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W Mass From LEP

W Mass From LEP. Fermilab Wine and Cheese Seminar 6th October, 2006 Ambreesh Gupta, University of Chicago. Outline . 1 . Introduction - W Boson in the Standard Model of Particle Physics 2 . W mass Measurement - Identifying and reconstructing W’s.

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W Mass From LEP

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  1. W Mass From LEP Fermilab Wine and Cheese Seminar 6th October, 2006 Ambreesh Gupta, University of Chicago

  2. Outline 1. Introduction - W Boson in the Standard Model of Particle Physics 2. W mass Measurement - Identifying and reconstructing W’s. - Mass extraction techniques used by LEP experiments 3. Systematic Uncertainties on W mass measurement 5. Summary I will show results from all the four experiments with details on OPAL analyses. Fermilab Wine & Cheese

  3. Standard Model of Particle Physics Our picture of the fundamental constituents of nature There are about 19 (+10) free parameters in the theory to be determined experimentally Standard Model predicts relationship between these parameters. Fermilab Wine & Cheese

  4. W H W W W t (running of a) f Standard Model Relations • Standard Model predicts relation between the parameters; W boson mass(MW) • and Fermi constant(GF), fine structure constant(), Z boson mass (MZ) • : electron g-2 0.004 ppm GF : muon life-time 9 ppm MZ : LEP 1 lineshape 23 ppm • Precision measurements require higher order terms in the theory and help • constraint the unknown pieces. Fermilab Wine & Cheese

  5. Precision EW top-quark mass “predicted” by electroweak corrections prior to direct discovery • The measured W mass precision is such that Top and Higgs loops required for consistency in the Standard Model (SM) •  This gives an indirect inference on the Higgs. •  Better precision on W mass constraints the Higgs • Indirect measurement of W mass • - W mass known to 20 MeV from indirect measurement (LEP1 + SLD +Tevatron). • - A direct measurement of W mass with similar precision is of great interest. • Measurement of the width of W boson can also be carried out at LEP providing further checks on consistency of the SM. Fermilab Wine & Cheese

  6. Large Electron Positron Collider (LEP) LEP I (1989-1993) : Z physics. 18 million Z bosons produced LEP II (1996-2000) : W physics. 80,000 W’s produced. (Energies from 161 GeV – 209 GeV) W’s produced in pairs. Fermilab Wine & Cheese

  7. The Four LEP Experiments ALEPH L3 Fermilab Wine & Cheese

  8. WW Production and Decay at LEP Backgrounds BR ~ 10% WW ll • W’s produced in pairs at LEP • - 700 pb-1/experiment; 40,000 WW BR ~44% WW qql Efficiency Purity l l70%90% qql85% 90% qqqq85% 80% BR ~ 46% WW qqqq Fermilab Wine & Cheese

  9. Event Selection • Event selection primarily based on multivariate relative likelihood discriminants OPAL Very good agreement between expected and observed. Fermilab Wine & Cheese

  10. W Mass at LEP • The WW cross section at s = 2Mw • sensitive to W mass • LEP experiments collected • 10 pb-1 data at s = 161 GeV • Combined Result : • Mw = 80.40  0.21 GeV • Most of LEP 2 data at higher energies - use direct reconstruction • There are two main steps in measuring W mass and width 1. Reconstruct event-by-event mass of W’s 2. Fit mass distribution  Extract MW and W. • However, jet energies poorly measured ( /E ~ 12% ), neutrinos unobserved. Kinematic fitting plays vital role Fermilab Wine & Cheese

  11. Kinematic Fitting • Mass Reconstruction • - Identify lepton and jets (DURHAM) • -- Energy flow techniques • - Kinematic fitting • -- Use LEP beam energy as constraint • -- Total Energy = s; Total Momem. = 0; • -- Additionally, apply equal mass constraint • mw+ - mw- = 0; •  Significantly improved mass resolution • Caveat- Photon radiation will change s s’ (photon energy)  Need good WW4f theory model (~0.5% theory error ) Fermilab Wine & Cheese

  12. Mass Reconstruction • qqqq channel • - Well constrained events • - But, ambiguity in assigning jets to • W’sCombinatorial Background • - 5-jet event: 10 comb., 4-jet: 3-comb. • qql channel • - 1 or 2 constraint kinematics fit • - Golden channel Fermilab Wine & Cheese

