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Single boson production at LEP

Single boson production at LEP. Guillaum e Leibenguth On behalf of the LEP experiments Institut de physique nucl éaire Universit é catholique de Louvain DIS’04, Štrbské Pleso. Two Channels. W exchange: Test of the Standard Model: IR Study of the trilinear gauge coupling

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Single boson production at LEP

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  1. Single boson production at LEP Guillaume Leibenguth On behalf of the LEP experiments Institut de physique nucléaire Université catholique de Louvain DIS’04, Štrbské Pleso

  2. Two Channels • W exchange: • Test of the Standard Model: IR • Study of the trilinear gauge coupling • Measurement of W mass at the next linear collider • Z/ exchange: • Electroweak Compton scattering process • Cross section refers to 48 Feynman Diagrams Diagrams involving Higgs boson are neglected Single Boson Production at LEP

  3. Single W Production: Context W pair production 96 Charged or Neutral Currents Single W production: fermion-antifermion comes from the W Boson, t-channel diagram Single Boson Production at LEP

  4. Single W production is in Yellow! Data taken between 161 and 209 GeV qq+isr: photon emitted along the beam pipe qq: little missing energy W pair: 4q: no missing energy, lqq: high energetic lepton visible fully leptonic: 2 leptons alone! Z pair: irreducible bkg ee single W boson (signal We!) Zee (second part of the talk) And Difficulties… Single Boson Production at LEP

  5. Signal Definition L3 (single W) Generated phase space = SIGNAL: • |cos e+ | > 0.997 • Min (Ef, Ef’) > 15 GeV • for ee only: |cos e- | < 0.75 for Z ()ee rejection Analysis relies on: • Two fermions and Pmiss MC: e+qq’ Angular acceptance of LEP detectors e+in the beam pipe Single Boson Production at LEP

  6. Hadronic channel Two acoplanar jets Ee.m > 15 GeV 0.3 < Evis/s < 0.65 ET > 0.2 Evis |cos miss | < 0.92 Acoplanarity > 11º No high Elepton Anti ZZ cut Neural Network (NN) Leptonic channel High energetic lepton No other activity in the detector L3 Single W Analysis Single Boson Production at LEP

  7. Signal Definition Aleph Generated phase space requirement: • |cos e | > 0.9994 • Mqq’ > 60 GeV/c² for eqq’ OR • |cos l | < 0.95 and El > 20 GeV for el Monte Carlo signal : Grc4f A candidate: Single Boson Production at LEP

  8. Hadronic channel |cos miss | < 0.90 Acollinearity between the two hemisphere momentum < 165º Energy of a  wedge of 30º centered on missing momentum direction Energy in a cone of 12º (E12) around the beam pipe < 0.025 s Invariant hadronic mass < 60 GeV/c² Neural Network High energetic isolated lepton No E12 energy, missing momentum unbalance ALEPH Single W Analysis Leptonic channel Single Boson Production at LEP

  9. Signal Definition Opal-Delphi Graph content (and not generated phase space): • t-channel diagram only • Mqq > 45 GeV/c2 • |cos e- | < 0.95, El+ > 20 GeV and |cos e+ | > 0.95 for ee Monte Carlo Signal: Wphact (Delphi), KoralW, Grc4f, Excalibur (Opal) MC Background: 2 fermions: KK2f (Delphi) KoralZ (Opal), e+e- : TEEGG & BHWIDE,  PYTHIA 6.143 & BDK (Delphi), Pythia, Herwig, Phojet, twogen, Phot02 (Opal) The LEP experiments have agreed to use this definition as LEP wide. Single Boson Production at LEP

  10. Hadronic selection Ee.m/s < 50 % Evis/s > 20 % ET > 0.2 Evis |cos miss | < 0.98 Acoplanarity > 10 deg No E.M Cluster > 45 GeV Multiplicity < 50 30 <Mvis < 110 GeV/c2 E(e,) < 7.5 (10) s % Neural Network Leptonic channel One Lepton, El (Et) /s< 45 (12)% | IPR | < 0.2 cm for muon DELPHI single W analysis Single Boson Production at LEP

