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Direct and Indirect Probes of EWSB in e + e - Annihilation (LEP and SLC)

Direct and Indirect Probes of EWSB in e + e - Annihilation (LEP and SLC). Outline :. Patrick Janot, CERN. First Lecture : Second Lecture : Third Lecture :. Higgs. EP. (and SLC too! ). Indirect Probes of EWSB at LEP and SLC. First Lecture Outline:

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Direct and Indirect Probes of EWSB in e + e - Annihilation (LEP and SLC)

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  1. Direct and Indirect Probes of EWSB in e+e- Annihilation (LEP and SLC) Outline: Patrick Janot, CERN First Lecture: Second Lecture: Third Lecture: Higgs EP (and SLC too!) Probes of Electroweak Symmetry Breaking at LEP and SLC

  2. Indirect Probes of EWSB at LEP and SLC • First Lecture Outline: • 1. Brief History & Overview • 2. Brief Theory Reminder: • Why is Precision needed? • 3. List of “Electroweak” Observables • 4. A few PrecisionMeasurements • Z Lineshape & Beam Energy (LEP) • Left-Right Asymmetry • and Beam Polarization (SLC) • Heavy Flavour Rates (LEP&SLC) • W mass (LEP) • 5. The top mass prediction • 6. The Higgs boson mass prediction • 7. Standard Model consistency • Common Goals of LEP and SLC: • Check the internal consistency of the Standard Model of Electroweak interactions (Z & W studies); • Test with precision the predictions of Electroweak Symmetry Breaking (mW vs mZ); • Predict heavy particle masses and new physics scale (mtop, mH,?) W Z H mW=mZcosW Probes of Electroweak Symmetry Breaking at LEP and SLC

  3. A Little Bit ofHistory 1967: Electroweak unification, with W, Z and H (Glashow, Weinberg, Salam); 1973: Discovery of neutral currents in e scattering (Gargamelle, CERN) 1983: W and Z discovery (UA1, UA2); LEP and SLC construction start; First Z detected in the world: UA2 - qq  Z  e+e- 1989: First collisions in LEP and SLC; Precision tests of the SM (mtop); 1995: Discovery of the top (FNAL); Precision tests of the SM (mH); 2000:First hints of the Higgs boson? 1974: Complete formulation of the standard model with SU(2)WU(1)Y (Illiopoulos) 1981: The CERN SpS becomes a pp collider; LEP and SLC approved before W/Z discovery; - Probes of Electroweak Symmetry Breaking at LEP and SLC

  4. LEPOverview:Luminosity,Energy,Precision Total Luminosity: 1000 pb-1 LEP 1 LEP1.5 LEP 2 89-95 95 96 97 98 99 00 Z Precision: 0.1% • Conventional collider e+e- ring; • Energy upgradeable; • Energy measurable; • Four detectors (A,L,D,O); • Large luminosity; • 20 Million Z events. W H Energy: 88 209.2 GeV Probes of Electroweak Symmetry Breaking at LEP and SLC ak Symmetry Breaking 4

  5. Stanford Linear Collider Overview • First high energy e+e- “linear” • collider (with arcs); • Reduced luminosity; • 73% polarized electron beam; • Small transverse beam sizes; • Small beam pipe; • Only one detector (MarkII, SLD) • 350,000 Z events COMPLEMENTARY to LEP 1992-1998 SLD Run Probes of Electroweak Symmetry Breaking at LEP and SLC

  6. Why is Precision Needed? Electroweak Observables (i.e., related to W and Z) sensitive to vacuum polarization effects: L Couplings (v+a)  R Couplings (v-a) 0.1% Precision needed! with • Determine  and sin2W from LEP/SLD data; • Predict mtop and mW; • Compare with direct measurements; • Predict mH; • Compare with direct measurements. eff Probes of Electroweak Symmetry Breaking at LEP and SLC

  7. Quantum Corrections to a(mZ) (I) Common quantum (energy-dependent) corrections to g and Z can be absorbed in a redefinition of the QED coupling constant a (also called “running”) e, m, t u,d,c,s,b top + + … + Small,  log mtop Calculate Precise QED Calculation e+e- hadrons at various s + QCD predictions Evaluate from: Probes of Electroweak Symmetry Breaking at LEP and SLC

