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

4/11/2007PHY 352 Seminar2 Outline Introduction to the Standard ModelIntroduction to the Standard Model Motivation of W mass measurementMotivation of W mass measurement Method (calibration, simulation … )Method (calibration, simulation … ) Result and discussionResult and discussion Future prospectsFuture prospects

4/11/2007PHY 352 Seminar3 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 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.)It is renormalizable (divergences can be absorbed into parameters such as masses and coupling strengths.) Encompasses Electroweak theory and QCDEncompasses Electroweak theory and QCD The only elementary particle theory that has been verified experimentally.The only elementary particle theory that has been verified experimentally.

4/11/2007PHY 352 Seminar4 The Standard Model (SM) Number of “ elementary particles ” in SM:Number of “ elementary particles ” in SM: Physical Quantity No. Mass of quark6 Mass of lepton3 Masses of W±,Z, Higgs3 Coupling strength2 Quark EWK mixing parameter4 Strong CP violation1 Neutrino mass3 Neutrino mixing parameter4 12 leptons + 36 quarks + 12 mediators + 1 Higgs = 61 Parameters needed to SM completely predictive:Parameters needed to SM completely predictive: Total = 26

4/11/2007PHY 352 Seminar5 Motivation W mass is a fundamental parameter in SM.W mass is a fundamental parameter in SM. Precise W mass and top quark mass values constrain the mass of undiscovered Higgs.Precise W mass and top quark mass values constrain the mass of undiscovered Higgs. With ultimate precision can set limits on new particles in loopsWith ultimate precision can set limits on new particles in loops (Higher order radiative corrections from loop diagrams involving other particles contribute to the observed W boson mass)

4/11/2007PHY 352 Seminar6 Radiative Corrections Top quark mass and the Higgs boson mass dominate radiative correctionsTop 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 relationshipCurrently W mass uncertainty dominates the above relationship

4/11/2007PHY 352 Seminar7 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.Loop effects of the masses of W and t to that of Higgs are quite different in size. W mass uncertainty dominates.

4/11/2007PHY 352 Seminar8 History of W Boson Study Experimental effortExperimental effort 1983 Discovery of the W at CERN ’ s proton-antiproton collider by UA1 & UA2 collaborations 1996 CERN ’ s e+e- collider LEP increased its c.m. energy above 161 GeV which is threshold for W pair production 1985 Tevatron, the second proton-antiproton collider, was commissioned at Fermilab 2000 four LEP experiments (ALEPH, DELPHI, L3, OPAL) ceased data taking 1987 Fermilab observed its first W candidate Now CDF and D0 at Fermilab are still running W boson mass has been measured with increasing precision by those experiments

4/11/2007PHY 352 Seminar9 Collider Detector at Fermilab (CDF) Muon Detector Central Hadronic Calorimeter Central Outer Tracker

4/11/2007PHY 352 Seminar10 The CDF Detector

4/11/2007PHY 352 Seminar11 The CDF Detector (Quadrant) Central Hadronic Calorimeter Central E&M Calorimeter Provides precise measurement of electron energy Provides precise measurement of track momentum Provides measurement of hadronic recoil objects

4/11/2007PHY 352 Seminar12 Particle Identification Particle detectors measure long-lived particles produced from high energy collisions: electrons, muons, photons and “ stable ” hadrons (protons, kaons, pions)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.Quarks and gluons do not appear as free particles, they hadronize into a jet.

4/11/2007PHY 352 Seminar13 W Boson Production Process a) dominates (80%), Process b) implies the existence of net transverse momentum. Lepton Pt carries most information of W mass

4/11/2007PHY 352 Seminar14 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 massInvariant mass of lepton-neutrino cannot be reconstructed since neutrino momentum in beam direction is unknown. However, we can use transverse mass 1). Relatively insensitive to the production dynamics of W. 2). Sensitive to detector response to recoil particles. Features of transverse mass spectrum: Angle between 2 pt

4/11/2007PHY 352 Seminar15 W Mass Measurement (2) Another way is to use transverse momentum spectrum of leptonAnother way is to use transverse momentum spectrum of lepton 1). Better resolution than neutrino pt 2). Sensitive to the W boson production dynamics Features of transverse momentum of lepton: → relatively insensitive to the recoil response of detector Sensitive to both W production dynamics & the recoil response A third way is to use transverse momentum spectrum of neutrinoA third way is to use transverse momentum spectrum of neutrino Features of transverse momentum of neutrino:

4/11/2007PHY 352 Seminar16 W Mass Measurement (3) Source: A. Kotwal 2007 Aspen talk

4/11/2007PHY 352 Seminar17 Tracker calibration EM Calorimeter calibration Detector CalibrationDetector Calibration W Mass Measurement Strategy Fast SimulationFast Simulation NLO event generator Detector response simulation Hadronic recoil modelling W mass templates, bule for 80 GeV, red for 81 GeV Data + Backgrounds Binned Likelihood FitW boson mass

4/11/2007PHY 352 Seminar18 Event Selection for W & Z Select clean W and Z samples to get maximum ratio of S/N.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 30GeV Z boson: two charged leptons Collected data used (02/ /2003) ~ 1/10 of data on tape. Number of W events comparable to 4 LEP experiments combined.

