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Cracking the CKM Triangle or What B A B AR Needs a Billion B ’s For

Cracking the CKM Triangle or What B A B AR Needs a Billion B ’s For. Masahiro Morii Harvard University B A B AR Collaboration Cornell, March 2003. Outline. Very brief introduction Measurement of angle a B 0  p + p - results from B A B AR and Belle Measurement of V ub

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Cracking the CKM Triangle or What B A B AR Needs a Billion B ’s For

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  1. Cracking the CKM TriangleorWhat BABAR Needsa Billion B’s For Masahiro Morii Harvard University BABAR Collaboration Cornell, March 2003

  2. Outline • Very brief introduction • Measurement of angle a • B0 p+p- results from BABAR and Belle • Measurement of Vub • Upcoming BABAR result using recoil of fully-reco’ed B • Future prospects • Where and how BABAR and Belle will get a billion B’s Masahiro Morii

  3. Wolfensteinparameters CKM Matrix • CKM matrix appears in the weak Lagrangian as • Unitary matrix translates mass and weak basis • 3 real parameters + 1 complex phase • CPV in the Standard Model is uniquely predictive • Attractive place to look for New Physics The only source of CPV in the Minimal SM Masahiro Morii

  4. Unitarity Triangle • V is unitary  Consider • Dividing by gives the familiar triangle • CP violation in B0 decays gives access to the angles of the Unitarity Triangle Masahiro Morii

  5. Unitarity Triangle and sin2b • sin2b has been measured to ±0.055 • You’ve just heard aboutit from Gabriella • Measured sin2b agreeswith indirect constraints • Angle b known betterthan any angle/side ofthe triangle • Shrinking s(sin2b)alone will not revealnew physics Masahiro Morii

  6. What To Do Next? • Must measure the other angles and the sides • B Factories good at 3 of them CPV in charmless hadronic B decays, e.g. B0 p+ p- Vub from charmless semileptonic B decays Bs mixing at Tevatron Very hard: many long-shot ideas Penguin decays, e.g. B0 f KS Masahiro Morii

  7. Measuring a • Time-dependent CP asymmetry in B0fCP is CKM phase appears here Easy! Masahiro Morii

  8. Penguin Pollution • Unlike J/yKS, the 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 0.3 from BR(B Kp)/BR(B  pp) • d = strong phase difference between P and T T = Tree P = Penguin Masahiro Morii

  9. Taming Penguins • Take advantage of the isospin symmetry Masahiro Morii

  10. assumed Bound on aeff – a • Full isospin analysis (Gronau & London, 1990)requires and separately • Too hard for BABAR/Belle  Upper limits on average BR • UseBR(p0p0) to put upper bound on aeff – a • Grossman and Quinn, 1998; Charles, 1998 Gronau, London, Sinha, SinhaPLB 514:315-320, 2001 BABARp0p0 limit Allowed Masahiro Morii

  11. B0 p0p0 Branching Ratio • CLEO: (PRD65:031103) 9.13 fb-1 • BR(p0p0) < 5.7×10–6(90% CL) • BABAR: (submitted to PRL) 87.9×106BB • BR(p0p0) < 3.6×10–6 (90% CL) • Belle: (PRD66:092002) 31.7×106BB • 2.4s “bump” in the signal • FittedBR = (3.2 ± 1.5 ± 0.7)×10–6 • BR(p0p0) < 6.4×10–6 (90% CL) • First observation may come soon • Will it be small enough to give us a good aeff – a bound? BELLE Masahiro Morii

  12. CP Asymmetry in B0 p+p- • Same method as the 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 Masahiro Morii

  13. Experimental Challenges • Specific to B0p+p- • Topology B0h+h-simple to reconstruct • But no strong kinematical constraints like the J/y mass • Significant background from continuum • Event-shape variables  Fisher discriminant • Particle ID must separate p± from K± • DIRC (BABAR) or Aerogel (Belle) • Common with other CP measurements • Flavor tagging, vertex reconstruction, etc. • And, of course, as much as possible Masahiro Morii

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

  15. 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 Masahiro Morii

  16. 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 Masahiro Morii

  17. 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 Masahiro Morii

  18. Event Sample – BABAR • BABAR: 87.9×106BB(PRL89:281802) • p+p- enhanced for these plots with a cut on Fisher Kp +continuum Masahiro Morii

