter level, oﬄine track quality reconstruction, vertex and mass cuts willbe applied to select candidates for specific exclusive or semi-inclusive decayssuch as J/Ψ → µ+µ− and B → µ+µ−(X). This strategy can be used atlow initial luminosity and also at nominal LHC 1034cm−2s−1 luminosity.4. Data SamplesA series of Monte Carlo data production, called Data Challenge (DC),has started in 2002. Their aim is the validation of the computing and soft-ware models. They also provide large data samples for the preparation ofthe physics analysis. The data sample for the present studies have beenproduced in the framework of the DC2 production, using the final ATLASdetector layout, simulation based on Geant4 and new GRID-oriented soft-ware ([14],[15],[16]).Below we introduce the basic results of the data analysis. The main the-oretical modeling basis for Monte Carlo generation of data samples is shownin Table 1. Pythia ([17]) and Evtgen ([18]) Monte Carlo software packageshave been used in the framework of ATLAS software for data production.The first level trigger cuts on muons and Inner Detector acceptance cutspseudorapidity (|η| < 2.5 and pT > 0.5GeV ) on stable hadrons and muonshave been applied at the generation level for all the decay channels. Thenumber of events expected for semileptonic dimuon decay channel after trig-ger and acceptance cuts (triggerable and reconstructible) events is shown inTable 4 for 30fb−1 integrated luminosity (3 years at 1033cm−2s−1).4 policicchio printed on September 22, 2006Decay channel Br. ratio Events in 30fb−1B+ → K+µ+µ− 3.5 × 10−7 40000Λb → Λµ+µ− 2.0 × 10−6 28000B0d → K0∗µ+µ− 1.3 × 10−6 120000B0s → φ µ+µ− ∼ 10−6 21000Table 2. Number of expected events in 30fb−1 integrated luminosity after first leveltrigger and Inner Detector acceptance cuts.The impact of trigger and acceptance cuts has been studied for Λb →Λµ+µ− channel. Trigger cuts slightly prefer higher dimuon invariant massand a slight suppression of |AFB | in low q2/M2Λb region has been found, butthey do not change significantly the dimuon invariant mass and the forward-backward asymmetry differential distributions. We have to point out thatthese distributions are very sensitive to SM extensions.4.1. Background SamplesThe semileptonic decay channel with cc¯ resonances (J/Ψ and Ψ(2S))decaying into two muons represent an irreducible background source forrare decays. Only a cut on dimuon invariant mass around mass peaks canremove it. B+ → K+J/Ψ(µ+µ−) and B+ → K+Ψ(2S)(µ+µ−) data sam-ple have been fully simulated and reconstructed in order to evaluate theresonances mass width. Semileptonic decays of both b and b¯ quarks anddouble semileptonic decays of b quarks (b → cµν, c → sµν) can be acombinatorial background source for rare semileptonic decays. A bulk ofbb¯ → µ+µ−X events with muons and hadrons satisfying the trigger and ac-ceptance cuts have been produced in order to evaluate combinatorial back-ground events after reconstruction cuts. Other specific decays channels canrepresent background sources for the decay channel under investigation. Inparticular kaons and pions misidentification as muons can generate back-ground events. In the current studies these decay channels have been notincluded.5. Simulation of rare semileptonic B-decays5.1. B+ → K+µ+µ− decayFor B+ → K+µ+µ− decay, the AFB vanishes in all the dimuon invariantmass spectrum in the SM framework. Non zero values for AFB are predictedin some beyond SM theories ([20],[21]). The forward-backward asymmetryis defined aspolicicchio printed on September 22, 2006 5AFB(sˆ) =∫ 10dΓdsˆzˆdzˆ − ∫ 0−1dΓdsˆzˆdzˆ∫ 1−1dΓdsˆzˆdzˆ(2)where zˆ = cosθ (θ is the angle between µ+ and K+ in the µ+µ− restframe) and sˆ = (pµ+ + pµ−)2/M2Λb = q2/M2Λb is the dimuon invariant mass.After trigger and oﬄine analysis cuts and considering a 75% first leveltrigger efficiency, 1500 signal events are expected in 30fb−1 integrated lu-minosity. Due to limited statistic of simulated background events available,only an upper limit of 10000 background events was determined. Theseresults will be soon updated. The simulation shows that the trigger and theoﬄine cuts do not change the shape of the dimuon mass spectrum and AFBdistribution, as shown in Figure 1.2+B/M2-µ+µM0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9FBA-0.4-0.3-0.2-0.1-00.10.20.30.4forward-backward asymmetry2+B/M2-µ+µM0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1entries/0.02210310dimuon mass distributionFig. 