The LHC experiments will perform sensitive tests of physics beyond the
Standard Model (BSM). The investigation of decays of beauty hadrons represents
an alternative approach in addition to direct BSM searches. The ATLAS and CMS
efforts concentrate on those B-decays that can be efficiently selected already
at the first and second level trigger. The most favorable trigger signature
will be for $B$-hadron decays with muons in the final state. Using this
trigger, ATLAS and CMS will be able to accommodate unprecedentedly high
statistics in the rare decay sector. These are purely dimuon decays, and
families of semimuonic exclusive channels. Already with data corresponding to
an integrated luminosity of \ensuremath{1 fb^{-1}}, the sensitivity in the
dimuon channels will be comparable to present measurements (world average). The
strategy is to carry on the dimuon channel program up to nominal LHC
luminosity. In particular the \ensuremath{B_s \to \mu\mu} signal with
\ensuremath{\sim}5 sigma significance can be measured combining low luminosity
\ensuremath{10^{33}cm^{-2} s^{-1}} samples with those of one year of LHC
operation at a luminosity of \ensuremath{10^{34}cm^{-2} s^{-1}Comment: Submitted for the SUSY07 proceedings, 4 pages, 2 figures, 4 table

Crosetti, G.

Policicchio, A.

English

arXiv

The LHC experiments will perform sensitive tests of physics phenomena beyond the Standard Model (BSM). Investigation of decays of beauty hadrons represents an alternative approach in addition to direct BSM searches. The ATLAS and CMS efforts concentrate on those B decays that can be selected already at the first and second trigger levels. The most favorable trigger signature will be for B hadron decays with muons in the final state. Using this trigger, ATLAS and CMS will be able to accommodate unprecedentedly high statistics in so called Golden LHC channel Bs->J/ÏÏ, as well as in the rare decays sector. These are purely di-muon decays, and families of semi-muonic exclusive channels. Already with 1 fb-1 the sensitivity in the di-muon channels will be comparable to today worlds statistics. The strategy is to carry on the di-muon channel programme up to nominal LHC luminosity. In particular the BsâÎ¼Î¼ signal with 4.3 sigma significance can be measured combining low luminosity samples with those of one year of LHC operation at a luminosity of 10^34cm-2 s-1. This precision allows excluding or confirming the SM unambiguously

