W Physics at LEP

1. W Physics at LEP Paolo Azzurri Pisa (ALEPH-CMS) Hanoi August 7, 2004 5th Rencontres du Vietnam New Views in Particle Physics

2. Outline • W-pair production at LEP2 • W Gauge Couplings • W Decay Couplings • Other four fermion processes • W mass P.Azzurri - W Physics at LEP

3. Introduction The study of W-pair production at LEP2 was a nice and exciting work that provided: • a clear evidence of the presence of Standard Model triple gauge couplings and of their SU(2)U(1) structure • the first direct measurements of the W decay branching ratios, and from these: • a direct test of the W lepton family and lepton-quark universality to 1% and 0.5% • a test of CKM unitarity and best current determination of the Vcs amplitude • an independent W mass measurement at production threshold (200 MeV accuracy) • a direct measurement of the W mass and width with 40 MeV and 90 MeV accuracy P.Azzurri - W Physics at LEP

4. W-pair production at LEP2 P.Azzurri - W Physics at LEP

5. LEP2 produced about 10000 WW events/exp. with ~700pb-1/exp. Expect: per experiment. W-pair production CC03 diagrams (not gauge invariant) P.Azzurri - W Physics at LEP

6. … sophisticated selections Channel efficiency purity bkg ll 50-80% 80-90% ,,ll eqq 75-90% ~90% qq,Zee qq 75-90% ~95% qq qq 50-80% 80-85% qq,We qqqq 80-90% 75-80% qq W-pair event selection Clear event decay ... P.Azzurri - W Physics at LEP

7. W-pair cross sections Clear proof of SU(2)xU(1) gauge couplings ! WW (pb) s (GeV) Precision better than 1% Test of the SM radiative corrections to the CC03 diagrams P.Azzurri - W Physics at LEP

8. W Gauge Couplings P.Azzurri - W Physics at LEP

9. triple and quartic SU(2) gauge boson self couplings The signature of the non-abelian SU(2) electroweak structure ! Triple Gauge Couplings Standard electroweak theory U(1) SU(2) P.Azzurri - W Physics at LEP

10. Triple Gauge Couplings The most general Lorentz Invariant WWV (V=,Z) vertex has 7 complex couplings P.Azzurri - W Physics at LEP

11. W-pair angular distributions The WWZ and WW gauge couplings can be measured in W-pair events, fitting the W-pair event rates and the W production and decay angular distributions. P.Azzurri - W Physics at LEP

12. Relaxing all constraints and fitting for any of the 28 WWZ and WW couplings one-dimensional fit results for the SM non-zero TGC values (ALEPH data only) … all other 24 couplings are consistent with zero (within 5-20%) ! Triple Gauge Couplings Standard measurements of three TGC couplings that conserve C and P, U(1)em and global SU(2)L⊗U(1)Y one dimensional fit results (LEP): P.Azzurri - W Physics at LEP

13. W Decay Couplings P.Azzurri - W Physics at LEP

14. Direct test of W lepton universality at the 1% precision level W leptonic couplings P.Azzurri - W Physics at LEP

15. Direct test of W quark-lepton universality at the 0.6% precision level W hadronic couplings P.Azzurri - W Physics at LEP

16. Other four fermion processes P.Azzurri - W Physics at LEP

17. Z-pairs No gauge self couplings involved in Standard Model Z-pair production In agreement with SM within 5% precision P.Azzurri - W Physics at LEP

18. Single W and Z In agreement with SM within 8% precision In agreement with SM within 7% precision P.Azzurri - W Physics at LEP

19. W mass P.Azzurri - W Physics at LEP

20. W mass Direct kinematic reconstruction of the W mass in qqqq, qql  (and l l ) final states Improve the W invariant mass resolution: kinematic fit energy-momentum conservation (equal mass constraints) • mW value extracted with different methods • Breit-Wigner fit (with bias correction) • Monte Carlo reweighting • (with different observables M1,δM) • Probability Density function P(M1,M2,..) Statistical power of the LEP2 data: ΔmW(stat)=21 MeV P.Azzurri - W Physics at LEP

21. mW(qqqq)= 80.420±107 MeV mW(qql )= 80.411± 44 MeV (ρ=0.18) ΔmW(qqqq-qql )= +22±43 MeV without CR and BE errors W mass systematics The fully hadronic channel is de-weighted to 0.10 for possible final state interconnection effects ! P.Azzurri - W Physics at LEP

22. Colour Reconnection effects: measure particle flow in regions between W’s / inside W’s upper limits: ki<2.13  Prob(CR)<0.65  ΔmW=90 MeV Hadronic final state interactions Bose-Einstein correlations: measure two particle correlations between W’s / inside W’s full effect ΔmW=35 MeV measured fraction ΔmW=15 MeV P.Azzurri - W Physics at LEP

23. CR insensitive jet reconstructions ? Prospects Can reduce the CR mass shifts by a factor 2-3 deteriorating the statistical precision by 20% CR systematics : ΔmW=90  40 MeV statistical error: ΔmW=35  40 MeV total mW(qqqq) error:ΔmW=110  60 MeV mW(qqqq) weight in combination : 0.10  0.30 combined mW error :42  38 MeV • Ways to reduce the CR bias: • Remove low energy particles (pcut) • Hybrid cone algorithm (R) P.Azzurri - W Physics at LEP

24. W mass and width W measured with the same methods used for the mW extraction mW= 80.412±0.042 GeV/c2 W= 2.150±0.91 GeV/c2 P.Azzurri - W Physics at LEP

25. Standard Model Fit All this and more in Roberto Tenchini’s talk tomorrow ! P.Azzurri - W Physics at LEP

26. Conclusions = Introduction The study of W-pair production at LEP2 was a nice and exciting work that provided: • a clear evidence of the presence of Standard Model triple gauge couplings and of their SU(2)U(1) structure • the first direct measurements of the W decay branching ratios, and from these: • a direct test of the W lepton family and lepton-quark universality to 1% and 0.5% • a test of CKM unitarity and best current determination of the Vcs amplitude • an independent W mass measurement at production threshold (200 MeV accuracy) • a direct measurement of the W mass and width with 40 MeV and 90 MeV accuracy P.Azzurri - W Physics at LEP