Search for the rare decays B 0 d μ + μ − and B S μ + μ - with LHCb

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Search for the rare decays B 0 d μ + μ and B S μ + μ - with LHCb Gaia Lanfranchi LNF-INFN on behalf of the LHCb Collaboration Les XXV Rencontres de Physique de la Vallee d’Aoste

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Search for the rare decays B 0 d  μ + μ − and B S  μ + μ - with LHCb. Gaia Lanfranchi LNF-INFN on behalf of the LHCb Collaboration. Les XXV Rencontres de Physique de la Vallee d’Aoste. Outline. Brief theoretical introduction Experimental status - PowerPoint PPT Presentation

Transcript of Search for the rare decays B 0 d μ + μ − and B S μ + μ - with LHCb

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Search for the rare decaysB0d + and BS +- with LHCbGaia Lanfranchi LNF-INFN on behalf of the LHCb Collaboration

Les XXV Rencontres de Physique de la Vallee dAosteOutlineBrief theoretical introductionExperimental statusLHCb skills for the search for rare decays Bs,dAnalysis strategyResultsOutlook

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s,db

The LHCb hunt for non-SM Higgs(es)

B(d,s) is the best way for LHCb to constrain the parameters of the extended Higgs sector in MSSM, fully complementary to direct searches2Double suppressed decay: FCNC process and helicity suppressed: very small in the Standard Model but very well predicted:

sensitive to New Physics contributions in the scalar/pseudo-scalar sector:Bs+-= (3.20.2)10-9 Bd+-= (1.00.1)10-10

()2()2MSSM, large tan approximationBuras et al., arXiv:1007.5291BR parameterization from a fully generic OPE of the Hamiltonian: CS,CP,CA and CA are the Wilson coefficients associated to all the possible operators that can contribute to this decay [arXiv 0801.1833 and references therein]: the scalar (Cs) and pseudo-scalar (CP) coefficients (eg extended Higgs sector)can be the dominant terms depending on how New Physics modifies the axial terms(helicity suppressed)

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5 discovery contours for observing the heavy MSSM Higgs bosons H, Ain the three decay channels H,A+- jets (solid line), jet+ (dashed line), Jet+e (dotted line) assuming 30-60 fb-1 collected by CMS.

Best fit contours in tan vs MA plane in the NUHM1 model[O. Buchmuller et al, arxiv:0907.5568]3 tan vs MA planeThe LHCb hunt for non-SM Higgs(es)Fit is done in the NUMHM1 model including the EW data, the anomalous magnetic moment of the muon,B observables as BR(b->s g) and BR(Bu->tau nu), Bs mixing, and limit on Bsmumu and lightest Higgs mass.4

The LHCb hunt for non-SM HiggsesRegions compatible with BR(Bs) = 2x10-8, 1x10-8,5x10-9 and SM.LHCb calculation using F. Mahmoudi, SuperIso, arXiv: 08083144 4 tan vs MA plane tan vs MA planeCurrent experimental resultsLimits from Tevatron @ 95% CL:CDF (~3.7 fb-1): BS (Bd) < 43 (7.6) 10-9D0 (~6.1 fb-1): BS < 5110-95

CDF, 3.7 fb-1, 25 years old 6.1 fb-1 PLB 693, 539 (2010)rifare6

LHCb, 10 months old37 pb-1 (33 pb-1 collected in15 days!)Current experimental resultsLimits from Tevatron @ 95% CL:CDF (~3.7 fb-1): BS (Bd) < 43 (7.6) 10-9D0 (~6.1 fb-1): BS < 5110-96

CDF, 3.7 fb-1, 25 years old 6.1 fb-1 PLB 693, 539 (2010)rifare7

pp250 mrad10 mradVELORICH1MAGNETT-StationsRICH2MUON SYSTEMECALHCAL TT .Bs,d @ LHCbLHCb skills for the search of the Bs,d: r Huge cross section: (ppbbX) @ 7 TeV ~ 300 b r Large acceptance ( bb are produced forward/backward): LHCb acceptance 1.90.56 GeV/cHLT1 single-: pT>0.8 GeV/c IP>0.11 mm IPS> 5HLT2Several dimuon lines with M cuts and/or displaced vertex+ Global Event Cuts for events with high multiplicity Half of the bandwidth (~1 kHz) given to the muon lines pT cuts on muon lines kept very low (trigger Bsd) ~ 90% Trigger rather stable during the whole period (despite L increased by ~105)2 kHz on tapeL0HLT1HLT2CDF does not reconstruct muons below 2 GeV/c pt, D0 below 1 GeV/c pT.11Analysis strategySoft selection: - reduces the dataset to a manageable level Discrimination between S and B via Multi Variate Discriminant variable (GL) and Invariant Mass (IM) - events in the sensitive region are classified in bins of a 2D plane Invariant Mass and the GL variables Normalization: Convert the signal PDFs into a number of expected signal events by normalizing to channels of known BR

