1c. mills (Harvard U.)20 September,

2010

W and Z Physics at ATLASW and Z Physics at ATLAS

Corrinne Mills

Harvard DOE Site Visit

20 September 2010

2c. mills (Harvard U.)20 September,

2010

W and Z at the LHCW and Z at the LHC• 5 months of 7 TeV collisions

• 5 months of coherent effort by Harvard group on muon-focused analysis

• Results presented at PLHC, ICHEP, HCP/SUSY conferences

• Work shown here to be submitted for publication

event selectionevent selection

lepton charge asymmetry

lepton charge asymmetry

cross section calculations

cross section calculations

muon definition & efficiency

muon definition & efficiency

data qualitydata qualityQCD backgroundQCD background

KashifKashifMillsMills

BelloniBelloni

SmithSmith

KaganKagan

Martinez-Martinez-OutschoornOutschoorn

PrasadPrasad

Zevi della PortaZevi della PortaJeantyJeanty

3c. mills (Harvard U.)20 September,

2010

• Combined muon: matched inner detector (ID) and muon spectrometer (MS) track

• Selection: pT (combined) > 15 GeV

pT (MS) > 10 GeV

|pT(MS) – pT(ID)| < 15 GeV

Muons in ATLASMuons in ATLAS

|| < 2.4 (trigger geometry)

• Trigger: L1 (hardware) pT > 6 GeV

reject decays in flight

4c. mills (Harvard U.)20 September,

2010

Muon Quality Criteria Muon Quality Criteria • Leverage knowledge

from studies of cosmic ray data

• Consistency requirement for combined muon kinematics: |pT(MS) – pT(ID)| < 15 GeV

5c. mills (Harvard U.)20 September,

2010

• Refine muon selection: pT > 20 GeV and relative track isolation < 0.2 SumpT of tracks in cone around muon of R < 0.4, divided by the muon pT

• Reduce backgrounds by requiring ETmiss > 25 GeV

Selecting the W signal (I)Selecting the W signal (I)

electron channel muon channel

6c. mills (Harvard U.)20 September,

2010

Selecting the W signal (II)Selecting the W signal (II)• Clean up sample with MT > 40 GeV

• Transverse mass

electron channel muon channel

€

MT = 2(pTμ )(ET

miss)(1− cos(ϕ μ −ϕ ETmiss

))

7c. mills (Harvard U.)20 September,

2010

W Cross SectionW Cross Section

• Measure cross section times branching ratio BR(W→l )

• Theoretical prediction: 10.46 ± 0.02 nb

• Luminosity uncertainty is 11%

channel int. lumi. Ncand Nbackground acceptance x efficiency

electron 315 nb-1 1069 59.9 ± 10.8 0.304 ± 0.048

muon 310 nb-1 1181 100.4 ± 11.2 0.364 ± 0.034

€

σ =Ncand −NbackgroundAW × εW × L dt∫

channel cross section (nb)

electron 10.52 ± 0.34 (stat) ± 0.81 (sys) ± 1.16 (lum)

muon 9.58 ± 0.30 (stat) ± 0.50 (sys) ± 1.05 (lum)

combined 9.96 ± 0.23 (stat) ± 0.50 (sys) ± 1.10 (lum)

S. Prasad thesis: graduation ~ May 2011S. Prasad thesis: graduation ~ May 2011

8c. mills (Harvard U.)20 September,

2010

W Cross Section in ContextW Cross Section in Context

9c. mills (Harvard U.)20 September,

2010

Charge AsymmetryCharge Asymmetry• W+ favored in proton-proton collisions

• Sensitive to valence quark PDFs

€

A =σ l + −σ l −

σ l ++σ l −

electron muon

integral result 0.200 ± 0.022 (stat) ± 0.006 (sys)

theory prediction 0.20

V. Martinez-Outschoorn thesis: graduation ~ May 2011

V. Martinez-Outschoorn thesis: graduation ~ May 2011

10c. mills (Harvard U.)20 September,

2010

• Oppositely-charged muon candidates

• pT > 20 GeV, range, quality requirements as with W analysis, including track isolation

• 66 GeV < Mll < 116 GeV

Selecting the Z → Selecting the Z → signal signal

muon channelmuon

11c. mills (Harvard U.)20 September,

2010

Z Cross SectionZ Cross Section

• Measure cross section times branching ratio BR(W→l )

• Theoretical prediction: 0.964 ± 0.039 nb

• Luminosity uncertainty is 11%

channel int. lumi. Ncand Nbackground acceptance x efficiency

electron 316 nb-1 70 1.18 ± 0.43 0.290 ± 0.066

muon 331 nb-1 109 0.25 ± 0.04 0.376 ± 0.045

€

σ =Ncand −NbackgroundAZ × ε Z × L dt∫

channel cross section (nb)

electron 0.75 ± 0.09 (stat) ± 0.08 (sys) ± 0.08 (lum)

muon 0.87 ± 0.08 (stat) ± 0.05 (sys) ± 0.10 (lum)

combined 0.83 ± 0.06 (stat) ± 0.04 (sys) ± 0.09 (lum)

L. Kashif thesis: graduation ~ Dec. 2010L. Kashif thesis: graduation ~ Dec. 2010

12c. mills (Harvard U.)20 September,

2010

Z Cross Section in ContextZ Cross Section in Context

13c. mills (Harvard U.)20 September,

2010

More Data in the PipelineMore Data in the Pipelinem

uo

n c

han

nel

14c. mills (Harvard U.)20 September,

2010

ConclusionConclusion• Establishing the W and Z samples at ATLAS

• Rapidly increasing dataset Better precision W/Z properties, differential cross sections

W pT (next talk)