Policicchio, Antonio

Crosetti, G

CERN Document Server

Study of DiMuon rare beauty decays with ATLAS and CMSSUSY 2007July 26 ­ August 1, 2007Karlsruhe, GermanyAntonio Policicchio ­ Giovanni CrosettiUniversità della Calabria & INFNon behalf of ATLAS and CMS B­Physics groups A. Policicchio and G. Crosetti – Università della Calabria & INFN CosenzaSUSY07– July 26 ­ August 1, 2007 – Karlsruhe (Germany) Slide 2OutlineIntroductionATLAS and CMS experiments at LHC from a B­Physics point of viewDiMuon triggerBs in CMS and ATLASb s  rare decays in ATLASConclusionsA. Policicchio and G. Crosetti – Università della Calabria & INFN CosenzaSUSY07– July 26 ­ August 1, 2007 – Karlsruhe (Germany) Slide 3B­Physics with ATLAS and CMSLHC: proton­proton collisions at √s=14TeV and bunch crossing rate 40MHzHigh bb production cross section: σ(bb)=500µb105 bb pairs/s at initial low luminosity ~1033cm­2s­1Bs system can be fully exploited at the LHCATLAS and CMS design dedicated to high­pT physicsmajority of B­events has low pT particlesa challenge for the trigger and the analysisB­decays with muons in the final state are the most promising B­Physics programmeCP violation (low luminosity, e.g. B  J/ X)Bs oscillation (low luminosity, e.g. Bs   Ds)rare decays (even with high luminosity, B   and b  s )A. Policicchio and G. Crosetti – Università della Calabria & INFN CosenzaSUSY07– July 26 ­ August 1, 2007 – Karlsruhe (Germany) Slide 4Rare decays (1)Bs   transition forbidden at the tree level, occur through loop diagramsvery low branching ratio in SMBR(Bs) = (3.35±0.32)⋅10­9sensitive to new physics: some BSM models predict a larger branching ratio than SM (e.g. in SUSY BR~(tanβ)6 about 100 times larger than SM)long­distance contributions expected to play a negligible rôleCDF measurements with ~1fb­1BR(Bs) < 8⋅10­8 @ 90% CLDØ measurements with ~2fb­1BR(Bs) < 7.5⋅10­8 @ 90% CLA. Policicchio and G. Crosetti – Università della Calabria & INFN CosenzaSUSY07– July 26 ­ August 1, 2007 – Karlsruhe (Germany) Slide 5Rare decays (2)b  s  FCNC transitions forbidden at the tree level, occur through loop diagramslow branching ratio (~10­7)sensitive to new physicsFor semimuonic exclusive decays (the only accessible at LHC), theorists suggest to measure variables describing dimuon systemforward­backward asymmetry (AFB) as a function of szero of the AFB curve: s0=2C7 / C9 (quite robust with respect to hadronic uncertainties)Current B­factory data for inclusive b  s  branching ratios and AFB are in accordance with SM, but suffer from large uncertaintiesLHC measurements will improve this situationRare decays need very precise measurements to find deviations from SM predictionsexcellent calibrations and efficient muon triggerA. Policicchio and G. Crosetti – Università della Calabria & INFN CosenzaSUSY07– July 26 ­ August 1, 2007 – Karlsruhe (Germany) Slide 6ATLAS and CMS detectorsATLAS and CMS are general purpose detectors|η|<2.5B­physics using trigger with relatively high pT muonsfor B­Physics ATLAS and CMS performances are similar (especially for vertexing and muon system acceptance)A. Policicchio and G. Crosetti – Università della Calabria & INFN CosenzaSUSY07– July 26 ­ August 1, 2007 – Karlsruhe (Germany) Slide 7Overview of ATLAS triggerThree level triggerLVL1coarse granularity calorimeter and muon informationidentifies Regions of Interest (RoI) for further processingLVL2full granularityconfirm LVL1 triggerprecision muon chamber and inner detector measurements in LVL1 RoIEvent Filter (EF)refine LVL2 selection using offline­like algorithmsalignment and calibration data availableB­Physics is accounted for 5­10% of total trigger resources: it must be fast, efficient and selectiveA. Policicchio and G. Crosetti – Università della Calabria & INFN CosenzaSUSY07– July 26 ­ August 1, 2007 – Karlsruhe (Germany) Slide 8Overview of CMS triggerTwo level triggerLevel 1 is based on muon detector and calorimeter measurements40MHz  → ~100kHzlatency 3.2µsHigh Level Trigger (HLT) with fast reconstructionuses reconstruction code and informations similar to off­line 100kHz  → ~150HzA. Policicchio and G. Crosetti – Università della Calabria & INFN CosenzaSUSY07– July 26 ­ August 1, 2007 – Karlsruhe (Germany) Slide 9ATLAS/CMS trigger for rare decaysATLAS dimuon triggerLVL12µ RoI with pT(µ)>6GeV (500Hz @ L=1033cm­2s­1)LVL2confirm each µ RoI with precision muon chamber and inner detector measurementsdimuon vertex reconstructionEFrefit inner detector tracks in LVL2 RoIdecay vertex reconstructionproper time cutangular distribution cutsOutput rate <10HzCMS dimuon triggerLevel 12µ with pT(µ)>3GeV HLTLevel 1 muon confirmation with full muon system and improvement of the momentum measurement with the trackerprimary vertex reconstruction with pixel detectoruse the three most probable vertexestrack reconstruction in cones around Level1­muon candidatespartial