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What we have learned from LEP and SLC?

Precision tests of electroweak interactions-. What we have learned from LEP and SLC?. Krzysztof Doroba, Warsaw University & DELPHI Collaboration. XXVIII Mazurian Lakes Conference on Physics, Aug 31 – Sep 7 2003. Outline of the talk:.

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What we have learned from LEP and SLC?

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  1. Precision tests of electroweak interactions- What we have learned from LEP and SLC? Krzysztof Doroba, Warsaw University & DELPHI Collaboration XXVIII Mazurian Lakes Conference on Physics, Aug 31 – Sep 7 2003

  2. Outline of the talk: • Strategy of the Standard Model tests • Radiative corrections • LEP/SLC and detectors • Z0 line shape • Z0 decays to heavy quarks • Asymmetries at the Z0 pole • Direct W mass measurement • Direct Higgs search • Global fit • Conclusions from the tests

  3. Strategy of the test. Minimal Standard Model (MSM) describes electroweak interactions of quarks (q), leptons (l) and Higgs boson(s) (h) by exchange of W first step: build LEP1 (SLC) collider at (with possible electron beam polarization at SLAC) second step: increase the energy to (LEP only) • Study W and Z production • Check model internal consistency • Look for Higgs boson(s) and • supersymetric particles and

  4. Input parameters of Minimal Standard Model (MSM) -electromagnetic fine structure constant -Fermi constant- determines charged current strength - Z0 boson mass, measured at LEP with high precision - strong coupling constant at (for quarks in final state) above parameters are sufficient to perform MSM calculations on the tree level. However due to high precision of the LEP/SLC measure- ments tree level is not sufficient and radiative corrections are required. This brings into the game more parameters: - fermion masses (mt) - Higgs boson mass

  5. Radiative corrections Pure QED corrections factorize from electroweak part QED: + .......... Electroweak part: Vacuum polarization Vertex correction

  6. This leads to improved Born approximation; the improved amplitude for the process has same form as Born amplitude for this process but with effective coupling constants: • The electroweak corrections dependence is: • quadratic on top quark mass • logarithmic on Higgs boson mass For electroweak corrections two loop level is achieved today for most of the processes. Numerical calculations are performed using the programmes TOPAZ0 and ZFITTER.

  7. LEP and detectors • Large Electron Positon collider • 27 km circumference • peak luminosity L=2.*1031cm-2s-1 (design value 1.6*1031) • maximum energy 208 GeV • beam energy known with precision of about 2 MeV (at Z0 peak) • To operate LEP special „LEP standard model” took into account • earth tides generated by moon and sun • rainfalls in Jura • Lake Geneva water level • leakage currents from trains Four experiments have been operating at LEP (ALEPH, DELPHI,L3 and OPAL). At Z0 peak ADLO collected about 17 M events.

  8. LEP I running at Z0 peak quark and antiquark fragment into two separete jets

  9. LEP II running at four jets in the final state

  10. SLAC Linear Collider • SLC, the first linear e+e- collider ever • operated with good luminosity and polarization from 1992 till 1998 • had worse then LEP beam energy resolution • run only at Z0 peak (600 k events) • But... • its electron beam was longitudinally polarized • its beam spot was much smaller (1.5μm*.7μm vs. 150μm*5μm) The designs of LEP and SLC detectors are quite similar. But, • for example, due to • lower repetition rate • smaller beam spot Slac Linear Detector (SLD) had better vertex reconstructiom (CCD vs. micro-strip)

  11. X-section formula at Z0 peak: calculated from SM, not fitted H(s,s’)-radiative function Fit performed to the hadron data: MZ, ΓZ, σ0had, Rl and to the lepton data: Γe, Γμ, Γτ, or (lepton universality) Γlept

  12. Values of Mz,Гz,Гμ,Гτ,Гe,Rl,... extracted with use of SM elements Observables  Pseudo-observables ADLO results (with lepton universality) SM expresion for

  13. The number of light neutrino families depends strongly on Predicted cross-section for two, three and four (massless) neutrino species with SM couplings

  14. Z0 decays to heavy quarks (charm and beauty) • two (or more) jets are formed in process, following the quark fragmentation into hadrons. • in the final state we observe hadrons, not quarks. How to select • Z0 decays into particular flavour Flavour tagging: • jet (initial quark) direction is • established from thrust axis. • heavy flavours tagged by leptons • (high p,pT), lifetime, secondary • vertex mass,.... Works well for b and c quarks. thanks to vertex detectors: b hadron on average travels 3 mm, position of the secondary vertex is measured with accuracy of 300 μm.