  11. Hadronic channel Two non collinear jets recoiling against the neutrino in the transverse plane 120 > Mqq > 40 GeV/c² 15 % < Ee.m/s < 50 % WW  eqq removed No lepton isolated |cos miss | < 0.96 Acoplanarity > 7.5 º Likelihood selection Leptonic selection very sophisticated OPAL Single W Analysis ee e Single Boson Production at LEP

  12. Take the Delphi-Opal definition (rescale the phase space cut L3, Aleph to it) Correlation between uncertainties are taken into account Only a preliminary analysis of 189 GeV data from OPAL Systematic errors are treated in 4 different ways! Several sources, but a few really contribute: Limited amount of MC stat Background estimate Luminosity For the muon channel, triggering issue (ALEPH and L3) 5 % accuracy of signal modelling Combining … Single Boson Production at LEP

  13. Accuracy of SM prediction Good agreement in all experiment between the expected and the observed number of events. Luminosity used by Aleph : 683.4 pb-1 Delphi : 619.3 pb-1 L3 : 675.5 pb-1 Opal : 174.7 pb-1 Single Boson Production at LEP

  14. Comparison Grc4f-Wphact Single Boson Production at LEP

  15. Sensitivity to  Assuming SU(2)xU(1) symmetry -> 3 couplings 0.90 <  < 1.32 (L3) The sensitivity to z is weaker. For a more complete discussion on TGC, see U. Parzelfall’s talk Single Boson Production at LEP

  16. Z/ exchange Main diagrams contributing to the “ single Z ” production Single Boson Production at LEP

  17. DELPHI Zee Analysis Requirement on the phase space (COMMON LEP Definition!) • un < 12 º, 12 º < sc<120º and Esc > 3 GeV • Mqq or M > 60 GeV/c2 Analysis based on: • A loose preselection • Identification of an isolated electron • Cut on Mqq or M Single Boson Production at LEP

  18. Hadronic channel A loose preselection Identification of an isolated electron with E> 3GeV cos e < 0.8 Qe cos z > -0.85 Qz cos miss > 0.9 Mhad > 50 GeV/c² Emiss / s > 25 % Leptonic channel Three Leptons Charge = +/- 1 ALEPH Zee Analysis Single Boson Production at LEP

  19. Hadronic channel An electron identified in the BGO with an energy of 3 GeV 50<Mhad < 130 GeV Qe cos z > -0.5 Qz cos miss > 0.94 Two jets (Durham) Opening angle of the two jets > 150 55 < M < 145 GeV/c² -0.5 < q cos  < 0.93, q cos miss > 0.5 Q cos z < 0.40 L3 Zee Analysis : Single Boson Production at LEP

  20. OPAL Zee Analysis Eqq Hadronic channel • Two jets • EM. Cluster larger than 1 GeV • dE/dx consistent with electron • Qe cos miss  0 • Mqq’ > 25 GeV/c² +Kin. Fit. • Eem/ s < 0.93 • |Pmiss| > 35 GeV/c • Isolation of the electron: nothing in 0.6 rad cone • Qe cos miss  0.95, Emax in forward detector < 30 GeV • - Qe cos e  0.65 or Ee  0.35 E Mqq Ee Single Boson Production at LEP

  21. Comparison Grc4f -Wphact Single Boson Production at LEP

  22. Conclusions • Standard model is in a very good shape • Even when these difficult corner of phase space are studied and tested to a 5% accuracy So • L3 has already published its final results • ALEPH and DELPHI are really close to finalizing their publications • OPAL has only analyzed 189 GeV data set • Final LEP word on this issue hopefully in summer 2004 And • The single W cross section is rising! These channels are of importance in view of the next linear collider Single Boson Production at LEP

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