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Recent Results from the BaBar Experiment.

Recent Results from the BaBar Experiment. Brian Meadows University of Cincinnati. Outline. CP Violation The BaBar Experiment B 0 – B 0 Mixing, Lifetime and sin 2  Measurements Summary. Why the B Factories Studied CP Violation.

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Recent Results from the BaBar Experiment.

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  1. Recent Results from the BaBar Experiment. Brian Meadows University of Cincinnati Brian Meadows, U. Cincinnati.

  2. Outline • CP Violation • The BaBar Experiment • B0 – B0 Mixing, Lifetime and sin 2 Measurements • Summary Brian Meadows, U. Cincinnati

  3. Why the B Factories Studied CP Violation • Understanding its origin is an intrinsically interesting goal • It had, thus far, only been seen in K0 decays • KL0->p+p- • Asymmetry in KL0 p§ l ¨n Over 40 years ago ! • It is an important ingredient in explaining baryon-antibaryon asymmetry in the universe • Standard Model has 3 quark generations with CPV built in • Unlikely to be sufficient to explain baryon-antibaryon asymmetry) • The B0-B0 system appeared to be an excellent laboratory for studying CPV in the Standard Model • K0’s are known to mix and exhibit CPV: • Could CPV be due to a new force that brings about DS = 2 ? • B0 mixing had also been discovered by then – seems like a place to look for DB = 2. Brian Meadows, U. Cincinnati

  4. What is Known About Matter Asymmetry? • It exists – well, at least locally • No isotropic, high energy g’s from e+e- annihilations • Cosmic rays do not contain anti-nuclei • Anti-protons abundances consistent with production in atmosphere • Baryon to CMB g ratio nB/ng ~ 109 • If nB = nB initially, for T < mp thermal equilibrium would lead to nB/ng = nB/ng ~ 10-20 • Simplest explanation depends on • Mechanism for baryon non-conservation • Mechanisms for C and CP violation • Departure of universe from thermal equilibrium so that collision time long in comparison with above. • Other models are less compelling or too complicated • nB = nB when t = “big bang” • Baryons and anti-baryons separated spacially. Brian Meadows, U. Cincinnati

  5. Why a “B Factory”? • Principal aim of experiment – study CP violation, principally in the B0-B0 system. • Requires examination of rare decay modes of B0 mesons • Need huge samples (at least 108) BB pairs • Original goal at SLAC was to build a machine with “luminosity” 3 x 1033 cm-2¢ sec-1¢ • Should provide 3 x 107 B pairs (30 fb-1) per year (107 secs.) • Produces BB pairs at a few Hz and other interesting physics events at ~ 100 Hz. • Tuned on the resonance Y (4S) ( BB) • Uses 9 GeV/c e- and 3.1 GeV/c e+ beams to provide way to measure time dependence in the laboratory (bg=0.56). Brian Meadows, U. Cincinnati

  6. The BaBar Detector at SLAC (PEP2) • Asymmetric e+e- collisions at (4S). •  = 0.56 (3.1 GeV e+, 9.0 GeV e-) 1.5 T superconducting field. Instrumented Flux Return (IFR) Resistive Plate Chambers (RPC’s): Barrel: 19 layers in 65 cm steel Endcap: 18 “ “ 60 cm “ Brian Meadows, U. Cincinnati

  7. Off • On PEP-II performances (2008) Peak Luminosity ~2 £ 1034 cm-2¢ s-1 • Approx. 600 fb-1 in runs 1-7 run7 run6 Data taken mostly at Y(4S) BUT ~ 12% below this: run5 run4 run3 run2 run1 Brian Meadows, U. Cincinnati

  8. Belle (KEK) performances (2008) Peak Luminosity ~2£1034 cm-2¢ s-1 Integrated luminosity Approx. 880 fb-1 You can “LIVE” event Displays at http://belle.kek.jp/evdisp/index.html Brian Meadows, U. Cincinnati

  9. Silicon Vertex Tracker (SVT) • 5 Layers double sided AC-coupled Silicon • Rad-hard readout IC (2 MRad – replace ~2005) • Low mass design • Stand alone tracking for slow particles • Point resolution z » 20 m • Radius 32-140 mm Brian Meadows, U. Cincinnati

