Measurement of W p and Z Production in pp Collisions at s · Introduction Signal De nition ATLAS...

Transcript of Measurement of W p and Z Production in pp Collisions at s · Introduction Signal De nition ATLAS...

Introduction

SignalDefinition

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e and γ ID

γ Isolation

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KinematicDistributions

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Measurement of Wγ and Zγ Production inpp Collisions at

√s = 7 TeV with the

ATLAS Detector

Andrea BocciOn behalf of the ATLAS Collaboration

Duke University

DIS 2011, April 14th, 2011
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Outline

1 IntroductionSignal Definition

2 ATLAS Detectore and γ IDγ Isolation

3 Data AnalysisEvent SelectionKinematic DistributionsSignal Yield

4 Cross SectionsCalculationResults

5 Conclusions

6 Backup
Introduction

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Introduction

The Wγ and Zγ productions are direct test of thenon-Abelian nature of the Electroweak theory

Probing the WWγ triple gauge boson coupling (TGC)

Probing the presence of ZZγ and Zγγ TGC forbidden(at the tree level) in the Standard Model

Highest cross sections among all diboson processes

Amplitude interferences between u− and t−channelsuppresses the Wγ production w.r.t. Zγ production

WWγ TGCq̄ γ

W

l

ν̄

q

W

ISR Production (u− and t−channel)q̄

q

γ

W (Z)

l

ν̄(l)q

q̄

q

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W (Z)l

ν̄(l)

q
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Introduction






WWγ TGCq̄ γ

W

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ν̄

q

W


q

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ν̄(l)q

q̄

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W (Z)l

ν̄(l)

q
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Introduction






WWγ TGCq̄ γ

W

l

ν̄

q

W


q

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l

ν̄(l)q

q̄

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ν̄(l)

q
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Introduction






WWγ TGCq̄ γ

W

l

ν̄

q

W


q

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W (Z)

l

ν̄(l)q

q̄

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W (Z)l

ν̄(l)

q
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Introduction






WWγ TGCq̄ γ

W

l

ν̄

q

W


q

γ

W (Z)

l

ν̄(l)q

q̄

q

γ

W (Z)l

ν̄(l)

q
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Signal Definition: lνγ and llγ final states

The experimental signature of these processes are thelνγ+X and llγ+X final states.

Besides the TGC and ISR contributions they include also:

QED FSR from W(Z) inclusive production

Dominating for EγT . 40 GeV

High order O(ααS) contributions (NLO corrections)Photons from fragmentation of jets produced inassociation with a W or a Z boson (W (Z)+jet events)

WWγ TGCq̄ γ

W

l

ν̄

q

W


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q̄

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Signal Definition: lνγ and llγ final states

The experimental signature of these processes are thelνγ+X and llγ+X final states.


QED FSR from W(Z) inclusive production

Dominating for EγT . 40 GeV


WWγ TGCq̄ γ

W

l

ν̄

q

W


q

γ

W (Z)

l

ν̄(l)q

q̄

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W (Z)l

ν̄(l)

q
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Signal Definition: lνγ and llγ final statesThe experimental signature of these processes are the

lνγ+X and llγ+X final states.


QED FSR from W(Z) inclusive productionDominating for EγT . 40 GeV


QED FSR

q̄

q

γ

W (Z) l

ν̄(l)q
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High order O(ααS) contributions (NLO corrections)

Photons from fragmentation of jets produced inassociation with a W or a Z boson (W (Z)+jet events)

QED FSR

q̄

q

γ

W (Z) l

ν̄(l)q
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QED FSR

q̄

q

γ

W (Z) l

ν̄(l)q

“Fragmentation” Photon Production

q̄

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Dg/γ(z,Q2)

q

g

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W

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Dq/γ(z, Q2)

q
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Fragmentation Photon ContributionPhotons from fragmentation of jets produced in association with a W ora Z boson (W (Z)+jet events)

Only the sum of the prompt and fragmentation components isphysically well defined ⇒ Part of the signalStrongly suppressed by the photon identification and isolationrequirements, but still significantly contributing

Because of collinear divergences the measurements are restrictedto events with ΣEhadT < 0.5 · EγTExperimentally challenging because of large uncertainties inestimating its contribution in data, and because of a very differentidentification efficiency w.r.t. “prompt” photons


q̄

q

γ

W

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ν̄

Dg/γ(z,Q2)

