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CP-Violating Asymmetries in Charmless B Decays: Towards a measurement of a

CP-Violating Asymmetries in Charmless B Decays: Towards a measurement of a. On behalf of the BaBar Collaboration. International Conference on High Energy Physics Amsterdam, July 24-31, 2002. James D. Olsen Princeton University. CP asymmetries in p + p - and K + p -

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CP-Violating Asymmetries in Charmless B Decays: Towards a measurement of a

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  1. CP-Violating Asymmetries in Charmless B Decays: Towards a measurement of a On behalf of the BaBar Collaboration International Conference on High Energy Physics Amsterdam, July 24-31, 2002 James D. Olsen Princeton University CP asymmetries inp+p-and K+p- Decay rates forp+p0and p0p0 CP asymmetries inr+p-andr+K- Submitted to Phys Rev (hep-ex/0207055) hep-ex/0207065 and hep-ex/0207063 hep-ex/0207068

  2. * VtdVtb * VudVub * VcdVcb CP Violation in the Standard Model • CP symmetry can be violated in any field theory with at least one irremovable complex phase in the Lagrangian • This condition is satisfied in the Standard Model through the three-generation Cabibbo-Kobayashi-Maskawa (CKM) quark-mixing matrix Unitarity Triangle • The angles (a,b,g) are related to CP-violating asymmetries in specific B decays • One down, two to go… a(f2) B0pp, rp g(f3) b(f1) B0DK B0J/yKS ICHEP 2002 J. Olsen

  3. Observing CP violation at the U(4S) • At the (4S), BBpairs are produced in a coherent P-wave • Three observable interference effects: • CP violation in mixing (|q/p| ≠ 1) • (direct) CP violation in decay (|A/A| ≠ 1) • (indirect) CP violation in mixing and decay (Iml≠ 0) Observable in time evolution of B0B0system (assume DG=0) direct CP violation C ≠ 0 indirect CP violation → S ≠ 0 J. Olsen

  4. CP Violation in B0 → p+p- With Penguins (P): Tree (T) Level: mixing decay Need branching fractions for p+p-, pp0, and p0p0 to get a from aeff→ isospin analysis J. Olsen

  5. Overview of Analyses • Analysis issues: charmless B decays • Rare decays! BR ~ 10-5-10-6→ need lots of data (PEP-II) • Backgrounds: • Large background from e+e- → qq → need background suppression • Modes with p0 suffer backgrounds from other B decays • Ambiguity between p and K → need excellent particle ID (DIRC) • Time-dependent CP analysis issues: • Need to determine vertex position of both B mesons → silicon • Need to know the flavor of “other” B → particle ID • We use maximum likelihood (ML) fits to extract signal yields and CP-violating asymmetries • Kinematic and topological information to separate signal from light-quark background • Particle ID to separate pions and kaons • The data sample corresponds to 87.9 million BB pairs J. Olsen

  6. K/p Separation with the DIRC • Cherenkov angle c used in the maximum likelihood fit to distinguish pions and kaons • Resolution and K- separation measured in data Kaon sample p hypothesis K hypothesis J. Olsen

  7. Analysis of B → pp, Kp, KK • Analysis proceeds in two steps: • Time-independent fit for yields and Kp charge asymmetry • Time-dependent fit for Spp, and Cpp • Kinematically select B candidates with mES, DE • Suppress qq background with Fisher discriminant • Fit yields and charge asymmetry p* q* p- p+ J. Olsen

  8. Branching Fraction Results 87.91.1 million BB Submitted to Phys Rev (hep-ex/0207055) Preliminary Projections in mES and DE pp pp Kp Kp Kp pp J. Olsen

  9. Vertex Reconstruction Dz resolution dominated by tag side → same resolution function as charmonium (sin2b) sample Exclusive Brec reconsctuction • Resolution function parameters obtained from data for both signal and background • Signal from sample of fully reconstructed B decays to flavor eigenstates: D*(p, r, a1) • Background from data sidebands BREC Vertex BREC daughters Average Dz resolution ~ 180mm Interaction Point Example in B → pp Beam spot BTAG Vertex e+e- → qq z BTAG direction TAG tracks, V0s B → pp J. Olsen

  10. b c s K- B Flavor Tagging • New tagging algorithm with physics-based neural networks • Inputs include leptons, kaons, slow-p (from D*), and high-momentum tracks • Outputs combined and categorized by mistag prob (w) • 5 mutually exclusive categories: • Lepton – isolated high-momentum leptons • Kaon I – high quality kaons or correlated K- and slow-p+ • Kaon II – lower quality kaons, or slow-p • Inclusive – unidentified leptons, poor-quality kaons, high-momentum tracks • Untagged – no flavor information is used ~7% improvement in Q = e(1-2w)2 J. Olsen

