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Measurement of the W Boson Mass

Measurement of the W Boson Mass. Yu Zeng Supervisor: Prof. Kotwal Duke University. Outline. Introduction to the Standard Model Motivation of W mass measurement Method (calibration, simulation … ) Result and discussion Future prospects. The Standard Model (SM).

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Measurement of the W Boson Mass

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  1. Measurement of the W Boson Mass Yu Zeng Supervisor: Prof. Kotwal Duke University

  2. Outline • Introduction to the Standard Model • Motivation of W mass measurement • Method (calibration, simulation …) • Result and discussion • Future prospects PHY 352 Seminar

  3. The Standard Model (SM) • It is a special relativity quantum field theory in which the dynamics is generated from the assumption of local gauge invariances. • It is renormalizable (divergences can be absorbed into parameters such as masses and coupling strengths.) • Encompasses Electroweak theory and QCD • The only elementary particle theory that has been verified experimentally. PHY 352 Seminar

  4. Total = 26 The Standard Model (SM) 12 leptons + 36 quarks + 12 mediators + 1 Higgs = 61 • Number of “elementary particles” in SM: • Parameters needed to SM completely predictive: PHY 352 Seminar

  5. Motivation • Precise W mass and top quark mass values constrain the mass of undiscovered Higgs. • W mass is a fundamental parameter in SM. (Higher order radiative corrections from loop diagrams involving other particles contribute to the observed W boson mass) • With ultimate precision can set limits on new particles in loops PHY 352 Seminar

  6. Radiative Corrections • Top quark mass and the Higgs boson mass dominate radiative corrections 13 MeV shift to Mass of W if △M_t≈2.1GeV Arouse few MeV shift to Mass of W • Currently W mass uncertainty dominates the above relationship PHY 352 Seminar

  7. Motivation cont’d Example: Relations among the masses of W, t and Higgs • Loop effects of the masses of W and t to that of Higgs are quite different in size. W mass uncertainty dominates. http://acfahep.kek.jp/acfareport/node181.html PHY 352 Seminar

  8. History of W Boson Study • Experimental effort W boson mass has been measured with increasing precision by those experiments PHY 352 Seminar

  9. Collider Detector at Fermilab (CDF) Muon Detector Central Hadronic Calorimeter Central Outer Tracker PHY 352 Seminar

  10. The CDF Detector PHY 352 Seminar

  11. Central Hadronic Calorimeter Central E&M Calorimeter Provides precise measurement of electron energy Provides measurement of hadronic recoil objects Provides precise measurement of track momentum The CDF Detector (Quadrant) PHY 352 Seminar

  12. Particle Identification • Particle detectors measure long-lived particles produced from high energy collisions: electrons, muons, photons and “stable” hadrons (protons, kaons, pions) • Quarks and gluons do not appear as free particles, they hadronize into a jet. PHY 352 Seminar

  13. W Boson Production Process a) dominates (80%), Process b) implies the existence of net transverse momentum. Lepton Pt carries most information of W mass PHY 352 Seminar

  14. Angle between 2 pt W Mass Measurement (1) • Invariant mass of lepton-neutrino cannot be reconstructed since neutrino momentum in beam direction is unknown. However, we can use transverse mass Features of transverse mass spectrum: 1). Relatively insensitive to the production dynamics of W. 2). Sensitive to detector response to recoil particles. PHY 352 Seminar

  15. Features of transverse momentum of lepton: 1). Better resolution than neutrino pt → relatively insensitive to the recoil response of detector 2). Sensitive to the W boson production dynamics • A third way is to use transverse momentum spectrum of neutrino Features of transverse momentum of neutrino: Sensitive to both W production dynamics & the recoil response W Mass Measurement (2) • Another way is to use transverse momentum spectrum of lepton PHY 352 Seminar

  16. W Mass Measurement (3) Source: A. Kotwal 2007 Aspen talk PHY 352 Seminar

  17. Tracker calibration EM Calorimeter calibration W Mass Measurement Strategy Data • Detector Calibration • Fast Simulation Binned Likelihood Fit W boson mass NLO event generator Detector response simulation Hadronic recoil modelling + Backgrounds PHY 352 Seminar W mass templates, bule for 80 GeV, red for 81 GeV