  8. Quantum Corrections to a(mZ) (II) • Results more and more precise: • More e+e- data at low energy (in particular with BES) • Better knowledge of QCD at low energy (proven by hadronic t decays studied at LEP) • Dahad(mZ) = 0.02738  0.00020 • a(mZ) = 1/128.968(27) Will improve soon with BaBar (5) (most precise, most up-to-date) Probes of Electroweak Symmetry Breaking at LEP and SLC

  9. PrecisionElectroweakObservables (I) LEP1 LEP2 Heavy Flavour Rates Probes of Electroweak Symmetry Breaking at LEP and SLC

  10. Dependence on mtop, mHof mW and Rb 1. W mass: Specific Vacuum Polarization (mW/mZ)2  (mW /mZ)2(1+Dr) (Different mtop, mH dependence) - 2. Z  bb decay rate: Specific Vertex Correction Rb Rb (1+dVb) (No mH !) 0.5% Precision needed Probes of Electroweak Symmetry Breaking at LEP and SLC

  11. PrecisionElectroweakObservables (II) Precision of the Detectors LEP eff 1-4|Qi|sin2qW eff sin2qW e.g. Precise Energy Zm+m- to High Luminosity 5.10-4 + b-Tagging t-selection mW ... ... Polarized Beam (SLC only) Consistency Checks! tpnt eff sin2W = 1 – mW2/mZ2(1+) Probes of Electroweak Symmetry Breaking at LEP and SLC

  12. The Z Lineshape at LEP At tree-level:  (1+) • Measure s and s • Correct for QED and QCD -30% for s0 +200 MeV for mZ +4% for Gqq • Fit for the Z parameters • (mass, total width, peak cross • section and partial widths) s = Probes of Electroweak Symmetry Breaking at LEP and SLC

  13. Measurement of the LEP Beam Energy (I) 1) The electrons get transversally polarized (i.e., their spin tends to align with B), but Approximation: LEP is a circular ring immersed in a uniform magnetic field: • Process very sensitive to imperfections • ( slow, typically hours, and limited • to o(10%) polarization) • Process very sensitive to beam-beam • interactions ( one beam, no polarization • in collisions) Bdipole  p e- R LEP (L = 2pR = 27km) E  p = e B R = (e/2p) B L In real life: B non-uniform, ring not circular To be measured Probes of Electroweak Symmetry Breaking at LEP and SLC

  14. Measurement of the LEP Beam Energy (II) 2) The spin precesses around B with a frequency proportional to B. The number of revolutions for each LEP turn is thus proportional to BL (in fact, to  B dl, and then to Ebeam) 3) Measure ns : 0 B 1 2 Bx: oscillating field with frequency n, in one point. 3 -Bx Bx Vary n until Polarization = 0 101.5 Peak-2 103.5 Peak 105.5 Peak+2 1993 Precision  210-6 DEbeam  100 keV ! Probes of Electroweak Symmetry Breaking at LEP and SLC

  15. Measurement of the LEP Beam Energy (III) A dispersion of 10 MeV is observed ( 100 keV) in the same machine conditions. Correlation with the moon found on 1992, Nov 11th: • At midnight, the electrons see • less magnetic field, E is smaller; • At noon, they see more magnetic • field, and E is larger. LEP at midnight Longer by 300 mm LEP at noon Shorter by 300 mm  S  U  N Prediction and data fit perfectly … However, the electron orbit length is fixed by the RF frequency: L = c  t Probes of Electroweak Symmetry Breaking at LEP and SLC

  16. Measurement of the LEP Beam Energy (IV) Bdipole RF frequency: 352 254 170 Hz (1 Hz) Electron orbit fixed by RF Probes of Electroweak Symmetry Breaking at LEP and SLC

  17. QUAD QUAD Dipole Dipole   Beam Orbit Monitors (BOM)   QUAD QUAD Probes of Electroweak Symmetry Breaking at LEP and SLC