  19. Event Sample – Belle • Belle: 85×106BB(hep-ex/0301032) Continuum Kp Masahiro Morii

  20. 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 PID in event selection, Fisher to bin the data • Add PDFs for background (Kp, KK, continuum) • Feed the candidates and turn the crank… Masahiro Morii

  21. CP Asymmetries – BABAR • No significant CP asymmetry  p+p- enhanced for these plots with a cut on Fisher Masahiro Morii

  22. CP Asymmetries – Belle • Large CP asymmetry • Rate difference (= Cpp) • Dt-dependent asymmetry  Subtract bkg Masahiro Morii

  23. CP Fit Results • Results made bigger splash than expected • BABAR and Belle disagree by 2.56 s • Belle result outside physical boundary BABAR Belle Masahiro Morii

  24. Brief History BABAR 33×106BB BABAR 60×106BB BABAR 88×106BB Spp stayed apart as the errors shrank Cpp looks consistent Belle 85×106BB Belle 45×106BB Masahiro Morii

  25. Crosschecks • Both experiments got into extensive crosschecks • What is special about pp compared to, e.g., J/yKS? • High background, dominated by Kp and continuum Asymmetry in the background? • Two-body decay topology Different vertexing systematics? • How well did the likelihood fit work? • Errors and the likelihood value reasonable? • How often should you get an unphysical result? • Just how significant are the results? Masahiro Morii

  26. Two-Body Vertexing • Measure lifetime and mixing in the data Belle BABAR Mixing inB0 Kp Looking good Masahiro Morii

  27. Is It Physical? • Belle’s 1s ellipse lies outside the physical boundary • How likely is this? • Suppose truth is onthe boundary • 60.1% of experimentsproduce unphysicalresults • 16.6% are moreunphysical than theactual Belle result That’s not so bad Masahiro Morii

  28. Averaging the Rivals • Both measurements seem OK • Agreement is ~1% level • Let’s try averaging • Errors largely uncorrelated • What can we learn from this? • Can we measure a? Masahiro Morii

  29. Combined result is 3.24s away from CP conservation • Cpp < 0 at 2.68s • Spp consistent with zero at 1.81s CP is violated in B0 p+p- at 99.9% CL Cpp Indication of direct CPV at 99.3% CL BABAR + Belle Significance of CPV • Belle rejects non-CPV (Spp = Cpp = 0) at 99.93% CL Belle Masahiro Morii

  30. Bound on a • We can’t do isospin analysis without BR(p0p0) • What else can we do? • Start with the following knowledge: • Spp = -0.49 ± 0.27, Cpp = -0.51 ± 0.19 • sin2b = 0.734 ± 0.055 • (P/T)pp = 0.28 ± 0.10 • d (strong phase difference) = no clue • Try to put them together • Convention by Gronau & Rosner (PRD65:093012) BABAR + Belle World average BR + theory + salt Masahiro Morii

  31. How They Fit Together • Decay amplitudes can be written as dT = strong phase of T dP = strong phase of P Masahiro Morii

  32. How They Look • Example: b = 23.6° and |P/T| = 0.28 • Large negativeCpp Large negative d • a seems to be near 105° • Does this allow us to determine a? a 75° 60° 90° 105° 120° + The fact that |Cpp| is large for the assumed value of |P/T| helps us to constrain a and d d = 0° - Masahiro Morii

  33. Fitting d vs. a • Fit for a, b and d • Inputs: measurements of Spp, Cpp and sin2b • |P/T| = 0.18–0.38 • It works! • At 2s, an islandgives a poor limit -ln(L). Contours at each s Masahiro Morii

  34. Unitarity Triangle • Let’s put it on the Unitarity Triangle 84° < a < 124° • Agrees with the other constraints! • Is this significant? • A few caveats … Masahiro Morii

  35. Caveat 1 – Degeneracy • I assumed in the fit that b was in the “right” branch • Belle did this in hep-ex/0301032 • But we measured only sin2b If we allow all branches A new local minimum appear with just as good likelihood,but not consistent with the SM Masahiro Morii

  36. Caveat 2 – Accuracy • We could fit a because |Cpp| was measured large • That’s why the Belle result, with poorer errors, gives more interesting fit than BABAR’s • Q: What will happen when we have more data? Belle BABAR Masahiro Morii

