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CP Violation Measuring matter/anti-matter asymmetry with BaBar

CP Violation Measuring matter/anti-matter asymmetry with BaBar. Wouter Verkerke University of California, Santa Barbara. Outline of this talk. Introduction to CP violation A quick review of the fundamentals. CP-violating observables Experiment and analysis techniques

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CP Violation Measuring matter/anti-matter asymmetry with BaBar

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  1. CP ViolationMeasuring matter/anti-matter asymmetry with BaBar Wouter Verkerke University of California, Santa Barbara Wouter Verkerke, UCSB

  2. Outline of this talk • Introduction to CP violation • A quick review of the fundamentals. • CP-violating observables • Experiment and analysis techniques • Accelerator and detector (PEP-II and BaBar) • Event selection, measuring time dependent CP asymmetries • Selection of (recent) BaBar CP violation results • The angle b • The angle a • The angle g Wouter Verkerke, UCSB

  3. Why is CP violation interesting? • It is of fundamental importance • Needed for matter/anti-matter asymmetry in the universe • Standard Model CP-violation in quark sector is far too small to explain matter asymmetry in the universe • History tells us that studying symmetry violation can be very fruitful • CP violating processes sensitive to phases from New Physics • Can CP-violation measurements at the B factories break the Standard Model in this decade? • Measure phases of CKM elements in as many ways as possible Wouter Verkerke, UCSB

  4. d s b u c t The Cabibbo-Kobayashi-Maskawa matrix • In the Standard Model, the CKM matrix elements Vij describe the electroweak coupling strength of the W to quarks • CKM mechanism introduces quark flavor mixing • Complex phases in Vij are the origin of SM CP violation Mixes the left-handed charge –1/3 quark mass eigenstates d,s,b to give the weak eigenstates d’,s,b’. l3 l l l2 l3 l2 l=cos(qc)=0.22 CP The phase changes signunder CP. Wouter Verkerke, UCSB Transition amplitude violates CP if Vub ≠ Vub*, i.e. if Vub has a non-zero phase

  5. d s b u c t The Unitarity Triangle – Visualizing CKM information from Bd decays • The CKM matrix Vij is unitary with 4 independent fundamental parameters • Unitarity constraint from 1st and 3rd columns: i V*i3Vi1=0 • Testing the Standard Model • Measure angles, sides in as many ways possible • SM predicts all angles are large CKM phases (in Wolfenstein convention) Wouter Verkerke, UCSB

  6. Bf Observing CP violation • So far talking about amplitudes, but Amplitudes ≠ Observables. • CP-violating asymmetries can be observed from interference of two amplitudes with relative CP-violating phase • But additional requirements exist to observe a CP asymmetry! • Example: process Bf via two amplitudes a1 + a2 = A. weak phase diff. g 0, no CP-invariant phase diff. Bf A=a1+a2 A=a1+a2 a2 A +g a1 a1 -g A a2 |A|=|A|  No observable CP asymmetry Wouter Verkerke, UCSB

  7. Bf Observing CP violation • Example: process Bf via two amplitudes a1 + a2 = A. weak phase diff. g 0, CP-invariant phase diff.d 0 Bf A=a1+a2 A=a1+a2 +g d d A -g a2 A a2 a1 a1 |A||A| Need also CP-invariant phase for observable CP violation Wouter Verkerke, UCSB

  8. f = b f = b CP violation: decay amplitudes vs. mixing amplitudes • Interference between two decay amplitudes gives two decay time independent observables • CP violated if BF(B  f) ≠ BF(B  f) • CP-invariant phases provided by strong interaction part. • Strong phases usually unknown  this can complicate things… • Interference between mixing and decay amplitudesintroduces decay-time dependent CP violating observables • Bd mixing experimentally very accessible: Mixing freq Dmd0.5 ps-1, t=1.5 ps • Interfere ‘B  B  f’ with ‘B  f’ • Mixing mechanism introduces weak phase of 2band a CP-invariant phase of p/2, so no large strong phases in decay required N(B0)-N(B0) N(B0)+N(B0) 2pDmd 2tB Wouter Verkerke, UCSB