  18. Event Selection for W & Z • Select clean W and Z samples to get maximum ratio of S/N. Trigger info: lepton Pt>18 GeV Central leptons selection: |eta|<1 Final Analysis: lepton Pt>30 GeV W boson further requires: u<15 GeV and missing Et>30GeV Z boson: two charged leptons Collected data used (02/2002-09/2003) ~ 1/10 of data on tape. Number of W events comparable to 4 LEP experiments combined. PHY 352 Seminar

  19. Detector Calibration • Tracker calibration 1). Calibration of COT using comic rays 2). J/psimu+mu- and Upsilonmu+mu- are used to scale COT momentum 3). Using Zmu+mu- invariant mass fit to further check • EM Calorimeter calibration 1). Using Ecal/p ratio to scale COT momentum 2). Using Ze+e- mass fit to further check calorimeter energy scale PHY 352 Seminar

  20. Backgrounds For Wmu nu • Largest background comes from Zmu+mu- • Wtau numu nu nu events • Cosmic rays • Kaon decays in flight • QCD jet events where one jet contains one non-isolated muon For We nu • Ze+e- • Wtau nue nu nu • QCD PHY 352 Seminar

  21. Transverse Mass Fitting results background background PHY 352 Seminar

  22. Transverse Mass Uncertainties Combined electron and muon uncertainty is 48 MeV PHY 352 Seminar

  23. Other W Mass Fits – Lepton Pt (Et) PHY 352 Seminar

  24. Other W Mass Fits – Neutrino Pt PHY 352 Seminar

  25. Combined Results • Combine all 6 fitting results: Best single precise measurement! PHY 352 Seminar

  26. Implications for Standard Model • Uncertainty down from 29 MeV to 25 MeV • Central value up from 80392 MeV to 80398 MeV • Previous SM Higgs mass prediction from • 95% CL upper limit on Higgs mass lowers from previous 199 GeV to 189 GeV PHY 352 Seminar

  27. The Implications for Tevatron In 2004, the estimated upper limit for Higgs mass is 250 GeV, however Tevatron only reach upper limit 170 GeV, people think Tevatron has no chance to find Higgs. Now Tevatron is back into the competition. PHY 352 Seminar

  28. Future Prospects at CDF For Example: • Mw uncertainties are dominated by statistics of calibration data. Current analysis only used 1/10th of data on tape. • Detailed study of PDFs (Parton Distribution Fuction) to reduce systematic uncertainties. • Magnetic field within COT is not uniform, need to fix that. • Calibrate sag of wires in COT due to gravity • … Goal: Delta_mw<25 MeV from 1.5 fb^-1 of CDF data PHY 352 Seminar

  29. References Acknowledgement Prof. Ashutosh Kotwal • Ashutosh Kotwal, Aspen Conference on Particle Physics (2007) • CDF Note 8665 • http://acfahep.kek.jp/acfareport/node181.html • William Trischuk, Collider 2 Cosmic Rays (2007) • Oliver Stelzer-Chilton, PhD thesis, University of Toronto (2006) • Andrew Gordon, PhD thesis, Harvard University (1998) • Al Goshaw, Phy346 Lecture notes, Duke University (2007) PHY 352 Seminar

  30. Backup Slides … PHY 352 Seminar

  31. Total = 26 Choices of SM Parameters (1) Can be chosen from: PHY 352 Seminar

  32. Choices of SM Parameters (2) Choice 1. Choice 1. Choice 2. Follow the pattern that parameters are masses and coupling constants. Choose parameters measured most precisely. PHY 352 Seminar

  33. Motivation • The EWK sector of SM is constrained by three precisely measured parameters: • At lowest order, these parameters are related by: PHY 352 Seminar

  34. Blind Analysis Technique • A random [-100,100] MeV offset is added in the likelihood fitter, thus all W mass fits are blinded • Blinding offset is removed after the analysis was frozon. • Benefit: allowing study data in detail while keeping W mass value unknown within 100 MeV. Helps to avoid biased analysis. PHY 352 Seminar

  35. e e e e ne e g Z0 W Why two coupling constants Thus, only two counpling constants: 1) a=e2/(4phc)=1/137; 2) aS for strong coupling PHY 352 Seminar

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