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g

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Dq/γ(z, Q2)

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Only the sum of the prompt and fragmentation components isphysically well defined ⇒ Part of the signal

Strongly suppressed by the photon identification and isolationrequirements, but still significantly contributing



q̄

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q̄

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γ

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q

g

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Dq/γ(z, Q2)

q
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Because of collinear divergences the measurements are restrictedto events with ΣEhadT < 0.5 · EγT

Experimentally challenging because of large uncertainties inestimating its contribution in data, and because of a very differentidentification efficiency w.r.t. “prompt” photons


q̄

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q̄

q

γ

W

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ν̄

Dg/γ(z,Q2)

q

g

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A ToroidaL LHC ApparatuS
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The Inner Detector

Pixel Tracker3 Barrel, 2×3 Endcap LayersResolution: 10µm (Rφ)60M Channels

SCT Tracker4 Barrel, 2×8 Endcap LayersDouble Stereo SidesResolution: 17µm (Rφ)6.3M Channels

TRT Tracker73, 2×160 LayersAbout 30 hits/trackResolution: 130µm (Rφ)PID Capability∼ 0.3M Channels

Silicon Pixel, Silicon Strips, TransitionRadiation Detectors

Coverage: |η| < 2.5 in 2T B-field

σ/p2T (GeV ) ∼ 3.8 · 10−4 ⊕ 0.015
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The Inner Detector

Pixel Tracker3 Barrel, 2×3 Endcap LayersResolution: 10µm (Rφ)60M Channels

SCT Tracker4 Barrel, 2×8 Endcap LayersDouble Stereo SidesResolution: 17µm (Rφ)6.3M Channels

TRT Tracker73, 2×160 LayersAbout 30 hits/trackResolution: 130µm (Rφ)PID Capability∼ 0.3M Channels

e/π Separation in TRT

Transition radiation depending on thecharge particle Lorentz factor γ
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The Electromagnetic Calorimeter

Liquid Argon and Lead with accordion geometryCoverage: |η| < 3.2 (with forward ECAL |η| < 4.9)

Three longitudinal samplings (plus a thin pre-sampler for |η| < 1.8)∆E/E ∼ 10%/

pE(GeV )
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The Electromagnetic CalorimeterLiquid Argon and Lead with accordion geometry

Coverage: |η| < 3.2 (with forward ECAL |η| < 4.9)Three longitudinal samplings (plus a thin pre-sampler for |η| < 1.8)

∆E/E ∼ 10%/pE(GeV )

EM GranularityLongitudinal segmentation for maximum

background rejection

∆ϕ = 0.0245

∆η = 0.02537.5mm/8 = 4.69 mm∆η = 0.0031

∆ϕ=0.0245x436.8mmx4=147.3mm

Trigger Tower

TriggerTower∆ϕ = 0.0982

∆η = 0.1

16X0

4.3X0

2X0

1500

mm

470 m

m

η

ϕ

η = 0

Strip cells in Layer 1

Square cells in Layer 2

1.7X0

Cells in Layer 3∆ϕ×�∆η = 0.0245×�0.05

Second layer collect most of the EM shower energy(cell ∆η × ∆φ = 0.025 × 0.025)

First (strip) later with high granularity

(event-by-event π0/γ discrimination)Third layer for tails of very high EM shower

Pre-sampler for energy loss corrections

π0/γ Discrimination

Single Photon

π0 Candidate
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Electron and Photon IdentificationEM object identification algorithm

fully exploits the longitudinal segmentation of the EM calorimeter, thetracking information and the TRT particle identification capability.

Identification DiscriminantsHadronic energy leakage

Middle layer energy and lateral width(Rφ, Rη)

Strip layer lateral shower

Strip layer first and second maximumenergy ratio

Track quality

Track/Calorimeter matching

First pixel layer hit

E/p ratio

Transition radiation probability

Three (Two) baseline set of identification cutsfor electrons (photons)

Optimized in ET and η binning for uniformefficiency. For photons optimized separately for

converted and unconverted photons
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Track quality



E/p ratio




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Track quality



E/p ratio





e: EM Fraction in Layer 1

e: EM Fraction in Layer 2
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Track quality



E/p ratio





γ unconverted: Rφ

γ converted: Rφ
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Photon Energy Isolation

Transverse energy in a cone with R = 0.4around the photon

Taken as a sum of uncalibrated cell energy

Central core 5× 7 not includedCorrections for out-of-core leakage

In principle sensitive to underlying event andpileup....