  18. Measurement of the LEP Beam Energy (V) • Other 10 MeV-ish effects understood even later: • Effect of the rain: water pressure in the mountains change LEP circumference; (controlled with the BOM’s) • Effect of the TGV: currents induced on the LEP beam pipe change the magnetic field (controlled by 16 NMR probes) Understood after one-day strike Understood after three rainy months Now: DEbeam 2 MeV Probes of Electroweak Symmetry Breaking at LEP and SLC

  19. Z Lineshape: Final State Identification (I) Probes of Electroweak Symmetry Breaking at LEP and SLC

  20. Z Lineshape: Final State Identification (II) - e) Z->nn - • Z  nn : • Not detectable. • Z  t+t- : Two low multiplicity jets + missing • energy carried by the decay neutrinos - • Z  qq :Two jets, large particle multiplicity. • Z  e+e-, m+m- :Two charged particles (e or .) Probes of Electroweak Symmetry Breaking at LEP and SLC

  21. Z Lineshape: Final State Identification (III) Leptonic decays: Low multiplicity, with (t) or without (e, m) missing energy 600,000 gg Collisions: Low multiplicity, Low mass 16 million • Selections with • High Efficiency; • High Purity; 600,000 Count events : Easy? Hadronic decays: High multiplicity High mass Systematic Uncertainty  0.1% Probes of Electroweak Symmetry Breaking at LEP and SLC

  22. Z Lineshape: Results (I) • s varied from 88 to 94 GeV; • Points are from data in both • coordinates; • Lines are from a standard model • fit to the Z parameters; MEASURE 10-5 Dominant (and common) error: LEP Beam Energy Measurement MEASURE Probes of Electroweak Symmetry Breaking at LEP and SLC

  23. Three Weeks after start Z Lineshape: Results (II) Nn= 2 13 October 1989 DELPHI L3 OPAL Nn= 3 Nn= 4 L3 L3: 2538 hadronic Z’s Nn= 3.420.48 ALEPH: 3112 hadronic Z’s ALEPH Nn= 3.270.30 OPAL: 4350 hadronic Z’s OPAL Nn = 2.984  0.008 Nn= 3.1  0.4 DELPHI: 1066 Hadronic Z’s MarkII, Aug. 1989, with 106 Z decays: Nn = 3.8  1.4. 13-Oct-1989: DELPHI Nn=3.16 0.20 Nn= 2.4  0.4 Probes of Electroweak Symmetry Breaking at LEP and SLC

  24. Z Lineshape: Results (III) (1998) 10-3 0 0 shad/sl 500 MeV in 1989 With this measurement alone: mtop  165  25 GeV/c2 GZ (1 + ) (+small sensitivity to mH) Probes of Electroweak Symmetry Breaking at LEP and SLC

  25. Heavy Flavour Rates: Identification (I) b- and c-hadrons decay weakly towards c- and s-hadrons, with a macroscopic lifetime (1.6 ps for b’s), corresponding to few mm’s at LEP 3d-vertexing determines secondary and tertiary vertices. High resolution is crucial. Impact parameters of reconstructed tracks allow b quarks to be tagged with very good purity. Mass of secondary vertex tracks is a very powerful discriminator of flavour (b, c, and light quarks): mb5 GeV/c2, and mc1.5 GeV/c2.  10 cm  1 cm Probes of Electroweak Symmetry Breaking at LEP and SLC

  26. Heavy Flavour Rates: Identification (II) • Vertex detectors (Si m-strips, CCD’s, pixels): • At LEP: inner radius 6 cm, good resolution; • At SLC: inner radius 2.3 cm, superior resolution. SLD can do both b- and c-tagging with good purity. Data Vertex Mass for Events With a Secondary Vertex In SLD b (MC) c (MC) uds (MC) Vertex Mass (GeV/c2) Probes of Electroweak Symmetry Breaking at LEP and SLC

  27. Heavy Flavour Rates: Results Use double-tag method to reduce uncertainties from the simulation, e.g., in bb events: Take ec, euds, and rb (all small) from simulation. Solve for eb and Rb ! - Rb =Gbb/ Ghad 3 10-3 With this measurement alone: mtop  150  25 GeV/c2 (dependence on mH, as, … cancel in the ratio) Probes of Electroweak Symmetry Breaking at LEP and SLC