  11. Tagging in Charmless B Decays 81/fb B→ h+h- sample split by tagging category • Tagging efficiency is very different for signal and bkg • Strong bkg suppression in categories with the lowest mistag prob (Lepton/Kaon) • Different bkg tagging efficiencies for pp, Kp, KK Tagging Efficiencies (%) Background J. Olsen

  12. Validation of Tagging, Vertexing, and ML Fit Fit projection in sample of Kp-selected events • Kp decays are self-tagging • T = tag charge • Q = kaon charge • Float t and Dmd in same sample used to extract CP asymmetries: J. Olsen

  13. CP Asymmetry Results Fit projection in sample of pp-selected events Preliminary qq + Kp Submitted to Phys Rev (hep-ex/0207055) J. Olsen

  14. Cross-checks App vs. mES in sample of pp-selected events • Inspect pp-selected sample • 2-param fit consistent with full fit • asymmetry vs. mES • Asymmetry in yields consistent with measured value of Cpp, but does not suggest large direct CP violation • Toy MC generated over all allowed values of Spp and Cpp • Expected errors consistent with data • No significant bias observed • Validated in large samples of signal and background MC events • Systematic errors dominated by uncertainty in PDF shapes signal bins J. Olsen

  15. But, need branching fractions for all three decay modes, and for B0 and B0 separately Taming the Penguins: Isospin Analysis Gronau and London, Phys. Rev. Lett. 65, 3381 (1991) • The decays B p+p-, p+p0, p0p0 are related by isospin • Central observation is that pp states can have I = 2 or 0 • (gluonic) penguins only contribute to I = 0 (DI = 1/2) • p+p0 is pure I = 2 (DI = 1/2) so has only tree amplitude  (|A+0|= |A-0|) • Triangle relations allow determination of penguin-induced shift in a J. Olsen

  16. The Base of the Isospin Triangle: B+→p+p0 • Analysis issues: • Usual charmless two-body; large qq background, p/K separation • Potential feeddown from r+p- • Minimize with tight cut on DE Fit region r+p- Simultaneous fit to pp0/Kp0 e+e- → qq Preliminary hep-ex/0207065 J. Olsen

  17. Next Side Please: B0→p0p0 • Analysis issues: • Small signal! • rp0 feeddown • Background suppression: • Event shape and flavor tagging to reduce qq • Cut on M(p+p0) and DE to reduce rp0 background, then fix in the fit Data after cut on probability ratio (e ~ 20%) hep-ex/0207063 Preliminary p0p0 rp0 Significance including systematic errors = 2.5s J. Olsen

  18. Setting a Bound on Penguin Pollution • Can still get information on a with only an upper bound on p0p0: • For example: Grossman-Quinn bound (assume only isospin) • Many other bounds on the market • Charles, Gronau/London/Sinha/Sinha, etc… Correlations and systematic errors included J. Olsen

  19. CP-Violating Asymmetries in B0 → r+p-,r+K- R. Aleksan et al., Nucl. Phys. B361, 141 (1991) • Opportunity and challenges • In principle, can measure a directly, even with penguins • Much more difficult than p+p- • Three-body topology with neutral pion (combinatorics, lower efficiency) • Significant fraction of misreconstructed signal events and backgrounds from other B decays • Need much larger sample than currently available to extract a cleanly • We perform a “quasi-two-body” analysis: • Select the r-dominated region of the p+p-p0/K+p-p0Dalitz plane • Use multivariate techniques to suppress qq backgrounds • Simultaneous fit for r+p- and r+K- J. Olsen

  20. Not a CP eigenstate, (at least) four amplitudes contribute: Time-integrated asymmetry:  CP CP Time evolution includes:  Q is the r charge rK is self-tagging: direct CP violation → ACP and C ≠ 0 indirect CP violation → S ≠ 0 Fit for: DC and DS are insensitive to CP violation J. Olsen

  21. Analysis • Multi-dimensional ML fit • mES, DE, Neural Net (NN), qc, Dt • Components • Signal rp and rK • Misreconstructed signal events • Mostly due to wrong photon(s) • B backgrounds • from b → c and charmless B decays • Same lifetime as signal • e+e- → qq • Fix B background yields, fit for signal yields and CP asymmetries signal misreconstructed signal Validation: J. Olsen

  22. Yields and Charge Asymmetries hep-ex/0207068 Preliminary J. Olsen

  23. B0 rp time-dependent asymmetry hep-ex/0207068 Preliminary Systematic error dominated by uncertainty on B backgrounds J. Olsen

  24. Summary • Hyperactive effort within BaBar to constrain, measure, and otherwise determine a • Charmless two-body decays: • No evidence for large direct or indirect CP violation in pp • Beginning to piece together the necessary inputs to the isospin analysis • Measurements of decay rates for pp0 and p0p0 (upper limit) • Too early for a significant constraint • Charmless three-body decays • First measurement of CP asymmetries in rp and rK The next few years will be interesting indeed! J. Olsen

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