  9. f = b f = b ACP(t) from interference between mixing+decay and decay • Time dependent CP asymmetry takes Ssin(Dmdt)+Ccos(Dmdt) form • C=0 means no CP violation in decay process • If C=0, coefficient S measures sine of mixing phase mixing decay If only single real decay amplitude contributes Wouter Verkerke, UCSB

  10. CKM Angle measurements from Bd decays • Sources of phases in Bd amplitudes* • The standard techniques for the angles: bu *In Wolfenstein phase convention. td B0 mixing + single bu decay The distinction between a and gmeasurements is in the technique. B0 mixing + single bc decay Interfere bc and bu in B± decay. Wouter Verkerke, UCSB

  11. The PEP-II B factory – specifications • Produces B0B0 and B+B- pairs via Y(4s) resonance (10.58 GeV) • Asymmetric beam energies • Low energy beam 3.1 GeV • High energy beam 9.0 GeV • Boost separates B and B and allows measurement of B0 life times • Clean environment • ~28% of all hadronic interactions is BB (4S) BB threshold Wouter Verkerke, UCSB

  12. Operates with 1600 bunches Beam currents of 1-2 amps! Continuous ‘trickle’ injection Reduces data taking interruption for ‘top offs’ High luminosity 6.6x1033 cm-2s-1 ~7 BB pairs per second ~135 M BB pairs since day 1. Daily delivered luminosity still increasing Projected luminosity milestone 500M BB pairs by fall 2006. The PEP-II B factory – performance Wouter Verkerke, UCSB

  13. The BaBar experiment • Outstanding K ID • Precision tracking (Dt measurement) • High resolution calorimeter • Data collection efficiency >95% Electromagnetic Calorimeter (EMC) 1.5 T Solenoid Detector for Internally reflected Cherenkov radiation (DIRC) SVT: 5 layers double-sided Si. DCH: 40 layers in 10 super- layers, axial and stereo. DIRC: Array of precisely machined quartz bars. . EMC: Crystal calorimeter (CsI(Tl)) Very good energy resolution. Electron ID, p0 and g reco. IFR: Layers of RPCs within iron. Muon and neutral hadron (KL) Drift chamber (DCH) Instrumented Flux Return (IFR) Silicon Vertex Detector (SVT) Wouter Verkerke, UCSB

  14. Silicon Vertex Detector Readout chips Beam bending magnets Beam pipe Layer 1,2 Layer 3 Layer 4 Layer 5 Wouter Verkerke, UCSB

  15. Čerenkov Particle Identification system • Čerenkov light in quartz • Transmitted by internal reflection • Rings projected in standoff box • Thin (in X0) in detection volume, yet precise… Wouter Verkerke, UCSB

  16. DE mes>5.27 GeV N= 1506 Purity = 92% mes mes (GeV) Selecting B decays for CP analysis • Exploit kinematic constraints from beam energies • Beam energy substituted mass has better resolution than invariant mass • Sufficient for relatively abundant & clean modes (mES) 3 MeV s(DE)  15 MeV 2 Wouter Verkerke, UCSB

  17. Measuring (time dependent) CP asymmetries • B0B0 system from Y(4s) evolves as coherent system • All time dependent asymmetries integrate to zero! • Need to explicitly measure time dependence • B0 mesons guaranteed to have opposite flavor at time of 1st decay • Can use ‘other B0’ to tag flavor of B0CP at t=0 Vertexing Tag-side vertexing ~95% efficient B-Flavor Tagging sz170 mm sz70 mm Dt=1.6 ps  Dz 250 mm Exclusive B Meson Reconstruction Wouter Verkerke, UCSB Dz/gbc

