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Precision electroweak measurements at the LEP e + e – collider Jan Timmermans / NIKHEF Amsterdam

Precision electroweak measurements at the LEP e + e – collider Jan Timmermans / NIKHEF Amsterdam. First study B. Richter 1976: C~43 km, E=2*100 GeV, L=10 32 cm -2 s -1 , 8 ip’s Les Houches study 1979: study of Z prod. and decay study of WW production Higgs search

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Precision electroweak measurements at the LEP e + e – collider Jan Timmermans / NIKHEF Amsterdam

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  1. Precision electroweak measurements at the LEP e+e– colliderJan Timmermans / NIKHEF Amsterdam Nijmegen Colloquium

  2. Nijmegen Colloquium

  3. Nijmegen Colloquium

  4. First study B. Richter 1976: C~43 km, E=2*100 GeV, L=1032 cm-2s-1, 8 ip’s • Les Houches study 1979: • study of Z prod. and decay • study of WW production • Higgs search • searches for new leptons, quarks • 3 and 4 jet structures, scaling • violations • 1983: C=27 km; start construction Nijmegen Colloquium

  5. Gauge bosons • , Z, W± (electroweak interaction) • gluons g (strong interaction) • Higgs boson H (resulting from EW symmetry breaking) • E.g. decays: Z +- , Z  bb , W+  e+e , W+  cs Particles in the Standard Model • Fermions (matter particles) leptons quarks Nijmegen Colloquium

  6. Some events at LEP1 e+ e–  2-fermions Z  + – Z  e+ e– Nijmegen Colloquium

  7. Z  q q Z  + – Nijmegen Colloquium

  8. 3 jets: Z  q q g (2 quarks and gluon) Nijmegen Colloquium

  9. e+ e–  W+W– q1q2q3q4 And some events at LEP2 e+ e–  4-fermions The first WW event! 1996: s = 161 GeV Nijmegen Colloquium

  10. e+ e–  ZZ+–qq e+ e–  Z**+– e+e– Nijmegen Colloquium

  11. LEP1 (1989-1995): ~200 pb-1/exp LEP2 (1996-2000): ~700 pb-1/exp • events/experiment:  4.5 M Z events •  10000 WW events •  600 ZZ events •  250 single-W events Nijmegen Colloquium

  12. e+ e+ Z  f f e- e- f f Z line shape and asymmetries pole cross-section: vector and axial-vector couplings partial width: total width: Nijmegen Colloquium

  13. Measure lepton partial width ll and F/B asymmetry AFB gives the couplings gVl and gAl (for l=e, , ) • Effective weak mixing angle: • Effective couplings contain radiative corrections, which depend on the top quark mass and Higgs boson mass Forward-backward asymmetry: At the Z pole: Nijmegen Colloquium

  14. e+ e+ e+  t f e- b f e- e- b f f Z t t H Some examples of radiative corrections: top quark loop W loop W t /Z Higgs loop Nijmegen Colloquium

  15. Z line shape Moriond 1990: mZ = 91171 ± 12 ± 32 (incl. ELEP) MeV Z = 2538 ± 26 ± 28 MeV N = 3.04 ± 0.12 first evidence 3 generations Now: mZ = 91187.5 ± 2.1 MeV Z = 2495.2 ± 2.3 MeV N = 2.9841 ± 0.0083 Nijmegen Colloquium

  16. Beam energy precision 0.2 MeV from resonant depolarisation But corrections needed due to: TIDES LEVEL LAKE TGV Nijmegen Colloquium

  17. gVf / gAf = 1 – 4 | Qf | sin2eff f Axial and Vector couplings for leptons Measured from partial widths ll,, forward-backward asymmetries AFB0,l and τ polarisation Couplings and lepton-universality established at 1 per-mille (gAl) and few % (gVl) Nijmegen Colloquium

  18. Vertex Detectors: Precise -lepton & b-quark studies Here DELPHI LEP2 version Nijmegen Colloquium

  19. Rb is also sensitive to top quark mass, but much less than sin2eff ! lept Rb = b / had and top quark mass • Summer 1992: • b = 3739 MeV  I3b = -1/2 • (370 MeV; for I3b =0: 24 MeV) • Top quark must exist Nijmegen Colloquium

  20. Prediction mtop from EW fit EPS93 Marseille: mtop = 166 –19 -22 GeV ICHEP94 Glasgow: mtop = 178 -11 -19 GeV and CDF saw excess due to top at mtop = 174 -10 -23 GeV EPS95 Brussels: both CDF and D0 observed top at predicted mass. +17 +19 +11 +18 +10 +13 Top mass: predicted by LEP Great success for the Standard Model Nijmegen Colloquium

  21. lept A puzzle left ….. : comparison sin2eff Two most precise sin2eff values from SLD ALR and LEP A0,b differ by 2.9 lept A0,b (and also A0,c ) prefer a high value for the Higgs boson mass FB FB FB SM: 0.1036 Nijmegen Colloquium