Extraction of the limit: - assign to each observed event a probability to be S+B or B-only as a function of the BR(Bs,d) value; exclude (observe) the assumed BR value at a given confidence level using the CLs binned method.11Soft selection:(pairs of opposite charged muons with high quality tracks, making a common vertex very displaced with respect to the PV and M in the range [4769-5969] MeV/c2)

1) Keeps high efficiency for signals: After selection Bs(Bd) events expected (if BR=BR(SM)): 0.3 (0.04)

3) Rejects most of the background ~ 3000 background events in the large mass range [4769-5969] MeV/c2 ~ 300 background events in the signal windows M(Bs,d) 60 MeV

Soft selectionBlinded regionSignal regionsblinded up to the analysis endMuonID performance & background compositionPerformance measured with pure samples of J/, Ks, KK, p MuonID efficiency vs p misidentification rate vs pDataMCDataMCWe are dominated by the bbX component (double semi-leptonic decays and cascade processes) fake+ ~ 10% and double fake ~0.3%Peaking background (Bhh) fully negligible ( < 0.1 events in signal regions)13In the Bq p range:(h) ~ (7.10.5) 10-3(hh) ~ (3.50.9) 10-5 () ~ (97.1 1.3)%MVA: Geometrical Likelihood (GL) Our main background is combinatorial background from two real muons:

reduce it by using variables related to the geometry of the event: (vertex, pointing, IPS, lifetime, mu-isolation) + pT of the B14

Input Variables to the GLvertexB IP IPSlifetime isolationB pT MC Bd,s - MC bbXGeometrical Likelihood (GL) Our main background is combinatorial background from two real muons:

reduce it by using variables related to the geometry of the event: (vertex, pointing, IPS, lifetime, mu-isolation) + pT of the B14

Input Variables to the GLvertexB IP IPSlifetime isolationB pT

signalbb backgroundGeometrical Likelihood (MC)Variables are decorrelated and a Multi Variate Variable is built: flat for signal peaked at zero for backgroundHere we have to notice that the largest sensitivity is given by the high values of GL (where background is zero)16Geometrical Likelihood (GL)15

signalbb backgroundGeometrical Likelihood (MC)Variables are decorrelated and a Multi Variate Variable is built: flat for signal peaked at zero for backgroundOptimization & training with MCCalibration of the shape with data(see later)Geometrical LikelihoodBDTFisher DiscriminantNNGL: Rejection vs Efficiency profileHere we have to notice that the largest sensitivity is given by the high values of GL (where background is zero)17Measure the BR/Upper limit:the CLs binned method1) Events are classified in 2D planeGL vs mass. 2) Signal regions are divided in bins:and for each bin the compatibility is computed with the: - S+B hypothesis [CLS+B] - B only hypothesis [CLB]

Geometrical Likelihood vs Mass 16Bs mass regionBd mass region- CLS= CLS+B/CLB = compatibility with the signal hypothesisUsed to compute the exclusion - CLB = (in)compatibility with the background hypothesis Used for observationCLs is more conservative as it takes into account the statistical fluctuations of the background.18Measure the BR/Upper limit:the CLs binned methodGeometrical Likelihood vs Mass 16Bs mass regionBd mass regionExpected background events: Use mass sidebands Expected signal events: Need PDFs (Mass and GL) and an absolute normalization factor (for a given BR)The normalization factor is needed to avoid the dependence with sigma and Luminosity.19Expected background in signal regionsThe expected background events in signal regions are extracted from a fit of the mass sidebands divided in GL bins Invariant mass in GL binsbin1bin2bin3bin4Background GL in mass sidebandsbin1bin2bin3bin4bin3bin2bin117Sidebands for bin1-bin2: 600 MeV around the Bs,d massSidebands for bin3-bin4: 1200 MeV around the Bs,d massThe errors on the estimate of the number of background events is computedby fluctuating with a Poissonian distribution the number of events measured in the left and right sidebands and by varying within 1 the value of the k parameters

20Expected background in signal regionsThe expected background events in signal regions are extracted from a fit of the mass sidebands divided in GL bins Invariant mass in GL binsbin1bin2bin3bin4bin3bin2bin117GL binBsBdbin1329.16.4 (335)351.66.6 (333)bin27.4 1 (7)8.3+1.1-1.0 (8)bin31.51+0.41-0.35 (1)1.85+0.45-0.39 (1)bin40.08+0.10-0.05 (0)0.13+0.13-0.07 (0)Expected (observed) background events in Bs,d mass regionsThe errors on the estimate of the number of background events is computedby fluctuating with a Poissonian distribution the number of events measured in the left and right sidebands and by varying within 1 the value of the k parameters

