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(Semi)-leptonic charm decays at BaBar

(Semi)-leptonic charm decays at BaBar. Paul D Jackson The Ohio State University Charm 2006, Beijing, China. June 5 th , 2006. Outline. D s + → μ + ν → extract Branching Fraction and f Ds D 0 →K - e + ν → measure form factor and q 2 D→Xl + l - → search for FCNC decays

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(Semi)-leptonic charm decays at BaBar

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  1. (Semi)-leptonic charm decays at BaBar Paul D Jackson The Ohio State University Charm 2006, Beijing, China. June 5th, 2006

  2. Outline • Ds+→μ+ν → extract Branching Fraction and fDs • D0→K-e+ν → measure form factor and q2 • D→Xl+l- → search for FCNC decays • Describe analysis strategies • Conclude 2 Paul D. Jackson (jack@slac.stanford.edu) Charm 2006

  3. l q2 nl D K FDπ(q2) q2=mD2 l D nl fD QCD Parameterization by Form Factors • Strong interaction is asymptotically free: • Large energies (small distances): αs(q2) small → perturbation in powers of αs • Large distances (size of hadron): αs≈1 → no perturbative treatment • Form factors F(q2,ε) parameterize soft QCD effects (hadronization) • Decay of pseudoscalar mesons Mq constant, ε=0: F(q2,ε) → fM 3 Paul D. Jackson Charm 2006

  4. Phasespace (disfavours l=τ) Helicity Suppression (disfavours l=e, favours l=τ) CKM Mixing nm m+ (disfavours M = B, Bs, D) DS+ (J=0) Measurement of fM (M+=π+,K+,D(s)+,B(s)+) Extraction of fM through leptonic decays: M+ → l+νl • Cleanest source • 1% measurements for π+,K+ • Heavy D(s)+,B(s)+: low branching ratio Partial Width of M+→l+ν 4 Paul D. Jackson Charm 2006

  5. Ds+→μ+ν Using charm tagging technique to extract fDs 5 Paul D. Jackson Charm 2006

  6. Analysis Overview Ds+→μ+ν • Goal: Identify Ds → μνμ decays in cc events • Identify cc events: ‘Charm-Tagging’ • Reconstruct charm mesons D0, D+, Ds+,and D*+ in hadronic mode – the ‘tag’ • High tag momentum close to kinematiclimit from B decays • Search for Ds*+→ γDs+ → γμ+νin recoil • Advantages: • Reduction of uds, BB,  background • Better ν resolution • Flavour correlation of tag and recoil • Disadvantages • Loss in efficiency due to tagging M(Ds*)-M(Ds) 6 Paul D. Jackson Charm 2006

  7. D0→ K-π+, K-π+π0, K-π+π+π- • D+→ K-π+π+(π0), KS0π+(π0), Ks0π+π+π-,K+K-π+, KS0K+ • DS+→ KS0K+, φρ+ • D*+→ D0π+, D0→KS0π+π-(π0), KS0K+K-, KS0π0 Modes allow identification of the charm quark flavour Tagging Strategy Ds+→μ+ν • Fully reconstructed D in 13 hadronic decay modes • Tag momentum above 2.35 GeV/c • Fit tag mass peak: estimate μ, σ • Define tag signal region μ±2σ, and sidebands between 3 and 6σ 7 Paul D. Jackson Charm 2006

  8. Photon energy pcorr = |pmiss|-|pν| Ds*+ momentum Signal Selection Ds+→μ+ν • Tagging removes bb, uds, and ττ background, left with signal and large cc background • Identify kinematic quantities which distinguish signal • Used also Emiss, angle (μ,Ds+), θν 8 Paul D. Jackson Charm 2006

  9. Control Samples Ds+→μ+ν • Validate signal simulation: • D0*→γD0→γK-π+ • Remove π+ and treat K- as μ- • In particular: pmiss reconstruction, neutrino fit (pcorr) • Validate electron efficiency correction: • D*+→π+D0→ K-l+ν,l+ = e+ and μ+ • Apply electron weighting (data PID tables) and phase space correction (3%) μ+ minus e+ subtracted ΔM distribution 9 Paul D. Jackson Charm 2006

  10. Signal Yield Ds+→μ+ν • Signal, leptonic background D→μν, and fake muon combinatoric background (shapes from simulation) remain • Binned 2-fit • Vary: signal and background yield • Fix: relative background sizes Nμν=489 ± 55 • 2/d.o.f. = 31/22 10 Paul D. Jackson Charm 2006

  11. Comparison to Previous Measurements Ds+→μ+ν • All (except BES) normalized to BR(Ds+→φπ+) = (3.6±0.9)% (PDG) • Average: fDs=267±33 MeV (6.1%stat+sys) • For comparison, our measurement (similarly normalized): fDs=248±35 MeV (6.5%stat+sys) • Dominated by BR(Ds+→φπ+) (12.5% on fDs) New average: fDs = (261 ± 31) MeV (6.1% → 4.4%stat+sys) (based on BR(Ds+→φπ+) = (3.6±0.9)%) 11 Paul D. Jackson Charm 2006

  12. D0→K-l+ν Measurement of form factor from semileptonic D decay 12 Paul D. Jackson Charm 2006

  13. Physics outline D0→K-e+ν • Several measurements need precise Lattice calculations of hadronic effects in weak interactions (fB,….) • Large effort from Lattice community (FNAL, MILC,…) to improve computation methods (unquenched, staggered fermions…) → impressive accuracy • Semileptonic decays of charm hadrons provide a way to validate those results through the measurement of form factors hadronic effects parameterized by form factors 13 Paul D. Jackson Charm 2006

