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

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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

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First Run II Measurement of the W Boson Mass by CDF

High Energy Physics Seminar Michigan State University Oliver Oliver Stelzer-Chilton Stelzer-Chilton University of Oxford

April 3rd, 2007

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HEP Seminar Oliver Stelzer-Chilton - Oxford 2

Outline

1. 1. Motivation Motivation 2. 2. W Production at the W Production at the Tevatron Tevatron 3. 3. Analysis Strategy Analysis Strategy 4. 4. Detector Calibration Detector Calibration

  • Momentum Scale

Momentum Scale

  • Energy Scale

Energy Scale

  • Recoil

Recoil 5. 5. Event Simulation Event Simulation 6. 6. Results Results 7. 7. Conclusions Conclusions

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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

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Introduction

  • Derive W mass from precisely measured electroweak quantities
  • where MW=MZcosθW
  • αEM(MZ)=1/127.918(18)
  • GF=1.16637(1) 10-5 GeV-2
  • MZ=91.1876(21) GeV
  • Δr: radiative corrections dominated by tb and Higgs loop

mW

2 =

em 2GF sin2W (1 r)

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Measured Top Mass

New Tevatron average (3 weeks ago): Top mass now measured to 1.8 GeV http://tevewwg.fnal.gov/top

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  • Progress on W mass uncertainty now has the biggest impact on

Higgs mass constraint

  • With improved precision also sensitive

to possible exotic radiative corrections

Motivation

Current top mass uncertainty 1.1% (1.8 GeV) → contributes 0.014 % (11 MeV) to δMW Before Winter 2007: W mass uncertainty 0.036% (29 MeV)

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Higgs Mass Prediction

Predicted Higgs mass from W loop corrections (LEP EWWG): mH=85+39

  • 28 GeV (<166 GeV at 95% CL)

direct search from LEP II: mH>114.4 GeV Before Winter 2007

http://lepewwg.web.cern.ch/LEPEWWG/

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Analysis Strategy

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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, ECM=1.96 TeV

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W Production

precise charged lepton measurement is the key (achieved ~0.03%) mT = 2pT

l pT (1 cosl )

Combine information into transverse mass mT: Recoil measurement (restricted to u<15 GeV) allows inference of neutrino pT pT

ν=|-u-pT l|

Quark-antiquark annihilation dominates (80%)

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W/Z Boson Production at the Tevatron

  • Initial state QCD radiation appears

as soft “hadronic recoil” in calorimeter

  • Pollutes W mass information

fortunately pT

W << MW

W W±

±

Z Z0

  • Can use Z→ll decays to calibrate recoil model
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W/Z Boson Production and Decay

σ(W→lν)=2775 pb After event selection ET(l,ν) > 30 GeV 51,128 W→µν candidates 63,964 W→eν candidates σ(Z→ll)=254.9 pb After event selection ET(l) > 30 GeV 4,960 Z→µµ candidates 2,919 Z→ee candidates From the high pT lepton triggers (pT>18 GeV)

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81 GeV 80 GeV

Measurement Strategy

W mass is extracted from transverse mass, transverse momentum and transverse missing energy distribution

Detector Calibration

  • Tracking momentum scale
  • Calorimeter energy scale
  • Recoil

Fast Simulation

  • NLO event generator
  • Model detector effects

W Mass templates + Backgrounds Data Binned likelihood fit W Mass

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W Mass Measurement

pT

W = 0

pT

W ≠ 0

measured

mT

  • Insensitive to pT

W to 1st order

  • Reconstruction of pT

ν sensitive

to hadronic response and multiple interactions

pT

  • Less sensitive to hadronic

response modeling

  • Sensitive to W production

dynamics

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CDF II Detector

θ

η η = 1.0 = 1.0 η η = 2.8 = 2.8 η η = 2.0 = 2.0

■ Silicon tracking detectors ■ Central drift chambers (COT) ■ Solenoid Coil ■ EM calorimeter ■ Hadronic calorimeter ■ Muon scintillator counters ■ Muon drift chambers ■ Steel shielding

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CDF II Detector

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Tracking Momentum Scale Calibration

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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→eν decays

  • Statistical uncertainty of

track-level corrections leads to systematic uncertainty ΔMW= 6 MeV

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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

