1. 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 武汉
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2. Outline Introduction WW Signal and Background Event selection Results
Observation and Expectation
Uncertainties
Cross-Section
Conclusion
Y. Wu
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3. Introduction Motivation WW Production at LHC
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
Use 35 pb-1 collision data collected during 2010 at ATLAS
Single lepton triggers are applied (pTm > 13 GeV, ETe > 15 GeV)
𝑞 𝑞 initial state: ~97%
gg initial state: ~3%
𝜎 𝑊 + 𝑊 − 𝑁𝐿𝑂 =44±3 𝑝𝑏 ( 𝑠 =7 𝑇𝑒𝑉)
s-channel contains TGCs
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4. ATLAS Detector Length: 44 m, Diameter: 25 m, Weight: 7000 t,
~108 electronic channels, 3000 km cables
To the center of LHC
To the sky q
𝜂=−ln tan⁡(𝜃/2)
Coordinate
Y. Wu

5. 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
Z+jets
Top
Diboson
One lepton from W decay
+ One jet faked lepton
+ MET
Less likely to pass lepton identification
Larger jet activity
Leptons from Z decay
+ MET from jet mis-measurement or Ztt
Has real Z in event, removed by Z-veto
Small MET, more jet
Leptons from W decays
+ MET
Have large jet activity, apply jet-veto can remove its majority
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
Y. Wu

6. Event Selection Physics Objects Pre-selection WW Selection
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
𝐸 𝑇,𝑅𝑒𝑙 𝑚𝑖𝑠𝑠 is used in analysis, calculated as
∆ 𝜙 𝑙,𝑗 is the minimum separation angle between 𝐸 𝑇 𝑚𝑖𝑠𝑠 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
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)
Require 𝐸 𝑇,𝑅𝑒𝑙 𝑚𝑖𝑠𝑠 > 40 GeV (ee, mm) and 𝐸 𝑇,𝑅𝑒𝑙 𝑚𝑖𝑠𝑠 > 20 GeV (em)
Require zero jet in the event (Jet-Veto)
Y. Wu

7. 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)
(mm)
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8. 𝐸 𝑇,𝑅𝑒𝑙 𝑚𝑖𝑠𝑠 after Z-Veto
The remaining Drell-Yan background after the Z- Veto cut can be effectively further removed by cutting on 𝐸 𝑇,𝑅𝑒𝑙 𝑚𝑖𝑠𝑠
(ee,mm)
(em)
Njets = 0
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9. Jet Multiplicity after 𝐸 𝑇,𝑅𝑒𝑙 𝑚𝑖𝑠𝑠 Cut
Most of the top background can be removed by Jet veto (Njets= 0)
WW signal dominates 0 jet bin.
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10. 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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11. Observations and Predictions
Observe 8 WW candidates in data (ee:1, mm:2, em:5)
Prediction: 7.1 signal events background events
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
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
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
0.38±0.04±0.04
Y. Wu

12. Systematics and Detection Sensitivity
Luminosity uncertainty ( 𝜎 𝑙𝑢𝑚𝑖 ): ~3.4%
Acceptance uncertainty ( 𝜎 𝑎𝑐𝑐 )
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 𝜎 𝑙𝑢𝑚𝑖 , 𝜎 𝑎𝑐𝑐 , 𝜎 𝑃𝐷𝐹 1% , 𝜎 𝐽𝑒𝑡−𝑉𝑒𝑡𝑜
Background: ~33% (Overall)
𝜎 𝑙𝑢𝑚𝑖 , 𝜎 𝑎𝑐𝑐 , 𝜎 𝑐𝑟𝑜𝑠𝑠−𝑠𝑒𝑐𝑡𝑖𝑜𝑛 , 𝜎 𝐽𝑒𝑡−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 ).
Y. Wu

13. 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%)
𝜎 𝑠𝑦𝑠 = ( Δ𝐴 𝐴 ) 2 + ( Δ 𝑁 𝑏 𝑁 𝑏 ) 2 =11.5%
𝜎 𝑙𝑢𝑚𝑖 =3.4%
𝜎 𝑊𝑊 = 41 − (𝑠𝑡𝑎𝑡.)±5(𝑠𝑦𝑠𝑡.)±1 𝑙𝑢𝑚𝑖. 𝑝𝑏
Y. Wu

14. 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:
Measured WW production cross-section is in agreement with the SM prediction of NLO) within the uncertainties.
𝜎 𝑊𝑊 = 41 − (𝑠𝑡𝑎𝑡.)±5(𝑠𝑦𝑠𝑡.)±1 𝑙𝑢𝑚𝑖. 𝑝𝑏
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15. 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
Y. Wu

16. 发表文章和会议报告
文章列表：
Measurement of the $W^+W^-$ cross section in $\sqrt{s}$ = 7 TeV $pp$ collisions with ATLAS, ATLAS Collaboration, Physics Review Letter, /PhysRevLett
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
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
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
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
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
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, (2013)
Diboson productions and aTGCs search at LHC，HCP2012国际会议论文，EPJ Web of Conferences 49, (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
Y. Wu

17. Backup
Y. Wu

18. ATLAS Detector Length: 44 m, Diameter: 25 m, Weight: 7000 t,
~108 electronic channels, 3000 km cables
To the center of LHC
To the sky q
𝜂=−ln tan⁡(𝜃/2)
Coordinate
Y. Wu

19. Physics Objects Muon Vertex Electron Jet Missing ET
“Combined (ID+MS)” muon
Momentum scale/resolution corrections applied properly.
PT>20GeV, | |<2.4
PTMS>10GeV, |ΔPTMS-ID/PTID|<0.5
Isolation: 𝑃 𝑇 (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
𝐸𝑇 miss = - 𝐸𝑇 (calorimeter clusters + muons)
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 : 𝐸 𝑇 (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
Y. Wu

20. Results Appendix I (Signal Acc., Bkg Prediction)
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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21. Systematics for acceptance uncertainties
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22. 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.
Y. Wu