1. Directed by : Dr. Lancon Eric and Dr. Zhengguo Zhao
Measurement of the ratio of the W and Z cross sections (Rjets) with exactly one associated jet in pp collisions at 𝑠 = 7 TeV with ATLAS
Chao Xu
Directed by : Dr. Lancon Eric and Dr. Zhengguo Zhao
To Obtain the degree of the Doctorate of Philosophy
University of Science and Technology of China ( USTC ) and
University of Paris Sud Orsay ( Particle, Noyaux, Cosmos (ED 517) )
Prepared at
USTC and IRFU/SPP/CEA-Saclay
Defend on June 5th 2012
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2. Outline Introduction Rjets Measurement Results and Conclusion
The Standard Model (SM)
The Large Hadron Collider (LHC)
W/Z + Jet Physics at LHC
The ATLAS Experiment
Rjets Measurement
Motivation
Strategy of Rjets Analysis
Data Samples
Object and Event Selection
Background Estimation
Correction of Rjets
Systematics Uncertainty
Results and Conclusion
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3. Introduction
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4. The Standard Model (SM)
Explains a wide variety of experimental results in particle physics over large energy scale
Particle Content
Fermions : three generations, six quarks, six leptons
Bosons : force mediators
Field Content
Electromagnetic interaction
Weak interaction
Strong interaction
Higgs : the last particle yet to be confirmed
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5. The Large Hadron Collider (LHC)
Largest and highest energy particle accelerator in world
27 km in circumference, mean depth of 100 m
Purpose for providing 14 TeV proton-proton collisions data
Designed luminosity 𝑐𝑚 −2 𝑠 −1
Running at 𝑠 =7 TeV, ~ 𝑐𝑚 −2 𝑠 −1 at 2010
CMS
LHC Tunnel
ATLAS
CERN
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6. W/Z + Jet Physics at LHC
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7. W/Z + Jet Physics at LHC Underlying Event Pile Up
Gluons and quarks not taking part in the hard scattering. Additional hadronic activity from the proton remnants results in hadronic deposits
in the detector
Pile Up
Providing high luminosity at LHC, multiple number of collisions in
one bunch crossing brings extra energy deposited in detector, the average interaction number in each bunch crossing < 4
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8. W/Z + Jet Physics at LHC
𝛼 𝑠
𝛼 𝑠
Leading order W/Z production process through annihilation of an incoming quark-antiquark pair.
To higher order W/Z bosons production processes in associated with one or several hard gluons or quarks, i.e. the hadronic jets in the detector
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9. The ATLAS Experiment A Toroidal LHC ApparatuS ( ATLAS )
A general purpose LHC detector
Four major components : Inner detector , Calorimeter ,
Muon Spectrometer, Magnet System(Solenoid, Toroid)
44 m long, 25 m high, 7000 tons
𝜂=− ln ( tan 𝜃 2 ) ; Δ 𝑅 𝑖𝑗 = ( 𝜂 𝑖 − 𝜂 𝑗 ) 2 + ( 𝜙 𝑖 − 𝜙 𝑗 ) 2
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10. The ATLAS Experiment
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11. The ATLAS Experiment Muon
ATLAS designed to have large rapidity coverage and be efficient to the detection of muons up to 1 TeV
Muon Trigger detectors (RPCs and TGCs) provide acceptance in the range |𝜂|< 2.4 and over the full 𝜙 range
Muon trigger system provides fast information on muon tracks traversing the detector, allowing the L1 trigger logic to pattern recognition.
