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Exploring CP Violation through the CKM Triangle: Insights from BABAR Collaboration

This presentation by Masahiro Morii from Harvard University delves into the CKM (Cabibbo-Kobayashi-Maskawa) triangle and its significant role in CP (charge parity) violation within the B meson system. He discusses the methodologies used by the BABAR experiment and PEP-II accelerator to make precise measurements of CKM parameters, focusing on angles α, β, and γ. With advancements in experimental techniques, Morii highlights future prospects for probing new physics alongside the promising results already obtained.

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Exploring CP Violation through the CKM Triangle: Insights from BABAR Collaboration

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  1. Cracking the CKM Triangle – BABAR’s Next Step – Masahiro Morii Harvard University BABAR Collaboration October 2003

  2. Outline • Very brief introduction • CKM triangle and CP violation in the B system • BABAR and PEP-II • What we can do today, and in 3 years • Measurements • Angle b from B  yKS/L , fKS , h’KS • Angle a from B  pp • Angle g from B  Dp, DK • |Vub| from B  Xuln decays • Summary 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. Why CP Violation? • Standard Model is unreasonably successful • It predicts everything we measure, while • We all know it’s wrong • BIG failing: Baryogenesis • Matter-dominant universe is created through: • CP violation • Baryon number violation • Non thermal equilibrium • SM prediction falls way short of reality • CKM mechanism was a postdiction of CPV • Never tested (before BABAR) its predictive power All three availablein the Standard Model Masahiro Morii

  5. Unitarity Triangle • V is unitary  Consider • Dividing by gives the familiar triangle • Non-zero angles  CP violation • All sides are O(1)  Can test closure with realistic experimental precision Masahiro Morii

  6. W+ s/b d W- d s/b Anatomy of B0System • B0-B0 system very similar to K0 system • Mixing through box diagrams • Coupling constants appear as • Mass eigenstates BH and BL are linear combinations • Lifetime GH and GL close  Ignore DG • Follow the time evolution… Mass difference Dmcauses mixing Masahiro Morii

  7. B0 Evolution and Decay • Starting from pure B0(B0) state, after time t • Now, consider decays into a CP eigenstate fCP • It’s trivial (just tedious) to calculate the decay rates Neat problem for anundergrad. QM exam Masahiro Morii

  8. mixing CP Asymmetry hf = CP eigenvalue of fCP Scenario 1: There is only one diagram   For B0J/yKS, -Im(lf)=sin2b • Scenario 2: • More than one diagram contribute • both sin and cos terms will survive • Sfdepends on Iml  UT angles • Cf depends on |l|  direct CP Masahiro Morii

  9. Measuring CP Violation • Experiments must do 3 things: • Produce and detect B  fCP events • Typical BR: 10-4 – 10-5 • Need a lot of lot of lot of lot of B’s • Separate B0 from B0 = “Flavor tagging” • Use U4SB0B0 and tag one B • Measure the decay time • Measure the flight length bgct • But B’s are almost at rest in U4S decays Solution: Asymmetric B Factory Masahiro Morii

  10. e+ Three Ingredients e- Ingredient #2: Flavor tagging Btag B0 e- U(4S) m+ B0 Breco Ingredient #1: Exclusive reconstruction p+ p- m- Dz~bgc Dt Ingredient #3:Dt determination Masahiro Morii

  11. Asymmetric B Factory • Collides e+e- at ECM = m(Y4S) but with E(e+) ≠ E(e-) • PEP-II: 9 GeV e- vs. 3.1 GeV e+ bg = 0.56 • The boost allows measurement of Dt • Collides lots of them: Ibeam = 1 – 3A • PEP-II luminosity 6.6 x 1033/cm2/s = 6.6 Hz/nb • That’s >2x the design  • KEKB has hit 1 x 1034/cm2/s  Masahiro Morii

  12. Integrated Luminosity • >100 fb-1/expt. accumulated • Physics results used 90-130fb-1 so far Masahiro Morii

  13. 600 15 10 400 Integrated luminosity [fb-1] Peak luminosity [1033] 5 200 Luminosity Projection – PEP-II • That’ll give BABARa billion B mesons to play with PEP-II plans todeliver 500 fb-1by end 2006 Masahiro Morii

  14. Luminosity Projection – KEKB KEKB also shoot for 500 fb-1 by end 2006 Masahiro Morii

  15. Detector: BABAR (or Belle) Photon energy with a CsI(Tl) crystal calorimeter Charged particle momentum with a drift chamber in a 1.5T field Muons detected after penetrating iron yoke Particle ID with a Cerenkov detector(DIRC in BABAR,aerogel in Belle) Precise vertex with a silicon strip detector Masahiro Morii