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Signal Invariant Mass calibration The Bs,d mass line shapes are described by Gaussian + Crystal Ball parameters (,) calibrated with Bhh and dimuon resonances

1) M(Bd), M(Bs) average values from BdK and BsKK samples

B0KpBspKB0ppBsKKB0pKBSKK18B0pKSignal Invariant Mass calibration Mass resolutions (M(Bd,s)) from :Bhh inclusive sample:Interpolation from dimuon resonances

19The family: (1S), (2S), (3S) J/, (2S) @ M(Bs,d)- similar kinematics/topology- Selection identical to the signal one:Avoid to use the PID and use only events triggered by the other b to avoid bias in the phase space [eg resolution]

Signal Invariant Mass calibration Mass resolutions (M(Bd,s)) from :Bhh inclusive sample:Interpolation from dimuon resonances

19The family: (1S), (2S), (3S) J/, (2S) @ M(Bs,d)The two methods give compatible results: (M) = 26.7 0.9 MeV/c2CDF (D0) : (M) ~ 24 (120) MeV/c2 Geometrical Likelihood calibration signalbackgroundGL shape for signal extracted from B hh is flat as expected.Systematic error dominated by the fit model.20Bin 3

Bin 4

B hh sample is also used to calibrate the GL shape with dataThe GL signal shape is given by the fractional yield of Bhh in each GL bin. L0*HLT1*HLT2 : too few events TOS + knowledge of (TOS): not feasible (discrepancies data/MC)25Analysis strategySoft selection: - reduces the dataset to a manageable level Discrimination between S and B via Multi Variate Discriminant variable (GL) and Invariant Mass (IM) - events in the sensitive region are classified in bins of a 2D plane Invariant Mass and the GL variables Normalization: Convert the signal PDFs into a number of expected signal events by normalizing to channels of known BR: selection as similar as possible with the signal to minimize systematic uncertainties.Extraction of the limit/measure the BR: - assign to each observed event a probability to be S+B or B-only as a function of the BR(Bs,d) value; exclude (observe) the assumed BR value at a given confidence level 21NormalizationThe signal PDFs can be translated into a number of expected signal events by normalizing to a channel with known BR

Three different channels used:

1) BR(B+J/(+-) K+) = (5.980.22) 10-5 3.7% uncertainty Similar trigger and PID. Tracking efficiency (+1 track) dominates the systematic in the ratio of efficiencies. Needs fd/fs as input: 13% uncertainty 2) BR(BsJ/(+ -) (K+K -)) = (3.350.9) 10-5 26% uncertainty Similar trigger and PID. Tracking efficiency (+2 tracks) dominates the systematic 3) BR(B0K+-) = (1.940.06) 10-5 3.1% uncertainty Same topology in the final state. Different trigger dominate the syst. Needs fd/fs22

Normalization Factors: breakdownThe normalization with three different channels is equivalent to perform threedifferent analyses with different systematic uncertaintiesWe use fd/fs=3.710.47, a recent combination of LEP+Tevatron data by HFAG, with 13% uncertainty, dominated by LEP measurementshttp://www.slac.stanford.edu/xorg/hfag/osc/end_2009/#FRACLHCb: 12000 B+ Jpsi K+ in 37 pb-1 (320/pb) LHCb has a factor 60 wrt CDFCDF: 20,000 B+ Jpsi K+ in 3.7 fb-1 (5,4/pb)D0: 46,000 B+Jpsi K+ in 6.1 fb-1 (6,5/pb)28

The three normalization channels give compatible results: Weighted average accounting for correlated systematic uncertainties Normalization: results24Look inside the box.1) count the events in the 4 GL bins and 6 mass bins,2) compare observed events with the expected number of signal and background eventsBs search windowBd search window25

