Precision electroweak physics

1. 5th Rencontres du Vietnam Particle Physics and Astrophysics Hanoi August 5 to August 11, 2004 Precision electroweak physics Roberto Tenchini INFN-Pisa

2. 21 Years of W and Z Physics s=1.5 GeV s=400 MeV s=100 MeV s=39 MeV (*) 2000 TeV LEP 1986 UA1 UA2 1990 UA2 CDF Only Published Results (*) Preliminary average 2004 : s=34 MeV, weight of LEP 2/3, Tevatron 1/3

3. 21 Years of W and Z Physics s=2.1 MeV s=1.7 GeV s=31 MeV 1986 UA1 UA2 1990 LEP Only Published Results

4. 21 Years of W and Z Physics 1986 2002 1986 2002 Strong Evidence of pure E.W. Higher Order Corrections E.W. Tree level SM relation (with running a QED)

5. Tree level not enough : more parameters • Radiative corrections in function of three more parameters Mtop, Mhiggs, as • An Observable is written as one loop radiative corrections Example :

6. Outlook of the rest of the talk NEW MEANS FIRST TIME AT THIS CONFERENCE • Z Couplings: asymmetries at the Z and measurement of sin2(q)W • (a crisis ?) • W Couplings:W Branching Ratios • (three LEP exp are final) • Measurement of the W mass • (some hot issues) • Constraints on the SM Higgs

7. Asymmetries at the Z pole • Z decay into a ff pair. With unpolarized beam from fermion direction and helicity 3 asymmetries Can measure for e,m,t,c,b Can measure with t’s • Z with beam polarization (SLC) :

8. Electroweak Couplings : from deep inelastic scattering to LEP-SLC Huge increase in precision

9. Asymmetriesat the Z and • Consistency is at 6.2% • Long standing difference between Alr and AFB(b) NEW FINAL

10. b anomalous couplings ?No independent evidence NEW FINAL measures measures Plots like this one include b asymmetry !

11. Again on the b asymmetry NEW FINAL • Two techniques • use semileptonic b decays • use weighted charge of particles in the hemisphere • Very different systematic effects • LEP average still statistically dominated Example: Stability of result as a function of the b purity

12. The b asymmetry: results LEP average NEW FINAL Leptons only Inclusive

13. My conclusion on “sin2(q) crisis” • There is no evidence of problems related to the b asymmetry measurement • Will solve the ALR-b asymmmetry discrepancy with future linear colliders if • Polarization of both beams available • Very high statistics run at the Z will take place

14. WW Production at LEP2 Three diagrams contribute at Born level (CC03 diagrams) : Actually look for subsequent W decay into lepton-(anti) neutrino and quark1- (anti) quark2. Real process is defined by exp. cuts

15. WW production at LEP2: the backgrounds • Selections needed to • Extract events from background • Classify WW decays to different channels (fully hadronic, fully leptonic, semileptonics) • (Semi)leptonic decays can further be separated in e,m,t channels

16. Hadronic (4q) Leptonic Semileptonic (qqln) q q l n n l q q q l q n 11% 44% 45% Event selection Lepton=e,m,t

17. Cross Sections in all channels EXAMPLE OF PROCEDURE • Njselected events in each selection (j) • L total luminosity • jexpected events function of • iWW for each channel (i) • ijefficiency matrix • jbkgbackground cross section • j4ffour fermion interference correction Likelihood

18. Four Fermion Interference Small Correction taken from Monte Carlo (~ 1% relative) Single W SIGNAL (following the CC03 definition) Most important(*) 4-f (non WW) processes can be directly measured (*) for the interference

19. Total WW Cross Section • Precision reached by LEP experiments a challenge to theoretical predictions • Predictions with enhanced O(a) radiative corrections are needed ! NEW 3 exp FINAL Strong evidence of Triple Gauge Couplings

20. W decays : Branching Ratios NEWADL FINAL Standard Model : 10.8% Standard Model : 67.5% Test of lepton universality: t result higher than e+m

21. W Leptonic Couplings • If electronand muon couplings are assumed to be the same and combined the t result is 3shigher • was 2.3s in summer 2003 • is 2.6sif final results only are used NEWADL FINAL

22. W Mass at LEP from direct reconstruction • Above threshold the W mass is measured from direct reconstruction of the jet-jet invariant mass in the fully hadronic and semileptonic channels • Event reconstructed as 2 (semileptonic candidate) or 4 (hadronic candidate) jets with iterative procedure • In the hadronic channel 3 jet-jet combinations from 4 jet • WW boson pairs at LEP • 161 – 209 GeV centre of mass energy • ~700 pb-1 by each experiments • ~4500 qqqq , ~4000 lnqq events for each experiments.