reconstruction using ≤6 hitsvertex fit2<20decay flight length > 150m mass windows for signalOutput rate <1.7Hz A. Policicchio and G. Crosetti – Università della Calabria & INFN CosenzaSUSY07– July 26 ­ August 1, 2007 – Karlsruhe (Germany) Slide 10Tracking performanceProper time resolutiona good proper time resolution is necessary for time dependent asymmetry and oscillation measurementsthe proper time resolution for ATLAS and CMS is comparable: ~95fs for Bs  decayMass resolution in MeV/c2ATLAS CMS~67 ~36~40~47B  B → K* µ+µ−Λb  Λ0 A. Policicchio and G. Crosetti – Università della Calabria & INFN CosenzaSUSY07– July 26 ­ August 1, 2007 – Karlsruhe (Germany) Slide 11Bs   in CMSLow branching ratio (3.5⋅10­9 in SM) requires a good background rejectioncombinatorial with muons mainly from b decaysrare B decaysmis­identified hadrons ­ e.g. B → pipi, Κpi, ΚΚAnalysis variables in CMSdecay flight length significance on transverse planemuon separation isolation of the muon pair in a cone with R=1 (and pT>0.9GeV)secondary vertex: pointing angle                    and  vertex fit with mass cut                                 for separation with Bd6.1±0.6(stat)±1.5(sys) signal events and 13.8+22.0­13.8 background events in 10fb­1Upper limit on branching ratio 1.4⋅10­8 @ 90% CL ∣M −MBs∣40MeV∣ s ∣A. Policicchio and G. Crosetti – Università della Calabria & INFN CosenzaSUSY07– July 26 ­ August 1, 2007 – Karlsruhe (Germany) Slide 12Bs   in ATLASAnalysis variables in ATLASMuon separation ∆R(µµ)<0.9Isolation cut: no charged tracks with pT>0.8 in a cone with θ=15°Decay flight length significance on transverse plane > 15Matching between the direction from the primary to secondary vertex and the dimuon momentum (pointing angle) α<1°vertex fit with χ2<15Mass cut7±2.6 signal events and 20 ±12 BG events expected in 10fb­1Upper limit on branching ratio 7⋅10­9 @ 90% CLATLAS expects to reach the sensitivity at the level of SM                                          predictions with 30fb­1 (3 years of data taking)Rare decays as background: studies limited to particle level show that the background is small in comparison with signal and negligible comparing to combinatorial backgroundM=MBs−70140MeVA. Policicchio and G. Crosetti – Università della Calabria & INFN CosenzaSUSY07– July 26 ­ August 1, 2007 – Karlsruhe (Germany) Slide 13Semileptonic rare decaysin ATLAS (1)B+  K+ , B+  K*+ , Bd, BsbBackground sourcesChannels with J/Ψ and Ψ(2S) resonances: irreducible background, cut on dimuon invariant massCombinatorial backgroundSemileptonic decays of both b and b­bar quarksDouble semileptonic decay of b quark  ( b­>cµν, c­>sµν )topological and vertex requirements to eliminate this background kaons and pions misidentification as muons (mainly at low pT) still missed in the present resultstheir contribution is expected poor with respect to combinatorial backgroundAnalysis variablesGood dimuon vertex with χ2/NDF<3Dimuon mass in kinematical allowed window and J/Ψ and Ψ(2S) areas excluded mµµ∉ [mΨ±3σ]Secondary hadron reconstruction with vertex χ2/NDF<2, pT>3GeV and mass in mh±3σGood B meson vertex with χ2/NDF<2b­hadron mass in mΒ±3σ and proper time > 0.5ps A. Policicchio and G. Crosetti – Università della Calabria & INFN CosenzaSUSY07– July 26 ­ August 1, 2007 – Karlsruhe (Germany) Slide 14Semileptonic rare decaysin ATLAS (2)Signal after 30fb­1Good sensitivity to forward backward asymmetry measurements for BdecaysStatistical error on B  K and    B  K*  branching ratio measurements ~3.5% and ~6.5%  respectivelyDecay Channel Signal Background4000 <100002300 <100002500 <10000900 <10000800 <4000B+  K+B+  K*+BdBsbBddecayA. Policicchio and G. Crosetti – Università della Calabria & INFN CosenzaSUSY07– July 26 ­ August 1, 2007 – Karlsruhe (Germany) Slide 15Semileptonic rare decaysin ATLAS (3)bdecay: after 3 years @ 1033cm­2s­1 ATLAS can distinguish MSSM (C7eff>0) from SM in the region with low values of dimuon massThe study on semileptonic rare decays     is yet limited by the size of thebackground MC sample Further BG and trigger efficiency studies are ongoing ATLAS statistical errors 6% in this area●ATLAS expected events after 3 years @ 1033cm­2s­1 ●ATLAS MC events generated with SM ●ATLAS MC events generated with MSSM (C7eff>0)A. Policicchio and G. Crosetti – Università della Calabria & INFN CosenzaSUSY07– July 26 ­ August 1, 2007 – Karlsruhe (Germany) Slide 16ConclusionsLHC potential for flavor physics is enormous high luminosity allows to study rare decaysATLAS and CMS will use this potential for precise measurements of quantities sensitive to New Physicsafter 3 years of data taking at L=1033cm­2s­1 there will be enough statistics to find deviations from SM predictions and to set strong limits on New Physics beyond SMmeasurements on rare decays will continue even at nominal  luminosity (L=~1034cm­2s­1) thanks to final state muons

