ATLAS探测器上WW过程产生截面测量
WW Production Cross-Section Measurement at
the ATLAS experiment
吴雨生 / 中国科学技术大学
导师：赵政国 教授， 周冰 教授（美: 密歇根大学）
刘建北（代表吴雨生作报告）
中国科学技术大学
晨光杯论文评选终审报告
2014.4.21 武汉
Outline
 Introduction
 WW Signal and Background
 Event selection
 Results
– Observation and Expectation
– Uncertainties
– Cross-Section
 Conclusion
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Introduction
 Motivation
– Test of SM electroweak theory at high energy frontier
– Probe new physics by anomalous triple-gauge-boson couplings (TGC)
– Dominant background for HW+W- search and some BSM searches
 WW Production at LHC
𝑁𝐿𝑂
𝜎𝑊
+ 𝑊 − = 44 ± 3 𝑝𝑏 ( 𝑠 = 7 𝑇𝑒𝑉)
𝑞𝑞 initial state: ~97%
s-channel contains TGCs
gg initial state: ~3%
 Use 35 pb-1 collision data collected during 2010 at ATLAS
 Single lepton triggers are applied (pTm > 13 GeV, ETe > 15 GeV)
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ATLAS Detector
Length: 44 m, Diameter: 25 m, Weight: 7000 t,
~108 electronic channels, 3000 km cables
Coordinate
To the sky
q
To the
center
of LHC
𝜂 = −ln tan(𝜃/2)
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WW Signal and Background
 Through WW leptonic decay channels (𝑊𝑙𝜐, 𝑊𝜏𝜐𝑙𝜐, 𝑙=
𝑒, 𝜇), final states would have 2 high-pT isolated leptons (ee, mm and
em channels), large missing energy (MET), and less jet activity
 Main background:
W+jets
One lepton from W decay
+ One jet faked lepton
+ MET
 Less likely to pass
lepton identification
 Larger jet activity
Z+jets
Leptons from Z decay
+ MET from jet mismeasurement or Ztt
 Has real Z in event,
removed by Z-veto
 Small MET, more jet
Top
Leptons from W decays
+ MET
 Have large jet
activity, apply jetveto can remove its
majority
Diboson
Includes WZ/ZZ/W,Z+g
Leptons from W/Z decays
or g-fake
+ MET from decays or 𝑒, 𝜇
escape
 Z-related processes
can be suppressed by
Z mass veto
 Others are less likely
to have 2 high-pT
isolated leptons
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Event Selection
 Physics Objects
– Collision vertex should associate with at least 3 tracks
– Leptons are selected with pT>20GeV,  constraint, identification, isolation, etc.
– Jets (Anti-Kt, R=0.4) are required to have pT>20GeV, ||<3.0
𝑚𝑖𝑠𝑠
– 𝐸𝑇,𝑅𝑒𝑙
is used in analysis, calculated as
∆𝜙𝑙,𝑗 is the minimum separation angle between 𝐸𝑇𝑚𝑖𝑠𝑠 and lepton, jet.
 Pre-selection
– Select events with good collision vertex (remove cosmic/ beam background)
– Reject events if have bad measured jets (otherwise MET will be affected)
– Select leptons as defined above
 WW Selection
–
–
–
–
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Require the event to have exactly two opposite sign leptons
Require 𝑀 𝑙 + 𝑙 − >15 GeV and 𝑀 𝑙 + 𝑙 − − 𝑀 𝑍 >10 GeV (Z-Veto) (ee, mm)
𝑚𝑖𝑠𝑠
𝑚𝑖𝑠𝑠
Require 𝐸𝑇,𝑅𝑒𝑙
> 40 GeV (ee, mm) and 𝐸𝑇,𝑅𝑒𝑙
> 20 GeV (em)
Require zero jet in the event (Jet-Veto)
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𝑀 𝑙+ 𝑙 − after di-lepton selection
 97% of the di-lepton events in ee, mm channels are
Drell-Yan background
 Those background events can be largely removed by
𝑀 𝑙 + 𝑙 − − 𝑀 𝑍 > 10 GeV (Z-Veto)
(ee)
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𝑚𝑖𝑠𝑠
𝐸𝑇,𝑅𝑒𝑙
after Z-Veto
 The remaining Drell-Yan background after the ZVeto cut can be effectively further removed by
𝑚𝑖𝑠𝑠
cutting on 𝐸𝑇,𝑅𝑒𝑙
(ee,mm)
(em)
(ee,mm)
(em)
Njets = 0
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Njets = 0
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𝑚𝑖𝑠𝑠
Jet Multiplicity after 𝐸𝑇,𝑅𝑒𝑙
Cut
Most of the top background can be removed by Jet veto
(Njets= 0)
WW signal dominates 0 jet bin.
