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

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 • gg • ZZ • Main source of background are: • mm, tt, qq, gg, ZZ, WW, Zee • 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/Zee Monte Carlo KK2F02 KoralW03 Phot02 and Pyth05 Pyth05 and KoralW03

9. Monte Carlo The signal simulation is performed using a dedicated 4-fermions final state MC generated by Koral produced to calculate the signal efficiency and MC measured 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

11. Cross-section Measurement (mm) 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 ± 0.004 pb

12. Selection Cuts For the qq final state: CLAS 16 preselection One isolated electron Qe cos qZ < -0.85 Qe cos qmiss > 0.9 Emiss/ ELEP > 0.25 Pmiss > 20 GeV All the definition cuts are also required

13. BKGND SIGNAL cos q Z0

14. BKGND SIGNAL cos q MISS

15. Isolated Electron • Only good tracks with the following requirements are considered: • Momentum p > 1GeV • d0 < 2 cm and z0 < 10 cm • At least 4 TPC hits • cos q< 0.975 • The most isolated track ID as electron is taken into account • For the electron bremadd is applied • The isolation cut on f = angle between the electron and • the closest charged track is: • cos f < 0.8 (f > 37°)

16. BKGND SIGNAL f ISO

18. Cross-section Measurement (qq) e = (22 ± 1) % Efficiency L = 683.434 pb-1Integrated Luminosity NDAT = 98 ± 10 events in the Data NBKG = 21.7 ± 2.0 estimated bkgnd  s = (NDAT - NBKGND) /e L = 0.50± 0.06 pb

19. Cross-section Measurement (qq) • (GeV) NDATA NBKGe %L(pb-1)s(pb)s-(pb)s+(pb)sMC(pb)

20. Other LEP MeasurementsL3 DELPHI • s(pb)s-(pb)s+(pb)sMC(pb)s(pb)s-(pb)s+(pb)sMC(pb)

22. Next Steps • Understand the small discrepancy at qq mass below the cut • Compare KoralW with Pythia05 for the ZZ/Zee Background • Possible review of thebackground estimation in the mm channel • Calculate the systematic errors in the qq channel