Measurement of time dependent CP violation in B φks. Mahalaxmi Krishnamurthy University of Tennessee. BaBar Collaboration Beauty 2003

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1 Measurement of time dependent CP violation in B φks Mahalaxmi Krishnamurthy University of Tennessee BaBar Collaboration Beauty 2003 CMU, Pittsburgh 1

2 b-> sss Decays CP violation from interference of decay amplitudes into CP eigenstate: B 0 φks and B 0 B 0 -bar φks Decays dominantly through penguin amplitudes (gluonic & EW), tree small: e.g. Decay exclusively through penguin : sin2β(φks) sin2β(j/ψks) < 4% Estimate hadronic pollution (rescattering) using flavor SU(3) relations (30% breaking) between (measured) decay rates of charmless quasi-twobody decays e.g. [Grossman, Ligeti, Nir, Quinn, hep-ph/ ] worst case scenario today: sin2β(φks) sin2β(j/ψks) < 25% With more data, smaller bounds: 1 ab -1 expect ~10% 2

3 BaBar Detector 110 fb-1 1.5T Magnet Luminosity fb-1 EMC e + delivered recorded DCH Off Peak SVT e - DIRC IFR 3

4 Measuring sin(2β) at the Asymmetric B-Factory µ +, e +, K + B tag : Xl +, XK +, Xπ s - (B flavor sample) e - : 9 GeV B tag B-Flavor Tagging e + 3 GeV B CP z φ K S π π + K + K Exclusive B Meson Reconstruction B CP : φks, φkl, J/ψ Ks.. Time dependent asymmetry: R R af ( t) = ( t) = C f cos( mdt) + S f sin( mdt) R + R 4

5 Event Variables Energy-substituted mass: M ES = [s/2 + p Υ p B ) 2 /E Υ2 -p B2 ] 1/2 Energy difference: E = E B * - s 0.5 /2 (E Υ, p Υ ), (E B,p B ) 4 momenta of Υ(4S) and B in laboratory frame, E B * reconstructed B energy in center of mass frame Events/(2.5 MeV/c 2 ) abs( E)< 0.06 GeV 5.27<M ES < 5.29 GeV/c 2 M ES (GeV/c 2 ) E (GeV) signal events : 70 ±9.0 purity : 61 ±0.1% Number of signal events is consistent with published branching fraction. 5

6 Event Variables Helicity angle: - angle(k, B) in φ-rest frame Fisher discriminant: linear combination of - cosθ T (thrust angle*) - cosθ B (emission angle* w.r.t. z-axis) - momentum-weighted Legendre polynomials side band data F = a 0 cosθ T + a 1 cosθ B + a 2 Σp i L 0 (θ i ) + a 3 Σp i L 2 (θ i ) Signal MC *Υ(4S) rest frame 6

7 Tagging Tagging performance evaluated from data using B flavor sample Tags grouped into 4 mutually exclusive categories: σ(s) ~ 1/ N Q Category Lepton Kaon I Kaon II Inclusive ε (%) ω (%) ω (%) efficiency mistag rate tag difference Q = Σ ε(1-2ω) 2 Q (%) φks signal events Total ± 0.7 Tagging optimized for maximal value of Q 7

8 Vertexing (ΦK % J/Ψ K) Opening angles quite different: φ Κ + Κ B->φ K : - large Q-value - helicity angle ~ cos 2 θ K + J/ψ l + l K - θ H φ in B (Y(4S) ) J/ψ Ks B vertex z resolutions : B cp ~ 50 µm B tag ~ 170 µm φks Resolution dominated by the tag side cosθ H 8

9 t Resolution Comparable σ( t) distributions in J/ψ Ks and φks J/ψKs φks Background Resolution Function: R( t) = [ G ( bias, scale ( σ ( t)); core i t) + G outlier (0 ps,8 σ( t) (ps) ps; t)] { f Signal Resolution Function: R( t) = (1 f tail + f f outlier tail G Exp ( t ) G tail core / τ ( bias B tag ) ( bias,3 ps) + + (1 f ) δ ( t)}, scale ( σ ( t)); t) f outlier G i outlier (0 ps,8 ps; t) - Established by extensive test with Monte Carlo (MC) events - Final parameters obtained from B flavor data in simultaneous fit with CP data 9

10 CP Fit Strategy Use B-flavor, 2138 φks(π + π - ) Maximum likelihood fit: L = L(M ES ) x L( E) x L(F ) x L(cosθ H ) x L( t) Fix parameters (yields) that depend exclusively on CP sample Float parameters that depend on B flavor sample Total 40 parameters: CP t Tag TOTAL S and C Sig. Resolution Bkg. Resolution Bkg. time evolution. Fractions Sig. Dilution Sig. dilution asym. Bkg. Dilution Bkg. dilution asym Tested with J/ψ Ks data, MC data mockfit and Toy MC. 10

11 Time Dependent CP Fit for φks S = 0.45 ± 0.43 C = ± 0.37 statistical error only Zoom into the signal region (m ES >5.27GeV, E < 0.06 GeV) Raw asymmetry: charmonium modes world average WA S =

12 Time Dependent CP Fit for φk + Use φk + as control sample signal events : 252 purity : 76 ±0.1% M ES (GeV/c 2 ) Standard Model: expect S=0 and C=0 S = 0.23 ± 0.24 C = ±

13 Systematic Uncertainties 13

14 The move.. Old result: Fit to φks, Ks π + π - and Ks π 0 π 0 sample combined (80 fb -1 ) S = ± 0.51 C = ± 0.38 Old Ks π + π only: S = ± 0.52 C = ± 0.41 Overlap between reprocessed (new) Run1+2 and old Ks π + π (80% overlap) Old Ks π + π : S = 0.02 ± 0.55 C = ± 0.44 New Ks π + π : S = 0.05 ± 0.51 C = ± 0.48 S ~ 0.0 C ~ 0.3 New+Run3 Ks π + π : S = 0.45 ± 0.43 (110 fb -1 ) C = ±

15 The Understanding % change in tag due to reprocessing : ~ 8% ~ 3 events flip tag around t = 0 cp - old sample : F + - F - = 5 events new sample : if all 3 flip events are B 0 or B 0 tags (F + - F - = 2 or 8 ) C ~ ±0.3 S ~ swap of the samples confirms the effect => change of result covered by statistical error. -> F ± integrated rate of B 0 /B 0 tagged events -> average dilution <D>=0.61±0.04 Low statistics channels are prone to fluctuations! 15

16 More results B (B 0 φk 0 ) = ( ± 0.3) x B (B + φk + ) = ( ± 0.5) x B (B + φπ + ) < 0.41 x 10-6 A cp (B + φk + ) = 0.04 ± 0.09 ± 0.01 in 80 fb -1 hep-ex/ time dependent: C (B + φk + ) = ± 0.18 A cp = 0.05 ± 0.07 the two independent methods give the same result of A cp in φk + C (B 0 φks) = ± 0.37 A cp = 0.14 ± 0.14 in 110 fb -1 16

17 Summary / Outlook - Measured time dependent asymmetry in B 0 -> φks(π + π - ). - More CP events are needed by increase of luminosity and by including other channels e.g. φkl, φks(π 0 π 0 ). - Expectation for the error in 300 fb -1 : σ (S) = 0.26 σ (C ) =

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19 Current status of S average 19

20 Time Dependent CP Fit for φks S = 0.45 ± 0.43 C = ± 0.37 S C 15% correlation 20