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ATLAS 探测器上 WW 过程产生截面测量

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

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ATLAS 探测器上 WW 过程产生截面测量

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  1. ATLAS探测器上WW过程产生截面测量 WW Production Cross-Section Measurement atthe 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

  2. Outline • Introduction • WW Signal and Background • Event selection • Results • Observation and Expectation • Uncertainties • Cross-Section • Conclusion Y. Wu

  3. 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 • Use 35 pb-1 collision data collected during 2010 at ATLAS • Single lepton triggers are applied (pTm> 13 GeV, ETe> 15 GeV) ( initial state: ~97% gg initial state: ~3% s-channel contains TGCs Y. Wu

  4. ATLAS Detector Length: 44 m, Diameter: 25 m, Weight: 7000 t, ~108electronic channels, 3000 km cables Coordinate To the sky q To the center of LHC Y. Wu

  5. WW Signal and Background • 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: Z+jets Diboson W+jets Top • Leptons from W decays • + MET • Have large jet activity, apply jet-veto can remove its majority • 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 • 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 Y. Wu

  6. 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 • 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 GeVand >10 GeV (Z-Veto) (ee, mm) • Require > 40 GeV (ee, mm) and > 20 GeV (em) • Require zero jet in the event (Jet-Veto) Y. Wu

  7. 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) Y. Wu

  8. after Z-Veto • The remaining Drell-Yan background after the Z- Veto cut can be effectively further removed by cutting on (ee,mm) (em) (ee,mm) (em) Njets= 0 Njets= 0 Y. Wu

  9. Jet Multiplicity after Cut • Most of the top background can be removed by Jet veto (Njets= 0) WW signal dominates 0 jet bin. Y. Wu

  10. Candidate Event Pt(m-)=67.8GeVPt(e+)=21.4GeVPt(e+,m-)=84.3GeVM(e+,m-)=46.1GeVMET=68.8GeV Y. Wu

  11. Observations and Predictions • 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 Y. Wu

  12. Systematics and Detection Sensitivity • 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 ). Y. Wu

  13. WWproduction 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%) Y. Wu

  14. Conclusion • 8 WW candidate events observed in 35 pb-1of data with 1.70.6 background eventspredicted, 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 (443pb@ NLO) within the uncertainties. Y. Wu

  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, 10.1103/PhysRevLett.107.041802 • 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, 112001 (2013) • 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 Y. Wu

  17. Backup Y. Wu

  18. ATLAS Detector Length: 44 m, Diameter: 25 m, Weight: 7000 t, ~108electronic channels, 3000 km cables Coordinate To the sky q To the center of LHC Y. Wu

  19. Physics Objects • Muon • “Combined (ID+MS)” muon • Momentum scale/resolution corrections applied properly. • PT>20GeV, | |<2.4 • PTMS>10GeV, |ΔPTMS-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) = 0.980.01 • Jet • Anti-Ktwith 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) • 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 : (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) Y. Wu

  21. Systematics for acceptance uncertainties Y. Wu

  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 3.0σ’s. Y. Wu

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