  22.  lepton: a puzzle solved • lifetime related to  lifetime,  mass,  mass and leptonic  branching ratio Up to 1992 a 2-2.3  discrepancy between the coupling constants g and g In 1992 a new, more precise  mass by BES Now - -e universality at few per-mille, thanks to high-precision Si vertex detectors Nijmegen Colloquium

  23. WW cross section CC03 diagrams: only  exchange No ZWW vertex Theory predictions include full O(aem) corrections; theory error  0.5% Clear evidence for the SU(2) x U(1) Gauge structure Exp. Errors 1-3% / energy point Nijmegen Colloquium

  24. DLO Single W WW Quartic Gauge Couplings Nijmegen Colloquium

  25. Mw2  Mw21 – ––––   –––– 1 – r  MZ2 GF2 t W W  Mw  Mt2 b H W W W  Mw  ln MH W mass measurement W mass defined by relativistic Breit-Wigner lineshape of the W propagator with s-dependent width • precision MW measurement 0.04% • gives handle on Higgs mass Nijmegen Colloquium

  26. qq qqqq before 4C fit qqe qq after 4C fit W mass • W masses reconstructed directly from the decay products • Constrained fits improve mass resolution from 8 GeV to 3 GeV Nijmegen Colloquium

  27. W mass: LEP (prel.) combined results • Single measurements are FINAL • Working on combination (systematics) * Apart from NuTeV result (from rates of CC and NC (anti-) scattering) very good agreement between direct and indirect measurements *2008 Tevatron average; now D0 alone: 80.401 ± 0.044 GeV

  28. No Higgses, 4th-generation, sleptons, squarks, charginos, neutralinos.... Large part of the MSSM parameter space excluded We searched….. and searched….. and searched….. and searched….. and searched….. Nijmegen Colloquium

  29. SM Higgs search • Branching ratios (mH=115 GeV): • H  bb (74%) • H   (8%) • H  WW (8%) Higgs strahlung: dominant WW fusion: small (can go beyond ‘kinematical limit’) • Final states: • bbqq (4-jet channel) • bb (missing energy channel) • bbee, bb (leptonic channel) • bb, bb (tau channel) • Selections: cut based or NN • select multi-hadronic events • look for b-tagged jets • lepton identification • apply constraint fits + sometimes W/Z mass constraints Nijmegen Colloquium

  30. Nijmegen Colloquium

  31. Mass distributions: • But not only mass information useful! • Combine all information in a discriminating variable (event likelihood or NN output) • 2-dim inputs to statistical analysis: • - invariant mass of bb jet combination • - discriminating variable (containing b-tags, kinematics, jet-properties) Nijmegen Colloquium

  32. CLs+b 1-CLb Likelihood test: sig+bkg  bkg • 1-CLb measures incompatibility with bkg • CLs+b a measure of compatiblity with s+b Nijmegen Colloquium

  33. FINAL 2003 • Small excess around 116 GeV but less than 2 (mainly coming from ALEPH candidates and from four-jet events) • Excess was 2.9 on 3/11/2000 • CLs = CLs+b / CLb • when CLs < 0.05 the hypothesis is rejected at 95% CL •  MH > 114.4 GeV (115.3 expected) Nijmegen Colloquium

  34. Global electroweak fit • Results of different fits: • using only Z pole data • using all data Nijmegen Colloquium

  35. Very good consistency between direct and indirect measurement of mt and mW • Both prefer low Higgs boson mass mH < 163 GeV at 95% CL (Tevatron excludes 160-170 GeV) Nijmegen Colloquium

  36. LEP was great! • Full gauge structure of the SM has been measured, and many measurements in good agreement with the SM prediction • Largest discrepancy between AbFB (LEP) and ALR (SLD) at ~3 but much less w.r.t. average in terms of sin2eff ! • SM Higgs boson not yet found: MH > 114.4 GeV (and MH outside 160-170 GeV window from Tevatron) A task for the Tevatron and Large Hadron Collider experiments lept • total output: ~ 300 journal papers/experiment • PhD theses in NL: ~50 (L3 + DELPHI) , ~half of which RUN Conclusions Nijmegen Colloquium

  37. Backup slides Nijmegen Colloquium

  38. No Higgses, 4th-generation, sleptons, squarks, charginos, neutralinos.... Large part of the MSSM parameter space excluded We searched….. and searched….. and searched….. and searched….. and searched….. But sometimes…… Nijmegen Colloquium

  39. ALEPH Why it is good to have more than one expt. • 1995: 130-136 GeV • 4-jet events • Sum of di-jet masses with smallest M • 16 events (8.3 exptd) • Prob. accumulation in 6.3 GeV bin: 0.01% Nijmegen Colloquium

  40. TF / CF QCD: test of gluon selfcoupling From fit to angular distributions in 4-jet events. Rel. strength of couplings qqg, ggg, gqq depends on gauge group through Casimir factors CF , CA , TF Nijmegen Colloquium

  41. QCD: running of s Results from fits to different event shape variables. Combined fit at all energies: s(MZ) = 0.1201  0.0003  0.0009  0.0009  0.0047 (stat) (exp.syst) (theor.hadr.) (theor.ev.shapes) Nijmegen Colloquium

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