W z g measuerment @ atlas
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W/ Z+ g measuerment @ ATLAS. Liang HAN. University of Science & Technology of China (USTC). On behalf of ATLAS Collaboration. ICHEP2012 Melbourne, Australia. Motivation. W( l n )/Z( ll )+ g production measurement:. ISR: t/u-channel. FSR. g luon/quark fragmentation. s-channel TGC.

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W z g measuerment @ atlas

W/Z+gmeasuerment @ ATLAS

Liang HAN

University of Science & Technology of China (USTC)

On behalf of ATLAS Collaboration


Melbourne, Australia

W z g measuerment atlas


  • W(ln)/Z(ll)+g production measurement:

ISR: t/u-channel


gluon/quark fragmentation

s-channel TGC

  • Searching anomalous triple gauge couplings (aTGC):

  • W magnetic dipole and electric quadrupole moment

  • ZZg/Zgg prohibited by SM

ZggaTGC: hg3/4

ZZgaTGC: hz3/4

WWgaTGC: Dkg,lg

  • Prior-LHC results

+ CDF, 5fb-1Zg, PRL107(2011)051802

+ D0, 4.2fb-1Wg, PRL107(2011)241803

+ D0, 6.2fb-1Zg, PRD85(2012)052001

[email protected]

W z g measuerment atlas

Event Selection

  • ATLAS 35pb-1 result asJHEP 1109,072

  • ATLAS 1.02fb-1 result as arXiv:1205.2531

  • ATLAS 7TeV 1fb-1(35pb-1) data:

  • Lepton :

  • + e/m pT>25GeV,detector fiducial || coverage; isolated in calorimeter;

  • + Tight electron identification

  • W/Z events : MET>25GeV, MT(ln)>40GeV;

  • M(ll)>40GeV

  • Photon :

  • + pT>15GeV, detectorfiducial || coverage;

  • + Isolated in calorimeter

  • + Tight photon identification

  • + FSR suppression dR(l,g)>0.7

  • + Simulation corrected to Zllgdata

  • Jet : pT>30GeV,||<4.4, dR(j,/lepton)>0.6

  • Inclusive (>=0jet) vs.

  • Exclusive (==0jet)

[email protected]

W z g measuerment atlas

  • ISR/FSR vs. pT(g) cut:


ISR: M(llg) > MZ




High photon pT cut to suppress FSR as

Zg : pT(g)>15, 60GeV; Wg : pT(g)>15,60, 100GeV

FSR: M(llg) ≤ MZ

[email protected]

W z g measuerment atlas

W(ln)+g control plot

  • Electroweak background derived from simulation

  • Dominant background, W+jethas to be estimated from data

[email protected]

W z g measuerment atlas

W/Z+jet background

  • 2D sideband jet“g”background estimation:

  • Photon Identification: based on calorimeter shower-shape

  • Photon Isolation:

[email protected]

W z g measuerment atlas

Jet+g background in Wg

  • Date-driven jet“e/m”estimation:

1) jet+g : real g; non-isolated letponfrom heavy b/c decay;

2) Control region : MET<20GeV to extract faked “e/m” isolation shape

W(en)g :MET vs. isolation 2-d sideband

[email protected]

W z g measuerment atlas

Signal event yield

  • Photon ET spectrum:



* Signal distribution normalized to the number of extracted data

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W z g measuerment atlas

  • Number of jet distribution:





Inclusive(>=0jet) vs. Exclusive(==0jet)

[email protected]

W z g measuerment atlas

Cross section measurement



± 3.8%

Experimental selection efficiency

Detector Acceptance

  • Unfold detector efficiency:

+ Correction factor CWg(Zg)~40-60%

+ Systematic dC~10%, dominated by photon identification & jet energy scale

[email protected]

W z g measuerment atlas

  • Unfold for detector acceptance :

+ Extend detector fiducial to a uniform lepton |h| coverage

Estimated from


+ Theoretical uncertainty on acceptance

dTheo. ~1-3%

Compared against MCFM

  • Compare to SM prediction:


[email protected]

W z g measuerment atlas

Differential cross sections

1) Photon pT>15,60,100GeV; 2) Inclusive(≥0jet) vs. Exclusive (=0jet)

  • The Exclusive measurements are consistent with MCFM predictions (SM NLO)

  • The Wg Inclusive are higher than MCFM, especially in high pT(g) region

  •  high order effects (NNLO and beyond)

[email protected]

W z g measuerment atlas

Anomalous couplings

+aTGChV3/4: ZVgelectric dipole / magnetic quadrupole transition moment

+non-zeroaTGCwill result in increasing of W/Z+g cross section,

especially in high photon pT region

ZZgaTGC: hz3/4


ZggaTGC: hg3/4

[email protected]

W z g measuerment atlas

  • Extract ZVgaTGC:

+Exclusive ET(g)>60GeVmeasurement against aTGC hypotheses

+Bayesian probability with nuisance parameters to set limits

[email protected]

W z g measuerment atlas

  • Extract WWgaTGC:

ExclusiveET(g)>100GeVmeasurement against aTGC hypotheses


WWgaTGC: Dkg,lg

WWgaTGC: Dkg,lg

[email protected]

W z g measuerment atlas


  • The differential cross section W(ln)g/Z(ll)gmeasured @ 1fb-1 7TeV ATLAS:

  • Exclusive (=0jet) measurement is consistent with SM NLO

  • High order effect (NNLO and beyond) is observed in

  • Wγinclusive (≥0jet) data, especially in high photon

  • pT(g)>60GeV region

  • Limits on anomalous TGC couplings derived from high photon pT spectrum

  • WWaTGC results better than the existing Tevatron limits

[email protected]

W z g measuerment atlas

Backup slides

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W z g measuerment atlas

Signal event yield

  • W(ln)g :

  • Dominate background as

  • W+jet(“g”),g+jet(“e”),

  • Z(ll)

  • Z(ll)g :

  • Dominate background as

  • Z+jet

[email protected]

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