Bs search windowGeometrical Likelihood BinsInvariant Mass bins (MeV/c2)31

Bd search windowInvariant Mass bins (MeV/c2)Geometrical Likelihood Bins32Results: Bs33Expected upper limit68% of possible experimentscompatible with expected limitObserved upper limit90% exclusion95% exclusion @ 90% [email protected] 95% CLLHCbObserved (expected), 37 pb-1< 43 (51) x10-9< 56 (65) x10-9D0World best published, 6.1 fb-1PLB 693 539 (2010)< 42 x10-9< 51 x10-9CDFPreliminary, 3.7 fb-1Note 9892< 36 x10-9< 43 x 10-9CLs vs BR(BS)28Results: B0d34Expected upper limit68% of possible experimentsCompatible with expected limitObserved upper limit90% exclusion95% exclusion @ 90% [email protected] 95% CLLHCbObserved (expected) 37 pb-1< 12 (14) x10-9< 15 (18) x10-9CDFWorld best, 2 fb-1PRL 100 101802 (2008)< 15 x10-9< 18 x10-9CDFPreliminary, 3.7 fb-1Note 9892< 7.6 x10-9< 9.1 x 10-9CLs vs BR(Bd)29In all cases the observed upper limits contains the uncertainty in the signal and background pdfs and normalization factors.The input parameters (+ normalization factor) are fluctuated according to their uncertainty before build the pdfs.34Bs: LHCb reach in 201137 pb-1500 pb-1With the data collected in 2011 we will be able to explore the very interesting region of BR~ 10-8 and below1x10-8 (automn 2011)2x10-8 (summer 2011 )305.6x10-8 (today)5x10-9 (end 2011?)1 fb-1BS exclusion @ 95% CLBs: LHCb reach in 201137 pb-1500 pb-1With the data collected in 2011 we will be able to explore the very interesting region of BR~ 10-8 and belowtan vs mA plane:1x10-8 (automn 2011)2x10-8 (summer 2011 )305.6x10-8 (today)5x10-9 (end 2011?)1 fb-1BS exclusion @ 95% CLBs: LHCb reach in 2011With the data collected in 2011-2012 we will be able to have a 5 discovery if BR>10-8tan vs mA plane:31Bs 3 observation/5 discovery 0.6 fb-12 fb-110-82x10-8ConclusionsWith only 37 pb-1 LHCb has shown an amazing potential to search for New Physics in the scalar/pseudo-scalar sector.

The LHCb results:

Paper to be submitted to Phys. Lett. B are very close to the best world limits from Tevatron with ~100 (CDF) -200 (D0) times less luminosity.BR(Bs) < 43 (56) x 10-9 @ 90% (95%) CLBR(B0d) < 12 (15) x 10-9 @ 90% (95%) CLThe 2011-2012 run will allow LHCb to explore the very interesting range of BR down to 5x10-9and possibly discover New Physics.STOPBs @ ATLAS/CMSExperimentN sigN bkg90% CL limit in absence of signalATLAS ( 10 fb-1 ) (bb)=500 ub5.6 events 14+13-10 events (only bb) -------CMS ( 1 fb-1 ) (bb)=500 ub2.36 events6.53 events (2.5 bb)< 1.6 x 10-8Cut based analysis: separate signal from background by using high discriminant variablessuch as pointing , isolation and secondary vertex displacement:

[CMS-PAS-BPH-07-001 (2009)]Eg: Distance of flight and distance of flight significance:[CERN-OPEN-2008-020)]

Ratio of fragmentation fractionsWe use fd/fs=3.710.47, a recent combinaton of LEP+Tevatron data by HFAG, with 13% uncertainty, dominated by LEP measurementsB speciesZ0 fractions [%]Tevatron fractions [%]B40.41.233.3 3.0B040.41.233.3 3.0Bs10.91.212.1 1.5b 8.32.021.4 6.8HFAG: http://www.slac.stanford.edu/xorg/hfag/osc/end_2009/#FRACTevatron results from PLB, 667,1 (2008)LHCb will measure them with semileptonic decays andhadronic B(s)Dh decays (Phys.Rev.D83, 014017 (2011) (REC)x(SEL)(TRIG) fd/fsNBRtotalBJ/ K4%5%13%3%4%15%BSJ/8%5%--9%26%(*)28%B0dK7%14%13%13%3%23%Normalization factors: systematic uncertainties(*) from Belle @ (5S): arXiv:0905.4345Geometrical LikelihoodHow the decorrelation is done:1). Input variables 2) Gaussian variables In this space the correlations are more linear: easier to decorrelate3) Decorrelation is applied and the variables are re-gaussianized

1) Tranformation under signal hypothesis: 2STransformation under background hypothesis: 2BDiscriminating variable: GL = 2S-2B kept flat for signalGaussian andindependentvariables: Build 22) 3) D. Karlen, Comp. Phys.12 (1998) 380Trigger configurations

Data samples grouped in 5 trigger categories: - Muon lines stable for 90% of the data set - Hadron lines: 80% of L taken with L0(h) ET>3.6 and SPD4.7GeV/cHLT2:L0:Background composition:peaking background from BhhThe fake rate probability has been convoluted with the p-spectrum of the dominant B hh modes. In all cases we expect