1 / 48

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.

Télécharger la présentation

Recent Results from the BaBar Experiment.

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript


  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

More Related