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W boson mass and width measurements at LEP2

W boson mass and width measurements at LEP2. Hugo Ruiz, CERN – Aleph On behalf of the LEP Collaborations. Outline. Introduction Measurement of M W (and G w ) by direct reconstruction Some relevant systematics: Bose-Einstein correlations Colour Reconnection Results and conclusions

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W boson mass and width measurements at LEP2

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  1. W boson mass and width measurements at LEP2 Hugo Ruiz, CERN – Aleph On behalf of the LEP Collaborations

  2. Outline • Introduction • Measurement of MW (and Gw) by direct reconstruction • Some relevant systematics: • Bose-Einstein correlations • Colour Reconnection • Results and conclusions • Prospects

  3. At LEP2, W’s produced in pairs: Introduction • mW in perspective: • From pp colliders (transverse mass spectra, single Ws,april 2004): mW = 80.452  0.059 GeV (CDF run1+D0 run1+UA2) • Prediction from EW fit: mW = 80.373  0.033 GeV(LEP1,SLD) mW = 80.386  0.023GeV (+Mtop) • mW measured from direct reconstruction of W decay products 1996-2000: ~40k WW evts precision measurement

  4. The measurement

  5. Hadronic (4q) Leptonic Semileptonic (qqln) q q l n n l q q q l q n 10% 44% 46% Event selection

  6. W reconstruction • Hadronic and semileptonic channels: • Reconstruct leptons and cluster jets • Apply a kinematic fit: • Constraints: • E,p conservation • In some cases, M1=M2 / b1=b2 • Effects: • alows ‘reconstruction’ of n in semileptonic channel • resolution ~7GeV  ~3GeV • decreases detector systematics • Hadronic channel: 3. Pair jets (there is a 3-fold ambiguity): • algorithms provide ~85% of good pairing

  7. Reconstructed MW

  8. mW and Gw extraction • Extraction by fitting distribution of reconstructed MW to either: • MC samples generated with different MW (and GW) values • Function convoluting BW and detector effects, and then use MC to correct for residual offsets Mass extraction • Assume SM relation between M and G • Perform 1-parameter fit Width extraction • Assume no relation between M and G • Perform 2-parameter fit • Rely on MC simulation: • Lots of data for tuning from LEP1 • Residual discrepancies  systematics

  9. next sections of the talk Systematic uncertainties for mW • Expected final statistical error for LEP2 ~ 25 MeV • Largest systematic uncertainties:

  10. Interconnection effects on hadronic events

  11. Hard process: e+e-4q q • parton shower (large Q2, pQCD) W- • hadronisation (phenomenological) e- _ q d~0.1 fm e+ W+ q _ q • Colour reconnection: hadronic interaction between W decays • d(W+,W-) < 1 fm Interconnection effects • Event simulation: • Fragmentation (quarks  hadrons): • Interconnection effects (not included in standard MC models): • Bose-Einstein correlations: momenta of identical bosons tend to be correlated.

  12. W1 • Intra-W: • not relevant for Minv • Inter-W: • Cause discrepancies data-MC in jet overlaps  jet clustering different data-MC  bias. W2 Bose-Einstein Correlations (BEC) • Pairs of p0p0, p+p+ andp-p- tend to be bunched. • For calculation of BEC, quantum phases and space-time structure are needed  only phenomenological models available. • Effect on MW:

  13. LEPWW/FSI/2002-02 L3 fraction of model seen BEC in W+W- events • BEC effects experimentally established in Z jets at LEP1 • Inter-W BEC? Analyses performed in 4 LEP experiments to search/limit them • Observable: distance in p-space between pairs of charged pions: Q2ij=-(pi-pj)2 0 1 Q(GeV) • Inter-W BEC correlations disfavoured • Limit on systematic: dMW ~ 15 MeV

  14. Colour Reconnection • Several phenomenological models used to study the effect on mW, amongst them: • At parton shower: • Ariadne2: formation of some inter-W dipoles, dMW ~ 70 MeV • At hadronisation: • Herwig-CR: hadrons created from inter-W parton pairs, dMW ~ 40 MeV • Rathsman: reduce string tension by reconnecting, dMW ~ 40 MeV • Jetset SK1: allow formation of inter-W strings, dMW up to 400 MeV, depending on a free parameter • Dedicated analyses try to observe / limit CR effects from data on 4 LEP experiments

  15. CR models predict a modified particle flow in W+W- events: • Data • - SK1 (extreme parameter) No CR: CR: A - Jetset W- W- C W+ W+ D B • Observable: ratio of particle flow between the inter and intra-W regions: (A + B) / (C + D) The particle flow analysis

  16. For SK1: • Extreme values discarded • Preferred value of the parameter corresponds to dMW ~ 100 MeV!! CR Prob Results from particle flow • ‘Asymmetry’ from experiments combined in a c2. • Cannot discard models like: • Ariadne2 • Herwig-CR • Rathsman  Try to make analyses more robust to CR effects

  17. Towards a less CR sensitive analysis

  18. Idea: modify clustering algorithm to dismiss information from those particles. This implies: • “purer” information • loss of statistical precision Principle • Interconnections mainly occur between low momentum particles in the inter-W region. • Many variations of jet algorithms tried, mainly: • Cones: perform angular cut around jet direction • P-cuts: remove low momentum particles

  19. Reduction of dMW • Example: SK1 • Good reduction factors are obtained for all available models • Example: Cone (R=0.5 rad), with a statistical loss of ~ 25%: K parameter

  20. DELPHI, Cone algorithm R=0.5 DMW • DELPHI preliminary: • Exclude extreme scenarios. • Minimum at ~1.3, P~0.5 A by-product: measure CR? • The difference between MW measured with ‘robust’ and standard analyses is sensitive to CR effects: MW (GeV) ALEPH SK1 k=2.13 Cone radius (rad)

  21. Results

  22. 4q Non-4q mW from direct reconstruction Results in CERN-EP/2003-091, LEPEWWG/2003-02 still with standard jet algorithms DmW = 22 ± 43 MeV

  23. mW at LEP2 • LEP2 combination: • World average: mW = 80.412 ± 0.042 GeV 0.029 stat , 0.031 syst mW = 80.425 ± 0.034 GeV

  24. Combination: GW = 2.150 ± 0.091GeV 0.068 stat, 0.060 syst • World average: GW = 2.133 ± 0.069 GeV Gw at LEP2 Detector: 29 MeV Frag: 30 MeV FSI: 37 MeV

  25. Conclusions • Consistency of mW within SM: • The combination of the results of the LEP experiments gives: mW = 80.412 ± 0.042 GeV GW = 2.150 ± 0.091 GeV

  26. Prospects • Publication status: • BEC studies already published for L3 Phys.Lett.B 547 (2002) and ready for OPAL (CERN-PH-EP/2004-008, to Eur. Phys. Journal C). • CR studies ready for publication for L3 (CERN-EP/2003-12, to Phys. Lett. B). • All the rest will be ready by this autumn/end of this year • Expected developments related with color reconnection: • Final estimation of effect • Analysis with improved robustness. If all experiments use them: • Total error in hadronic channel: ~90  ~60 MeV. • Total error from decrease by ~3 MeV • Weight of hadronic channel in combination: 0.1%  0.3%. + Learn something about colour reconnection

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