  13. 80.33 81.33 Fit Methods • LEP experiments used three likelihood methods to extract W mass and width from the reconstructed mass spectrum. 1.Re-weighting 2. Breit-Wigner 3.Convolution The primary difference between the methods is the amount of information they try to use for the best measurement. • Re-weighting - Weight fully simulated events to create sample with new W mass and width parameter - No external bias correction needed - Need large event sample to derive stable weights Fermilab Wine & Cheese

  14. Fit Methods (continued) Fitted Function (70-88) GeV mass • Breit-Wigner • - Fit to W mass spectrum with Breit-Wigner function • - Width adjusted to account for resolution and ISR effects. • - Bias corrected by comparing to fully simulated MC. • Convolution - P(m1,m2|Mw,Gw)R(m1,m2) - Build event-by-event Likelihood - Maximize statistical sensitivity - Need bias correction as in BW Fermilab Wine & Cheese

  15. Likelihood Variables • - Likelihood built using three variables -- both in qqlv, qqqq channels • ~ 400 events per bin for stable fit • Fit for eight energy point, four channels, then combine => lots of MC needed. • OPAL variables • ALEPH also • 3-d fit 5C fit mass error Hadronic 4C mass 5C fit mass 4C fit mass difference 5C fit mass error 5C fit mass Fermilab Wine & Cheese

  16. Performance of Likelihood Fucntion • Check bias and pulls distributions. . .below a typical example OPAL • Test the central value modeling with bias plot • Test the uncertainty on central value with pull distribution. Fermilab Wine & Cheese

  17. MwMw (Stat.+Syst.) CV 80.416  0.053 RW 80.405  0.052 BW 80.390  0.058 OPAL W mass • Very good agreement between three methods in channel and year • Strong correlation between methods => Combining them had only small stat. gain • CV, which has the smallest expected statistical uncertainty is used as the main method. • Use ofmomentum cut analysismakes significant reductionin FSI uncertainty. • Final W mass and total uncertainty from the three methods on OPAL - Fermilab Wine & Cheese

  18. qqqq combined qql Source 19 5 8 35 79 11 Hadronisation QED(ISR/FSR) Detector Colour Reconnection Bose-Einstein Correlation LEP Beam Energy Other 14 7 10 9 2 10 4 13 8 10 0 0 9 3 44 40 59 Total Systematics Statistical Total 21 30 36 22 25 33 LEP W Mass Channel weights qqlv : 76% qqqq : 22% xs : 2% • The combined preliminary LEP W mass • MW = 80.376  0.025 (stat)  0.022 (syst) GeV • Systematics on W mass (MeV) Fermilab Wine & Cheese

  19. LEP W Width • The combined preliminary LEP W width • W = 2.196  0.063(stat)  0.055(syst) GeV • Systematics on W width qql + qqqq (MeV) Source Hadronisation QED(ISR/FSR) Detector Colour Reconnection Bose-Einstein Correlation LEP Beam Energy Other 40 6 22 27 3 5 19 Total Systematics Statistical Total 55 63 84 Fermilab Wine & Cheese

  20. LEP Beam Energy • LEP beam energy used in event kinematics fit  DMW/MW DELEP/ELEP • Beam energy calibrated using • - Resonant De-Polarization (41- 60 GeV.) • - Extrapolated to LEP II energies NMR probes • - Main systematic error due toextrapolation • Extrapolation checked with • 1. Flux Loop • 2. Spectrometer • 3. Synchrotron Oscillation • Final results on LEP beam energy ( Eur. Phys. J., C 39 (2005), 253 ) • - Reduction of beam energy uncertainty used in earlier W mass combination • - old : DEbeam= 20-25 MeV  DMW = 17 Mev • - new : DEbeam = 10-20 MeV  DMW ~ 10 Mev • -- OPAL Final 9 MeV Fermilab Wine & Cheese

  21. LEP Beam Energy Cross Check with Data • LEP beam energy can be estimated using radiavtive return events - Z mass precisely known - Measured mass in radiative events sensitive to beam energy • Result consistent with zero within experimental errors Fermilab Wine & Cheese

  22. Detector Systematics from MC Modeling • Main Sources • - QED/EW radiative effects • - Detector Modeling • Hadronisation Modeling • - Background Modeling • - Final State Interaction MC modeled to represent data; Disagreements  Systematic error Fermilab Wine & Cheese

  23. Photon Radiation • KoralW’s O(3) implementation adequate, • but misses • - WSR • - interference between ISR,WSR & FSR • KandY includes • - O() corrections • - Screened Coulomb Correction Error ~ 7 MeV Fermilab Wine & Cheese