  18. Flavor tagging Determine flavor of Btag BCP(Dt=0)from partial decay products Leptons : Cleanest tag. Correct >95% Full tagging algorithm combines all in neural network Four categories based on particle content and NN output. Tagging performance e- e+ W- W+ n n b b c c Kaons : Second best. Correct 80-90% efficiency mistake rate W- W+ c c K- s s b K+ b W- u u = 28% W+ d d Wouter Verkerke, UCSB

  19. B0(Dt) B0(Dt) ACP(Dt) = Ssin(DmdDt)+Ccos(DmdDt) sin2b Dsin2b Putting it all together: sin(2b) from B0 J/y KS • Effect of detector imperfections • Dilution of ACP amplitude due imperfect tagging • Blurring of ACP sine wave due to finite Dt resolution • Measured & Accounted for in simultaneously unbinned maximum likelihood fit to control samples • measures Dt resolution and mistag rates. • Propagates errors Imperfect flavor tagging Finite Dt resolution  Actual sin2b result on 88 fb-1 Wouter Verkerke, UCSB Dt Dt

  20. * Vcb c b J/Y f = 0 W+ c B0 Vcs s f = b Ks d d f = b B-factory ‘flagship’ measurement: sin2b from J/y KS • Interference between mixing and single real decay • Interfering amplitudes of comparable magnitude  the observable asymmetry is large (ACP of order 1) • Extraordinarily clean theory prediction (~1% level) • Single real decay amplitude  all hadronic uncertainty cancel • ACP(t) = sin(2b) sin(Dmd t) • Experimentally easy • ‘Large’ branching fraction O(10-4) • Clear signature (J/y l+l-and KS p+p-) Decay B0 Mixing……followed by………Decay d Ks s Vcs * c W+ J/Y c f = 0 Vcb Wouter Verkerke, UCSB

  21. Combined result (88 fb-1, 2001) sin2b = 0.741  0.067  0.034 |l| = 0.948  0.051  0.030 (stat) (syst) sin2b = 0.76  0.074 ‘Golden’ measurement of sin2b B0 (cc) KS (CP=-1) No evidence for cos(DmDt) term sin2b = 0.72  0.16 B0 (cc) KL (CP=+1) Wouter Verkerke, UCSB

  22. r = r(1-l2/2) h = h(1-l2/2) Standard Model interpretation Constraints on the apex of the Unitarity Triangle. h r Wouter Verkerke, UCSB Method as in Höcker et al, Eur.Phys.J.C21:225-259,2001

  23. Standard Model interpretation 4-fold ambiguity because we measure sin(2b), not b One solution for b is very consistent with the other constraints. 2 1 Latest results including the Belle experiment. h • The CKM model for CP • violation has passed its first precision test! 3 There is still room for improvement: measurement is statistics dominated Summer ’04 data  2-3 x 88fb-1 4 r Wouter Verkerke, UCSB Method as in Höcker et al, Eur.Phys.J.C21:225-259,2001

  24. B-factory measurements of sin2b • Going beyond the ‘golden’ modes • Consistency requires S=sin2b, C=0for all B0 decay modes for whichthe weak phase is zero. • Decay modes dominated by the bs penguin may meet these criteria • Measure ACP(t) from interference between mixing + bs decayand bs decay • Loop diagrams are sensitive to contributions from new physics • Look for deviations of S=sin2b f=0 f=0 f=??? f=??? Wouter Verkerke, UCSB

  25. u/ u/ Standard model expectation for sin(2b) from bs penguins I Experimentally best modes: B0fK0 B0h’K0 B0p0K0 (I, II & III) (II & III) f = g / 0 (I) • SM contributions that spoil S = sin2b • u-quark penguin (weak phase = g!) but relative CKM factor of ~0.02 • u-quark tree (different phase) II f = g / 0 these limits will improve with additional data f = g III *Grossman, Ligeti, Nir, Quinn. PRD 68, 015004 (2003) and Gronau, Grossman, Rosner hep-ph/0310020

