ATLAS 探测器上 WW 过程产生截面测量

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

WW Production Cross-Section Measurement at the ATLAS experiment

Y. Wu 2

Outline

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Introduction

WW Signal and Background

Event selection

Results

– Observation and Expectation

– Uncertainties

– Cross-Section

Conclusion

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Introduction

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Motivation– Test of SM electroweak theory at high energy frontier– Probe new physics by anomalous triple-gauge-boson couplings (TGC)– Dominant background for HW+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, ET

e > 15 GeV)

initial state: ~97% gg initial state: ~3%

(

s-channel contains TGCs

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ATLAS Detector

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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

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WW Signal and Background

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Through WW leptonic decay channels (, , = ), 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 Ztt 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 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

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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

– is used in analysis, calculated as

is the minimum separation angle between 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 >15 GeV and >10 GeV (Z-Veto) (ee, mm)

– Require > 40 GeV (ee, mm) and > 20 GeV (em)

– Require zero jet in the event (Jet-Veto)

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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 > 10 GeV (Z-Veto)

(ee) (mm)

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after Z-Veto

The remaining Drell-Yan background after the Z- Veto cut can be effectively further removed by cutting on

(ee,mm)

(ee,mm)

(em)

(em)

Njets = 0 Njets = 0

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Jet Multiplicity after 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

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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

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Observe 8 WW candidates in data (ee:1, mm:2, em:5)

Prediction: 7.1 signal events + 1.7 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 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

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Systematics and Detection Sensitivity

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Luminosity uncertainty (): ~3.4%

Acceptance uncertainty ()– 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 – Background: ~33% (Overall)

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 ).

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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%)–

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Conclusion

8 WW candidate events observed in 35 pb-1 of data with 1.70.6 background events predicted, corresponding to a WW signal significance of ~3 .s

WW production cross-section at 7 TeV measured to be:

Measured WW production cross-section is in agreement with the SM prediction of (443pb@ NLO) within the uncertainties.

𝜎 𝑊𝑊=41−16+20 (𝑠𝑡𝑎𝑡 .)±5 (𝑠𝑦𝑠𝑡 .)±1 (𝑙𝑢𝑚𝑖 . ) 𝑝𝑏

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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,

HWW, HZZ … ）

– 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

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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.107.0418022. 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)0603. 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) 289–3084. 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

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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

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Physics ObjectsVertex

– Ntracks>=3

– Vertex with the maximum sum of track PT

2 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 : (cone0.3)<6GeV– Impact parameters w.r.t. PV satisfy d0/σd0<10 && |z0|<10mm

– ε(data)/ε(MC) = 0.970.03

Muon– “Combined (ID+MS)” muon– Momentum scale/resolution

corrections applied properly.– PT>20GeV, | |<2.4

– PTMS>10GeV, |ΔPT

MS-ID/PTID|<0.5

– Isolation: (cone0.2)/PTm<0.1

– Impact parameters w.r.t. PV satisfy d0/σd0<10 && |z0|<10mm

– ε(data)/ε(MC) = 0.980.01

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

– miss = - (calorimeter clusters + muons)

More powerful in background rejection2014.4.21

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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 sensitivityTo 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 1.410-3 corresponding to a significance of 3.0 σ’s.

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