Max Baak1 Impact of Tag-side Interference on Measurement of sin(2 + ) with Fully Reconstructed B 0 D (*) Decays Max Baak NIKHEF, Amsterdam For the BaBar Collaboration APS Meeting Philadelphia, 8 April 2003
Max Baak2 CP Violation in B 0 D (*) Four final states: D - +, D + - ; D *- +, D *+ - (not CP eigenstates) Each pair accessible to B 0 and B 0 CP violation through interference Expected CP violation small (2%) Suppressed amplitude through b u transition Dominant amplitude strong phase difference ( 3 ) CKM Unitarity Triangle
Max Baak3 Time-Dependent sin(2 + ) Measurement: Old School Time evolution for B 0 decays (R unmix ) and B 0 decays (R mix ) to D - + For D + - : sin(2 + - ) sin(2 + + ) r = 0.0 r = 0.1, = 0 Simultaneous determination of r and challenging Estimate (and fix) r from B 0 D s + - [1] r (D ) = r (D * ) = Similar for D* [1] hep-ex/ – submitted to PRL
Max Baak4 Common Analysis Technique in BaBar Fully reconstruct one B in state D or D * Determine flavor of other B meson B TAG (“tagging”) Reconstruct vertex of B TAG and compute proper time difference t (4s) Tag B Reco B K+K+ ++ zz K+K+ t z/c = 0.55 At time of B TAG decay, the 2 B’s are in opposite flavor states z Coherent B 0 B 0 production -s-s --
Max Baak5 Subtlety of B Flavor Tagging Many B 0 DX modes (X a hadron) ‘kaon tag modes’ At BR levels of O(10 -4 ) intrinsic mistagging because of b u transitions in B 0 decays Effect always assumed tiny, and accounted for by mistag fractions. Hidden assumption: Individual tagging states dominated Use charge correlation between final state and B flavor by single B 0 decay amplitudes Not True! Works well for lepton tags!
Max Baak6 B0B0 B0B0 Doubly-Cabibbo suppressed decays on the tag side 1. (4s) B 0 B 0 system symmetric in two B’s 2.System evolves coherently in time 3.On reco-side b u interference is used for sin(2 + ) measurement. Induced time-dependent effects of order V ub V cd /V cb V ud = 0.02 b u Interference at tag-side B induces time-dependent effect, just like reco-side! Lepton tags unaffected. Kaon tags have problems. Change the time-dependent pdf’s! Long, Baak, Cahn, Kirkby hep-ex/ 2% for sin2 with J/ K s 100% effect for D (*) ! (4s)
Max Baak7 How the decay distributions change for D (*) For sin(2 + ) measement (b u transition) tag-side problem of similar size compared to signal, cannot be ignored! How do the time-dependent decay distributions change? One kaon tag mode with (unknown!) amplitude ratio r’ and strong phase ’ Similar for D + - r=0.1, =0, r’=0 r=0.1, r’=0.1, =0, ’= r=0, r’=0 Equations pick up sine terms
Max Baak8 Change of Variables Flavor-tag symmetric lepton tags Handle on phase -Only 3 independent parts -The b parameter does not dilute in mistagging! Split off the strong phases and ’ for better parametrization set Reco side Tag side (`primed’) r’ unknown
Max Baak9 Fit Technique Unbinned maximum likelihood fit to t spectra of D and D * samples. Fit Parameters # a,b,c parameters13 Resolution Function8 Mistag fractions12 Background modelling15 m d and B fixed (PDG) - Free arameters48 2a(D ) and a(D* ) common for all tag. cats. 2c LEP (D ) and c LEP (D* ) 3b pars. : for each kaon tag. category, common to D and D* 6c pars. : for each kaon tagging cat. of D and D* Most sensitive to 2 + BaBar uses 1 lepton and 3 kaon tag. categories! Similar to other mixing measurements In practice numerous kaon tag-modes No reason tag-side parameters r’ and ’ to be identical for each tag category Use different b and c tag-parameters for each kaon tag. category
Max Baak10 a(0)=0.000 c(0)=0.001 r’(generated) c LEP (fit) – c LEP (gen) r’(generated) (c LEP ) (a) D* a(fit) – a(gen) Toy MC Validation No observed biases in signal parameters! r = 0.02, , ’ = 0.00 a=0.04, c=0.00 Toys correspond to 82 /fb of data+background (D plus D* )
Max Baak11 Data Sample [ ]: 82 fb -1 on (4s) Resonance N(D ) = 5207 87 Purity = 85 % N(D* ) = 4746 78 Purity= 94 % LeptonKaonIKaonIIOther DD 704 44 D* 644 40 breakdown in tagging categories Fully reconstructed B D (*) normally used for mistag fractions in sin2 analysis
Max Baak12 Sensitivity to after tag-side effect Analysis not yet finished With current D (*) data set, what is loss & sensitivity to ? No tag-side interference (c c LEP ) With tag-side interference Signal parameters: (a[D ]) 0.04 (c[D ]) 0.04 (a[D ]) 0.04 (c LEP [D ]) 0.07 Stat. sensitivity (sin(2 + )) ~ 0.X loss due to kaon tags Identical errors for D * sample B A B AR 82 fb B A B AR 82 fb Dominant Systematic uncertainties: 1) Uncertainty in r3) Detector effects 2) Monte Carlo4) Background Systematics (sin(2 + )) ~ 0.5 x stat.
Max Baak13Conclusion Tag-side Doubly-Cabibbo-Suppressed decays sofar neglected in most time-dependent analyses (using coherent B-decays) Complications for sin(2 + ) measurements using D (*) / /a 1 Effect cannot be ignored! Using correct parametrization, can work around problem. Some loss in sensitivity to First BaBar Result coming soon! 2% for sin2 with J/ K s 100% effect for D (*) (stay tuned) Long, Baak, Cahn, Kirkby hep-ex/
Max Baak14 Backup Slides
Max Baak15 B Flavor Tagging Methods In BaBar tagging is handled with Neural Nets Information used: Primary lepton Secondary lepton Kaon(s) Soft pions from D * decays Fast charged tracks Tagging category Fraction of tagged events (%) Wrong tag fraction w (%) Q = (1-2w) 2 (%) Lepton 9.1 0.3 Kaon+Kpi 16.7 0.4 Kaon+Spi 19.8 0.4 Inclusive 20.0 0.2 ALL 65.6 0.7 Smallest mistag fraction The errors on a,c LEP scale with quality Q Mistag fraction w determined from cosine coefficients
Max Baak16 Multiple tag-side final states In practice there are numerous tag-modes Combining these leads to average, effective r’ eff and ’ eff No reason to expect r’ and ’ to be identical for each tag category Use different b and c parameters for each category Hardly anything known about values of r’ i or ’ i Uncertainty on r’ eff : 0 – r’ max No sensitivity to 2 + from kaon tag-side parameters b and c