  26. bs penguin measurements • Experimentally more difficult • Branching fractions smaller, more irreducible background B0 fKS B0 KSp0 B0 h’KS Wouter Verkerke, UCSB

  27. ’ sin2b from bs penguin measurements h’Ks BaBar 0.02  0.34  0.03 fKs BaBar 0.45  0.43  0.07 p0Ks BaBar 0.48 (+0.38)  0.11 –0.47 bs penguin average Babar 0.27  0.22 Wouter Verkerke, UCSB sin2b from B0 (cc) KS

  28. ’ sin2b from bs penguin measurements (My naïve averages) h’Ks BaBar 0.02  0.34  0.03 Belle 0.43  0.27  0.05 Ave 0.27  0.21 fKs BaBar 0.45  0.43  0.07 Belle –0.96  0.50 (+0.09) Ave –0.14  0.33 –0.11 p0Ks Babar 0.48 (+0.38)  0.11 –0.47 K+K-Ks non-resonant Belle 0.51  0.26  0.05 (+0.18) –0.00 bs penguin average Babar and Belle 0.27  0.15 Wouter Verkerke, UCSB sin2b from B0 (cc) KS

  29. sin2b : bs penguin modes • Current naïve world averages S = 0.27 ± 0.15 (~3s below J/yKs S = 0.74 ± 0.05). C = 0.10 ± 0.09 • Still very early in the game • Measurements are statistics limited. Errors smaller by factor 2 in 2-3 years. • Standard Model pollution limits from SU(3) analysis will also improve with more data. Wouter Verkerke, UCSB

  30. f = b f = b The angle a from B  pp • Determination of a: Observe ACP(t) of B0 CP eigenstate decay dominated by bu • Interference between mixing+bu decay and bu decay • Textbook example is B0 p+p-. • If the above bu tree diagram dominates the decay ACP(t)=sin(2a)sin(DmdDt). B0 Mixing bu decay Vub f = g sin2a Wouter Verkerke, UCSB

  31. The angle a - the penguin problem • Turns out the dominant tree assumption for p+p- is bad. • There exists a penguin diagram for the decay as well • Magnitude of penguin can be estimated from B  K+p-(dominated by SU(3) variation of this penguin) • Penguin amplitude is large, contribution to B  p+p- could be ~30%! • Including the penguin component (P) in l • Coefficients from time-dependent analysis tree decay penguin decay s Vtd/Vts / K+ Vub f = 0 f = 0 f = g Ratio of amplitudes |P/T|and strong phase difference dcan not be reliably calculated Unknown phase shift Wouter Verkerke, UCSB

  32. Disentangling the penguin: determining 2k • Gronau & London: Use isospin relations • Measure all isospin variations of B  pp • B0 p+p- , B0  p+p-, B0  p0p0 , B0  p0p0 B-  p-p0 = B+  p+p0 • Weak phase offset 2k can bederived from isospin triangles • Complicated… 2k - Wouter Verkerke, UCSB

  33. Disentangling the penguin: the Grossman-Quinn bound • Easy alternative to isospin: Grossman-Quinn bound • Look at isospin triangles and construct upper limit on k • Minimum required input: BF(B pp0) and limit on BF(B0 p0p0) • Works best if B0 p0p0 is small • Experimental advantage: no flavor tagging in Bp0p0 • Measure B0  p0p0! ‘~10-5’ ‘~10-6’ Wouter Verkerke, UCSB

  34. the Grossman-Quinn bound on k for B0 pp • B0 p0p0 is observed! (4.2s) • GQ Bound using world averages • p0p0: (1.9±0.5)x10-6 • p±p0: (5.3±0.8)x10-6 • p0p0 large, thus GQ bound not very constraining • Isospin analysis required for p0p0! Plots are after cut on signal probability ratio not including variable shown, optimized with S/sqrt(S+B) . [BELLE: (1.7±0.6±0.2)x10-6, 3.4s] Wouter Verkerke, UCSB