Study of dimuon rare beauty decays with ATLAS and CMS

The LHC experiments will perform sensitive tests of physics beyond the Standard Model (BSM). Investigation of decays of beauty hadrons represents an alternative approach in addition to direct BSM searches. The ATLAS and CMS efforts concentrate on those B-decays that can be easily selected already at the first and second level trigger. The most favorable trigger signature will be for $B$-hadron decays with muons in the final state. Using this trigger, ATLAS and CMS will be able to accommodate unprecedentedly high statistics in the rare decay sector. These are purely dimuon decays, and families of semimuonic exclusive channels. Already with \ensuremath{1fb^{-1}} the sensitivity in the dimuon channels will be comparable to today world statistics. The strategy is to carry on the dimuon channel program up to nominal LHC luminosity. In particular the \ensuremath{B_s \to \mu\mu} signal with \ensuremath{\sim}5 sigma significance can be measured combining low luminosity \ensuremath{10^{33}cm^{-2} s^{-1}} samples with those of one year of LHC operation at a luminosity of \ensuremath{10^{34}cm^{-2} s^{-1}}. This precision will allow to exclude or to confirm the SM unambiguously.The LHC experiments will perform sensitive tests of physics beyond the Standard Model (BSM). The investigation of decays of beauty hadrons represents an alternative approach in addition to direct BSM searches. The ATLAS and CMS efforts concentrate on those B-decays that can be efficiently selected already at the first and second level trigger. The most favorable trigger signature will be for $B$-hadron decays with muons in the final state. Using this trigger, ATLAS and CMS will be able to accommodate unprecedentedly high statistics in the rare decay sector. These are purely dimuon decays, and families of semimuonic exclusive channels. Already with data corresponding to an integrated luminosity of \ensuremath{1 fb^{-1}}, the sensitivity in the dimuon channels will be comparable to present measurements (world average). The strategy is to carry on the dimuon channel program up to nominal LHC luminosity. In particular the \ensuremath{B_s \to \mu\mu} signal with \ensuremath{\sim}5 sigma significance can be measured combining low luminosity \ensuremath{10^{33}cm^{-2} s^{-1}} samples with those of one year of LHC operation at a luminosity of \ensuremath{10^{34}cm^{-2} s^{-1

