Measurement of CP Violation in B Decays with the BaBar detector Shahram Rahatlou University of California, San Diego UCSD HEP Seminar La Jolla, 25 September 2001

B mesons and CP violation in the Standard Model Outline What is CP symmetry? B mesons and CP violation in the Standard Model How can we measure the CP Violation? Brief introduction to the BaBar detector Overview of the measurement technique B reconstruction Lifetime measurement Measurement of B0B0 Mixing Frequency Extraction of CP Violating Asymmetry Results and Prospects 25 September 2001 Sh. Rahatlou

Discrete Symmetries in Nature In general if a physical law is symmetric under a transformation, then there is a conserved quantity 3 important discrete symmetries in Particle Physics Parity, P Parity reflects a system through the origin. Converts right-handed coordinate systems to left-handed ones. Vectors (momentum) change sign but axial vectors (spin) remain unchanged x  -x L  L Charge Conjugation, C turns a particle into its anti-particle e+  e- g  g Time Reversal, T Changes, the sign of the time; t  -t all time dependent quantities, e.g. momentum, change sign +  25 September 2001 Sh. Rahatlou

Weak Interactions and Symmetry Violation In 1957 violation of parity was observed Asymmetry in b decays of 60Co  60Ni + e- + n Electrons produced mostly in one hemisphere C is violated too! only left-handed neutrinos and right-handed anti-neutrinos In 1964 CP violation was observed in the weak decay of neutral K mesons Ks => p+ p- (CP = 1) Kl => p0 p0 p0 (CP = -1) Observed Kl => p+ p- (0.2%) => CP violation! Theoretically difficult to precisely interpret CP violation results in neutral K systems B Mesons expected to show CP violation good testing ground for possible sources of CP violation 25 September 2001 Sh. Rahatlou

CP Violation in the Standard Model The weak interaction between quarks regulated by the Cabibbo-Kobayashi-Maskawa matrix With 3 generations of quarks, the SM can accommodate CP violation through complex coupling constants: 3 angles and a complex phase Unitarity of the CKM Matrix b a g Vtd Vtb * Vud Vub Vcd Vcb Unitarity Triangle 25 September 2001 Sh. Rahatlou

Unitarity of the CKM Matrix The sides and the angles of this triangle can be determined experimentally in the B decays 25 September 2001 Sh. Rahatlou

Direct and Indirect CP Violation Mechanisms Direct CP Violation: Interference of two decay amplitudes Can occur in both neutral and charged B decays Total amplitude for a decay and its CP conjugate have different magnitudes Large hadronic uncertainties => difficult measure CKM matrix elements Relatively small asymmetries expected in B decays Indirect CP Violation Only in neutral B decays Charge asymmetry in semileptonic decays Expected to be small in Standard Model B0 b d u,c,t W- Mixing Diagram d b W- u s p+ K- B0 Decay Diagram 25 September 2001 Sh. Rahatlou

CP Violation in interference between Mixing and Decay Time evolution of initial pure B0/B0 states: Mixing Mass eigenstates: fCP is a CP eigenstate B0(t) fCP B0 Initial state Flavor eigenstate B0(t) fCP B0 Initial state Flavor eigenstate 25 September 2001 Sh. Rahatlou

Time evolution of B0 mesons into a final CP eigenstate Amplitude ratio Weak Phase In order to have CP Violation 25 September 2001 Sh. Rahatlou

Time-dependent CP Asymmetry From the time evolution of the B0 and B0 states we can define the time-dependent asymmetry to be Probe of direct CP violation since it requires Sensitive to the phase of l even without direct CP Violation 25 September 2001 Sh. Rahatlou

Golden Decay Mode: B0 ® J/y K0S K0 mixing u,c,t W- Theoretically clean way to measure the phase of l (sin2b) Clean experimental signature Large branching fraction compared to other CP eigenstates “Golden Modes” hCP = -1 B0  J/ K0S B0  (2s) K0S Time-dependent CP asymmetry hCP = +1 B0  J/ K0L 25 September 2001 Sh. Rahatlou

What does one expect to see? Dt spectrum and the observed asymmetry for a perfect detector (assuming sin2b = 0.6) Visible difference between B0 and B0 decay rates sin 2b In this ideal case, the amplitude of the oscillation is the CP Asymmetry time-integrated asymmetry is 0 t 25 September 2001 Sh. Rahatlou