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Momentum Scale from J/Ψ

  • J/ψ mass independent of pT
  • Slope affected by energy loss

modelling

  • Measurement dominated by

systematic uncertainties → QED and energy loss model

  • Data
  • Simulation

J/ψ→µµ

default material scaled to 0.94 to tune energy loss

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  • Υ provide invariant mass inter-

mediate between J/Ψ and Z’s

  • Υ are all primary tracks: can be

beam-constrained, like W tracks

Momentum Scale from Υ

  • Test beam constraint by measuring

mass using unconstrained tracks

  • Correct by half the difference

between fits and take corrections as systematic uncertainty

  • Data
  • Simulation
  • Data
  • Simulation
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Δp/p = (-1.50 ± 0.20) x 10-3

  • Systematic uncertainties:

Combined Momentum Scale from Quarkonia

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Momentum Scale Cross-Check

Z→µµ Apply momentum scale to Z→µµ sample Z mass in good agreement with PDG (91188±2 MeV) ΔMW= 17 MeV All momentum scales consistent

  • Data
  • Simulation
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EM Calorimeter Scale Calibration

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Calorimeter Energy Calibration

  • Transfer momentum calibration to calorimeter by fitting peak
  • f E/p distribution of electrons from W decay
  • Additional physics effects beyond those for muon tracks
  • photon radiation and conversion

E p

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Full Electron Simulation

Response and resolution In EM calorimeter Energy loss into hadronic calorimeter Energy loss in solenoid Track reconstruction In outer tracker Bremsstrahlung and Conversions in silicon

Electromagnetic Calorimeter

t

z[m] r[m]

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Energy Scale Calibration

W→eν

  • Calibrate calorimeter energy with peak of E/p distribution
  • Energy Scale SE set to SE=1±0.00025stat±0.00011X0±0.00021Tracker
  • Setting SE to 1 using E/p calibration

Calorimeter Energy< Track Momentum: Energy loss in Hadronic calorimeter Calorimeter Energy> Track Momentum: Energy loss in tracker

  • Data
  • Simulation
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Consistency of Radiative Material Model

  • Excellent description of E/p tail
  • Radiative material tune factor:

Smat=1.004±0.009stat±0.002bkg

  • Z mass reconstructed from

electron track momenta only geometry confirmed: Smat independent of |η| Measured value in good agreement with PDG

  • Data
  • Simulation
  • Data
  • Simulation

Smat |ηi|

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  • Fit Z Mass using scale from E/p calibration
  • Measure non-linearity through E/p fits in bins of ET in W→eν and

Z→ee data and apply correction to simulation

  • Include Z→ee mass for final energy scale (30% weight)

ΔMW= 30 MeV

Z Mass Cross-Check and Final Energy Scale

Z→ee Z mass in good agreement with PDG (91188±2 MeV)

  • Data
  • Simulation
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  • Tracking resolution parametrized in fast Monte Carlo by
  • Drift chamber hit resolution σh=150±3stat µm
  • Beamspot size σb=39±3stat µm
  • Tuned on widths of Z→µµ and Υ→µµ distribution

ΔMW= 3 MeV

  • Electron cluster resolution parametrized by 13.5%/√ET ⊕ κ
  • primary electron constant term: κ=0.89±0.15stat%
  • secondary photon resolution: κ=8.3±2.2stat%
  • Tuned on the widths of the E/p peak and Z→ee peak

(selecting radiative electrons) ΔMW= 9 MeV

Detector Resolutions

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Hadronic Recoil Model

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Hadronic Recoil Definition

Recoil definition: → Energy vector sum over all calorimeter towers, excluding:

  • lepton towers
  • towers near beamline

(“ring of fire”)

  • Lepton removal also removes

underlying event  Need to measure recoil under lepton

  • Recoil under lepton depends
  • n lepton tower definition
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Lepton Removal

Electrons: Remove 7 towers keystone (shower) ΔMW= 8 MeV

  • Estimate removed recoil energy using towers separated in Φ
  • Model tower removal in simulation

Muons: Remove 3 towers (MIP) ΔMW= 5 MeV

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Hadronic Recoil Simulation

Recoil momentum vector u has two components:

  • Soft spectator interaction component, randomly oriented
  • modelled using minimum bias data with tuneable magnitude
  • A hard ‘jet’ component, directed opposite the boson pT
  • pT-dependent response and resolution parametrization
  • Hadronic response R=(umeas/utrue)
  • R parametrized as a logarithmically increasing function of boson pT

motivated by Z boson data

  • Data
  • Simulation
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Hadronic Recoil Response Calibration