Study of DiMuon Rare Beauty Decays with ATLAS and CMSA. Policicchio1 a and G. Crosetti1Universita’ della Calabria and INFN Cosenza21 September 2007Abstract. The LHC experiments will perform sensitive tests of physics beyond the Standard Model(BSM). Investigation of decays of beauty hadrons represents an alternative approach in additionto direct BSM searches. The ATLAS and CMS efforts concentrate on those B-decays that can beefficietly selected already at the first and second level trigger. The most favorable trigger signaturewill be for B-hadron decays with muons in the final state. Using this trigger, ATLAS and CMS willbe able to accommodate unprecedentedly high statistics in the rare decay sector. These are purelydimuon decays, and families of semimuonic exclusive channels. Already with 1fb−1 the sensitivityin the dimuon channels will be comparable to today world statistics. The strategy is to carry onthe dimuon channel program up to nominal LHC luminosity. In particular the Bs → µµ signalwith ∼5 sigma significance can be measured combining low luminosity 1033cm−2s−1 samples withthose of one year of LHC operation at a luminosity of 1034cm−2s−1.PACS. 13.30.Ce Leptonic, semileptonic, and radiative decays – 13.20.He Decays of bottom mesons1 IntroductionRare leptonic and semileptonic B-decays, producedby FCNC transitions, are forbidden at the tree levelin the Standard Model (SM). These decays occur atthe lowest order only through one-loop “penguin” and“box” diagrams. The branching ratios of these decaysare very small: from 4 × 10−5 for rare radiative de-cay B0d → K∗γ to 10−15 for rare Cabibbo suppressedleptonic decay B0d → e+e−.The careful investigation of rare B-decays is manda-tory for testing ground of the Standard Model and offera complementary strategy in the search of new physics.The probing of loop-induced couplings provide meansof testing the detailed structure of the SM at the levelof radiative corrections. In particular, FCNC involv-ing b → s, d transitions and B → ll decays, providean excellent probe of new indirect effects by yieldinginformations on the masses and coupling of the vir-tual particles running in the loops. In SUSY models,the branching fraction for B0s(d) → µ+µ− has a strongdependence on tanβ. A precise measurement of suchdecays will allow to constrain the supersymmetric ex-tensions of the SM.To date the decay modes B0s(d) → µ+µ− have notyet been observed. The current best upper limits onbranching ratio come from D0 [1] and CDF [2] collab-orations and are 9.3×10−8 and 5.8×10−8 respectivelyat 95%CL. The searches for rare B decays at the B-factories CLEO, Belle and BaBar have not sensitivityto Bs decays.a Email: antonio.policicchio@cern.chTable 1. Branching ratios in SM for rare B-decays withµ+µ− pair in final states.Decay channel Br. ratio Ref.B0s → µ+µ− 3.4 × 10−9 [6]B+ → K+µ+µ− 3.5 × 10−7 [7]B+ → K∗+µ+µ− ∼ 10−6 [7]Λb → Λµ+µ− 2.0 × 10−6 [8],[9]B0d → K0∗µ+µ− 1.3 × 10−6 [10],[11]B0s → φ µ+µ− ∼ 10−6 [10],[11]In the last years the B-factories BaBar and Bellepresented the first results for B → (K∗, K)l+l− branch-ing ratios and forward-backward asymmetry (AFB) inthese rare semileptonic decays [3],[4],[5] but still af-fected by large statistical errors.In this report we will pay our attention to the(semi)leptonic decays with µ+µ− pair in final statewhere ATLAS and CMS can give a significant con-tribution. We discuss the simulation results and theperspectives of measurements. The branching ratiosin SM of the decays studied can be found in Table 1.2 Theoretical descriptionFrom the theoretical point of view, the b → q (q = s, d)transitions are described using the effective Hamilto-nianATL-PHYS-CONF-2007-01904 December 2007Flavor Physics Contributed TalkHeff = −4GF√2VtbV∗tq10∑i=1Ci(µ)Oi(µ) (1)in the form of Wilson expansion (see e.g. [12]).The set of Wilson coefficients Ci(µ) depends on thecurrent model and contains the lowest order modelcontributions and perturbative QCD corrections. Thescale parameter µ is approximately equal to the massof b-quark (∼ 5GeV ). This parameter separates theperturbative and nonperturbative contributions of thestrong interactions. The nonperturbative contributionis contained in the matrix elements of basis operatorsOi(µ) between the initial and final hadronic states. Forthe calculation of these matrix elements it is necessaryto use different decay-specific nonperturbative meth-ods: QCD Sum Rules, Heavy Quarks Effective The-ory, Quark Models and Lattice calculations. The ac-curacy in nonperturbative calculations depends on themethod, but it is not less than 15%. The accuracy ofthe Wilson coefficient for with NLO and NNLO QCDcorrections [13] is not greater than 15% if µ parameterranges in [mb/2, 2mb].In the SM the decay width of the rare muonic de-cays is:Γ (B0q → µ+µ−) =G2F α2em16pi3|V ∗tqVtb|2 · (2)(fBqmµC10A)2√M2Bq − 4m2µ.This expression contains only one nonperturbativeconstant fBq . The value of this constant is known withhigh accuracy (about 5-10%). Furthermore the WilsonCoefficient C10A in the NLO approach is not depen-dent on the scale parameter µ, and does not add anyuncertainty in the theoretical predictions.3 Trigger strategies for rare decaysDetails of the ATLAS and CMS experiments can befound in Refs. [14,15].3.1 The ATLAS triggerATLAS has a three level trigger system [16] which re-duces the 40MHz bunch crossing rate to about 100Hzof events to be recorded. The first level trigger (LVL1)is hardware-based and makes a fast decision (in 2.5µs)about which events are interesting for further process-ing. Coarse granularity informations from calorimeterand muon spectrometer are used to identify region ofinterest (RoI) of the detector which contain interestingsignals (high energy electrons, muons and taus and jetswith large transverse or missing energy). The RoIs areused to guide the later stages of the trigger. After LVL1the trigger rate will be reduced to less than 75kHz.The high level trigger (HLT) is software-based andis split into two levels. At the level 2 (LVL2) the fullgranularity of the detector is used to confirm the LVL1decisions and then to combine informations from dif-ferent sub-detectors within the LVL1 RoIs. Fast algo-rithms are used for the reconstruction at this stage andthe rate is reduced to ∼2kHz with an average time ofexecution of ∼10ms. At the level 3, the Event Filter(EF), the whole event is available and oﬄine-like algo-rithms are used with better alignment and calibrationinformations to form the final decision. The rate is re-duced to 100Hz with an execution time of ∼1s.3.1.1 ATLAS trigger for rare decaysThe B-trigger is expected to account for 5-10% of thetotal trigger resources. The core of the B-trigger is theLVL1 muon trigger which is based on the measurementof the muon transverse momentum (pT ). The efficiencyof the muon trigger is expected at about 85% at theplateau. The dimuon LVL1 trigger (two muons with pTabove 6GeV), used for rare decay selection, is expectedto have a rate of about 500Hz.The LVL1 dimuons will be confirmed at the LVL2firstly in the muon system by means of the precisiontracking chamber and then by combining muon andinner detector tracks. Finally the two muons can becombined and mass cuts are applied. At the EF thetracks are refitted in the RoIs and vertex reconstruc-tion is performed. Cuts are applied on decay lengthand invariant B-hadron mass. For B0s → µ+µ− eventscontaining two muons with pT >6GeV, efficiencies of60-70% are expected.3.2 The CMS triggerCMS has a two level trigger [15] which reduces thebunch crossing rate down to about 150Hz for record-ing. The Level-1 (L1) trigger uses muon detector andcalorimeter informations and is hardware-based withan output rate of about 100kHz and a latency of 3.5µs.The HLT is software-based with the required 150Hzoutput rate. It uses reconstruction algorithms similarto the oﬄine with a mean execution time per event ofabout 40ms. To speed up reconstruction in the HLT,a partial track reconstruction is performed: the trackresolution becomes asymptotic after 5-6 hits are usedin the track fit.3.2.1 CMS trigger for rare decaysAs in ATLAS, CMS trigger for B events uses single anddimuon triggers. The L1 dimuon trigger has a low pTthreshold of 3GeV which ensures a high efficiency forevents with two muons in the final state with a rate of0.9kHz at 2×1033cm−2s−1. At the HLT the L1 decisionwill be confirmed using the full muon system and animproved momentum measurement with the tracker.Primary vertex (PV) reconstruction is available fromthe pixel detector and also track reconstruction is per-formed in cones around the L1 muons using the partialA. Policicchio and G. Crosetti Study of DiMuon Rare Beauty Decays with ATLAS and CMSTable 2. Mass and proper time resolution obtained on theB0s → µ+µ− signal MonteCarlo event sample for CMS andATLAS.Experiment Mass res.