  35. Alternatives to B  pp for determination of a • There are other final states of bu tree diagram, e.g. • B  rp (Dalitz analysis required) • B  rr (Vector-vector  multiple amplitudes) • B  r+r- analysis • 3 helicity amplitudes: Longitudinal (CP-even), 2 transverse (mixed CP) • Looks intractable, but entirely longitudinally polarized*! • r+r- is basically a CP-even state with same formalism as p+p-. Wouter Verkerke, UCSB *As predicted by G.Kramer, W.F.Palmer, PRD 45, 193 (1992). R.Aleksan et al., PLB 356, 95 (1995).

  36. the Grossman-Quinn bound for B0 rr • The Grossman-Quinn bound for B0 rr (BaBar) (Belle) (assuming full longitudinal polarization) Wouter Verkerke, UCSB

  37. Alpha summary • The pp system: large penguin pollution • We have seen B0p0p0! • Current GQ bound: • Full isospin analysis required! • The rr system: small penguin pollution • Polarization is fully longitudinal (as predicted). • Current GQ bound: • Bound may improve as additional data becomes available • Time-dependent r+r- results (measures sin(2a+2k)) coming soon. • There are more techniques than pp and rr • e.g. Dalitz analysis of rp Wouter Verkerke, UCSB

  38. The angle g • Measuring g = Measuring the phase of the Vub • Main problem: Vub is very small: O(l3) • Either decay rate or observable asymmetry is always very small. • Conventional wisdom: measuring g at B factories is difficult/impossible. • Gamma is the least constrained angle of the Unitarity Triangle • Current attitude: we should try. • There are new ideas to measure g (Dalitz decays, 3-body decays,…) • New experimental data suggest color suppression is less severe, which eases small rate/asymmetry problem somewhat • B-Factories produce more luminosity than expected(BaBar & Belle approaching O(200) fb-1 by Summer ’04 time ) Wouter Verkerke, UCSB

  39. The angle g: B  DK • Strategy I: interfere bu and bc decay amplitudes • D0/D0 mustdecay to common final stateto interfere • Ratio of decay B amplitudes  rb is small: O(10-1) • rb isnot well measured, but important • rb large  more interference  more sensitivity to g f=g color suppression f=0 Ru is the left side of the Unitarity Triangle (~0.4). FCS is (color) suppression factor([0.2-0.5], naively1/3) Wouter Verkerke, UCSB

  40. g from B  DK – Two approaches • Approach I: D0/D0 decay to common CP eigenstate • ‘Gronau, London & Wyler’ • D0/D0 decay rate same • Approach II: D0/D0 decay to common flavor eigenstate • ‘Atwood, Dunietz & Soni’ • Use D0/D0 decay rate asymmetry to compensate B decay asymmetry` • Complementary in sensitivity • GLW:large BF: O(1±rb), small ACP: O(rb) • ADS:small BF: O(rb2),large ACP: O(1) Branching fractionssmall (0.1%-1%) CKM favored Doubly Cabibbo suppressed (by factor O(100)) Wouter Verkerke, UCSB

  41. B  DK Observables – Gronau-London-Wyler • There are more observables sensitive to g than ACP • Absolute decay rate also sensitive to g, but hard to calculatedue to hadronic uncertainties • GLW: measure ratio of branchingfractions: hadronic uncertainties cancel! • Experimental bonus: many systematic uncertainties cancel as well • Bottom line: 2 observables each for CP+ and CP- decays • 3 independent observables (R+, R-, A+=-A-), 3 unknowns (rb, db, g) Wouter Verkerke, UCSB

  42. B  DK : GLW results GLW method: large BF, small ACP • Result for B-  D0 K- in 115 fb-1 • Results for CP-odd modesin progress (R-, A-) D0p- background Wouter Verkerke, UCSB