  • Project vector sum of pT(ll) and u on orthogonal

axes defined by lepton directions

  • Use Z balancing to calibrate recoil energy scale
  • Mean and RMS of projections as a function of

pT(ll) provide information for model parameters Hadronic model parameters tuned by minimizing χ2 between data and simulation ΔMW= 9 MeV

  • Data
  • Simulation
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Hadronic Recoil Resolution Calibration

µ µ η

u

ΔMW= 7 MeV Resolution at low pT(Z) dominated by underlying event Resolution at high pT(Z) dominated by jet resolution

  • Data
  • Simulation

ξ

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Recoil Model Checks

  • Apply model to W sample to

check recoil model from Z’s

  • Recoil projection along lepton

direction u|| → directly affects mT fits → Sensitive to: lepton removal, efficiency model, scale, resolution, W decay

  • Recoil projection perpendicular

to lepton direction u⊥ → Sensitive to resolution model

  • Data
  • Simulation
  • Data
  • Simulation
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Recoil Model Checks

  • Recoil distribution

→ Sensitive to recoil scale, resolution and boson pT

  • Recoil model validation plots

confirm the consistency of the model

  • Data
  • Simulation
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Event Simulation

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Signal Simulation and Template Fitting

  • All signals simulated using a fast simulation
  • Generate finely-spaced templates as a function of fit variable
  • perform binned maximum-likelihood fits to the data
  • Custom fast simulation makes smooth, high statistics templates
  • provides analysis control over key components of simulation
  • We will extract the W mass from six kinematic distributions:

mT, pT and ET for muon and electron channel

81 GeV 80 GeV

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Generator-level Signal Simulation

  • Generator-level input for W&Z simulation provided by RESBOS

[Balazs et.al. PRD56, 5558 (1997)]

  • Radiative photons generated according to energy vs angle lookup

table from WGRAD [Baur et.al. PRD59, 013002 (1998)]

  • Simulate FSR (ISR, photons off the propagator, ΔMW< 5 MeV)
  • Apply 10% correction for 2nd photon

[Calame et.al. PRD69, 037301 (2004)] and take 5% systematic uncertainty ΔMW= 11 (12) MeV for e (µ) RESBOS WGRAD

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Boson pT Model

  • Model boson pT using RESBOS

generator

  • non pertubative regime at low

pT parametrized with g1, g2, g3 parameters [Landry et.al. PRD67, 073016 (2003)]

  • g2 parameter determines

position of peak in pT distribution

  • Measure g2 with Z boson data

(other parameters negligible)

  • Find: g2 = 0.685±0.048

ΔMW= 3 MeV

  • Data
  • Simulation
  • Data
  • Simulation
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Parton Distribution Functions

  • Affect W kinematic lineshape through acceptance cuts

(only use |η|<1)

  • We use CTEQ6M as the default
  • Use CTEQ6 ensemble of 20 ‘uncertainty PDFs’:

[Pumplin et.al. JHEP, 0207 (2002)]  20 free parameters in global fit  compute δMW contribution from each error PDF

  • Using CTEQ prescription and interpreting ensemble as 90% CL

ΔMW= 11 MeV

  • Cross-check: Fitting MC sample generated with MRST2003

[Martin et.al. Eur. Phys. Jour. C28, 455 (2003)] with default CTEQ6M template yields a 8 MeV shift in W mass

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Backgrounds

  • Backgrounds have very different lineshapes compared to W signal
  • distributions are added to template
  • QCD measured with data
  • EWK predicted with Monte Carlo

0.93±0.03 0.89±0.02 W→τν 0.24±0.04 6.6±0.3 Z→ll

  • 0.05±0.05

Cosmic Rays

  • 0.3±0.2

Decay in Flight 0.25±0.15 0.1±0.1 Hadronic Jets %(Electrons) %(Muons) Background

ΔMW= 8 (9) MeV for e (µ)

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W Boson Mass Fits

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Transverse Mass Fit (Muons)

  • Data
  • Simulation
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Transverse Mass Fit (Electrons)

  • Data
  • Simulation

Muon and Electron combined: MW=80417±48 MeV P(χ2)=7%

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Transverse Momentum Fit (Muons)

  • Data
  • Simulation
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Transverse Energy Fit (Electrons)

  • Data
  • Simulation

Muon and Electron combined: MW=80388±59 MeV P(χ2)=18%

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Missing Transverse Energy Fit (Muons)

  • Data
  • Simulation
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Missing Transverse Energy Fit (Electrons)

  • Data
  • Simulation

Muon and Electron combined: MW=80434±65 MeV P(χ2)=43%

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Systematic Uncertainty

Systematic uncertainty on transverse mass fit ⇒ Combined Uncertainty: 48 MeV for 200 pb-1

