9 April 2009Nijmegen Colloquium1 Precision electroweak measurements at the LEP e + e – collider Jan Timmermans / NIKHEF Amsterdam

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9 April 2009Nijmegen Colloquium4 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 searches for new leptons, quarks 3 and 4 jet structures, scaling violations 1983: C=27 km; start construction

9 April 2009Nijmegen Colloquium5 Particles in the Standard Model Fermions (matter particles) leptons quarks 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

9 April 2009Nijmegen Colloquium6 Some events at LEP1 e + e –  2-fermions Z   +  – Z  e + e –

9 April 2009Nijmegen Colloquium7 Z   +  – Z  q q

9 April 2009Nijmegen Colloquium8 3 jets: Z  q q g (2 quarks and gluon)

9 April 2009Nijmegen Colloquium9 And some events at LEP2 e + e –  4-fermions The first WW event! 1996:  s = 161 GeV e + e –  W + W –  q 1 q 2 q 3 q 4

9 April 2009Nijmegen Colloquium10 e + e –  ZZ   +  – qqe + e –  Z *  *   +  – e + e –

9 April 2009Nijmegen Colloquium11  events/experiment:  4.5 M Z events  WW events  600 ZZ events  250 single-W events  LEP1 ( ): ~200 pb -1 /exp LEP2 ( ): ~700 pb -1 /exp

9 April 2009Nijmegen Colloquium12 Z line shape and asymmetries e+e+ e-e- f f Z e+e+ e-e- f f  pole cross-section: partial width: total width: vector and axial- vector couplings

9 April 2009Nijmegen Colloquium13 Forward-backward asymmetry: At the Z pole: Measure lepton partial width  ll and F/B asymmetry A FB gives the couplings g Vl and g Al (for l=e, ,  ) Effective weak mixing angle: Effective couplings contain radiative corrections, which depend on the top quark mass and Higgs boson mass

9 April 2009Nijmegen Colloquium14 Some examples of radiative corrections: e+e+ e-e- b b  /Z/Z t t W e+e+ e-e- f f Z H e+e+ e-e- f f t t Higgs loop top quark loop W loop

9 April 2009Nijmegen Colloquium15 Z line shape Moriond 1990: m Z = ± 12 ± 32 (incl.  E LEP ) MeV  Z = 2538 ± 26 ± 28 MeV N = 3.04 ± 0.12 first evidence 3 generations Now: m Z = ± 2.1 MeV  Z = ± 2.3 MeV N = ±

9 April 2009Nijmegen Colloquium16 Beam energy precision  0.2 MeV from resonant depolarisation But corrections needed due to: TIDES LEVEL LAKE TGV

9 April 2009Nijmegen Colloquium17 Axial and Vector couplings for leptons Measured from partial widths  ll,, forward-backward asymmetries A FB 0,l and τ polarisation g Vf / g Af = 1 – 4 | Q f | sin 2  eff f Couplings and lepton-universality established at 1 per-mille (g Al ) and few % (g Vl )

9 April 2009Nijmegen Colloquium18 Vertex Detectors: Precise  -lepton & b-quark studies Here DELPHI LEP2 version

9 April 2009Nijmegen Colloquium19 R b =  b /  had and top quark mass Summer 1992:  b = 373  9 MeV  I 3 b = -1/2 (370 MeV; for I 3 b =0: 24 MeV)  Top quark must exist R b is also sensitive to top quark mass, but much less than sin 2  eff ! lept

9 April 2009Nijmegen Colloquium20 Top mass: predicted by LEP Prediction m top from EW fit EPS93 Marseille: m top = 166 – GeV ICHEP94 Glasgow: m top = GeV and CDF saw excess due to top at m top = GeV EPS95 Brussels: both CDF and D0 observed top at predicted mass Great success for the Standard Model

9 April 2009Nijmegen Colloquium21 Two most precise sin 2  eff values from SLD A LR and LEP A 0,b differ by 2.9  lept FB SM: A 0,b (and also A 0,c ) prefer a high value for the Higgs boson mass FB A puzzle left ….. : comparison sin 2  eff lept

9 April 2009Nijmegen Colloquium22  lepton: a puzzle solved  lifetime related to  lifetime,  mass,  mass and leptonic  branching ratio Up to 1992 a  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

9 April 2009Nijmegen Colloquium23 only exchange No ZWW vertex WW cross section Clear evidence for the SU(2) x U(1) Gauge structure Exp. Errors 1-3% / energy point CC03 diagrams: Theory predictions include full O(  em ) corrections; theory error  0.5%

9 April 2009Nijmegen Colloquium24 Single W WW  DLO Quartic Gauge Couplings

9 April 2009Nijmegen Colloquium25 W mass measurement M w 2  1 – ––––   ––––  1 –  r  Mw2Mw2 MZ2MZ2  GF 2GF 2 WW t b W WW H  M w  ln M H  M w  M t 2 W mass defined by relativistic Breit-Wigner lineshape of the W propagator with s-dependent width   M top  M W Direct 1.3 GeV25 MeV Indirect~10 GeV32 MeV precision M W measurement 0.04% gives handle on Higgs mass

9 April 2009Nijmegen Colloquium26 W mass W masses reconstructed directly from the decay products Constrained fits improve mass resolution from  8 GeV to  3 GeV before 4C fit after 4C fit qqqq qq  qq  qqe

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: ± GeV

9 April 2009Nijmegen Colloquium28 We searched….. and searched….. No Higgses, 4 th -generation, sleptons, squarks, charginos, neutralinos.... Large part of the MSSM parameter space excluded

9 April 2009Nijmegen Colloquium29 SM Higgs search Higgs strahlung: dominantWW fusion: small (can go beyond ‘kinematical limit’) Branching ratios (m H =115 GeV): H  bb (74%) H   (8%) H  WW (8%) 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

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9 April 2009Nijmegen Colloquium31 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)

9 April 2009Nijmegen Colloquium32 CL s+b 1-CL b Likelihood test: sig+bkg  bkg 1-CL b measures incompatibility with bkg CL s+b a measure of compatiblity with s+b

9 April 2009Nijmegen Colloquium33 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 CL s = CL s+b / CL b when CL s < 0.05 the hypothesis is rejected at 95% CL  M H > GeV (115.3 expected) FINAL 2003

9 April 2009Nijmegen Colloquium34 Global electroweak fit Results of different fits: using only Z pole data using all data

9 April 2009Nijmegen Colloquium35 Very good consistency between direct and indirect measurement of m t and m W Both prefer low Higgs boson mass m H < 163 GeV at 95% CL (Tevatron excludes GeV)

9 April 2009Nijmegen Colloquium36 Conclusions  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 A b FB (LEP) and A LR (SLD) at ~3  but much less w.r.t. average in terms of sin 2  eff !  SM Higgs boson not yet found: M H > GeV (and M H outside 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

9 April 2009Nijmegen Colloquium37 Backup slides

9 April 2009Nijmegen Colloquium38 We searched….. and searched….. But sometimes…… No Higgses, 4 th -generation, sleptons, squarks, charginos, neutralinos.... Large part of the MSSM parameter space excluded

9 April 2009Nijmegen Colloquium : 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% ALEPH Why it is good to have more than one expt.

9 April 2009Nijmegen Colloquium40 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 C F, C A, T F T F / C F

9 April 2009Nijmegen Colloquium41 QCD: running of  s Results from fits to different event shape variables. Combined fit at all energies:  s (M Z ) =    (stat) (exp.syst) (theor.hadr.) (theor.ev. shapes)