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 CutMost 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 methodWW 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 MCBkg 0.17±0.11±0.08 0.25±0.31±0.15 1.26±0.17±0.31 1.68±0.37±0.42Top 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 ~3s.

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– PT

MS>10GeV, |ΔPTMS-ID/PT

ID|<0.5– Isolation: (cone0.2)/PT

m<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 InclusiveWW Signal 0.85±0.02±0.13 1.74±0.04±0.24 4.81±0.06±0.68 7.40±0.07±1.05Bkgs 0.17±0.11±0.09 0.26±0.31±0.15 1.29±0.17±0.32 1.72±0.37±0.45Top 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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