4/11/2007PHY 352 Seminar19 Detector Calibration Tracker calibrationTracker 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 calibrationEM Calorimeter calibration 1). Using Ecal/p ratio to scale COT momentum 2). Using Z  e+e- mass fit to further check calorimeter energy scale

4/11/2007PHY 352 Seminar20 Backgrounds Largest background comes from Z  mu+mu-Largest background comes from Z  mu+mu- W  tau nu  mu nu nu eventsW  tau nu  mu nu nu events Cosmic raysCosmic rays Kaon decays in flightKaon decays in flight QCD jet events where one jet contains one non-isolated muonQCD jet events where one jet contains one non-isolated muon For W  mu nu For W  e nu Z  e+e-Z  e+e- W  tau nu  e nu nuW  tau nu  e nu nu QCDQCD

4/11/2007PHY 352 Seminar21 Transverse Mass Fitting results background

4/11/2007PHY 352 Seminar22 Transverse Mass Uncertainties Combined electron and muon uncertainty is 48 MeV

4/11/2007PHY 352 Seminar23 Other W Mass Fits – Lepton Pt (Et)

4/11/2007PHY 352 Seminar24 Other W Mass Fits – Neutrino Pt

4/11/2007PHY 352 Seminar25 Combined Results Combine all 6 fitting results:Combine all 6 fitting results: Best single precise measurement!

4/11/2007PHY 352 Seminar26 Implications for Standard Model Uncertainty down from 29 MeV to 25 MeVUncertainty down from 29 MeV to 25 MeV Central value up from MeV to MeVCentral value up from MeV to MeV Previous SM Higgs mass prediction fromPrevious SM Higgs mass prediction from 95% CL upper limit on Higgs mass lowers from previous 199 GeV to 189 GeV95% CL upper limit on Higgs mass lowers from previous 199 GeV to 189 GeV

4/11/2007PHY 352 Seminar27 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.

4/11/2007PHY 352 Seminar28 Future Prospects at CDF Mw uncertainties are dominated by statistics of calibration data. Current analysis only used 1/10 th of data on tape.Mw uncertainties are dominated by statistics of calibration data. Current analysis only used 1/10 th of data on tape. Detailed study of PDFs (Parton Distribution Fuction) to reduce systematic uncertainties.Detailed study of PDFs (Parton Distribution Fuction) to reduce systematic uncertainties. Magnetic field within COT is not uniform, need to fix that.Magnetic field within COT is not uniform, need to fix that. Calibrate sag of wires in COT due to gravityCalibrate sag of wires in COT due to gravity … Goal: Delta_mw<25 MeV from 1.5 fb^-1 of CDF data For Example:

4/11/2007PHY 352 Seminar29 References Ashutosh Kotwal, Aspen Conference on Particle Physics (2007)Ashutosh Kotwal, Aspen Conference on Particle Physics (2007) CDF Note 8665CDF Note William Trischuk, Collider 2 Cosmic Rays (2007)William Trischuk, Collider 2 Cosmic Rays (2007) Oliver Stelzer-Chilton, PhD thesis, University of Toronto (2006)Oliver Stelzer-Chilton, PhD thesis, University of Toronto (2006) Andrew Gordon, PhD thesis, Harvard University (1998)Andrew Gordon, PhD thesis, Harvard University (1998) Al Goshaw, Phy346 Lecture notes, Duke University (2007)Al Goshaw, Phy346 Lecture notes, Duke University (2007) Acknowledgement Prof. Ashutosh Kotwal

4/11/2007PHY 352 Seminar30 Backup Slides …

4/11/2007PHY 352 Seminar31 Choices of SM Parameters (1) Physical Quantity No. Fermion masses (6 quark + 3 lepton)9 Higgs Boson1 Quark weak mixing parameter4 Strong CP violation parameter1 Strong interaction coupling constant1 Fundamental EWK parameters3 Neutrino masses3 Neutrino mixing parameter4 Total = 26 Can be chosen from:

4/11/2007PHY 352 Seminar32 Choices of SM Parameters (2) Follow the pattern that parameters are masses and coupling constants. Choice 1.Choice 2. Choice 1. Choose parameters measured most precisely.

4/11/2007PHY 352 Seminar33 Motivation The EWK sector of SM is constrained by three precisely measured parameters:The EWK sector of SM is constrained by three precisely measured parameters: At lowest order, these parameters are related by:At lowest order, these parameters are related by:

4/11/2007PHY 352 Seminar34 Blind Analysis Technique A random [-100,100] MeV offset is added in the likelihood fitter, thus all W mass fits are blindedA 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.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.Benefit: allowing study data in detail while keeping W mass value unknown within 100 MeV. Helps to avoid biased analysis.

4/11/2007PHY 352 Seminar35 Why two coupling constants ee  e e W e e  Thus, only two counpling constants: 1)  e 2 /(4  hc)=1/137;  2)  S for strong coupling