First Run II Measurement of the W Boson Mass by CDF Oliver - PowerPoint PPT Presentation

First Run II Measurement of the W Boson Mass by CDF Oliver Stelzer-Chilton Stelzer-Chilton Oliver University of Oxford High Energy Physics Seminar Michigan State University April 3 rd , 2007

Outline 1. Motivation 1. Motivation 2. W Production at the Tevatron Tevatron 2. W Production at the 3. Analysis Strategy 3. Analysis Strategy 4. Detector Calibration 4. Detector Calibration • Momentum Scale • Momentum Scale • Energy Scale • Energy Scale • Recoil • Recoil 5. Event Simulation 5. Event Simulation 6. Results 6. Results 7. Conclusions 7. Conclusions HEP Seminar Oliver Stelzer-Chilton - Oxford 2

The W Boson and the Standard Model • 1930’s: Fermi explains nuclear β -decay as 4-point interaction • 1960’s: Glashow, Weinberg and Salam → unify electromagnetic and weak interaction → explain interaction by exchange of massive vector bosons • Became foundation of the Standard Model • W boson mass is fundamental parameter HEP Seminar Oliver Stelzer-Chilton - Oxford 3

Introduction • Derive W mass from precisely measured electroweak quantities �� em 2 = m W 2 G F sin 2 � W (1 � � r ) • where M W =M Z cos θ W • α EM (M Z )=1/127.918(18) • G F =1.16637(1) 10 -5 GeV -2 • M Z =91.1876(21) GeV • Δ r: radiative corrections dominated by tb and Higgs loop HEP Seminar Oliver Stelzer-Chilton - Oxford 4

Measured Top Mass New Tevatron average (3 weeks ago): Top mass now measured to 1.8 GeV http://tevewwg.fnal.gov/top HEP Seminar Oliver Stelzer-Chilton - Oxford 5

Motivation Current top mass uncertainty 1.1% (1.8 GeV) → contributes 0.014 % (11 MeV) to δ M W Before Winter 2007: W mass uncertainty 0.036% (29 MeV) • Progress on W mass uncertainty now has the biggest impact on Higgs mass constraint • With improved precision also sensitive to possible exotic radiative corrections HEP Seminar Oliver Stelzer-Chilton - Oxford 6

Higgs Mass Prediction Before Winter 2007 Predicted Higgs mass from W loop corrections (LEP EWWG): m H =85 +39 -28 GeV (<166 GeV at 95% CL) direct search from LEP II: m H >114.4 GeV http://lepewwg.web.cern.ch/LEPEWWG/ HEP Seminar Oliver Stelzer-Chilton - Oxford 7

Analysis Strategy HEP Seminar Oliver Stelzer-Chilton - Oxford 8

Tevatron Collider • Tevatron is a proton antiproton collider with ~1 TeV per beam • Currently the only place in the world where W and Z bosons can be produced directly • 36 p and pbar bunches, 396 ns between bunch crossing, E CM =1.96 TeV HEP Seminar Oliver Stelzer-Chilton - Oxford 9

W Production Quark-antiquark annihilation dominates (80%) precise charged lepton measurement is the key (achieved ~0.03%) Recoil measurement (restricted to u<15 GeV) allows inference of neutrino p T p T ν =|-u-p T l | Combine information into transverse mass m T : l p T � (1 � cos � l � ) m T = 2 p T HEP Seminar Oliver Stelzer-Chilton - Oxford 10

W/Z Boson Production at the Tevatron • Initial state QCD radiation appears as soft “hadronic recoil” in calorimeter • Pollutes W mass information fortunately p T W << M W ± W ± 0 Z 0 W Z • Can use Z → ll decays to calibrate recoil model HEP Seminar Oliver Stelzer-Chilton - Oxford 11

W/Z Boson Production and Decay σ (W → l ν )=2775 pb σ (Z → ll)=254.9 pb From the high p T lepton triggers (p T >18 GeV) After event selection After event selection E T (l, ν ) > 30 GeV E T (l) > 30 GeV 51,128 W → µ ν candidates 4,960 Z → µµ candidates 63,964 W → e ν candidates 2,919 Z → ee candidates HEP Seminar Oliver Stelzer-Chilton - Oxford 12

Measurement Strategy W mass is extracted from transverse mass, transverse momentum and transverse missing energy distribution Fast Simulation • NLO event generator • Model detector effects W Mass templates Detector Calibration • Tracking momentum scale • Calorimeter energy scale 81 GeV • Recoil 80 GeV Data + Backgrounds Binned likelihood fit W Mass HEP Seminar Oliver Stelzer-Chilton - Oxford 13

W Mass Measurement p T W = 0 p T W ≠ 0 measured m T p T • Less sensitive to hadronic • Insensitive to p T W to 1 st order response modeling • Reconstruction of p T ν sensitive • Sensitive to W production to hadronic response and dynamics multiple interactions HEP Seminar Oliver Stelzer-Chilton - Oxford 14