  14. Principle of the Analysis D0→K-e+ν From continuum events: e+e- cc( =1.3 nb) Using the D*+ D0 + decay channel Determine two constrained fits q2 = (pD – pK )2 = (pl + pn )2 Reduce the background Fisher discriminant (bb and cc events) Extract the form factor Unfolding: SVD method 14 Paul D. Jackson Charm 2006

  15. Form factor in D0→K-e+ν • Analysis is based on the reconstruction of D*+ mesons produced in cc events and in which the D0 meson decays semileptonically. • In this decay one can define a q2 as following: 15 Paul D. Jackson Charm 2006

  16. Form factor in D0→K-e+ν • A preliminary q2 distribution of the form factor, corrected for effects from reconstruction efficiency and finite resolution. mpole = (1.854 ± 0.016 ± 0.020) GeV/c2 αpole=0.43 ± 0.03 ± 0.04 αpole(lattice) = 0.50 ± 0.04 16 Paul D. Jackson Charm 2006

  17. D→Xl+l- Searches for Flavour Changing Neutral Currents with di-lepton in final state 17 Paul D. Jackson Charm 2006

  18. Why FCNC charm decays? FCNC decays only occur in loop diagrams in SM: Charm decays heavily GIM suppressed in SM: BF(cull)~10-8 New physics can introduce new particles into loop Some models increase BF(cull) to 10-6—10-5 In range of possible measurement! 18 Paul D. Jackson Charm 2006

  19. Theory predictions Several papers have studied SM and NP (mainly SUSY) contributions to the D+p+l+l– decay mode D+p+e+e– D+p+µ+µ– PRD 66, 014009 (2002) f SM with LD effects RPV SUSY r/w f SM+SUSY r/w (1/G)dG/dm2 (1/G)dG/dm2 (1/G)dG/dm2 SM, short distance only SM with LD effects m(µ+µ–) (GeV/c2) m(e+e–) (GeV/c2) Significant LD contributions from intermediate resonances This analysis excludes the region around f resonance 19 Paul D. Jackson Charm 2006

  20. Decay modes investigated PDG UL FCNC decays: Both quarks change flavor: BF (x10-6) PDG UL BF (x10-6) DØ: <4.7 * * * * * * * * * *Lepton-flavor violating decays * PRL93, 191801 (2004) BaBar has previously searched forD0e+e–, e+µ–andµ+µ– with BF limits of 1.2x10-6, 0.8x10-6 and 1.3x10-6 respectively 20 Paul D. Jackson Charm 2006

  21. Background Suppression Total E vs pT of event Reconstruct h+l+l-candidates Stringent lepton PID Require large pCM(h+l+l–) Semileptonic bb decays suppressed by event shape and vertexing requirements Radiative Bhabhas and other QED events suppressed by event shape variables Remove f's from p+l+l– decays p+e+e– Signal MC bb MC Number of Tracks p+e+e– Signal efficiency 0.3%-4.5%, depending on decay mode 21 Paul D. Jackson Charm 2006

  22. Normalization Calculate BR by normalizing signal yields to hadronic charm decays: Decays selection same as for signal modes except PID - Cancels most non-PID systematic errors BF (x10-3) (K+K- ) Lc+p+l+l- selection D+p+l+l- selection pK-p+ p+K+K- Ds+ D+ 22 Paul D. Jackson Charm 2006

  23. Signal Fits 23 Paul D. Jackson Charm 2006

  24. Unblinded Mass Distributions D+p+e+e– D+p+µ+µ– No signal for FCNC charm decays found Ds+K+e+e– Ds+K+µ+µ– Limits on yields extracted with unbinned likelihood fits Largest “signal” is ~1.5s in Lc+p+µ+µ–decays Lc+p+e+e– Lc+p+µ+µ– 24 Paul D. Jackson Charm 2006

  25. BF Upper Limits Yields converted to limits on branching fractions: BaBarBF (x10-6) Limits (90%CL) Existing BF(x10-6) Limits (90%CL) <7.4 CLEO-c <11 <8.8 FOCUS <24 <34 E791<11 <34 E791<6 <270 E791<8 <26 FOCUS<19 <610 E791<22 <610 E791<14 <6.2 CLEO-c<5 <9.2 FOCUS <14 <68 E791<4 <68 E791<4 <1600 E791<7 <36 FOCUS<25 <630 E791<6 <630 E791<4 - <4 <340 E653<40 - <9 - <8 25 Paul D. Jackson Charm 2006

  26. Branching Fraction Limits (preliminary) Upper limits on BF (x10-6) at 90% CL Improved limits in 17 modes, more than order magnitude in 12 modes 26 Paul D. Jackson Charm 2006

  27. Summary • Rich and varied program in (semi)leptonic charm decays • Measured Ds+→μ+ν and normalise to Ds+→φπ • Extract fDs to ~6% • Measured D0→K-e+ν and form factors • Searched for 20 FCNC D→Xl+l- modes. No measurements but improved limits in 17 cases. 27 Paul D. Jackson Charm 2006

  28. Backup Slides 28

  29. Unfolding Procedure Extraction of the q2 dependence of the form factor: Unfolding the measured q2 distribution MC Single Value Decomposition* approach:  SVD of the reconstruction matrix S[q2rec,q2sim]  algorithm imposing a minimum curvature condition  results tested on a toy generator * SVD; A. Höcker, V. Kartvelishvili [hep-ph/9509307] 29 Paul D. Jackson Charm 2006

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