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Measurements of sin2 a from B-Factories

Measurements of sin2 a from B-Factories. Masahiro Morii Harvard University The BABAR Collaboration BEACH 2002, Vancouver, June 25-29, 2002. Introduction. CP violation in B 0 decays gives access to the angles of the Unitarity Triangle

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Measurements of sin2 a from B-Factories

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  1. Measurements of sin2afrom B-Factories Masahiro Morii Harvard University The BABAR Collaboration BEACH 2002, Vancouver, June 25-29, 2002

  2. Introduction • CP violation in B0 decays gives access to the angles of the Unitarity Triangle • sin2b measured to ±0.08 dominated by B0 J/y KS • Where does this leave us? See D. Marlow’s talk M. Morii, Harvard University

  3. Unitarity Triangle and sin2b • Measured sin2b agrees with indirect constraints • Shrinking s(sin2b) alonemay not reveal new physics • Must measure the sidesand the other angles Next possibility at the B Factories? M. Morii, Harvard University

  4. Measuring sin2a • Time-dependent CP asymmetry in B0fCP is CKM phase appears here Easy! M. Morii, Harvard University

  5. Penguin Pollution • Unlike J/yKS, p +p - mode suffers from significant pollution from the penguin diagrams with a different weak phase • To estimate aeff – a, we need: • P/T ratio – about 1/3 from BR(B Kp)/BR(B  pp) • d = strong phase difference between P and T T = Tree P = Penguin M. Morii, Harvard University

  6. Taming Penguins • Take advantage of the isospin symmetry All preliminary M. Morii, Harvard University

  7. B0 p0p0 Branching Ratio • BABAR:Preliminary54 fb-1 • BR(p 0p 0) < 3.3×10–6 (90% CL) • Belle:Preliminary31.7 M BB • 2.2s “bump” in the signal • FittedBR= (2.9 ± 1.5 ± 0.6)×10–6 • BR(p 0p 0) < 5.6×10–6 (90% CL) • CLEO:9.13 fb-1 • BR(p 0p 0) < 5.7×10–6(90% CL) BELLE Expect first observation in the near future M. Morii, Harvard University

  8. CP Asymmetry in B0 p+p- • Same method as sin2b measurements • Difference: the direct CP term cannot be neglected Tagusing l±, K± Btag 9 GeV 4S 3.1 GeV Moving with bg = 0.55 CP finalstate BCP # of events with M. Morii, Harvard University

  9. Challenges • Specific to B0p+p- • Topology B0h+h-simple to reconstruct • Particle ID must separate p± from K± • DIRC (BABAR) and Aerogel (Belle) • Significant background from continuum • Event-shape variables  Fisher discriminant • Common with other CP measurements • Flavor tagging • Vertex reconstruction • And, of course, as much as possible M. Morii, Harvard University

  10. B0 Reconstruction • mbc (or mES) and DE peak cleanly for the two-body signal • Kp and KK peaks shifted in DE Additional discrimination BELLE BELLE p+p- MC p+p- MC K+p-MC off-resonance data M. Morii, Harvard University

  11. Continuum Background • Most of the background come from continuum • Use event shape variables that represent “jettiness” to suppress them Signal udsc background The other B decays spherically Whole event is jetty Examples M. Morii, Harvard University

  12. Sphericity Angle • Angle qS between the sphericity axes of the B candidate and the rest of the event • Cut at 0.8 removes 83% ofthe continuum background BABAR reject background p+p- MC M. Morii, Harvard University

  13. BABAR uses the “CLEO” Fisher Momentum flow in 9 cones around the candidate axis Output of Fisher goes into the likelihood fit Fisher Discriminant D0p+data p+p- MC mES sideband data Bkg MC M. Morii, Harvard University

  14. Fisher Discriminant • Belle’s Fisher discriminant uses: • Modified Fox-Wolfram moments • B flight direction • Output is turned intoa likelihood ratio R • Cut at 0.825 removes95% of continuumbackground p+p- MC reject off-res. data D0p+data Bkg MC M. Morii, Harvard University