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Results

  • Combining all six mass fits yields:

MW=80413±48 MeV (stat+syst), P(χ2)=44%

  • New CDF result is the world’s most precise single measurement
  • World average increases:

80392 to 80398 MeV

  • Uncertainty reduced ~15%

(29 to 25 MeV)

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Previous MW vs Mtop

Summer 2006

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Updated MW vs Mtop

Winter 2007 New CDF W Mass

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Latest Higgs Constraint

March 2007 New top mass

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Standard Model Higgs Constraint

  • Summer 2006 SM Higgs fit: (LEP EWWG)
  • MH = 85+39
  • 28 GeV
  • MH < 166 GeV (95% CL)
  • MH < 199 GeV (95% CL) Including LEPII direct exclusion
  • Updated preliminary SM Higgs fit: (With new CDF W Mass)
  • MH = 80+36
  • 26 GeV (M. Grünewald, private communication)
  • MH < 153 GeV (95% CL)
  • MH < 189 GeV (95% CL) Including LEPII direct exclusion
  • Updated preliminary SM Higgs fit: (With new Tevatron top mass)
  • MH = 76+33
  • 24 GeV
  • MH < 144 GeV (95% CL)
  • MH < 182 GeV (95% CL) Including LEPII direct exclusion
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Progress since 1995

2007 indirect mt and mw Winter 2007 direct mt and mw 1995 direct mt and mw 1995 indirect mt and mw

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Projection

  • Projection from previous Tevatron measurements
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Summary

  • W boson mass remains a very interesting parameter to measure

with increasing precision

  • CDF Run II measurement is the most precise single measurement

MW = 80413 ± 34 ± 34 MeV = 80413 ± 48 MeV (preliminary)

  • New preliminary Higgs constraint MH = 76+33
  • 24 GeV (LEP EWWG)

(including new CDF W boson mass and new top quark mass average) → Mass has moved further into the directly excluded region Looking forward: → Expect ΔMW < 25 MeV with >1.5 fb-1 already collected by CDF

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Higgs

What is the Higgs mass?

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Backup Slides

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Systematic Uncertainty

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Signed χ

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Latest Higgs Constraint

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Higgs Sensitivity

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Consistency Checks of Results

  • Use BLUE method to combine results and check consistency
  • List of obtained χ2 and probabilities for several combinations:
  • two transverse mass fits: χ2/dof = 3.2/1, prob = 7%
  • charged lepton fits: χ2/dof = 1.8/1, prob = 18%
  • two MET fits: χ2/dof = 0.6/1, prob = 43%
  • all three fits for electrons: χ2/dof = 1.4/2, prob = 49%
  • all three fits for muons: χ2/dof = 0.8/2, prob = 69%
  • all six fits, both channels: χ2/dof = 4.8/5, prob = 44%
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Tevatron Run I Uncertainties

84 113 144 Total 10 10 10 Γ(W) 12 11 11 QED rad. Corrections 8 15 15 Parton dist. Functions 9 5 25 Backgrounds 12

  • 18

Selection bias 15 15 20 pT(W) 35 37 35 Recoil model 19 25 20 Lepton resolution 56 75 85 Lepton energy scale 60 65 100 Statistics D0 e CDF e CDF µ

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Energy Loss Model

  • Use GEANT to parametrize energy loss in solenoid and leakage

into hadronic calorimeter

  • Energy loss in hadronic calorimeter
  • Relevant for E/p lineshape
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Measurement of EM Calorimeter Non-Linearity

  • Perform E/p fit-based calibration in bins of electron ET
  • Parametrize non-linear response as SE=1+ξ(ET/GeV-39)
  • Apply energy dependent scale to simulated electron and photon
  • Tune W and Z data: ξ=(6±7)x10-5

SE SE ET (e) (GeV) ET (e) (GeV)

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Tracker Alignment

Central Outer Tracker: Open-cell drift chamber

  • Use clean sample of cosmic

rays for cell-by-cell internal alignment

  • Fit COT hits on both sides

simultaneously to a single helix

  • Measure cell tilts and shifts
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Alignment Example

Cell shift (microns) Final relative alignment of cells ~5µm (initial alignment ~50µm)

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COT Wire Alignment

  • Fit separate helices to cosmic ray tracks on each side
  • Compare track parameters of the two tracks
  • Measure of track parameter bias

Curvature: Z[cm]

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Material Distribution