Required Experimental Ingredients Produce many B’s because of the small branching fraction Need a B-Factory Determine the flavor of the initial B meson to separate B0 from B0 (Tagging) Production: B-B pairs in a coherent state Reconstruction: Good particle identification for Kaons and leptons Define and measure a ‘time’ in order to study the time-dependent asymmetry Production: Coherent B-B pair produced with an initial boost in the LAB Reconstruction: A high precision tracking system to measure the distance between the B decay points BaBar Detector @ Asymmetric B-Factory 25 September 2001 Sh. Rahatlou

PEP-II Asymmetric B-Factory at SLAC B B~ production threshold 9 GeV e- on 3.1 GeV e+ U(4S) boost in lab frame bg = 0.56 25 September 2001 Sh. Rahatlou

B-Factory Performance as Today Records from Yesterday! PEP-II top luminosity: 4.15 x 1033cm-2s-1 (design 3.0 x 1033) Top recorded Lumi/24h: 246 pb-1 Top recorded Lumi/8h: 84 pb-1 BABAR logging efficiency: > 96% 30/fb analysed for CP September 25, 2001 October 99 PEP-II delivered: 49.1 fb-1 BABAR recorded: 46.5 fb (includes 5.1 fb-1 off peak) 87 million B’s available !! 25 September 2001 Sh. Rahatlou

The BaBar Detector e+ (3.1 GeV) e- (9 GeV) Electromagnetic Calorimeter 6580 CsI(Tl) crystals 1.5 T solenoid e+ (3.1 GeV) Cerenkov Detector (DIRC) 144 quartz bars 11000 PMs e- (9 GeV) Drift Chamber 40 stereo layers Instrumented Flux Return iron / RPCs (muon / neutral hadrons) Silicon Vertex Tracker 5 layers, double sided strips SVT: 97% efficiency, 15 mm z hit resolution (inner layers, perp. tracks) SVT+DCH:(pT)/pT = 0.13 %  pT + 0.45 % DIRC: K- separation 4.2  @ 3.0 GeV/c  2.5  @ 4.0 GeV/c EMC: E/E = 2.3 %E-1/4  1.9 % 25 September 2001 Sh. Rahatlou

B Decay Topology in the Boosted (4S) Environment z Tag vertex reconstruction Flavor Tagging (bg)U(4S) = 0.56 Start the Clock Coherent BB pair Exclusive B Meson and Vertex Reconstruction 25 September 2001 Sh. Rahatlou

Increasing complexity Analysis Strategy Factorize the analysis in building blocks Measurements B±/B0 Lifetimes B0 B0-Mixing CP-Asymmetries Analysis Ingredient Reconstruction of B mesons in flavor eigenstates B vertex reconstruction Flavor Tagging + a + b Reconstruction of neutral B mesons in CP eigenstates + a + b + c Increasing complexity Higher precision 25 September 2001 Sh. Rahatlou

Measurement of the B0 and B+ Lifetime U(4s) bg = 0.56 Tag B sz ~ 110 mm Reco B sz ~ 65 mm p+ Dz Dt @ Dz/gbc K0 g D- p- K+ 3. Reconstruct Inclusively the vertex of the “other” B meson (BTAG) Fully reconstruct one B meson in flavor eigenstate (BREC) Reconstruct the decay vertex 4. compute the proper time difference Dt 5. Fit the Dt spectra 25 September 2001 Sh. Rahatlou

Fully-Reconstructed B sample Flavor eigenstates Bflav : for lifetime and mixing measurements Cabibbo-favored hadronic decays “Open Charm” decays 30 fb-1 Neutral B Mesons Hadronic decays into final states with Charmonium Charged B Mesons [GeV] 25 September 2001 Sh. Rahatlou

Vertex and Dt Reconstruction Beam spot Interaction Point BREC Vertex BREC daughters BREC direction BTAG direction TAG Vertex TAG tracks, V0s z Reconstruct Brec vertex from charged Brec daughters Determine BTag vertex from charged tracks not belonging to Brec Brec vertex and momentum beam spot and U(4S) momentum High efficiency (97%) Average Dz resolution is 180 mm (<|Dz|> ~ bgct = 260 mm) Dt resolution function measured from data 25 September 2001 Sh. Rahatlou

tB Measurement in BaBar e-t/t true Dt B production point known eg. from beam spot LEP/SLD Dt resolution measured Dt Resolution function lifetime  = e-|Dt|/t Either Brec or Btag can decay first (this analysis) BaBar Resolution Function + Lifetime  = Need to disentangle resolution function from physics ! 25 September 2001 Sh. Rahatlou