(GeV) Proper time res.(fs)ATLAS 0.084 91CMS 0.036 95track reconstruction. The exclusive rare decay is thenreconstructed and cuts on invariant mass, vertex fitquality and decay length are applied.4 Muonic decays in ATLAS and CMSAs purely leptonic B-decays are theoretically very clean,they provide an ideal channel for seeking indirect hintsof new physics effects. However they are very diffi-cult to observe because of their small branching ratio(see Table 1). Most probably they will not be visiblebefore the LHC data taking. ATLAS and CMS willstart sensitive measurements at 1033cm−2s−1. Even athigh design luminosity (1034cm−2s−1) the trigger forB → µ+µ− decay is not problematic.A good background rejection is necessary for thesignal selection. The main contributions to the back-ground [17] come from the processes bb¯(bb¯bb¯, bb¯cc¯) →Xµ+µ− with the muons originating mainly from semi-leptonic b and c quark decays. This background can beestimated extrapolating the Tevatron data on heavyquark production to the LHC energies. It would be re-marked that both ATLAS and CMS analysis are lim-ited by the statistics of the MonteCarlo backgroundsample.The event selection relies on topological variablesrelated to the PV, the muon candidates and the Bssecondary vertex and is very similar in both the ex-periments [18,19]. Simple cuts can be applied to dis-tinguish combinatorial background from the signal:– B-hadron invariant mass;– secondary vertex length and quality;– pointing of B-hadron momentum to PV;– track isolation.Table 2 summarizes the mass resolution and theproper time resolution obtained on the signal Monte-Carlo event sample for CMS and ATLAS.The main result of the analysis is the expectednumber of signal and background events after 30fb−1as summarized in Table 3. This will allow to set strin-gent constraint on new physics models.Figure 1 shows the ATLAS expectation for B0s →µ+µ− branching ratio as a function of the integratedluminosity (or equivalently as a function of the time).The SM expectation can be reached with ∼5 sigma sig-nificance combining low luminosity 1033cm−2s−1 sam-ples with those of one year of LHC operation at thenominal luminosity of 1034cm−2s−1.Table 3. Expected number of B0s → µ+µ− and backgroundevents after 30fb−1.Experiment signal events BG eventsATLAS 21.0 60±36CMS 18.3 42+66−42Fig. 1. ATLAS perspective of measurements for B0s →µ+µ− branching ratio as a function of the integrated lumi-nosity (or equivalently as a function of the time).5 SemiMuonic decays in ATLASThanks to the dimuon final state (see Table 1), thesemimuonic decays, as the purely dimuonic ones, areeasy to select at the trigger level. The observation ofsemileptonic decays give access to a number of ob-servables. The precise measurements of such observ-ables could give very interesting informations for newphysics reach. AFB is one of the most promising pa-rameters.The small branching ratios of semimuonic decaysrequire a powerful background rejection. Semileptonicdecays with cc¯ resonances decaying into two muonsrepresent an irreducible background source. A cut ondimuon invariant mass around nominal values for reso-nances removes this background. Combinatorial back-ground arises from muons originating mainly from semi-leptonic decays of b and c quarks. Specific decay chan-nels can represent background sources due mainly tohadron misidentification as muons, but their contribu-tion is expected to be less important than the previoustwo sources.The event selection [20,21] is related to topologicalvariables:– vertex quality and invariant mass of the dimuonsystem;– displacement and quality of vertices and mass ofthe secondary hadrons;Flavor Physics Contributed TalkTable 4. Expected statistics for semimuonic decays eventsand background in ATLAS after 30fb−1.Decay Signal events Background eventsB+ → K+µ+µ− 4000 <10000B+ → K∗+µ+µ− 2300 <10000Λb → Λµ+µ− 800 <4000B0d → K0∗µ+µ− 2500 <10000B0s → φ µ+µ− 900 <10000Fig. 2. Forward-backward asymmetry for Λb → Λµ+µ−after 30fb−1 (see Section 5).– pointing of B hadron momentum to PV.Statistics expected after three years of data takingat low luminosity (30fb−1) are summarized in Table 4.The background level estimation is only limited by thelow MonteCarlo statistics available at the moment. Itshould be pointed out that, thanks to the muon pair inthe final state, semimuonic decays will be also studiedat high luminosity, so that a larger statistics than thoseof Table 4 will be collected.The expected precision on AFB after 30fb−1 is pre-sented on Figure 2 for Λb → Λµ+µ− decay. The threedots with error bars correspond to simulated data afteroﬄine analysis. Upper point set corresponds to theo-retical SM predictions, and lower set corresponds toMSSM predictions. The statistical error in the lowdimuon invariant mass region is at level of 6%. Thestatistical errors expected on branching ratio measure-ments are at the level of 3.5% and 6.5% respectivelyfor B → Kµ+µ− and B → K∗µ+µ− decays. Theseerrors on the branching ratio measurements are muchsmaller than the current experimental and theoreticalones.6 ConclusionsThe results obtained for B0s → µ+µ− by ATLAS andCMS are comparable and promise an interesting startupanalysis with the possibility of setting tight constraintson new physics models beyond the SM. The simulationstudies show that ATLAS detector will be capable toextract signals of semimuonic B-decays and reach agood sensitivity to new physics beyond the SM.7 AcknowledgmentsWe would thank ATLAS and CMS Collaborations, andespecially Maria Smizanska, Sergey Sivoklokov, PavelReznicek from ATLAS and Christina Eggel and UrsLangenegger from CMS for their useful suggestions inthe preparation of this work.References1. The D0 Collaboration, D Note 5344-CONF (2007)2. The CDF Collaboration, CDF Public Note 8956(2007)3. B. Aubert et al. (The BaBar Collaboration), Phys.Rev. D 73 (2006) 0920014. A. Ishikawa et al. (The Belle Collaboration), Phys.Rev. Lett. 96 (2006) 2518015. K. Abe et al. (The Belle Collaboration), hep-ex/0410006 (2004)6. A. J. Buras, Phys. Lett. B 566 (2003) 1157. A. Ali et al., Phys. Rev. D 66 (2002) 0340028. C. H. Chen et al., Phys. Rev. D 64 (2001) 0740019. T. M. Aliev et al., Nucl. Phys. B 649 (2003) 16810. D. Melikhov et al., Phys. Rev. D 57 (1998) 681411. D. Melikhov et al., Phys. Rev. D 62 (2000) 01400612. G. Buchalla et al., Rev. Mod. Phys.68 (1996) 112513. C. Bobeth et al., Nucl. Phys. B 574, (2000) 29114. The ATLAS Collaboration, CERN-LHCC-1999-14/15 (1999)15. The CMS Collaboration, CERN-LHCC-2006-001(2006)16. The ATLAS Collaboration, CERN-LHCC-2003-22(2003)17. N. Nikitin et al., ATL-PHYS-PUB-2007-009 (2007)18. P. Reznicek, Nucl. Phys. B Proceedings Supplements167 (2007) 24419. C. Eggel et al., CMS-AN-2006-097 (2006)20. A. Policicchio and G. Crosetti, Acta Phys. Pol. B 38(2007) 94721. A. Policicchio and G. Crosetti, ATL-PHYS-PUB-2007-017 (2007)