  43. B  DK : The Atwood-Dunietz-Soni method • Two observables, similar to GLW technique • Ratio of branching fractions and ACP • D0 K+p-: 2 observables (A, R), 3 unknowns (rb, db+dd, g) • Insufficient information to solve for g • Can add other D0 decay modes, e.g. D0 K+p-p0  4 observables (2xA, 2xR), 4 unknowns (rb, db+dDKp, db+dDKpp0, g) • Expected BF is ~510-7 – very hard! • Expect observable O(10) events in 100M BB events • Unknown values of g, rb, db add O(10) uncertainty of BF estimate • Measurement not attempted until now Wouter Verkerke, UCSB

  44. g from Atwood-Dunietz-Soni method: B- [K+p-]D0 K- : results MC yield prediction with BF=7x10-5: 12 evts ADS method: small BF, large ACP • Newly developed background suppression techniques give us sensitivity in BF = O(10-7) range BF 5x10-7 ~10 events • But we don’t see a signal! • Destructive interference, rb is small, or just unlucky? • Cannot constrain g with this measurement… • But BF proportional to rb2 results sets upper limit on rb Yield in 115 fb-1 of data:1.1  3.0 evts No assumptions: rb < 0.22 (90% C.L.) • from CKM fit : rb < 0.19 (90% C.L.) (95% C.I. region) Wouter Verkerke, UCSB

  45. B  DK : prospects for B-factories at 500 fb-1 • Combine information ong from various sources • Example study • Assume g=75o, db=30o, dd=15o • Consider various scenarios • GLW alone g=75o, db=30o, dd=15o Dc2 rb=0.3 3s 2s GLW 1s g Wouter Verkerke, UCSB

  46. B  DK : prospects for B-factories at 500 fb-1 • Combine information ong from various sources • Scenarios • GLW alone • GLW+ADS(Kp) • GLW+ADS(Kp)+dd from CLEO-c • ADS/GLW combination powerful • There are additional information not usedin this study, e.g. • GLW: D*0K,D0K*,D*0K* • ADS: Kpp0,K3p • sin(2b+g) from D*p, D0K0, DKp,… g=75o, db=30o, dd=15o Dc2 rb=0.3 3s GLW+ADS+CLEO-c GLW+ADS 2s 11o GLW 1s g Wouter Verkerke, UCSB

  47. B  DK : prospects for B-factories at 500 fb-1 • Combine information ong from various sources • rb is critical parameter g=75o, db=30o, dd=15o 3s rb=0.1 2s 67o 1s 3s rb=0.2 2s D2 1s 23o 3s rb=0.3 2s 11o 1s g Wouter Verkerke, UCSB

  48. Gamma summary • The B  DK program is underway • Measurements for GLW methods in progress (B  D(*)0 K(*)-) • First measurement of ADS method (B  [K+p-]K-) • ADS and GLW techniques powerful when combined • Final results depends strongly on rb • Other g methods in progress as well • Dalitz analyses of B-  D0(KSp+p-)K-, B  DKp • Time dependent analysis of B  D*p- (mixing + Vub decay) • |sin(2b+g)|>0.57 (95% C.L.)) • Analysis of B0  D(*)0 K(*)0 • There is no ‘golden’ mode to measure g • All techniques are difficult and to 1st order equally sensitive. • Combine all the measurements and hope for the best Wouter Verkerke, UCSB

  49. Concluding remarks • The CKM model for CP violation passed it’s first test (sin2b). • Future measurements of sin2b from B0  (cc)KS will continue improve constraints on apex of unitarity triangle • The bs penguin measurement of sin2b offers a window to new physics. • Another 2-3 years worth of data will clarify current 3s discrepancy • We are cautiously optimistic that we can measure a now that B  rr decay turns out have little penguin pollution • Measurement of g just starting. Success depends on many unknowns… • BaBar is projected to double its current dataset by 2006 Wouter Verkerke, UCSB

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