Dt Resolution Function sDz event-by-event s(Dt) from vertex errors Lifetime-like bias to Small correlation between the lifetime and the RF parameters ~0.6 ps Signal MC (B0) tracks from long-lived D’s in tag vertex asymmetric Resolution Function Dt (meas-true)/sDt 25 September 2001 Sh. Rahatlou

Lifetime Likelihood Fit Simultaneous unbinned maximum likelihood fit to B0/B+ samples Use data to extract the properties of background events Mass distribution provides the signal probability Use the events in the sideband (mES < 5.27) to determine the Dt structure of the background events under the signal peak 19 free parameters t(B+) and t(B0) 2 Dt signal resolution 5 empirical background 12 description B0 mES B0 Bkg Dt 25 September 2001 Sh. Rahatlou

B Lifetime Fit Results B0/ B0 B Precision measurements 2 % statistical error 1.5% systematic error Main source of systematic error Parameterization of the Dt resolution function Description of events with large measured Dt (outliers) 20 fb-1 B0/ B0 B signal + bkg Submitted to PRL t0 = 1.546  0.032  0.022 ps PDG: 1.548  0.032 ps t = 1.673  0.032  0.022 ps PDG: 1.653  0.028 ps t/t0 = 1.082  0.026  0.011 PDG: 1.062  0.029 background Dt (ps) 25 September 2001 Sh. Rahatlou

Analysis Strategy (II) Measurements B±/B0 Lifetimes B0 B0-Mixing CP-Asymmetries Analysis Ingredient Reconstruction of B mesons in flavor eigenstates B vertex reconstruction Flavor Tagging + a + b Reconstruction of neutral B mesons in CP eigenstates + a + b + c ü 25 September 2001 Sh. Rahatlou

Measurement of B0B0 Mixing U(4s) bg = 0.56 Tag B sz ~ 110 mm Reco B sz ~ 65 mm p+ Dz Dt @ Dz/gbc K0 g D- p- K+ 3. Reconstruct Inclusively the vertex of the “other” B meson (BTAG) ü 4. Determine the flavor of BTAG to separate Mixed and Unmixed events 1. Fully reconstruct one B meson in flavor eigenstate (BREC) ü 2. Reconstruct the decay vertex ü 5. compute the proper time difference Dt ü 6. Fit the Dt spectra of mixed and unmixed events 25 September 2001 Sh. Rahatlou

Dt Spectrum of Mixed and Unmixed Events perfect flavor tagging & time resolution realistic mis-tagging & finite time resolution + _ w: the fraction of wrongly tagged events Dmd: oscillation frequency 25 September 2001 Sh. Rahatlou

Extraction of Dmd and mistag fraction Fraction of Mixed Events Sensitive to mistag fraction measurement because the mixing has not started yet At t=0 the observed ‘mixed’ events are only due to wrongly tagged events B0 B+ Sensitive to Dmd measurement when the amplitude of the oscillation is at its maximum 25 September 2001 Sh. Rahatlou

B Flavor Tagging Methods Hierarchical Tagging Categories For electrons, muons and Kaons use the charge correlation b c d l- n B0 D, D* W- Lepton Tag b d B0 W- W+ c s K*0 Kaon Tag NN output Not Used Multivariate analysis exploiting the other kinematic information of the event, e.g., Momentum spectrum of the charged particles Information from non-identified leptons and kaons Soft p from D* decay Neural Network Each category is characterized by the probability of giving the wrong answer (mistag fraction w) 25 September 2001 Sh. Rahatlou

Flavor Tagging Performance The large sample of fully reconstructed events provides the precise determination of the tagging parameters required in the CP fit Tagging category Fraction of tagged events e (%) Wrong tag fraction w (%) Q = e (1-2w)2 (%) Lepton 10.9 0.3 8.9  1.3 7.4  0.5 Kaon 35.8 0.5 17.6  1.0 15.0  0.9 NT1 7.8 0.3 22.0  2.1 2.5  0.4 NT2 13.8 0.3 35.1  1.9 1.2  0.3 ALL 68.4 0.7 26.1  1.2 The error on sin2b the quality factor Q Highest “efficiency” Smallest mistag fraction 25 September 2001 Sh. Rahatlou

Dt Resolution Function Core Tail Outlier Use the event-by-event uncertainty on Dt Dt Residual (ps) R(dDt) B0 flavour sample CP sample sDt (ps) Different bias For each tagging category 25 September 2001 Sh. Rahatlou