  15. Event Sample – BABAR • BABAR 55.6 fb-1 preliminary • p+p- enhanced for these plots with a cut on Fisher Kp +continuum M. Morii, Harvard University

  16. Event Sample – Belle • Belle 41.8 fb-1 Kp Continuum M. Morii, Harvard University

  17. Maximum Likelihood Fit • Start from the physics function: • Fold in Dt resolution and mis-tag probabilities • Multiply by PDFs for mES, DE • BABAR uses particle ID and Fisher in the fit • Belle uses these variables in event selection • Add PDFs for background (Kp, KK, continuum) • Feed the candidates and turn the crank… M. Morii, Harvard University

  18. BABAR BELLE CP Fit Results • BABAR and Belle disagree by >2s • Belle 1.2s outside the physical boundary • Is there any problem? • Crosscheck systematics Belle uses M. Morii, Harvard University

  19. CP Asymmetries – BABAR • p+p- enhanced for these plots with a cut on Fisher • No significant asymmetry M. Morii, Harvard University

  20. CP Asymmetries – Belle • Rate difference (= Cpp) • Dt-dependent asymmetry (= Spp and Cpp) Subtract bkg M. Morii, Harvard University

  21. Crosschecks • Both experiment made extensive crosschecks, e.g. • Asymmetry in background? • Look for asymmetriesin Kp or mass sideband • Vertex resolution of the2-body decays? • Measure B lifetime with pp, Kp • Measure mixing with Kp • Likelihood values and errors? • Toy Monte Carlo studies BELLE M. Morii, Harvard University

  22. Monte Carlo Fit Test • Generate ~1000 “toy” experiments • Belle used (-0.7, -0.7)for the central values • Fit and compare: • Likelihood values • Pull distributions • Errors • Lowest probability: 5.4% BABAR BABAR MC Measured s(Cpp) s(Spp) BELLE BELLE Everything looks reasonable Measured M. Morii, Harvard University

  23. BABAR BELLE Interpretation • How well do we know a?(*Gronau and Rosner, PRD65, 093012) • Average BABAR and Belle • Assume b = 26°, P/T = 0.28 NB: Large uncertainty M. Morii, Harvard University

  24. Interpretation • Measured Spp±1scorresponds to Indirect: Accuracy comparableto the indirect constraints BABAR + Belle Gronau and RosnerPRD65, 093012 We are starting to measurea M. Morii, Harvard University

  25. Summary • BABAR and Belle measured sin2aeff using B0p+p- • Direct constraint on ais reaching useful accuracy • Things to watch out for: • sin2aeff with higher statistics  Resolve “discrepancy” • BR(B0p0p0)  Better bound on aeff – a M. Morii, Harvard University

  26. was assumed Bound on aeff – a • Full isospin analysis (Gronau & London, 1990)requires and separately • Too hard for BABAR/Belle  Upper limits on average BR • Use BR(p 0p 0) to put upper bound on aeff – a • Grossman and Quinn, 1998; Charles, 1998 Gronau, London, Sinha, SinhaPLB 514:315-320, 2001 Allowed M. Morii, Harvard University

  27. GLSS Bound on aeff – a • If I use and • GLSS bound weakerfor smaller BR(p+p0) • Better measurements ofBR(p+p0) and BR(p0p0) will give us a betterhandle on the penguinsin the near future Small p+p0 Large p+p0 BABAR 90% CL M. Morii, Harvard University

  28. B Flight Direction • Angle qB of the B candidate momentum relative to the beam axis • Signal • Background ~flat BELLE M. Morii, Harvard University

  29. Systematic Errors • BABAR: • Dominated by the shape of the particle ID variable • Belle: • Uncertainties of the background fractions • Fit bias near the physical boundary for Spp • Wrong tag fraction for Cpp • All measurements are statistically limited M. Morii, Harvard University

  30. Interpretation • Measurements favormaximally negative Cpp • Corresponds to d = -90° • Maybe a good news • No discrete ambiguity! • Time will tell b = 26°, P/T = 0.28 Gronau and RosnerPRD65, 093012 M. Morii, Harvard University

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