  28. Prediction of mtopfrom EW Measurements A top mass of 177 GeV/c2 was predicted by LEP & SLC with a precision of 10 GeV/c2 in March 1994. / SLD One month later, FNAL announced the first 3 evidence of the top. / SLD / SLD In 2001: / SLD (actually 2.9s) Perfect consistency between prediction and direct measurement. Allows a global fit of the SM (with mH) to be performed. Probes of Electroweak Symmetry Breaking at LEP and SLC

  29. Asymmetries: Measurement of ALRat SLD (I) (e- beam polarization) • ALR( Ae ~ 14% if Pe = 100%) is 10 times more sensitive to sin2W than AFB( AeAl ~ 1-2%); • ALR is independent of the final state (Z  hadrons, +-, +-); • ALR is independent of the detector acceptance; • Most of the theoretical corrections, uncertainties (QED, QCD, …) cancel in the ALR ratio. lept Statistical & Systematic uncertainties compete with LEP asymmetry measurements (unpolarized) Probes of Electroweak Symmetry Breaking at LEP and SLC

  30. Asymmetries: Measurement of ALRat SLD (II) • Condition # 1: Have a longitudinally 100% polarized electron beam. • Get longitudinally polarized e- by illuminating a GaAs photo cathode with circularly polarized Lasers (frequency: 2  60 = 120 Hz) • In principle, Pe  100% can be reached. In practice, 80% was achieved. • Change the sign of the polarization on a random basis to ensure that equal amount of data are taken with both signs, and that the luminosity is not tied to any periodic effects in SLC. • Transport, accelerate and collide the polarized electrons, with enough care to keep the same polarization at the interaction point. SLC was designed to do so from the beginning (unlike LEP). Probes of Electroweak Symmetry Breaking at LEP and SLC

  31. Asymmetries: Measurement of ALRat SLD (III) Condition # 2: Measure the e- polarization Pe with 0.5% accuracy. • Collide 45.6 GeV long. polarized e- with 2.33 eV(532 nm)circularly polarized photons every 7th bunch(17 Hz); • Detect Compton back-scattered e- as a function of their energy after a bend magnet; • s(E) =s0 [1 + A(E) PePg] s0 A(E) s0and A(E) are theoretically well known (pure QED process) Cerenkov Detectors Probes of Electroweak Symmetry Breaking at LEP and SLC

  32. Asymmetries: Measurement of ALRat SLD (IV) • Reverse on a random basis the sign of the photon polarization Pg (close to 100%, optically measured with filters); • Count the number of e- detected in each of the Cerenkov channels (about a hundred electrons per channel at each beam crossing) • Deduce the e- beam polarization from the asymmetry Probes of Electroweak Symmetry Breaking at LEP and SLC

  33. Asymmetries: Measurement of ALRat SLD (V) • Cross-checks: • Count Compton-scattered gammas at the kin. threshold with two additional calorimeters. Agree with the main measurement to 0.4%. • Measure the positron polarization to be 0.0% with an accuracy better than 0.1%. Condition # 3: Count events in SLD Complete data set: • e.g., in 1997-98: • NL = 183,355; NR = 148,259 • Pe = 72.92%  Ae = 0.1491  0.0024 (stat.)  0.0010 (syst.) Probes of Electroweak Symmetry Breaking at LEP and SLC

  34. eff Asymmetries: Results for sin2W Leptons: 0.23100.0002 ??? Quarks: 0.23230.0003 Still statistically acceptable, but a couple of add’l years at LEP and SLD would have helped… 5 10-4 ALL: 0.231520.00017 Probes of Electroweak Symmetry Breaking at LEP and SLC

  35. Asymmetries: Results for al and vl Axial and vector couplings (al, vl) from ALR (SLC) and Z  l+l-(LEP) Before LEP&SLC After LEP&SLC Errors 10  200 Precision on sin2W: 5 10-4, adequate to become sensitive to mH. Probes of Electroweak Symmetry Breaking at LEP and SLC