Fit Parameters Mixing Likelihood Fit Unbinned maximum likelihood fit to flavor-tagged neutral B sample Fit Parameters Dmd 1 Mistag fractions for B0 and B0 tags 8 Signal resolution function(scale factor,bias,fractions) 9 Empirical description of background Dt 16 B lifetime fixed to the PDG value tB = 1.548 ps 34 total free parameters All Dt parameters extracted from data 25 September 2001 Sh. Rahatlou

B0B0 Mixing Fit Result 20 fb-1 C.L. 28 % Dmd = 0.519 ± 0.020 (stat) ± 0.016 (syst) h ps-1 Preliminary 25 September 2001 Sh. Rahatlou

Dmd Measurement in Comparison preliminary Precision Dmd measurement 4% statistical error 3% systematic error dominated by MC correction 25 September 2001 Sh. Rahatlou

Analysis Strategy (III) Measurements B±/B0 Lifetimes B0 B0-Mixing CP-Asymmetries Analysis Ingredient Reconstruction of B mesons in flavor eigenstates B vertex reconstruction Flavor Tagging + a + b Reconstruction of neutral B mesons in CP eigenstates + a + b + c ü ü 25 September 2001 Sh. Rahatlou

Measurement of CP Asymmetry U(4s) bg = 0.56 Tag B sz ~ 110 mm CP B sz ~ 65 mm m+ Dz Dt @ Dz/gbc K0 g p+ p- Ks0 m- 3. Reconstruct Inclusively the vertex of the “other” B meson (BTAG) ü 4. Determine the flavor of BTAG to separate Mixed and Unmixed events ü 1. Fully reconstruct one B meson in CP eigenstate (BCP) 2. Reconstruct the decay vertex ü 5. compute the proper time difference Dt ü 6. Fit the Dt spectra of B0 and B0 tagged events 25 September 2001 Sh. Rahatlou

The fully Reconstructed CP Sample J/y KS KS  p0 p0 J/y KS KSp+ p- Before tagging requirement 1999-2001 data 32 x 106 BB pairs 29 fb-1 on peak cc1 KS y(2S) KS Sample tagged events Purity CP [J/, (2S), cc1] KS 480 96% -1 J/ KL 273 51% +1 J/ K*0(KSp0) 50 74% mixed Full CP sample 803 80% J/y KL J/y K* After tagging 25 September 2001 Sh. Rahatlou

Dt Spectrum of CP Events perfect flavor tagging & time resolution realistic mis-tagging & finite time resolution Mistag fractions w And resolution function R CP PDF Mixing PDF determined by the flavor sample 25 September 2001 Sh. Rahatlou

Fit Parameters Sin2b Likelihood Fit Combined unbinned maximum likelihood fit to Dt spectra of flavor and CP sample Fit Parameters sin2b 1 Mistag fractions for B0 and B0 tags 8 Signal resolution function 16 Empirical description of background Dt 20 B lifetime fixed to the PDG value tB = 1.548 ps Mixing Frequency fixed to the PDG value Dmd = 0.472 ps-1 Global correlation coefficient for sin2b: 13% Different Dt resolution function parameters for Run1 and Run2 tagged flavor sample tagged CP samples 45 total free parameters All Dt parameters extracted from data Correct estimate of the error and correlations 25 September 2001 Sh. Rahatlou

sin2b = 0.59 ± 0.14 Sin2b Fit Results Combined fit to all modes Phys. Rev. Lett. 87 091801 (2001) Cross-checks: Null result in flavor samples Consistency of CP channels P(c2) = 8% Goodness of fit(CP Sample): P(Lmax>Lobs) > 27% Combined fit to all modes sin2b = 0.59 ± 0.14 25 September 2001 Sh. Rahatlou

Raw CP Asymmetry Raw ACP f = -1 events sin2b=0.56 ± 0.15 All tags Kaon tags f = -1 events sin2b=0.56 ± 0.15 Raw ACP sin2b=0.59 ± 0.20 25 September 2001 Sh. Rahatlou

Consistency Checks sin2b measured in several Dt bins Combined CP=-1 sin2b measured in several Dt bins sin2b vs. J/ decay mode and tagging category and flavor for  = -1 events 25 September 2001 Sh. Rahatlou

