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Single Intermediate Neutral Vector Boson Production at LEP 2 Paolo Spagnolo INFN- Pisa

Single Intermediate Neutral Vector Boson Production at LEP 2 Paolo Spagnolo INFN- Pisa. Zee Diagrams. Introduction on Zee. The measurement of this cross section is an important test of the SM The process is also called the electroweak Compton scattering

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Single Intermediate Neutral Vector Boson Production at LEP 2 Paolo Spagnolo INFN- Pisa

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  1. Single Intermediate Neutral Vector Boson Production at LEP 2Paolo SpagnoloINFN-Pisa

  2. Zee Diagrams

  3. Introduction on Zee • The measurement of this cross section is an important test of the SM • The process is also called the electroweak Compton scattering • It is a mix between a Bhabha and a Z production • It is the dominant source of Z bosons at linear e+e- colliders • with E>500 GeV • At LEP however the cross-section is nearly two orders • of magnitude below the radiative return process gZ: • s(e+e- Z e+e-)~ 1 pb • The cross-section is almost independent on the LEP energy (Ds/s <5%)

  4. Zee Signal Definition only eeqq and eemmfinal states with the following cuts: M(qq) or M(mm) > 60 GeV q(e-) <12° and 12° < q(e+) < 120° with E(e+) > 3 GeV or q(e+) >168° and 60° < q(e-) < 168° with E(e-) > 3 GeV

  5. Zee Topology • The signature of the events is clear: • one electron is lost in the beam-pipe • the other electron is isolated and soft • the Z recoils against the visible electron e+/- Soft electron Z qq/mm (70% / 3%) Missing electron Beam pipe direction

  6. Zee Signal Definition • With this definition the following MC events contribute to the signal: • Zee • g*ee • ggmm < 2% in the mm final state • ZZ < 1 % .. .. .. .. .. .. .. .. .. • Main source of background are: • mm, tt, gg, ZZ, WW • All the data collected at energies between 183 and 207 • are taken into account in this analysis

  7. e+ e- e- e- Z gg qq / mm qq / mm e+ e+ Z e- e n e- W qq / mm n e+ qq e+ W g Other Diagrams

  8. Monte Carlo Generators Channel mm/tt/qq/ee WW gg ZZ/Wen/Zee Monte Carlo KK2F KoralW Phot02 Pythia05

  9. Monte Carlo limits Is known that Pythia05 doesn’t generate events around some “divergency poles” of the space-phase. For this reason an additional 4-fermions final state MC generated by Koral has been produced to compare the signal efficiency and the cross-section ( THANKS TO BRIGITTE !!! ) Koral has also the advantage to take properly into account of the interference of all diagrams contributing to the 4-f (eemm) final state The internal cuts of the Koral generator do not affect the analysis since they are beyond the cuts of signal definition

  10. Monte Carlo limits For issance: Koral cuts on the sum over the 4 fermions (PT )2 >25 (or 600) GeV2 MC Zee  (PT )2

  11. Pythia05+Phot02 s = 0.0452± 0.0001 pb e = (38± 2) % Nexp = 12± 2 signal events Monte Carlo Comparison Koralw s = 0.0423 ± 0.0002 pb e = (36± 2) % Nexp = 11 ± 2 signal events It seems that Koral generator is equivalent to Pythia+Phot02

  12. Energy of the visible electron (GeV) • Koral Pythia+Phot

  13. Energy of the invisible electron (GeV) • Koral Pythia+Phot

  14. Invariant mass of the di-muon system (GeV) • Koral Pythia+Phot Zee gg  mm ZZ

  15. cosqfor muons from Z decays Koral Pythia+Phot Muons outside the accettance could explain the slight difference in signal efficiency ….

  16. Background estimation Channel mm tt ZZ WW ggmm Total Number of events 0.83 0.158 0.548 0.242 0.09 1.868 ± 0.221

  17. Selection Cuts For the mm final state: Loose lepton ID Qe cos qZ < -0.8 Qe cos qmiss > 0.8 Nch = 3 QTOT = ± 1 NVDET> 0 for each track

  18. Lepton ID For the mm final state: Require only 2 (of 3) leptons ID At least 1 lepton ID as muon The 3rd lepton ID is deduced The charge combination has to be consistent For the electron bremadd is applied

  19. BKGND SIGNAL cos q Z0

  20. BKGND SIGNAL cos q MISS

  21. Cross-section Measurement e = (36± 2) % Efficiency L = 683.434 pb-1Integrated Luminosity NDAT = 16 ± 4 events in the Data NBKG = 1. 87 ± 0.22estimated bkgnd  s = (NDAT - NBKGND) /e L = 0.056± 0.014 pb

  22. Systematic errors Signal efficiency ± 0.003 pb Bkg level (±20%)± 0.002 pb Integrated Luminosity ± 0.0006 pb Lepton Id. efficiency < 0.001 pb  s = (NDAT - NBKGND) /e L = 0.056± 0.014 ± 0.004 pb

  23. LEP AVERAGE ALEPH 0.056± 0.014 ± 0.004 pb DELPHI0.070± 0.016 ± 0.003 pb AVERAGE 0.063 ± 0.011 pb THEORY 0.044 ± 0.002 pb

  24. Next Step • Perform the analysis for the qq final state • A kinematic fit will be probably necessary • An increase of a factor 20 in satistics is expected

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