“Ambient” energy density measuredby looking at underlying event inadditional cones in the same event(method proposed by Cacciari,Salam, Sapeta, and Soyez)http://arxiv.org/abs/0912.4926

Photon energy isolation different fordirect photons and photon fromfragmentation

http://arxiv.org/abs/0912.4926
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Event Selection

Require an high ET photon on W(Z) candidate events

W Selection

One lepton with ET > 20GeV

|η| < 2.47 (e) or |η| < 2.4 (µ)EmissT > 25 GeV

mWT > 40 GeV

Veto on a second lepton

Z Selection

Two leptons with ET > 20GeV

|η| < 2.47 (e) or |η| < 2.4 (µ)Mll > 40 GeV

Photon Selection

One photon with ET > 15GeV and |η| < 2.37∆R(l, γ) > 0.7

Isolation Energy EisoT < 5GeV

Number of Candidatesin 35 pb−1

Wγ: 19295 (eνγ) + 97 (µνγ)

Zγ: 4825 (e+−γ) + 23 (µ+µ−γ)
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Event Selection


W Selection



mWT > 40 GeV


Z Selection


|η| < 2.47 (e) or |η| < 2.4 (µ)Mll > 40 GeV

Photon Selection




Wγ: 19295 (eνγ) + 97 (µνγ)

Zγ: 4825 (e+−γ) + 23 (µ+µ−γ)
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Event Selection


W Selection



mWT > 40 GeV


Z Selection


|η| < 2.47 (e) or |η| < 2.4 (µ)Mll > 40 GeV

Photon Selection




Wγ: 19295 (eνγ) + 97 (µνγ)

Zγ: 4825 (e+−γ) + 23 (µ+µ−γ)
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Event Selection


W Selection



mWT > 40 GeV


Z Selection


|η| < 2.47 (e) or |η| < 2.4 (µ)Mll > 40 GeV

Photon Selection




Wγ: 19295 (eνγ) + 97 (µνγ)

Zγ: 4825 (e+−γ) + 23 (µ+µ−γ)
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Candidate Event Kinematic Distributions

Wγ Candidates: Photon ET Distribution
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Wγ Candidates: ∆R(l, γ) Distribution
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Wγ Candidates: mT (l, γ) Distribution
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Wγ Candidates: mT (l, EmissT , γ) Distribution
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Zγ Candidates: ml,l,γ Distribution
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Zγ Candidates: ml,l,γ vs. ml,γ
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Signal Yield Extraction

A 2D-sideband method is used to extract the signal yielddirectly from Wγ and Zγ candidate eventsDiscriminating Variable: Isolation Energy

Isolation Energy Distributions

Wγ Events Zγ Events

Isolation background shape from data.Normalization from the tail of the isolation distribution.
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A 2D-sideband method is used to extract the signal yielddirectly from Wγ and Zγ candidate eventsDiscriminating Variable: Isolation Energy

Isolation Energy Distributions

Wγ Events Zγ Events

Isolation background shape from data.Normalization from the tail of the isolation distribution.
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To model the background isolation energy distribution:reverse some photon ID cuts

Photons are required to pass the ID cuts except the striplayers variables

Assuming no/little correlation between photon ID variablesand isolation

Strip variables fairly uncorrelated with isolation energy
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Method applied only to the Wγ analysis(very limited statistics on Zγ)

Corrections for signal presence in control region applied

Contributions from other processes (W → τν, tt̄, Z → ee,etc..) estimated from MC.

Systematics due to the assumptions of the method and thedefinition of the control regions carefully estimated.
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Results: signal purity in selected events ∼ 80%
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Cross Section Calculation

σpp→lνγ(l+l−γ) =N sigWγ(Zγ)

CWγ(Zγ) · LWγ(Zγ) ·AWγ(Zγ)

Nsig is the number of the extracted signal events

L is the the integrated luminosity

C summarizes the reconstruction and identification efficiency

A is the acceptance of the “total” cross section

Cross section measured in two phase-space regions:

Fiducial Phase-space where the measurement has been performed(defined by the cuts reported earlier in the “selection slide”)

Total Phase-space extended to regions outside experimentalacceptance:fiducial total extrapolation done with Monte Carlo“Total” phase-space definition:

EγT > 10 GeV, ∆R(l, γ) > 0.5, ΣEhadT < 0.5 · E

γT
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Cross Section Calculation