  24. Raw  Corrected Detector Systematics Jet energy resolution • Z0 calibration data recorded annually provides • a control sample of leptons and jets (~ 45 GeV). • Data/Mc comparison used to estimate corrections for • - Jet/Lepton energy scale/resolution • - Jet/Lepton energy linearity • - Jet/Lepton angular resolution/biases • - Jet mass • Error is assigned from the error on correction Jet energy scale LEP Combined: qqlv qqqq Combined 10 MeV 8 MeV 10 MeV Fermilab Wine & Cheese

  25. Detector Systematics: Breakdown OPAL Fermilab Wine & Cheese

  26. Hadronization Modeling • MC programs (JETSET,HERWIG,ARIADNE) model production of hadrons but difference in particles and their distributions • The difference interplays with detector response - particle assignment to jets - cuts applied to low momentum particles - low resolution for neutral particles - assumptions made on particle masses at reco. • JETSET used by all LEP experiment with parameters tuned with Z peak data systematic shift estimated from shift with other hadronization models. LEP Combined: qqlv qqqq Combined 13 MeV 19 MeV 14 MeV Fermilab Wine & Cheese

  27. Final State Interactions The Basic Problem: If products of hadronically decaying W’s (~0.1 fm) interact before hadronization (~1.0 fm) Can create a mass bias. Two known sources that could potentially bias W mass and width measurement 1.ColorReconnection - color flow between W’s could bias their masses - only phenomenological models exist. - Most sensitive variable to CR is W mass itself 2. Bose-Einstein Correlation. - coherently produced identical pions are closer in phase space. - BE correlation between decay products of same W established - Does the effect exist between W’s? Fermilab Wine & Cheese

  28. Color reconnection • Only phenomenological models exist. - SK1 model produces largest shift  CR strength parameter (ki) • LEP experiments estimate effect of color reconnection • Measure particle flow in the inter-jet regions of the • W’s • - Extreme values of CR disfavored by data • but it does not rule out CR • - A 68% upper limit on ki is used to set a data driven • uncertainty on W mass. • - Combined LEP value of ki = 2.13 • For this Reco. Prob., CR error ~ 120 MeV (OPAL) Fermilab Wine & Cheese

  29. Cuts and Cones: Reducing CR effect • CR affects mostly soft particles between jetschanges jet direction • Re-calculate Jet direction 1.Within cone of radius R 2.Cut on particle momentum P 3.Weighted particle momentum |P| • A,D,L,O use varations of below - OPAL Uses P-cut2.5 GeVfor qqqq - ~18% loss in statistics. • - Much reduced CR systematics • 125  41 MeV (ki =2.3) OPAL • - A worthwhile tradeoff! • - ALEPH28 MeV, L338 MeV Final CR error in qqqq 35 MeV Fermilab Wine & Cheese

  30. BEC in WW events • 2.5 GeV P-cut to redefine jet direction also reduces BEC W mass bias - OPAL (default) 46 MeV (P-cut) 24 MeV. • LEP experiments have measured BEC between W’s - Using “mixing method” - A,D,L,O: only a fraction of Full BEC seen in data (0.1713) • A 68% upper limit on BEC fraction seen in data (OPAL), used to set W mass systematics MW = ( 0.33 + 044)  MW (Full BEC) = 19 MeV • L3 18 MeV, ALEPH 2 MeV Final BEC error in qqqq 7 MeV Fermilab Wine & Cheese

  31. Results: qqqq and qqlv channels Fermilab Wine & Cheese

  32. Results: LEP W mass and Width mW (LEP) = 80.376 ± 0.033 GeV GW (LEP) = 2.196 ± 0.083 GeV Fermilab Wine & Cheese

  33. W’s as Calibration Sample at LHC “Yesterdays sensation is today’s calibration and tomorrows background” - Telegdi • W’s from top decay are foreseen to provide the • absolute jet scale. • - Fast simulation studies in the past showed feasibility • Select samples with a four jets and lepton • (electron,muon) with two jets b-tagged. • estimated 45K events from 10 fb-1 • Cross check with Z/ +Jet sample • Sattistics not the issue but understanding • the physics of the events. Fermilab Wine & Cheese

  34. Summary • Final results from all the four LEP experiments • Final LEP combination will use combined FSI error •  Much reduced FSI error in final results • A new preliminary LEP combination • Total LEP W mass uncertainty decreased to 33 MeV • It took about five years after LEP shut down to get final W mass • results from all the four experiment. Now it is up to Tevatron to • better the W mass precision before LHC turns on. Fermilab Wine & Cheese

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