  37. 4 × Data  1/2 × Errors • Suppose we shrank the error on Spp and Cpp by 1/2 • Does the error on a shrinkby half? • Short answer: No • 1s bound go from84° – 124° to 91° – 120° • i.e. only 1/1.38 • Long answer: Depends • Where will the centralvalue go? Masahiro Morii

  38. 4 × Data  1/2 × Errors (contd.) • Try moving Cpp by the current 1s error • Very different errors on a depending on the central value • Is Nature kind enough to give us maximum direct CPV? Masahiro Morii

  39. Current Status of a • Combined measurements of Spp and Cppcan be interpreted as a 1s bound: 84° < a < 124° • This relies on |Cpp| being large • Future improvement of the bound unclear • Still important to pursue the isospin approach Masahiro Morii

  40. Outline • Very brief introduction • Measurement of angle a • B0 p+p- results from BABAR and Belle • Measurement of Vub • Upcoming BABAR result using recoil of fully-reco’ed B • Future prospects • Where and how BABAR and Belle will get a billion B’s Masahiro Morii

  41. Why Vub Is Interesting • Measurement of sin2b is more accurate than the indirect constraint • Width of the indirectellipse determined by|Vub/Vcb| Better measurement of |Vub| More stringent test of the Unitarity Triangle Masahiro Morii

  42. Measuring Vub • Measure the rate of charmless semileptonic decays • Catch: charm background • There are many techniques • Exclusive: • Inclusive: El endpoint, Mx cut, etc. That’s not a good sign… Masahiro Morii

  43. BABARVub Measurements • BABAR has released two measurements • Electron endpoint (hep-ex/0208081) ICHEP 2002 • (hep-ex/0301001) submitted to PRL • I’m NOT talking about these measurements • Instead, I will talk about an upcoming measurement • Can’t show the details – Wait for Moriond Masahiro Morii

  44. Why Vub Is Hard Error in extrapolation to full acceptance Poor S/B ratio PDG 2002, p. 706 Inclusive Exclusive S/B better Model-dependent prediction of BR Masahiro Morii

  45. What Can We Do? • Let’s consider an inclusive measurement • Smaller theoretical errors • Goal: better S/B ratio + larger kinematical acceptance • Minimize charm background • Reduce extrapolation • Best-chance variable: mX = mass of X in B Xln • Used at LEP with moderate success • Poor S/B limited their systematic errors • We need another trick to improve S/B Masahiro Morii

  46. Exclusively Reco’ed B Events • We have a large sample of U(4S)  BB events with one B fully reconstructed • ~1000 decay channels • Efficiency ~0.2%/B • Look at the other B inthese events (“recoil” B) • Pure B0, B± samples withknown momentum • Look for leptons with pl > 1 GeV • For B±, take only right-sign leptons Y± is any combinationof p±, K±, KS and p0 Our “golden”B Xln sample Masahiro Morii

  47. Measuring mX • The X in B Xln is the leftover of the event • Charged tracks and unmatched calorimeter clusters • Impose 4-momentum conservation, same B masses,zero missing mass  2-C fit Measured vs. generated mX in MC. Resolution ~350 MeV. Almost no bias Masahiro Morii

  48. Cleaning Up • A few more cuts to improve S/B • One and only one lepton • Charge conservation • Small missing mass • Special cut using soft pions to reduce B D*ln • Kaons come mostly from charm decays • Enrich (deplete) b u decays by vetoing (requiring) them • Enriched sample for the measurement • Depleted sample for MC vs. data agreement studies Masahiro Morii

  49. Signal • Fit mES to get the number of BXln events • Very clean signal  S/B > 1 • Comparable to exclusive measurements b uln enriched(~60%) All events mX < 1.55 GeV Normalizationfrom this fit Masahiro Morii

  50. What’s Coming • To get the branching fraction: • Cut on mES and fit mX distribution to get the yield • Estimate efficiencies and background • Done, but can’t show yet • Conference paper in circulation inside BABAR • What to expect: • Statistical error ~ best existing measurement (8% on Vub) • Low background  Very small systematics • Extrapolation error ~ LEP measurements (10% on Vub) Masahiro Morii

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