  36. Asymmetries: Prediction of mW PredictmW in the SM: mW2 = mZ2(1+ )cos2W eff Direct Measurements* Precision Measurements mH dependence in the SM through quantum corrections (see later) * Coming now for mW at LEP; See Y.-K. Kim lecture for mtop and mW at the Tevatron. Probes of Electroweak Symmetry Breaking at LEP and SLC

  37. W mass at LEP 2: Production and Decay s  2mW 45.6% 10.8% 43.8% - - - - W+W- q1q2ln: Two hadronic jets, One lepton, missing energy. - W+W- l1n1l2n2: Two leptons, missing energy W+W- q1q2q3q4: Four well separated jets. Probes of Electroweak Symmetry Breaking at LEP and SLC

  38. W mass at LEP 2: Threshold cross section mW (thresh.) = (80.40  0.22) GeV/c2 Probes of Electroweak Symmetry Breaking at LEP and SLC

  39. W mass at LEP 2: Direct measurement (I) 0 unknowns, 5C fit 5 Constraints: Fitted mass (GeV) • Initial State Radiation; • (change the effective beam energy) • Detector Resolution; • (change the jet directions) • Other four-fermion diagrams • (create non-Breit-Wigner bkgds) 3 unknowns, 2C fit Probes of Electroweak Symmetry Breaking at LEP and SLC

  40. W mass at LEP 2: Direct measurement (II)  Mostly rely on simulated mass distributions to determine mW 10,000 W pairs per experiment LEP Combination: mW(qqqq) = 80.448  0.043 GeV/c2 mW(qql) = 80.457  0.062 GeV/c2 Good compatibility between the two final state: Dm = 9  46 MeV/c2 Probes of Electroweak Symmetry Breaking at LEP and SLC

  41. W mass at LEP 2: Result and Uncertainties • Good consistency between experiments; • Good consistency with hadron colliders • (see Y.-K. Kim lecture); • Good consistency with Z data (LEP/SLD). mW(LEP) = 80.450  0.025  0.030 (stat.) (syst.) Limited by systematic uncertainties on final state interaction (Bose-Einstein Correlation and Colour Reconnection). Probes of Electroweak Symmetry Breaking at LEP and SLC

  42. W mass at LEP 2: Final State Interactions • Interactions between W hadronic decay products may • cause a shift between the mass from fully hadronic and • semileptonic events: • Colour Reconnection: QCD interaction between • quarks from different W’s; • Bose-Einstein Correlations between identical • hadrons (pions, kaons), well established in single • W or Z decays. q q SMALL q SMALL q Use the data to constrain them ! + = Without FSI Probes of Electroweak Symmetry Breaking at LEP and SLC

  43. Global Fit of the Standard Model to mH(I) Knowing mtop, most electroweak observables have a sensitivity to mH through Dr Probes of Electroweak Symmetry Breaking at LEP and SLC

  44. Global Fit of the Standard Model to mH(II) Global fit of mH and mtop: Probes of Electroweak Symmetry Breaking at LEP and SLC

  45. Global Fit of the Standard Model to mH(III) Internal Consistency of the Standard Model? Pull distribution = Normal Gaussian? Mean: 0.22  0.28 Sigma: 1.1  0.4 Largest discrepancy (-2.9s) well inside statistical expectation; 2 probability = 8%. Just fine. Probes of Electroweak Symmetry Breaking at LEP and SLC

  46. Global Fit of the Standard Model to mH(IV) What Next? 1st Lecture Conclusions • LEP and SLD allowed the internal consistency of the standard model to be tested with great precision; • LEP and SLD checked the predictions of the Electroweak Symmetry Breaking mechanism (e.g., mW = mZcosqW); • LEP and SLD allowed the mass of the top quark to be predicted several years before it was discovered in 1995 at the Fermi National Laboratory; • LEP and SLD measurements led to the prediction of a relatively small Higgs boson mass (around 100 GeV/c2); • Future Machines (Tevatron, LHC, LC) will be important for EW Physics. • Now: dmtop =  5.1 GeV, dmW =  34 MeV • With dmtop =  2 GeV • With dmW =  15 MeV • With dmtop =  2 GeV and dmW =  15 MeV Probes of Electroweak Symmetry Breaking at LEP and SLC

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