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Candidate Event
Pt(m-)=67.8GeV
Pt(e+)=21.4GeV
Pt(e+,m-)=84.3GeV
M(e+,m-)=46.1GeV
MET=68.8GeV
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Observations and Predictions
 Observe 8 WW candidates in data (ee:1, mm:2, em:5)
 Prediction: 7.1 signal events + 1.7 background events
Final State
ee
mm
em
combined
method
WW Signal
0.82±0.02±0.09
1.68±0.04±0.15
4.63±0.06±0.46
7.12±0.07±0.70
MC
Bkg
0.17±0.11±0.08
0.25±0.31±0.15
1.26±0.17±0.31
1.68±0.37±0.42
Top
0.04±0.02±0.02
0.14 ±0.06±0.07
0.35±0.10±0.19
0.53±0.12±0.28
MC
W+jets
0.08±0.05±0.03
0.00±0.29±0.10
0.46±0.12±0.17
0.54±0.32±0.21
Data
DY
0.00±0.10±0.07
0.01±0.10±0.07
0.23±0.05±0.02
0.23±0.15±0.17
MC/Data
Diboson
0.05±0.01±0.01
0.10±0.01±0.01
0.23±0.05±0.02
0.38±0.04±0.04
MC
– Scale factors are applied to compensate acceptance difference between data and MC
– WW signal acceptance is about 4%, 9% and 12% for ee, mm, em channel, respectively
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Systematics and Detection Sensitivity
 Luminosity uncertainty (𝜎𝑙𝑢𝑚𝑖 ): ~3.4%
 Acceptance uncertainty (𝜎𝑎𝑐𝑐 )
– contributed from trigger and lepton ID efficiency uncertainties
– overall ~4.3%
 Jet-Veto cut efficiency uncertainty
– Signal: 6%, Top: 40%
 Systematic uncertainty calculation
– WW signal: ~10%, quadratic sum of 𝜎𝑙𝑢𝑚𝑖 , 𝜎𝑎𝑐𝑐 , 𝜎𝑃𝐷𝐹 1% , 𝜎𝐽𝑒𝑡−𝑉𝑒𝑡𝑜
– Background: ~33% (Overall)
 𝜎𝑙𝑢𝑚𝑖 , 𝜎𝑎𝑐𝑐 , 𝜎𝑐𝑟𝑜𝑠𝑠−𝑠𝑒𝑐𝑡𝑖𝑜𝑛 , 𝜎𝐽𝑒𝑡−𝑉𝑒𝑡𝑜
 For top, additional term for ISR/FSR uncertainties are considered
 Systematics for DY and W+jets are derived from data
 With 8 observed events and 1.68±0.56 background, detection
sensitivity is ~ 3.0 s (p-value 1.2 × 10−3 ).
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WW production cross-section
 The combined WW production cross-section is determined using
the maximum likelihood method. The likelihood function based on
Poisson statistics is constructed as
 The systematics: (~12%)
– 𝜎𝑠𝑦𝑠 =
Δ𝐴
𝐴
Δ𝑁𝑏 2
) =
𝑁𝑏
( )2 +(
11.5%
– 𝜎𝑙𝑢𝑚𝑖 = 3.4%
 𝜎𝑊𝑊 = 41+20
−16 (𝑠𝑡𝑎𝑡. ) ± 5(𝑠𝑦𝑠𝑡. ) ± 1 𝑙𝑢𝑚𝑖. 𝑝𝑏
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Conclusion
 8 WW candidate events observed in 35 pb-1 of data with 1.70.6
background events predicted, corresponding to a WW signal
significance of ~3s.
 WW production cross-section at 7 TeV measured to be:
𝜎𝑊𝑊 = 41+20
−16 (𝑠𝑡𝑎𝑡. ) ± 5(𝑠𝑦𝑠𝑡. ) ± 1 𝑙𝑢𝑚𝑖. 𝑝𝑏
 Measured WW production cross-section is in agreement with the
SM prediction of (443pb@ NLO) within the uncertainties.