  10. The BaBar Collaboration Brian Meadows, U. Cincinnati

  11. Drift Chamber 40 layer small cell design 7104 cells He-Isobutane for low multiple scattering dE/dx Resolution »7.5% Mean position Resolution 125 m Brian Meadows, U. Cincinnati

  12. Particle ID - DIRC Detector of Internally Reflected Cherenkov light • Measures Cherenkov angle in quartz • Photons transported by internal refl. • Detected at end by » 10,000 PMT’s 144 quartz bars Brian Meadows, U. Cincinnati

  13. Particle ID - DIRC It Works Beautifully! Provides excellent K/ separation over the whole kinematic range Brian Meadows, U. Cincinnati

  14. Particle ID - DIRC D0 D0 Brian Meadows, U. Cincinnati

  15. Electromagnetic Calorimeter • CsI (doped with Tl) crystals • Arranged in 48()£120() • » 2.5% gaps in . • Forward endcap with 8 more  rings (820 crystals). Brian Meadows, U. Cincinnati

  16. d d d d Weak Decays • Two kinds of diagrams are prevalent in weak decays Tree: Penguin: • These interfere with one another. c c J/y b s W B0 K0 J/y c c g b s B0 K0 W Brian Meadows, U. Cincinnati

  17. Weak Decays • These decays are actually more complicated: • Each diagram represents the weak decay only • This occurs over a very short distance scale • It is represented by an amplitude – Weif This is followed by: • “Hadronization” • When the quarks emerge, they interact strongly in a “sea” of gluons and form hadrons that scatter off each other. • This process is represented by an amplitude – Seid • The nett result is represented by the amplitude A e i(d+f) where (|A| = W x S) Brian Meadows, U. Cincinnati

  18. CP Violation • CP violation is manifest when a process involving particles occurs at a different rate to that with anti particles: (B ! f)  (B ! f) • Under CP transformation, amplitudes A have weak phases  that reverse sign but strong ones  that do not A = a exp{i(+ )}! A = a e{i(- )} • If two amplitudes A1 and A2 contribute to a process, the rates are:  = |A1+ A2|2 = a12+ a22+ 2a1a2 cos( + )  = |A1+ A2|2 = a12+ a22+ 2a1a2 cos( - ) •  (CP Violation)when  (´2-1)  0 and (´2-1) 0. CP CP violation is maximum when a1 = a2 ! Brian Meadows, U. Cincinnati

  19. CPV in the Standard ModelCabbibo-Kobayashi-Maskawa Matrix • In the Standard Model, weak decays allow quarks to change flavor in transitions from charge +2/3 to charge –1/3. • The couplings are defined by the CKM matrix • The matrix in unitary, so is defined by • Three angles (real) • One complex phase • Phase is real – cannot be removed by re-phasing the quark fields. Brian Meadows, U. Cincinnati

  20. d s b u c t The Unitarity Triangles (K system) d•s* = 0 (Bs system) s•b* = 0 (Bd system) d•b* = 0 These three triangles (and the three triangles corresponding to the rows) all have the same area. A nonzero area is a measure of CP violation and is an invariant of the CKM matrix. apply unitarity constraint to pairs of columns From P. Burchat Brian Meadows, U. Cincinnati

  21. d s b u c t The Usual Unitarity Triangle Vtb*Vtd Vub*Vud    Vcb*Vcd Orientation of triangle has no physical significance. Only relative angle between sides is significant. apply unitarity constraint to these two columns From P. Burchat Brian Meadows, U. Cincinnati

  22. d s b u c t (, ) Vtb*Vtd Vcb*Vcd Vub*Vud Vcb*Vcd    (1, 0) (0, 0) The Usual Unitarity Triangle apply unitarity constraint to these two columns From P. Burchat Brian Meadows, U. Cincinnati

  23. J=Vqq’ q’  (1- 5) q q’ q W CP Violation in the Standard Model • The phase in the CKM quark mixing matrix can give rise to CP Violation. • CKM imparts a phase to weak currents that cannot be removed by re phasing the quark fields. • Interference between a tree and a penguin process can give direct CP Violation but information on strong phases is required to interpret it. • Decays of B0 to CP eigenstates f accessible also to B0 can occur directly or through mixing. Allows interpretation without knowing strong phases Brian Meadows, U. Cincinnati