σpp→lνγ(l+l−γ) =N sigWγ(Zγ)

CWγ(Zγ) · LWγ(Zγ) ·AWγ(Zγ)

Nsig is the number of the extracted signal events

L is the the integrated luminosity

C summarizes the reconstruction and identification efficiency

A is the acceptance of the “total” cross section

Cross section measured in two phase-space regions:

Fiducial Phase-space where the measurement has been performed(defined by the cuts reported earlier in the “selection slide”)

Total Phase-space extended to regions outside experimentalacceptance:fiducial total extrapolation done with Monte Carlo“Total” phase-space definition:

EγT > 10 GeV, ∆R(l, γ) > 0.5, ΣEhadT < 0.5 · E

γT
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Efficiency Values and UncertaintiesCWγ (e channel) = 36% CWγ (µ channel) = 46%

CZγ (e channel) = 29% CZγ (µ channel) = 43%

Efficiency Uncertainties

Main contributions to the total uncertainty are:

Fragmentation photon contribution in data (6% for both photonID and isolation efficiency)

Amount of material knowledge (particualry at high |η|)Data/MC discrepancies for the EM shower shape distributions
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Efficiency Values and UncertaintiesCWγ (e channel) = 36% CWγ (µ channel) = 46%CZγ (e channel) = 29% CZγ (µ channel) = 43%




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Efficiency Values and UncertaintiesCWγ (e channel) = 36% CWγ (µ channel) = 46%CZγ (e channel) = 29% CZγ (µ channel) = 43%




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Cross Section Measurements and Comparisonwith Standard Model Theory

Fiducial Cross Sections

Total Cross Sections

All cross section measurements are consistent within theiruncertainties with the Standard Model expectations
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Cross Section Measurements and Comparisonwith Standard Model Theory

Fiducial Cross Sections

Total Cross Sections

All cross section measurements are consistent within theiruncertainties with the Standard Model expectations
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Conclusions and Future Prospects

First measurement of Wγ and Zγ production cross sectionsat√s = 7 TeV performed by ATLAS using ∼ 35 pb−1 of

integrated luminosity

Experimental measurements are in agreement with StandardModel expectations within error

With more data available this year the Triple GaugeCouplings will be probed with unprecedented precision

Searches will be extended also to Wγ and Zγ resonancedecay of particles beyond Standard Model

Stay Tuned for New Results Very Soon!
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25


Assuming no/littlecorrelation between photonID variables and isolation:

ND

NB=NC

NA

for background events

With small signalcontamination in the controlregions,NB , NC , and ND from data⇒ amount of background inNA

Iden

tific

atio

nSt

anda

rd P

hoto

nId

entif

icat

ion

"Low

Qua

lity"

Pho

ton

A

C D

B

(Control Region) (Control Region)

(Control Region)(Signal Region)

(Isolated)

Photon IDQuality

(Not Isolated)

Isolation Energy [GeV]65
Introduction

SignalDefinition

ATLASDetector

e and γ ID

γ Isolation

Data Analysis

Event Selection


Signal Yield

CrossSections

Calculation

Results

Conclusions

Backup

25


Assuming no/littlecorrelation between photonID variables and isolation:

ND

NB=NC

NA



Iden

tific

atio

nSt

anda

rd P

hoto

nId

entif

icat

ion

"Low

Qua

lity"

Pho

ton

A

C D

B



(Isolated)

Photon IDQuality

(Not Isolated)


Method applied only to the Wγ analysis (very limited statistics on Zγ)

Corrections for signal presence in control region applied

Systematics due to the assumptions of the method and the definition ofthe control regions carefully estimated.
Introduction

SignalDefinition

ATLASDetector

e and γ ID

γ Isolation

Data Analysis

Event Selection


Signal Yield

CrossSections

Calculation

Results

Conclusions

Backup

25

Signal Yield ExtractionAssuming no/littlecorrelation between photonID variables and isolation:

ND

NB=NC

NA



Iden

tific

atio

nSt

anda

rd P

hoto

nId

entif

icat

ion

"Low

Qua

lity"

Pho

ton

A

C D

B



(Isolated)

Photon IDQuality

(Not Isolated)


Signal purity in selected events ∼ 80%

Introduction

Measurement of W p and Z Production in pp Collisions at s · Introduction Signal De nition ATLAS...

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