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结语
 报告中所述工作已发表在 Phys.Rev.Lett. 107 (2011) 041802
– WW过程截面测量在LHC标准模型物理分析中具有重大意义
 首次在ATLAS实验上探测到有质量玻色子对产生过程
 为以后基于双玻色子道的各种物理分析研究奠定了基础（WZ, ZZ,
HWW, HZZ …）
– 本人为文章主要贡献者之一
 文章发表于2011年
 在2012/2013年，参与并发表基于此分析道的另两篇文章（PLB，PRD）
 博士期间参加多项物理分析工作以及探测器刻度工作，文章及会议报告见
下一页
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发表文章和会议报告
文章列表：
1. Measurement of the $W^+W^-$ cross section in $\sqrt{s}$ = 7 TeV $pp$ collisions with ATLAS, ATLAS
Collaboration, Physics Review Letter, 10.1103/PhysRevLett.107.041802
2. Measurement of the W->ln and Z/r*->ll production cross sections in proton-proton collisions at sqrt(s)=7TeV with the
ATLAS detector，Journal of High Energy Physics，JHEP12(2010)060
3. Measurement of the WW cross section in sqrt(s)=7 TeV pp collisions with the ATLAS detector and limits on anomalous
gauge couplings, Physics Letters B, Physics Letters B 712 (2012) 289–308
4. Measurement of the WZ production cross section and limits on anomalous triple gauge couplings in proton-proton
collisions at sqrt(s)=7 TeV with the ATLAS detector, Physics Letters B, Physics Letters B 709 (2012) 341–357
5. Measurement of WZ production in proton-proton collisions at sqrt(s)=7 TeV with the ATLAS detector, The European
Physical Journal C, Eur. Phys. J. C (2012) 72:2173
6. Search for the Standard Model Higgs boson in the decay channel H->ZZ->4l with 4.8fb-1 of pp collision data at
sqrt(s)=7 TeV with ATLAS，Physics Letter B, Physics Letters B 710 (2012) 383–402
7. Measurement of WW production in pp collisions at sqrt(s)=7 TeV with the ATLAS detector and limits on anomalous
WWZ and WWg couplings, Physical Review D, Phys. Rev. D 87, 112001 (2013)
8. Diboson productions and aTGCs search at LHC，HCP2012国际会议论文，EPJ Web of Conferences 49, 14006 (2013)
国际会议：
• 美国物理学年会 APS2011(Orange County, CA): WW Cross-Section Measurement at ATLAS
• 美国物理学年会 DPF2011(Brown Univ.): WZ Cross-Section Measurement at ATLAS
• HCP2012 (Kyoto): Diboson Results from LHC
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Backup
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ATLAS Detector
Length: 44 m, Diameter: 25 m, Weight: 7000 t,
~108 electronic channels, 3000 km cables
Coordinate
To the sky
q
To the
center
of LHC
𝜂 = −ln tan(𝜃/2)
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Physics Objects
 Muon
Vertex
– Ntracks>=3
– Vertex with the maximum sum of track
PT2 selected as the primary vertex
– Pile-up MC reweighted to reproduce the
vertex multiplicity in data. Systematics
arising from the reweighting ~ 0.5%
Electron
– Energy scale/resolution corrections
applied properly
– ET>20GeV, ||<1.37 or 1.52<||< 2.47
– “Tight” electron identification
– Isolation : 𝐸𝑇 (cone0.3)<6GeV
– Impact parameters w.r.t. PV satisfy
d0/σd0<10 && |z0|<10mm
– ε(data)/ε(MC) = 0.970.03
More powerful in background rejection
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– “Combined (ID+MS)” muon
– Momentum scale/resolution
corrections applied properly.
– PT>20GeV, | |<2.4
– PTMS>10GeV, |ΔPTMS-ID/PTID|<0.5
– Isolation: 𝑃𝑇 (cone0.2)/PTm<0.1
– Impact parameters w.r.t. PV satisfy
d0/σd0<10 && |z0|<10mm
– ε(data)/ε(MC) = 0.980.01
 Jet
– Anti-Kt with R=0.4
– PT>20GeV, ||<3.0, ΔR(Jet, e)>0.3
– Jet veto ε(data)/ε(MC) =0.97 0.06
 Missing ET
– 𝐸𝑇miss = - 𝐸𝑇 (calorimeter clusters +
muons)
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Results Appendix I (Signal Acc., Bkg Prediction)
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Final State ee
mm
em
Inclusive
WW Signal
0.85±0.02±0.13
1.74±0.04±0.24
4.81±0.06±0.68
7.40±0.07±1.05
Bkgs
0.17±0.11±0.09
0.26±0.31±0.15
1.29±0.17±0.32
1.72±0.37±0.45
Top
0.04±0.02±0.03
0.15±0.06±0.08
0.36±0.10±0.19
0.55±0.12±0.30
W+jets
0.08±0.05±0.03
0.00±0.29±0.10
0.46±0.12±0.17
0.54±0.32±0.21
DY
0.00±0.10±0.07
0.01±0.10±0.07
0.23±0.06±0.15
0.24±0.15±0.17
Diboson
0.05±0.01±0.01
0.10±0.01±0.01
0.24±0.05±0.03
0.39±0.04±0.06
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Systematics for acceptance uncertainties
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W+W- Detection sensitivity
To estimate the statistical significance of the signal
detection, Poisson distributed pseudo-experiments are
generated with the expected background varying according
to its uncertainty.
The probability to observe 8 or more
events in the absence of a signal (i.e.
background only hypothesis) is 1.410-3
corresponding to a significance of 3.0 σ’s.
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