  16. Harvard Group At Work New 3D track trigger system for better backgd. rejectionat higher luminosity “ZPD” Masahiro Morii

  17. b B0 Charmonium + K0 • The “golden mode” • Theoretically clean • Only the tree diagram matters • Experimentally clean • “Large” BF (~10-4) • CP sample in 89 M BB pairs ACP(t) = sin2b sinDmDt Masahiro Morii

  18. b CP=+1 CP=-1 sin2b= 0.741 ± 0.067stat± 0.034syst BABAR 81 fb-1 PRL 89, 201802 (2002) CP Fit  sin2b Masahiro Morii

  19. b Unitarity Triangle • World average: sin2b = 0.736 ± 0.049 • Excellent agreement with SM constraint from indirect (non-CPV) data • Does that mean no New Physics? sin2b vs. indirect constraints Observed CP asymmetry consistent withCKMmechanism being the dominant source ofCPV Masahiro Morii

  20. Next Step • We’ve achieveds(sin2b) < 5% • Already more precise than the indirect constraints • Use it as the reference! • B0 yK0 is a tree decay • New Physics may be hidingin more suppressed diagrams • Strategy: Measure “other aspects” of the CKM triangle • Modes with different Feynman diagrams and • Clean theoretical interpretations  Look for inconsistencies= New Physics Masahiro Morii

  21. Cracking the CKM Triangle • Let’s measure everything we can! sin2a from B  pp decays Vub from charmless semileptonic B decays Bs mixing at Tevatron g from B  Dp and B  DK decays sin2b from penguin decays, e.g. B  fK Masahiro Morii

  22. b sin2b Measurements • “Redundant” measurements using other decay modes • Different diagrams  Different sensitivity to New Physics • Loop diagrams are particularly interesting • Theoretically clean modes are more useful • Single-diagram decays preferred • Modes under study Clean Less clean Masahiro Morii

  23. b B0  f/h’ KS • Dominated by b  sss penguin • ACPSM = sin2b = ACP(J/y KS) • New Physics may enter the loop • fKS is pure-penguin • As clean as J/yKS • Small BR: 7.6 x 10-6 • h’KS has tree diagrams too • Cabbibo- and color-suppressed • |A/Ā| ~ 1 within a few % • Larger BR: 5.5 x 10-5 • h’ decays harder to reconstruct Masahiro Morii

  24. b Penguin Signals Preliminary LP’03 BABARfKS Belle BABARh’KS Masahiro Morii

  25. b Penguin CPV Results Preliminary LP’03 Something very strange is happening here… Masahiro Morii

  26. b B0 f KS CP Fits Preliminary LP’03 BABAR Belle Low-purity tags High-purity tags B0 tags B0 tags ACP Masahiro Morii

  27. b B0 h’ KS CP Fits Preliminary LP’03 BABAR Belle B0 tags Low-purity tags B0 tags High-purity tags ACP Masahiro Morii

  28. b Status of Penguins • B0fKS • Belle 3.3s from J/yKS • Prob. < 0.1% • BABAR – Belle = 2.1s • Prob. = 3.6% • Average = –0.14 ± 0.33 • 2.6s from J/yKS • B0 h’KS • Average = 0.27 ± 0.21  2.2s from J/yKS • Do we have a hint of New Physics? • Theorists “told you so” for this very decay mode • Too early to tell  4x more data will settle the case Masahiro Morii

  29. Cracking the CKM Triangle a from B  pp decays |Vub| from charmless semileptonic B decays  g from B  Dp and B  DK decays sin2b from penguin decays, e.g. B  fK Masahiro Morii

  30. a Measuring Angle a • Tree diagram of B0 p+p- should give us sin2a • But there are penguin diagrams T = Tree P = Penguin Masahiro Morii

  31. a Taming Penguins • To estimate aeff – a, we need: • P/T ratio – about 0.3 from G(B  Kp)/G(B pp) • d = strong phase difference between P and T • Gronau & London (1990) suggested using isospin relations Measure BF for all modes and combine  Extract aeff – a from data T P C Masahiro Morii

  32. a assumed Allowed Isospin Analysis • Isospin analysis requires BF(B0 p0p0) separately for B0 and B0 • Too hard for BABAR/Belle  Only average measured • UseBF(B0 p0p0) to put upper bound on aeff – a • Grossman and Quinn, 1998; Charles, 1998 Gronau, London, Sinha, SinhaPLB 514:315-320, 2001 Masahiro Morii

  33. a Observation of B0 p0p0 • BABAR has observed B0 p0p0at 4.2s significance • Weak limit on aeff – a hep-ex/0308012 2(aeff – a) Allowed Masahiro Morii

