w and z physics at atlas
DESCRIPTION
W and Z Physics at ATLAS. Corrinne Mills Harvard DOE Site Visit 20 September 2010. W and Z at the LHC. 5 months of 7 TeV collisions 5 months of coherent effort by Harvard group on muon-focused analysis - PowerPoint PPT PresentationTRANSCRIPT
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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
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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
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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
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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
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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
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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
))
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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
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8c. mills (Harvard U.)20 September,
2010
W Cross Section in ContextW Cross Section in Context
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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
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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
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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
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12c. mills (Harvard U.)20 September,
2010
Z Cross Section in ContextZ Cross Section in Context
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13c. mills (Harvard U.)20 September,
2010
More Data in the PipelineMore Data in the Pipelinem
uo
n c
han
nel
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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
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15c. mills (Harvard U.)20 September,
2010
BackupBackup
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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
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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
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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)
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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)
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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
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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
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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