Study of DiMuon Rare Beauty Decays with ATLAS and CMS

A. A.Policicchio

G. Crosetti

Springer

The LHC experiments will perform sensitive tests of physics beyond the standard model (BSM). The investigation of decays of beauty hadrons represents an alternative approach in addition to direct BSM searches. The ATLAS and CMS efforts concentrate on those B-decays that can be efficiently selected already at the first and second level trigger. The most favorable trigger signature will be for B-hadron decays with muons in the final state. Using this trigger, ATLAS and CMS will be able to accommodate very high statistics in the rare decay sector. These are purely dimuon decays, and families of semimuonic exclusive channels. Already with data corresponding to an integrated luminosity of 1 fb-1, the sensitivity in the dimuon channels will be comparable to present measurements (world average). The strategy is to carry on the dimuon channel program up to nominal LHC luminosity. In particular the Bs→μμ signal with ∼5 sigma significance can be measured combining low luminosity 1033 cm-2 s-1 samples with those of one year of LHC operation at a luminosity of 1034 cm-2 s-1

A. Policicchio

EDP Sciences OAI-PMH repository (1.2.0)

Springer - Publisher Connector

Eur. Phys. J. C 55, 173–176 (2008) THE EUROPEAN
PHYSICAL JOURNAL C
DOI 10.1140/epjc/s10052-008-0594-6
Special Article – Scientiﬁc Note
Study of dimuon rare beauty decays with ATLAS and CMS
A. Policicchioa, G. Crosetti
Universita` della Calabria and INFN Cosenza, Ponte P. Bucci Cubo 31c, 87036 Arcavacata di Rende – Cosenza, Italy
representing the ATLAS and CMS Collaborations
Received: 7 January 2008 /
Published online: 19 April 2008 − © Springer-Verlag / Societa` Italiana di Fisica 2008
Abstract. The LHC experiments will perform sensitive tests of physics beyond the standard model (BSM).
The investigation of decays of beauty hadrons represents an alternative approach in addition to direct BSM
searches. The ATLAS and CMS eﬀorts concentrate on thoseB-decays that can be eﬃciently selected already
at the ﬁrst and second level trigger. The most favorable trigger signature will be for B-hadron decays with
muons in the ﬁnal state. Using this trigger, ATLAS and CMS will be able to accommodate very high statis-
tics in the rare decay sector. These are purely dimuon decays, and families of semimuonic exclusive channels.
Already with data corresponding to an integrated luminosity of 1 fb−1, the sensitivity in the dimuon chan-
nels will be comparable to present measurements (world average). The strategy is to carry on the dimuon
channel program up to nominal LHC luminosity. In particular the Bs→ µµ signal with ∼ 5 sigma signif-
icance can be measured combining low luminosity 1033 cm−2 s−1 samples with those of one year of LHC
operation at a luminosity of 1034 cm−2 s−1.
PACS. 13.30.Ce; 13.20.He
1 Introduction
Rare leptonic and semileptonic B-decays, produced by
FCNC transitions, are forbidden at the tree level in the
standard model (SM). These decays occur at the low-
est order only through one-loop “penguin” and “box” di-
agrams. The branching ratios of these decays are very
small: from 4×10−5 for the rare radiative decayB0d→K
∗γ
to 10−15 for the rare Cabibbo suppressed leptonic decay
B0d → e
+e−.
The careful investigation of rare B-decays is manda-
tory for testing ground of the standard model and oﬀers
a complementary strategy in the search of new physics.
The probing of loop-induced couplings provide a means
of testing the detailed structure of the SM at the level
of radiative corrections. In particular, FCNC involv-
ing b→ s, d transitions and B → ll decays, provide an
excellent probe of new indirect eﬀects by yielding in-
formations on the masses and coupling of the virtual
particles running in the loops. In SUSY models, the
branching fraction for B0s(d)→ µ
+µ− has a strong depen-
dence on tanβ. A precise measurement of such decays
will allow to constrain the supersymmetric extensions of
the SM.
To date the decay modes B0s(d) → µ
+µ− have not
yet been observed. The current best upper limits on the
branching ratio come from the D0 [1] and CDF [2] col-
laborations and are 9.3× 10−8 and 5.8× 10−8 respec-
a e-mail: antonio.policicchio@cern.ch
tively at 95% C.L. The searches for rare B decays at the
B-factories CLEO, Belle and BaBar have no sensitivity to
Bs decays.
In the last years the B-factories BaBar and Belle pre-
sented the ﬁrst results for B→ (K∗,K)l+l− branching ra-
tios and forward–backward asymmetry (AFB) in these rare
semileptonic decays [3–5] but those are still aﬀected by
large statistical errors.
In this report we pay attention to the (semi)leptonic
decays with µ+µ− pairs in ﬁnal states where ATLAS and
CMS can give a signiﬁcant contribution. We discuss the
simulation results and the perspectives of measurements.
The SM branching ratios of the decays studied can be
found in Table 1. The perspective of measurement of rare
beauty decays have been presented during the SUSY07
conference also by the LHCb collaboration [6].
Table 1. Standard model branching ratios for rare B-decays
into a µ+µ− ﬁnal state
Decay channel Branching ratio Ref.
B0s → µ
+µ− 3.4×10−9 [7]
B+→K+µ+µ− 3.5×10−7 [8]
B+→K∗+µ+µ− ∼ 10−6 [8]
Λb→ Λµ
+µ− 2.0×10−6 [9, 10]
B0d →K
0∗µ+µ− 1.3×10−6 [11, 12]
B0s → φµ
+µ− ∼ 10−6 [11, 12]
174 A. Policicchio, G. Crosetti: Study of dimuon rare beauty decays with ATLAS and CMS
2 Trigger strategies for rare decays
Details of the ATLAS and CMS experiments can be found
in [13, 14].
2.1 The ATLAS trigger
ATLAS has a three level trigger system [15] which re-
duces the 40MHz bunch crossing rate to about 100Hz of
events to be recorded. The ﬁrst level trigger (LVL1) is
hardware-based and makes a fast decision (in 2.5 µs) about
which events are interesting for further processing. Coarse
granularity informations from calorimeter and muon spec-
trometer are used to identify region of interest (RoI) of
the detector which contain interesting signals (high energy
electrons, muons and taus and jets with large transverse or
missing energy). The RoIs are used to guide the later stages
of the trigger. After LVL1 the trigger rate will be reduced
to less than 75 kHz.
The high level trigger (HLT) is software-based and is
split into two levels. At the level 2 (LVL2) the full gran-
ularity of the detector is used to conﬁrm the LVL1 de-
cisions and then to combine informations from diﬀerent
sub-detectors within the LVL1 RoIs. Fast algorithms are
used for the reconstruction at this stage and the rate is
reduced to ∼ 2 kHz with an average time of execution of
∼ 10ms. At the level 3, the event ﬁlter (EF), the whole
event is available and oﬄine-like algorithms are used with
better alignment and calibration informations to form the
ﬁnal decision. The rate is reduced to 100Hz with an execu-
tion time of ∼ 1 s.
2.1.1 ATLAS trigger for rare decays
The B-trigger is expected to account for 5%–10% of the
total trigger resources. The core of the B-trigger is the
LVL1 muon trigger which is based on the measurement of
the muon transverse momentum (pT). The eﬃciency of the
muon trigger is expected at about 90% above pT > 7 GeV.
The dimuon LVL1 trigger (two muons with pT > 6 GeV/c),
used for rare decay selection, is expected to have a rate of
about 500Hz at 1033 cm−2 s−1.
The LVL1 dimuons will be conﬁrmed at the LVL2
ﬁrstly in the muon system by means of the precision track-
ing chamber and then by combining muon and inner de-
tector tracks. Finally the two muons can be combined and
mass cuts are applied. At the EF the tracks are reﬁt-
ted in the RoIs and vertex reconstruction is performed.
Cuts are applied on decay length and invariant B-hadron
mass. For B0s → µ
+µ− events containing two muons with
pT > 6 GeV/c, the expected trigger eﬃciency is 46%.
2.2 The CMS trigger
CMS has a two level trigger [14] which reduces the bunch
crossing rate down to about 150Hz for recording. The
level-1 (L1) trigger uses muon detector and calorimeter
informations and is hardware-based with an output rate
of about 100 kHz and a latency of 3.5 µs. The HLT is
software-based with the required 150Hz output rate. It
uses reconstruction algorithms similar to the oﬄine with
a mean execution time per event of about 40ms. To speed
up reconstruction in the HLT, a partial track reconstruc-
tion is performed: the diﬀerence in track resolution be-
comes negligible after 5–6 hits are used in the track ﬁt.