• W and Z data at the LHC will illuminate the Standard Model in a new momentum regime

• And pave the way to find what may lie beyond it key to validation of high-pT leptons and ET

miss

• Harvard role Developing baseline muon selection for high-pT muon

analysis Driving W and Z cross section analyses, W lepton charge

asymmetry in muon channel Major contributor to 310 nb-1 paper, to be submitted soon

15c. mills (Harvard U.)20 September,

2010

BackupBackup

16c. mills (Harvard U.)20 September,

2010

WW event selection event selectiongood run list, filled bunch crossing, jet cleaning (data only)

vertex with ≥ 3 matched tracks and |z| < 150 mm exists

passed trigger (via pT cut on matched L1 trigger object) L1_MU6

at least one combined muon with pT > 15, || < 2.4 exists

muon spectrometer pT > 10 GeV/c

| pT (spectrometer) – pT (ID) | < 15 GeV/c

combined muon |z0 - z(pv)| < 10 mm

muon combined pT > 20 GeV/c

muon || < 2.4

(track iso (cone 0.4))/pT() < 0.2

MET > 25 GeV

transverse mass > 40 GeV

PR

ES

EL

EC

TIO

NW

SE

LE

CT

ION

17c. mills (Harvard U.)20 September,

2010

Backgrounds to W → Backgrounds to W → • Z → , W → , Z → , ttbar: 77.6 ± 5.4 (stat+sys) events

From simulation

• QCD: 21.1 ± 9.8 (stat+sys) events “Matrix Method”

Solve for NQCD using number of candidates with and without isolation req. (Nloose = 1272, Nisol = 1181)

Measure non-QCD = 0.984 ± 0.01 from Z’s

Measure QCD in data with 15 < pT < 20 GeV (get 0.292 ±

0.004) extrapolate to pT

> 20 GeV by scaling based on simulated dijet events (get 0.227)

• Cosmics: 1.7 ± 0.8 event Consideration of empty and unpaired bunch crossings

18c. mills (Harvard U.)20 September,

2010

QCD BG: Matrix Method (1)QCD BG: Matrix Method (1)• Solve for NQCD in isolated candidate sample

• Nisol (1181) and Nloose (1272) are number of W candidates with and without isolation cut

• QCD and non-QCD are efficiency of isolation cut for QCD and prompt muons Measure non-QCD = 0.984 ± 0.01 in tag-and-probe with Z’s

Measure QCD in QCD-dominated data: candidate events with 15 < pT

< 20 GeV

extrapolate to pT > 20 GeV by scaling by (pT

> 20 GeV)/(15 < pT

< 20 GeV) as measured in the MC (more on next slide)

19c. mills (Harvard U.)20 September,

2010

QCD BG: Matrix Method (2)QCD BG: Matrix Method (2)• Measure QCD in QCD-dominated data: candidate events

with 15 < pT < 20 GeV (get 0.292 ± 0.004)

extrapolate to pT > 20 GeV by scaling by (pT

> 20 GeV)/(15 < pT

< 20 GeV) as measured in the MC

(0.238 ± 0.005)/(0.307 ± 0.003) = 0.776 ± 0.017• Uncertainties

systematic from 100% uncertainty on extrapolation

stat. uncert. from non-QCD also significant

Bottom line21.1 ± 4.5 (stat) ± 8.7 (sys)21.1 ± 4.5 (stat) ± 8.7 (sys)

20c. mills (Harvard U.)20 September,

2010

Backgrounds to ZBackgrounds to Z• Predicted total backgrounds:

electron: 1.18 ± 0.11 (stat) ± 0.41 (syst) muon: 0.25 ± 0.01 (stat) ± 0.04 (syst) compare to 3 (0) same-sign events in electron (muon)

channel 2.8 same-sign events from Z → ee signal are expected

• Magnitude is small (<1% relative to expected signal)

• ttbar

• Z → • W → e/• QCD (muon channel)

• QCD (electron channel) Sideband subtraction for loose-loose electron-positron pairs Apply loose medium “rejection factor” measured in data

from simulation

21c. mills (Harvard U.)20 September,

2010

Electrons in ATLASElectrons in ATLAS• EM calorimeter cluster

matched to inner detector (ID) track

• ET > 20 GeV, || < 2.47 exclude gap between

barrel and endcap 1.37 < || < 1.52

• “Loose” selection shower shape in middle

layer of calorimeter

• “Medium” selection

add fine-granularity shower shape and track match

“Tight” selection add E/p, more track quality, high-threshold TRT hits, conversion veto

• Trigger: Level 1 (hardware) requires coarse-granularity cluster with || < 2.5 ET > 5 GeV

22c. mills (Harvard U.)20 September,

2010

More on ElectronsMore on Electrons• Trigger: sliding-window algorithm using reduced-granularity clusters x

= 0.1 x 0.1

• Offline reconstruction: sliding window of 3x5 cells or 0.075 x 0.125 in x Electron = cluster with ET > 2.5 GeV and matched track with pT > 0.5 GeV

• Reconstruction: exact requirements vary with ET and ||, but three categories:

• Loose electrons Fiducial: || < 2.37 and exclude 1.37 < || < 1.52 Shower shape in middle (largest) layer of calorimeter: cluster width in Hadronic leakage: ET(innermost later of HCAL) / cluster ET

• Medium electrons: loose += Shower shape in innermost (finely segemented in ) layer of calorimeter Track match () Track quality (pixel, SCT hits and impact parameter)

• Tight electrons: medium += High-threshold hits in transition-radiation tracker (TRT); hit in innermost pixel

layer E/p

• http://cdsweb.cern.ch/record/1273197/files/ATLAS-CONF-2010-005.pdf