CDF II Detector ■ Silicon tracking detectors ■ Central drift η = 1.0 = 1.0 η chambers (COT) ■ Solenoid Coil η = 2.0 = 2.0 η ■ EM calorimeter ■ Hadronic θ = 2.8 η = 2.8 calorimeter η ■ Muon scintillator counters ■ Muon drift chambers ■ Steel shielding HEP Seminar Oliver Stelzer-Chilton - Oxford 15

CDF II Detector HEP Seminar Oliver Stelzer-Chilton - Oxford 16

Tracking Momentum Scale Calibration HEP Seminar Oliver Stelzer-Chilton - Oxford 17

Tracker Alignment • Internal alignment is performed using a large sample of cosmic rays → Fit hits on both sides to one helix • Determine final track-level curvature corrections from electron-positron E/p difference in W → e ν decays • Statistical uncertainty of track-level corrections leads to systematic uncertainty Δ M W = 6 MeV HEP Seminar Oliver Stelzer-Chilton - Oxford 18

Momentum Scale Measurements • Template mass fits to J/ Ψ→ µµ , Υ→ µµ , Z → µµ resonances • Fast simulation models relevant physics processes - internal bremsstrahlung - ionization energy loss - multiple scattering • Simulation includes event reconstruction and selection • First principles simulation of tracking • Detector material model - Map energy loss and radiation lengths in each detector layer (3D lookup table in r, ϕ and z) • Overall material scale determined from data HEP Seminar Oliver Stelzer-Chilton - Oxford 19

Momentum Scale from J/ Ψ J/ ψ→ µµ • J/ ψ mass independent of p T default material scaled to • Slope affected by energy loss 0.94 to tune energy loss modelling • Measurement dominated by systematic uncertainties → QED and energy loss model  Data  Simulation HEP Seminar Oliver Stelzer-Chilton - Oxford 20

Momentum Scale from Υ • Υ provide invariant mass inter- mediate between J/ Ψ and Z’s • Υ are all primary tracks: can be  Data beam-constrained, like W tracks  Simulation • Test beam constraint by measuring mass using unconstrained tracks • Correct by half the difference  Data between fits and take corrections  Simulation as systematic uncertainty HEP Seminar Oliver Stelzer-Chilton - Oxford 21

Combined Momentum Scale from Quarkonia Δ p/p = (-1.50 ± 0.20) x 10 -3 • Systematic uncertainties: HEP Seminar Oliver Stelzer-Chilton - Oxford 22

Momentum Scale Cross-Check Z → µµ Apply momentum scale to Z → µµ sample Z mass in good agreement with PDG (91188 ± 2 MeV)  Data  Simulation All momentum scales consistent Δ M W = 17 MeV HEP Seminar Oliver Stelzer-Chilton - Oxford 23

EM Calorimeter Scale Calibration HEP Seminar Oliver Stelzer-Chilton - Oxford 24

Calorimeter Energy Calibration • Transfer momentum calibration to calorimeter by fitting peak E of E/p distribution of electrons from W decay p • Additional physics effects beyond those for muon tracks - photon radiation and conversion HEP Seminar Oliver Stelzer-Chilton - Oxford 25

r[m] z[m] Full Electron Simulation Response and resolution Energy loss into In EM calorimeter hadronic calorimeter Electromagnetic Calorimeter Energy loss in solenoid Track reconstruction In outer tracker Bremsstrahlung and t Conversions in silicon HEP Seminar Oliver Stelzer-Chilton - Oxford 26

Energy Scale Calibration • Calibrate calorimeter energy with peak of E/p distribution • Energy Scale S E set to S E =1 ± 0.00025 stat ± 0.00011 X0 ±0.00021 Tracker • Setting S E to 1 using E/p calibration W → e ν Calorimeter Energy> Calorimeter Energy<  Data  Simulation Track Momentum: Track Momentum: Energy loss in Energy loss in tracker Hadronic calorimeter HEP Seminar Oliver Stelzer-Chilton - Oxford 27

Consistency of Radiative Material Model • Excellent description of E/p tail • Radiative material tune factor: S mat =1.004 ± 0.009 stat ± 0.002 bkg • Z mass reconstructed from  Data electron track momenta only  Simulation geometry confirmed: S mat independent of | η | S mat  Data  Simulation Measured value in good | η i | agreement with PDG HEP Seminar Oliver Stelzer-Chilton - Oxford 28

Z Mass Cross-Check and Final Energy Scale • Fit Z Mass using scale from E/p calibration • Measure non-linearity through E/p fits in bins of E T in W → e ν and Z → ee data and apply correction to simulation Z → ee Z mass in good agreement with PDG (91188 ± 2 MeV)  Data  Simulation • Include Z → ee mass for final energy scale (30% weight) Δ M W = 30 MeV HEP Seminar Oliver Stelzer-Chilton - Oxford 29