  24. B Mixing • Principal standard-model mechanism dNB0 exp(–t/B) ( 1 ± cos(mt) ) Brian Meadows, U. Cincinnati

  25. B0 – B0 Mixing md , B andsin 2 Measurements Brian Meadows, U. Cincinnati

  26. An Early BaBar B Mixing Measurement hep-ex/0207071 (ICHEP) (2002) Dmd=0.492±0.018±0.013ps-1 +0.024 -0.023 B0=1.523±0.022ps correlation coefficient (m, B0) = -0.22 • “In a few years we might: • anticipate < 1% uncertainty in B0 mixing • possibly measure  • (test CPT limits directly).” Brian Meadows, U. Cincinnati

  27. Most Recent BaBar B Mixing Measurement Phys.Rev.D 73 012004 (2006) +0.007 -0.006 Dmd=0.511±0.007 ps-1 +0.018 -0.013 B0=1.504±0.013 ps • “We HAVE: • acheived~ 1% uncertainty in B0 mixing Brian Meadows, U. Cincinnati

  28. Increase in precision of B lifetimes and mixing frequency B0 Lifetime (ps) 1.548  0.032 1.530  0.009 Ratio of B+ to B0 Lifetime 1.060  0.029 1.071  0.009 B0 Mixing Frequency ( x 1012 s-1) 0.472  0.017 0.507  0.005 PDG2000 18 measurements 12 measurements 10 measurements PDG2008 New measurements: 5 B Factory 5 TeVatron 3 B Factory 3 TeVatron 6 B Factory 1 TeVatron • Uncertainties limited by: • knowledge of t resolution function • B (for mixing). • BABARMeasured both together using the copious B0!D*lmode. Brian Meadows, U. Cincinnati

  29. f B0 Mixing B0 Mixing Induced CP Violation • If final state f is accessible for both B0 and B0 decay then mixing will interfere with direct decays • If f is a CP eigen-state, amplitudes <f|T|B0> and <f|T|B0> have: • identicalstrong phases • identical weak phases, but with opposite signs • the same magnitudes. • Therefore • The CP violation is maximized. • The strong phases cancel in any interference observed • A Bonus: • CP violation has a time structure emanating from B0 mixing Brian Meadows, U. Cincinnati

  30. p+ B0 / B0 e+ e- — B0 / B0 e ±, m ±, K± tag Dz =c t Dz ~ 255 mm for PEP-II: 9.0 GeV on 3.1 GeV ~ 200 mm for KEKB: 8.0 GeV on 3.5 GeV The Asymmetric-Energy B Factories (4S) Brian Meadows, U. Cincinnati

  31. dN/dt/ e - |t|/£ [1 §Ccos(mt) ¨S sin(mt)] C = (1 - |f|2) / (1 + |f|2) S = Im{ f }/ (1 + |f|2) • For b!ccs decays (in the SM): • f = e 2ib Mixing Decay Time-Dependence of B0 Decay Modified by Interference between two direct decay modes such as P and T Interference between mixing and decay.  is one of the angles in a unitarity triangle -- • AND Penguin has the same weak phase Brian Meadows, U. Cincinnati

  32. Dt distributions with NO experimental effects Flavor states sorted by mixing status CP states sorted by B tag flavor B0B0 or B0 B0 Btag= B0 Btag= B0 B0B0 or B0 B0 B Mixing dN exp(–|Dt|/tB) ( 1 ± cos(DmDt) ) CP violation dN exp(–|Dt|/tB) ( 1 ± sin2b sin(DmDt) ) With NO penguins Brian Meadows, U. Cincinnati

  33. unmixed – mixed unmixed + mixed Asymmetry = ~ (1 – 2w)  (1 – 2w) cos(DmdDt) ~ p / Dmd Perfect flavor tagging and time resolution Realistic mis-tag and finite time resolution - unmixed - unmixed - mixed - mixed Brian Meadows, U. Cincinnati

  34. Results to Date (2003) Brian Meadows, U. Cincinnati

  35. Sin 2 Primary result comes from charmonium decay modes. Simultaneously fit flavour specific modes to determine flavour tagging quality and  resolution. Additional information from modes which include penguin (P) in addition to tree (T) modes. Vtb*Vtd  Vcb*Vcd Brian Meadows, U. Cincinnati