1. Dimuon mass spectrum and AFB distribution after trigger and oﬄine anal-ysis cuts for B+ → K+µ+µ− decay. In dimuon mass spectrum, the dotted and con-tinuous lines represent respectively events at the generation level and after analysiscuts on reconstructed events. In the AFB distribution, the circular and the squaredpoints are events at the generation level and events after analysis cuts respectively.In Figure 2(a) the AFB distribution is presented. The point in red are theexperimental Belle collaboration measurements ([7]) while the point in blueare the ATLAS expected measurements after 30fb−1 integrated luminosity.ATLAS will give an important contribution to AFB measurements.5.2. Λb → Λµ+µ− decayFor Λb → Λµ+µ− decay, the AFB is very sensitive to SUSY both forsmall and large value of dimuon invariant mass. After trigger and oﬄineanalysis cuts and considering a 75% first level trigger efficiency, for 30fb−1about 800 signal and 4000 upper limit background events are expected.The dimuon invariant mass q2/M2Λb is divided into three region intervals:6 policicchio printed on September 22, 2006the first interval is from (2mµ/M2Λb) to the zero-point, the second intervalis from the zero-point to lower boundaries of cc¯ resonances and the last isfrom resonances area to (MΛb−MΛ)2/M2Λb . The results are shown in Figure2(b). Upper point set corresponds to the theoretical SM predictions ([2])and the lower set corresponds to one of the possible non-SM prediction ([3]).ATLAS will be able to separate SM from some of its extensions with highconfidence level.Fig. 2. Forward-backward asymmetry for (a) B+ → K+µ+µ− decay (see Section5.1) and (b) Λb → Λµ+µ− decay (see Section 5.2).5.3. B0d → K0∗µ+µ− and B0s → φ µ+µ− decaysThe branching ratios and the AFB differential distributions of suchsemileptonic decays are very sensitive to SM extensions. After trigger andoﬄine analysis cuts and considering a 75% first level trigger efficiency, weexpect about 2500 signal events for B0d → K0∗µ+µ− and 900 signal eventsfor B0s → φ µ+µ− after 30fb−1 integrated luminosity. A upper limits of10000 events was determined for background events for both decays. InFigure 3 the AFB is presented for B0d → K0∗µ+µ− decay. Solid line cor-responds to SM predictions, according to [4]. Dashed and dotted lines areboundaries for MSSM predictions ([19]). The points in black with largeerror bars are the experimental Belle collaboration measurements ([6]) andthe points with small arror bars the ATLAS expected measurements after30fb−1. The bars in abscissa coordinate represent the kinematic intervalsand the cc¯ resonances kinematic area have been excluded. We can see, itwill be enough statistic after 3 years of LHC operations for SM confirma-tion and setting strong limits on SM extensions or registration of beyondSM contributions.policicchio printed on September 22, 2006 7Fig. 3. Forward-backward asymmetry for B0d→ K0∗µ+µ− decay (see Section 5.3).6. ConclusionsA full simulation of rare semimuonic B-decays for ATLAS detector withrealistic final detector layout has been performed. The corresponding back-ground has been simulated even if more statistic is needed and some specificbackground source have to be included. In Table 6 the number of eventsexpected after 30fb−1 integrated luminosity for the semileptonic dimuondecay channels is shown. Also the number of background events and thesignal reconstruction efficiency have been reported. The simulation stud-ies show that ATLAS detector will be capable to extract signals of all raresemimuonic B-decays investigated until now and achieve the sensitivity al-lowing to identify presence of the SUSY contributions.Decay channel signal eff. expected signal events expected BG eventsB+ → K+µ+µ− 0.05 1500 < 10000Λb → Λµ+µ− 0.038 800 < 4000B0d → K0∗µ+µ− 0.03 2500 < 10000B0s → φ µ+µ− 0.04 900 < 10000Table 3. Number of expected signal and background events in 30fb−1 integratedluminosity and signal reconstruction efficiency.REFERENCES[1] A.Ali et al., Phys. Rev. D 66 034002, (2002).8 policicchio printed on September 22, 2006[2] C.H.Chen et al., Phys. Rev. D 64 074001, (2001).[3] T.M.Aliev et al., Nucl. Phys. B 649, 168 (2003).[4] D.Melikhov et al., Phys. Rev. D 57 6814, (1998).[5] D.Melikhov et al., Phys. Rev. D 62 014006, (2000).[6] A. Ishikawa et al. (The Belle Collaboration), Phys. Rev. Lett. 96, 251801(2006).[7] K. Abe et al. 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Rare semileptonic beauty decays in ATLAS

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