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9 April 2009Nijmegen Colloquium1 Precision electroweak measurements at the LEP e + e – collider Jan Timmermans / NIKHEF Amsterdam
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9 April 2009Nijmegen Colloquium2
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9 April 2009Nijmegen Colloquium3
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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
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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
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9 April 2009Nijmegen Colloquium6 Some events at LEP1 e + e – 2-fermions Z + – Z e + e –
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9 April 2009Nijmegen Colloquium7 Z + – Z q q
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9 April 2009Nijmegen Colloquium8 3 jets: Z q q g (2 quarks and gluon)
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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
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9 April 2009Nijmegen Colloquium10 e + e – ZZ + – qqe + e – Z * * + – e + e –
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9 April 2009Nijmegen Colloquium11 events/experiment: 4.5 M Z events 10000 WW events 600 ZZ events 250 single-W events LEP1 (1989-1995): ~200 pb -1 /exp LEP2 (1996-2000): ~700 pb -1 /exp
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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
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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
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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
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9 April 2009Nijmegen Colloquium15 Z line shape Moriond 1990: m Z = 91171 ± 12 ± 32 (incl. E LEP ) MeV Z = 2538 ± 26 ± 28 MeV N = 3.04 ± 0.12 first evidence 3 generations Now: m Z = 91187.5 ± 2.1 MeV Z = 2495.2 ± 2.3 MeV N = 2.9841 ± 0.0083
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9 April 2009Nijmegen Colloquium16 Beam energy precision 0.2 MeV from resonant depolarisation But corrections needed due to: TIDES LEVEL LAKE TGV
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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 )
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9 April 2009Nijmegen Colloquium18 Vertex Detectors: Precise -lepton & b-quark studies Here DELPHI LEP2 version
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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
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9 April 2009Nijmegen Colloquium20 Top mass: predicted by LEP Prediction m top from EW fit EPS93 Marseille: m top = 166 –19 -22 GeV ICHEP94 Glasgow: m top = 178 -11 -19 GeV and CDF saw excess due to top at m top = 174 -10 -23 GeV EPS95 Brussels: both CDF and D0 observed top at predicted mass. +17 +19 +11 +18 +10 +13 Great success for the Standard Model
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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: 0.1036 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
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9 April 2009Nijmegen Colloquium22 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
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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%
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9 April 2009Nijmegen Colloquium24 Single W WW DLO Quartic Gauge Couplings
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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
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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
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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
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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
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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 Colloquium30
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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)
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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
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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 > 114.4 GeV (115.3 expected) FINAL 2003
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9 April 2009Nijmegen Colloquium34 Global electroweak fit Results of different fits: using only Z pole data using all data
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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 160-170 GeV)
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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 > 114.4 GeV (and M H 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
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9 April 2009Nijmegen Colloquium37 Backup slides
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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
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9 April 2009Nijmegen Colloquium39 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% ALEPH Why it is good to have more than one expt.
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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
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9 April 2009Nijmegen Colloquium41 QCD: running of s Results from fits to different event shape variables. Combined fit at all energies: s (M Z ) = 0.1201 0.0003 0.0009 0.0047 (stat) (exp.syst) (theor.hadr.) (theor.ev. shapes)
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