23. Systematics: • At LEP2: • Ebeam~20MeV (E/E~10-4)  mW~17MeV • Error coming mainly from extrapolation. LEP2 Energy RECENT FINAL • Kinematic fit imposing energy-momentum conservation  Resonant depolarization: • only works up to 60GeV  extrapolation

24. Reconstructed MW L3 tnqq OPAL mnqq DELPHI enqq

25. LEP: Systematic Uncertainties for MW • QED effects (ISR, FSR, etc.) • Fragmentation • Detector effects • Uncertainty on the LEP beam energy • Colour Reconnection • Bose-Einstein correlations (Weight of qqqq in LEP combination: 0.09) Effort to reduce this error by designing 4q analyses less affected by CR effects

26. Final State Interactions Separation of W decays vertex at LEP2 ~0.1 fm Þ small with respect to the hadronization scale ~1fm Interconnection phenomena : Final State is no longer factorized into two separated W’s Þ can bias the W mass in the fully hadronic channel NEED DATA to control these non-perturbative effects

27. Present LEP result: Colour Reconnection constrained by the particle flow analysis • Data • -SK1(extreme parameter) No CR: CR: A -Jetset W- W- C W+ W+ D B • The ratio of particle flow between the inter and intra-W regions is built: (A + B) / (C + D) • Most CR models predict a modified particle flow in W+W- events: • Measurement sensitive only to extreme scenarios, Colour Reconnection Systematic error ~ 90 MeV

28. Final LEP result: Colour Rec. constrained by Mass variation when soft particles are excluded Example: difference between Mass measured in standard analysis and Mass measured using only particles in the core of the jet • This correlation is a general feature of All Models : CR is expected to affect mainly • low momentum particles • particles away from the jet core Models Mass Shift predicted by model • By studying Mass Stability (or Measuring the W mass only with particles in the jet core ) expect to reduce CR Systematic Error to ~ 50 MeV

29. MW at LEP : 4q and lnqq 80.411±0.032(stat) ±0.030(syst)GeV/c2 80.420±0.035(stat) ±0.101(syst)GeV/c2 mass difference (no FSI syst) DMW(qqqq-lnqq)= +2243 MeV

30. W mass from hadron colliders: the past and the near future ! • Before LEP:measurements at hadron colliders (SppS, Tevatron Run I) • After LEP : measurements at hadron colliders (Tevatron Run II, LHC) • Drell-Yan single W production (quark-antiquark annihilation) • W decay to leptons (e or m) + neutrino • Fit to MWT , the transverse mass distribution -

31. TeVatron RUN IIW and Z cross sections

32. Present MW at LEP and TeVatron

33. ElectroWeak fit results • Electroweak theory tested at one loop level • Indications for a light Higgs Winter 2004 Summer 2003

34. Conclusions • Asymmetries at the Z are final (including quarks) • W cross sections at LEP are essentially final • The LEP W mass result is still PRELIMINARY • LEP collaborations are still working on this measurement, final results for the end of this year • Activities to reduce the uncertainty due to CR effects and gain information from the 4q channel • Reducing the LEP final error (~ 35 MeV ) will be the challenge for Tevatron II and LHC • In spite of the increased top mass there is still evidence for a light Higgs

35. Backup Slides

36. AFB(b) from semileptonic decays Impact parameter Secondary vtx • Can fit independently b and c asymmetries, mixing and background composition Transverse momentum Momentum

37. AFB(b) QCD corrections : cross-check No evidence of bias due to gluon emission

38. W Leptonic Branching Ratios • If the Branching Ratios to electronand muon are assumed to measure the same quantity and combined the t result is 3shigher • was 2.3s in summer 2003 • is 2.6sif final results only are used NEWADL FINAL

39. eg BEB BEI Final DMWdown from ~35 to ~15 MeV Measuring BE in W+W- events • Inter W, BEI confirmed • Between W’s, BEB, disfavoured

40. Error on W mass (Run I) These errors are determined using CDF/D∅ data,scale with luminosity Detector improvements for Run II will also help Theory improvements Improve PDF constraints with measurements (W charge asymmetry, Z rapidity distribution) 40 MeV per experiment with 2 fb-1 feasible

41. Running of a QED: a(MZ)

42. Mw from the threshold method at 161 GeV LEP average:

43. Kinematic Fits Kinematic fit used to improve reconstructed four momenta

44. Energy loss Based on the frequency of the field provided to beam to compensate from syncroton radiation. Spectrometer Based on measurement of lepton bending angle Radiative return E loss by sync. Systematics: LEP Energy (2) • 3 methods used to cross-check extrapolation:

46. Removing low energy neutrals (<1.5 GeV , and <2 GeV if mixed) Jet boost (and mass) now well calibrated ! Jet velocity & mass Last jet reconstruction (with ECAL cleaning) Z peak jets boost (T>0.8)

47. Systematic uncertainties Radiative Z peak Unbinned likelihood fit to a pdf built with a fit to the MC reference

49. The pull of individual pseudo-observables in the global fit Forward-backward Asymmetry of Prob(c2) 42% 14% Deep Inelastic nN scattering Prob(c2 ) 14 % 1.7%