2.2.1 CMS trigger for rare decays
As in ATLAS, the CMS trigger for B events uses single
and dimuon triggers. The L1 dimuon trigger has a low
pT threshold of 3 GeV/c which ensures a high eﬃciency
for events with two muons in the ﬁnal state with a rate
of 0.9 kHz at 2×1033 cm−2 s−1. At the HLT the L1 de-
cision will be conﬁrmed using the full muon system and
an improved momentum measurement with the tracker.
Primary vertex (PV) reconstruction is available from the
pixel detector and also track reconstruction is performed in
cones around the L1 muons using the partial track recon-
struction. The exclusive rare decay is then reconstructed
and cuts on the invariant mass, vertex ﬁt quality and decay
length are applied. The expected global trigger eﬃciency
for B0s → µ
+µ− selection is about 41%.
3 Muonic decays in ATLAS and CMS
As purely leptonic B-decays are theoretically very clean,
they provide an ideal channel for seeking indirect hints of
new physics eﬀects. However, they are very diﬃcult to ob-
serve because of their small branching ratio (see Table 1).
Assuming the standard model branching ratio, most prob-
ably they will not be observed by other experiments before
the LHC data taking. ATLAS and CMS will start sensitive
measurements at 1033 cm−2 s−1. Even at high design lumi-
nosity (1034 cm−2 s−1) the trigger for B→ µ+µ− decay is
not problematic.
A good background rejection is necessary for the signal
selection. The main contributions to the background [16]
come from the processes bb¯(bb¯bb¯, bb¯cc¯)→Xµ+µ− with the
muons originatingmainly from semi-leptonic b and c quark
decays. This background can be estimated by extrapo-
lating the Tevatron data on heavy quark production to
the LHC energies. In addition to the combinatorial back-
ground, two and three body decays of B meson where two
of the ﬁnal state particles areK±, π± or µ±, may leak into
the signal region. Although the rate for misidentiﬁcation of
kaons or pions as muons, due to punchthrough or decay in
ﬂight, is low (in ATLAS is of the order of 0.5%), the inves-
tigation of such decays has been done, because of the small
branching ratio of the B→ µ+µ− decay.
The event selection relies on topological variables
related to the PV, the muon candidates and the Bs
secondary vertex and is very similar in both experi-
ments [17, 18]. Simple cuts can be applied to distinguish
the combinatorial background from the signal:
– B-hadron invariant mass;
– secondary vertex length and quality;
A. Policicchio, G. Crosetti: Study of dimuon rare beauty decays with ATLAS and CMS 175
– pointing of B-hadron momentum to PV;
– track isolation.
Table 2 summarizes the mass resolution and the proper
time resolution obtained on the signal Monte Carlo event
sample for CMS and ATLAS.
The expected number of signal and background events
after 30 fb−1 are summarized in Table 3. This will allow to
set a stringent constraint on new physics models.
Figure 1 shows the ATLAS expectation for the meas-
urement of the B0s → µ
+µ− branching ratio as a function
of the integrated luminosity (or equivalently as a function
of time). The SM expectation can be reached with a ∼ 5
Table 2. Mass and proper time resolution obtained on the
B0s → µ
+µ− signal Monte Carlo event sample for CMS and
ATLAS
Experiment Mass res. (GeV/c2) Proper time res. (fs)
ATLAS 0.084 91
CMS 0.036 95
Table 3. Expected number of B0s → µ
+µ− signal and back-
ground events after 30 fb−1
Experiment Signal events BG events
ATLAS 21.0 60±36
CMS 18.3 42+66−42
Fig. 1. ATLAS perspective for the measurement of the B0s →
µ+µ− branching ratio as a function of the integrated luminos-
ity (or equivalently as a function of the time). The shaded band
shows the uncertainty in the BG level estimation
sigma signiﬁcance combining low luminosity 1033 cm−2 s−1
samples with those of one year of LHC operation at the
nominal luminosity of 1034 cm−2 s−1.
4 Semimuonic decays in ATLAS
Thanks to the dimuon ﬁnal state, the semimuonic decays,
as the purely dimuonic ones, are easy to select at the
trigger level. The observation of semileptonic decays gives
access to a number of observables. The precise measure-
ments of such observables could give very interesting in-
formations for new physics reach. AFB is one of the most
promising parameters.
The small branching ratios of semimuonic decays re-
quire a powerful background rejection. Semileptonic de-
cays with cc¯ resonances decaying into two muons rep-
resent an irreducible background source. A cut on the
dimuon invariant mass around the nominal values for res-
onances removes this background. Combinatorial back-
ground arises from muons originating mainly from semi-
leptonic decays of b and c quarks. The non-resonant
background B → Kπµµ, which is an irreducible back-
ground for B→K ∗µµ decays, has not been taken into
account in the present study. The cut on the invari-
ant mass should remove a large fraction of this back-
ground. Speciﬁc decay channels can represent background
sources due mainly to hadron misidentiﬁcation as muons,
but their contribution is expected to be less important
than the previous two sources. Further studies and back-
ground productions are already ongoing and results will
be updated, taking into account the missing background
sources.
The event selection [19, 20] is related to topological
variables:
– vertex quality and invariant mass of the dimuon sys-
tem;
– displacement and quality of vertices and mass of the
secondary hadrons;
– pointing of B hadron momentum to PV.
The number of events expected after three years of
data taking at low luminosity (30 fb−1) are summarized in
Table 4. The background level estimation is only limited by
the low Monte Carlo statistics available at the moment. It
should be pointed out that, thanks to the muon pair in the
ﬁnal state, semimuonic decays will be also studied at high
Table 4. Expected number of events from semimuonic de-
cays events and expected background events in ATLAS after
30 fb−1
Decay Signal events Background events
B+→K+µ+µ− 4000 < 10000
B+→K∗+µ+µ− 2300 < 10000
Λb→ Λµ
+µ− 800 < 4000
B0d →K
0∗µ+µ− 2500 < 10000
B0s → φµ
+µ− 900 < 10000
176 A. Policicchio, G. Crosetti: Study of dimuon rare beauty decays with ATLAS and CMS
Fig. 2. Forward–backward asymmetry for Λb → Λµ
+µ− as
a function of the dimuon invariant mass after 30 fb−1 (see
Sect. 4)
luminosity, so that a larger statistics than those of Table 4
will be collected.
Experimentally, the forward–backward asymmetry has
been deﬁned as
AFB =
NF[s1,s2]−NB[s1,s2]
NF[s1,s2]+NB[s1,s2]
, (1)
where NF[s1,s2] and NB[s1,s2] are the numbers of posi-
tive leptons moving in the forward and backward direc-
tions of the B meson in the dimuon rest frame, respec-
tively, in the the range of the squared dimuon mass [s1, s2].
The expected precision on AFB after 30 fb
−1 is presented
on Fig. 2 for Λb → Λµ+µ− decay. The three dots with
error bars correspond to simulated data after oﬄine an-
alysis. The upper point set corresponds to the theoretical
SM prediction, and the lower set corresponds to a pre-
diction within the MSSM [9]. The statistical error in the
low dimuon invariant mass region, where the sensitivity of
AFB on the form factor calculation is negligible, is at level
of 6%.
5 Conclusions
The results obtained for B0s → µ
+µ− by ATLAS and CMS
are comparable and promise an interesting startup analy-
sis with the possibility of setting tight constraints on new
physics models beyond the SM. The simulation studies
show that the ATLAS detector will be capable to extract
signals of semimuonic B-decays and reach a good sensitiv-
ity to new physics beyond the SM.
Acknowledgements. We would like to thank the ATLAS and
CMS Collaborations, and especially Maria Smizanska, Sergey
Sivoklokov, Pavel Reznicek from ATLAS and Christina Eggel
and Urs Langenegger from CMS for their useful suggestions in
the preparation of this work.
References
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092001 (2006)
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251801 (2006)
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17. P. Reznicek, Nucl. Phys. B Proc. Suppl. 167, 244 (2007)
18. C. Eggel et al., CMS-AN-2006-097 (2006)
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(2007)


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Study of DiMuon Rare Beauty Decays with ATLAS and CMS

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