  34. a CPV in B0 p+p- • B-Physics News of 2002:Belle “discovery” of large CPVin B0 p+p- • PRD 68 (2003) 012001 • CPV ≠ 0 at >99.9% CL • Not seen by BABAR 2.6s discrepancy • Did more data help? BABAR Belle Masahiro Morii

  35. a New world average: Spp = -0.58±0.20 Cpp = -0.38±0.16 Preliminary LP’03 BABAR 123x106 BB Brief History BABAR 33×106BB BABAR 60×106BB BABAR 88×106BB Belle 85×106BB Belle 45×106BB Masahiro Morii

  36. a CP Asymmetries BABAR preliminary Belle PRD 68 (2003) 012001 B0 tags B0 tags B0 tags B0 tags Masahiro Morii

  37. a Status of aeff • BABAR-Belle compatibility improved from 2.6s to 2.0s • Waiting for new result from Belle • Interpretation of aeff a remains murky • BF(B0 p0p0) too large for useful limit on |aeff – a| • Full isospin analysis beyond reach of existing B factories • Several model-dependent analysis proposed Masahiro Morii

  38. a Status of a • Different interpretationswith various assumptions“agree” with the indirectconstraint of a • Theoretical error??? • Experimental efforts areshifting towardB0 rp, rr final states • Interference betweenresonances give extra information for |P/T| and d • Broad r on top of non-resonant ppmakes analyses incredibly complicated indirect Masahiro Morii

  39. Cracking the CKM Triangle  a from B  pp decays |Vub| from charmless semileptonic B decays  g from B  Dp and B  DK decays sin2b from penguin decays, e.g. B  fK Masahiro Morii

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

  41. Vub Measuring |Vub| • Measure the rate of charmless semileptonic decays • Catch: charm background • There are many techniques • Exclusive: • Better S/B • Inclusive: Lepton endpoint spectrum, etc. • Better efficiency That’s not a good sign… Masahiro Morii

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

  43. Vub How To Improve |Vub| • Measure inclusive G(B  Xuln) • Exclusive G needs better FF calculation  Lattice QCD • Goal: better S/B ratio + larger kinematical acceptance • Minimize charm background • Reduce extrapolation • Three kinematical variables in the Xuln final state • We need a large sample of clean, isolated, and unbiasedB decays  “B beam” Want to use all of them for optimal efficiency and S/B Masahiro Morii

  44. Vub Tagged-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) • Almost-pure B0, B± withknown momentum • Purity known from mES fit • Subtract background using sideband Y± is any combinationof p±, K±, KS and p0 Ideal sample for branching fraction measurements Masahiro Morii

  45. Vub BF(B  Xuln) • Start from the recoil-B sample • Find a lepton with p > 1 GeV • Calculate mass of the remaining system “X” • Know pB, missing (n) mass = 0 • 2-C fit improves s(mX) from 500  350 MeV • Normalization from fitting tag-B mass All events with a lepton Mostly B  Xcln mX > 1.55 GeV Xuln enriched to ~60% Masahiro Morii

  46. Vub BF(B  Xuln)  |Vub| • Fit the mX distribution to extract BR • Use OPE calculation hep-ex/0307062 submitted to PRL OPE mb Masahiro Morii

  47. Vub Theoretical Errors • Signal efficiency depends on (El, q2, mX) distribution • Differential rate predicted at parton level to O(as) • Depends on the b quark mass and its Fermi motion Leading systematics for |Vub| • Parameterized in • Same parameters determine (El, q2, mX) for B  Xcln • Above values come from CLEO’s electron spectrum • We should be able to improve! • Measure both El and mX spectra in B  Xcln • mX requires the recoil-B technique Masahiro Morii

  48. Vub Hadron Mass Moment • Start from recoil-B with lepton • Calculate mX as in the |Vub| analysis • Subtract (small) B  Xuln contribution • Calculate moments <mX> <mX2> <mX3> <mX4> • Vary the lepton p cut from 0.9 to 1.6 GeV BABAR preliminary Coming Soon: Combine with Ee spectrum and fit  determine L and l1 Masahiro Morii

  49. Vub Status of |Vub| • BABAR/Belle/CLEO are working hard on |Vub| • BABAR is leading with the recoil-B technique • Improvements continue • El and mX spectra  Better L and l1 • Cut on q2 Reduced theoretical error • Tag B with semileptonic decays  higher efficiency Masahiro Morii

  50. Cracking the CKM Triangle   a from B  pp decays |Vub| from charmless semileptonic B decays  g from B  Dp and B  DK decays sin2b from penguin decays, e.g. B  fK Masahiro Morii

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