  36. Charmonium Modes for sin 2 b c , c, c One dominant decay amplitude !theoretically clean. (Penguin has same phase!) c B0 s KS , L d d Both BABAR and Belle use six charmonium modes: BJ/Ks0, Ks0p+p-, p0p0 BJ/KL0 B(2S) Ks0 Bc1Ks0 BJ/K*0, K*0 Ks0 BcKs0 Simultaneously measure self tagging modes to determine  and (t). Brian Meadows, U. Cincinnati

  37. Sin2b Data Samples in BABAR Bflav Mixing sample ccKs modes B0D(*)-p+/ r+/ a1+ Ntagged= 23618 Purity= 84% • Data • Data Signal J/y KL J/y Bkg Fake J/y Bkg (MeV) Brian Meadows, U. Cincinnati

  38. hep-ex/ 0207042 (PRL) Ks modes KL modes 81 fb-1 (88 M BB) 2641 tagged events with Dt measured (78% purity; 66% tagging e) sin2b = 0.741  0.067  0.034 || = 0.948  0.051  0.030 effective tagging eff: e=(28.1  0.7)% Brian Meadows, U. Cincinnati

  39. Golden modes with a lepton tag The best of the best! Ntagged = 220 Purity = 98% Mis-tag fraction 3.3% sDt 20% better than other tag categories background sin2b = 0.79  0.11 Brian Meadows, U. Cincinnati

  40. sin2b measurement history • “Osaka 2000” measurement • (hep-ex/0008048) • Only J/y Ks and y(2s) Ks. • 1st Paper (PRL 86 2515, 2001) • Added J/y KL. • Simultaneous sin2b and mixing fit. • 2nd Paper (PRL 87 201803, 2001) • Added J/y K*0 and c Ks. • Better vertex reconstruction. • Better SVT alignment and higher Ks efficiency for new data. • Winter 2002 (hep-ex/0203007) • Improved event selection. • Reprocessed 1st 20 fb-1. • e) Current measurement (hep- ex/0207042, PRL) • Improved flavor tagging. • One more CP mode: hcKs. (compiled by Owen Long) d e c b a Brian Meadows, U. Cincinnati

  41. Decrease in Statistical Uncertainty • Curves represent 1/sLdt. • Improvements in statistical uncertainty due to • adding new B decay modes, • improved vertex reconstruction, • improved SVT alignment, • improved tagging performance. Brian Meadows, U. Cincinnati

  42. hep-ex/0208025, sub to PRD RC Belle 78 fb-1 (85 M BB) 2958 events (81% purity) effective tagging efficiency: e=(28.8  0.6)% sin2b = 0.719  0.074  0.035|| = 0.950  0.049  0.025 Brian Meadows, U. Cincinnati

  43. Constraints on upper vertex of Unitarity Triangle from all measurements EXCEPT sin2b b Regions of >5% CL A. Höcker, H. Lacker, S. Laplace, F. Le Diberder, Eur. Phys. Jour. C21 (2001) 225, [hep-ph/0104062] Brian Meadows, U. Cincinnati

  44. World Average (2003) sin2b = 0.78  0.08 The Standard Model (and the CKM paradigm, in particular) wins again … at least at the current level of experimental precision, in this decay mode. Brian Meadows, U. Cincinnati

  45. World Average (2008)sin2b = 0.67  0.02 Brian Meadows, U. Cincinnati

  46. s s s t t d Other studies of sin2 B0Ks b s   s b s B0 s B0 K0 K0 d d d • Pure penguin ! • time-dependent asymmetry in B0Ks measures sin2. • direct charge asymmetry in B+K+ sensitive to new physics. Brian Meadows, U. Cincinnati

  47. B0Ks Old Result (2002)  NEW PHYSICS ??? 51 signal events hep-ex/0207070 (ICHEP2002) +0.52 - 0.50 sin2b = -0.19  0.90 • c.f. world average: sin2 = 0.67 ± 0.02 • >2 difference. • (over) stimulating theoretical interest Brian Meadows, U. Cincinnati

  48. B0fKs Most Recent Result (2005)  SM is FINE !! ~120 signal events Phys.Rev.D71:091102,2005 +0.07 - 0.04 sin2b = +0.50  0.25 • c.f. world average: sin2 = 0.67 ± 0.02 • <1 difference. Brian Meadows, U. Cincinnati

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