  15. secondary vertex mass and/or high p, pT allows to distinguish between b anc c hadrons. Different methods use different tags combinations to establish flavour of the initial (heavy) quark . • For tagged sample one has to know: • purity (up to 96%) • efficiency (up to 26%) usually requires very good Monte-Carlo program Most precise – double tag method Pseudo-observables: Most recent values: EPS Aachen 2003

  16. Asymmetries at Z0 pole Z0 couplings to right-handed and left-handed fermions are different. for even for unpolarized e beams Z0 is polarized along beam direction (LEP) forward (F) – e- beam direction. R (L) means right (left) handed fermions in final state For polarized electron beam (SLC): r(l) means right (left) handed electron beam polarization. <P> - mean beam polarization

  17. asymmetry parameter for fermion f At the Z0 pole: When the couplings conform to the SM structure: Studies of asymmetry parameters provide very sensitive measurement of the ,particulary good for Particulary cute- ALR at SLAC precise, direct measurement of Ae with hadron events

  18. Another precise measurements: EPS, Aachen 2003 combined Standard Model LEP SLC vs. LEP and SLAC measurements of Ab are consistent. But the combined Ab value seems to disagree with SM prediction. LEP Ab (and Ac ) result can be expresed in terms of

  19. Direct W mass and width measurement. From CDF and D0 experiments at 1 Tev proton antiproton collider at Fermilab: From direct measurements at LEP 2: cross section for process at the treshold (161 GeV) • study of decay channels: or • important corrections coming from: • Bose-Einstein correlations • color reconnection LEP 2 result:

  20. Very good agreement between electron and hadron colliders! Combined result: But NuTeV experiment measures from the ratio of the neutral to charged current interactions in and beams: Using MZ from LEP I This indirect measurement differs more then 3σ from direct one !

  21. Standard Model Higgs Search The production (and decay) of Higgs particle is predicted in the SM as a function of its (unknown) mass. main production channel Background: WW,ZZ,2f ZH decay channels For mH=115 GeV b-tagging plays essential role in Higgs search!

  22. At LEP I serches in fully hadronic channels excluded by background LEP I serches in other channels - negative At LEP II main sources of background in Higgs search: • Selection of Higgs candidate events: on the Monte-Carlo basis • topology • btag

  23. Does the data sample contains signal and background or only background ? • for each candidate i introduce the likelihoods • ratio: • Qi is estimated from topology combined with • mass information. • MC determines expected Qi distributions • the global likelihood: s and s+b equally likely for -2ln(Q)=0

  24. ADLO result by M.Duehrssen, EPS, Aachen 2003 green and yellow bands indicate 1σ and 2σ limits of backround only hypotesis. Conclusion from further statistical analysis: mH<114.4 GeV is excluded @ 95% CL

  25. The Global Fit Fit of the five Standard Model parameters to all available electroweak results. Runing coupling constant -from dispersion integral and low energy e+e- data. • The purpose of the fit • check internal consistency of the Standard Model • constrain the Higgs mass Some fit results already presented:

  26. If in the global fit replace 6 parameters with value fitted to the above parameters then for global fit -probability=13% -very precise measurement at low <Q2>~20 GeV2 3 σ from Standard Model prediction ! Removing from fit changes χ2 probability (to 28%) but does not influence SM parameters values much. Global EW fit with average and without problem remains... OK. for global fit but

  27. Conclusions from the tests • this talk is ,by all means, not exhaustive. Supersymmetry, • Grand Unification, Multi doublet Higgs Models, MSSM, TGC,... • were left behind. • precision (above tree level) predictions of the Standard Model • have been compared with experimental results from LEP and • SLC. • Standard Model looks fine after that comparison. SM is a well • established (effective) theory. • no need for New Physics. • where is (if at all) the Higgs boson(s)? • further measurements of MW, mt, (mH? .....) will make tests • more stringent and perhaps will show the road to New Physics. • Tools: Tevatron (Run II) + ......... • Large Hadron Collider (2007) • Next Linear Collider

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