Sources of Systematic Error Error/Sample KS KL K*0 Total Statistical 0.15 0.34 1.01 0.14 Systematic 0.05 0.10 0.16 Signal resolution and vertex reconstruction 0.03 Resolution model, outliers, residual misalignment of the Silicon Vertex Detector Tagging 0.03 possible differences between BCP and Bflavor samples Backgrounds 0.02 (overall) Signal probability, fraction of B+ background in the signal region, CP content of background Total 0.09 for J/Y KL channel; 0.11 for J/Y K*0 Total = 0.05 for total sample 25 September 2001 Sh. Rahatlou

The Unitarity Triangle One solution for b is consistent with measurements of sides of Unitarity Triangle BaBar sin2b (with 30/fb) Error on sin2b is dominated by statistics  will decrease as Method as in Höcker et al, hepex/0104062 (also other recent global CKM matrix analyses) 25 September 2001 Sh. Rahatlou

Summary and Outlook sin2b = 0.59 ± 0.14 ± 0.05 BaBar observes CP violation in the B0 system at 4s level Probability to observe an equal or larger value if no CP violation exists is < 3 x 10-5 Corresponding probability for the hCP = -1 modes only is 2 x 10-4 sin2b = 0.59 ± 0.14 ± 0.05 t0 = 1.546  0.032  0.022 ps t = 1.673  0.032  0.022 ps t0 /t = 1.082  0.026  0.011 Dmd = 0.519 ± 0.020 ± 0.016 h ps-1 New precision measurements of B0/B+ lifetimes and B0B0 mixing frequency Dmd With anticipated 100 fb-1 by next summer, we expect the error in sin2b to be 0.08 25 September 2001 Sh. Rahatlou

Backup Slides Why cc is OK Run1 vs. Run2 World Average J/psi KL plot Direct CP B->p-p+ 25 September 2001 Sh. Rahatlou

Is it possible to measure a large asymmetry ? The answer is… yes! Suppose at a given time t’ you have Nevents < 0 is possible in a likelihood fit The signal PDF can be negative in some regions Requires having NO OBSERVED event in those regions The only constraint on the PDF is the normalization 25 September 2001 Sh. Rahatlou

Large sin2b in B0  C1KS Kaon tags Lepton tags B0 tags B0 tags fit for B0/B0 Dt PDFs, not for ACP Large sin2b possible , because No events where PDF<0 (eg. lepton tags) Sum of signal + background PDFs positive (eg. Kaon tags) Note: a single lepton B0-tag at Dt = -p/2Dm would bring sin2b from 2.6 to ~1/(1-2wlep)  1.1 Measure sin2b unbiased for low stat. samples and probability to obtain sin2b2.6 when true value 0.7 is 1~2% Kaon tags Lepton tags B0 tags B0 tags B0 tags Dt [ps] Dt [ps] 25 September 2001 Sh. Rahatlou

Changes between Run1 and Run2 First publication in March 2001 Changes since then: More data (run 2): 23 32 BB pairs Improved reconstruction efficiency Optimized selection criteria takes into account CP asymmetry of background in J/KL Additional decay modes C1KS and J/K*0 Improved vertex resolution for reconstructed and tag B sin(2b) = 0.34 ± 0.20 (stat) ± 0.05 (syst) PRL 86 (2001) 2515 25 September 2001 Sh. Rahatlou

Consistency Check: Run1 vs. Run2 Difference for modes used in the old PRL: 1.8 s Run 1 Run 2 25 September 2001 Sh. Rahatlou

New sin2b world average is 8s significant The New World Average New sin2b world average is 8s significant Measurements assumed to be uncorrelated 25 September 2001 Sh. Rahatlou

Raw CP Asymmetry for J/y KL sin2b=0.70±0.34 Background contribution 25 September 2001 Sh. Rahatlou

Search for Direct CP Violation Contribution from at least 2 amplitudes with a weak phase difference |l| might be different from 1 (tree amplitude and leading penguin amplitude for B J/y KS have same weak phase in SM) Probing new physics (only use hCP=-1 sample that contains no CP background) |l| = 0.93 ± 0.09 (stat) ± 0.03 (syst) No evidence of direct CP violation due to decay amplitude interference (SCP unchanged) 25 September 2001 Sh. Rahatlou

B0  p+p- Asymmetry Result Decay distributions f+(f-) when tag = B0(B0) Preliminary A = N(K-p+)-N(K+p-)/N(K-p+)+N(K+p-) Measurement compatible with no CP in B0  p+p- Statistically limited due to small branching fraction Need ~500/fb for s(Spp) ~ 0.10-0.15 25 September 2001 Sh. Rahatlou