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12. The ATLAS Experiment Combined Muon, Standalone Muon available
σ=2.06 ±0.07 𝐺𝑒𝑉
Combined Muon, Standalone Muon available
The combined muon utilizes the extrapolation from spectrometer tracks to the inner tracks to provide good muon reconstruction efficiency and momentum measurement
σ=3.27 ±0.07 𝐺𝑒𝑉
σ=2.36 ±0.07 𝐺𝑒𝑉
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13. The ATLAS Experiment
Jet
As the product of fragmentation of gluons and (light) quarks in QCD scattering, Jet most often observed at LHC
Two algorithms, Cone and KT (Anti – KT )are being used in ATLAS
The jet is reconstructed using the input of Topological calorimeter cell clusters
Default jet used is AntiKt4 with R = 0.4
The jet energy applied to global calibration
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14. The ATLAS Experiment 𝐸 𝑇 𝑚𝑖𝑠𝑠 = 𝐸 𝑥 2 + 𝐸 𝑦 2 𝐸 𝑇 𝑚𝑖𝑠𝑠
𝐸 𝑇 𝑚𝑖𝑠𝑠 = 𝐸 𝑥 2 + 𝐸 𝑦 2
𝐸 𝑇 𝑚𝑖𝑠𝑠
Missing Transverse Energy is critical at ATLAS for many physics channels
The 𝐸 𝑇 𝑚𝑖𝑠𝑠 reconstruction in ATLAS based on the reconstructed calorimeter objects and on the reconstructed muons
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15. Rjets Measurement
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16. X ranging from 30 GeV to 200 GeV in steps of 10 GeV
Motivation
What is Rjets : Measurement of W/Z cross section as function of hadronic activity ( to leptonic decaying )
X ranging from 30 GeV to 200 GeV in steps of 10 GeV
Each bin includes all selected events with exactly one jet above the jet pT threshold
Advantage : Cancellation of Systematic
Luminosity systematics
Jet systematics
Lepton systematics (partial)
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17. Motivation Why it is interesting at ATLAS
Intrinsic cancellation of various systematics allows stringent test of perturbative QCD (pQCD)
Improving our understanding of the V+jet production as the main source of background to new physics searching at LHC
Can be used to develop data-driven background prediction at ATLAS
Rjets measurement at ATLAS : first kind of such measurement
Inclusive ratio R measured at
Tevatron : R = ± 0.15(stat) ± 0.14(sys)
( Phys.Rev.Letter.94 (2005) , )
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18. Strategy of Rjets Analysis
𝑅 𝑗𝑒𝑡𝑠 = 𝑁 𝑙, 𝑊 𝑁 𝑙, 𝑍 × 𝐶 𝑗𝑒𝑡 𝑙
ℒ canceled
𝑁 𝑙,𝑉 : The number of events after applying lepton correction and missing transverse momentum ( 𝐸 𝑇 𝑚𝑖𝑠𝑠 ) correction
𝐶 𝑗𝑒𝑡 𝑙 : Jet correction factor, the remaining effects related to the jet kinematics which has not been canceled in ratio
Each term measured as function of jet pT threshold
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19. Strategy of Rjets Analysis
𝑁 𝑙, 𝑉 = 𝑁 𝑑𝑎𝑡𝑎 ∗ 1− 𝑓 𝑄𝐶𝐷 ∗(1− 𝑓 𝑒𝑤𝑘 ) 𝜖 𝑡𝑟𝑖𝑔 𝑙 ∗ 𝜖 𝑙 ∗ 𝐶 𝑉 𝑙
𝑁 𝑑𝑎𝑡𝑎 : The number of selected events
𝑓 𝑄𝐶𝐷 : QCD background fraction in data
𝑓 𝑒𝑤𝑘 : Fraction of background from electroweak processes after QCD background subtraction
𝜖 𝑡𝑟𝑖𝑔 𝑙 : Lepton trigger efficiency
𝜖 𝑙 : Lepton identification efficiency
𝐶 𝑉 𝑙 : Boson Reconstruction Correction, correcting the observed phase
space to fiducial phase space, accounting for resolution of leptons and 𝐸 𝑇 𝑚𝑖𝑠𝑠
Each term measured as function of jet pT threshold
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20. Data Samples
Collision Data
The data used is collected by ATLAS experiment from March 2010 to October at a center-of-mass energy 7 TeV, corresponding to an integrated luminosity 𝑝𝑏 −1 in 𝜇 channel after filtered by Good Run List (GRL) provided by ATLAS to ensure the detector condition.
Simulated Samples
NLO perturbative QCD predictions for the W/Z + jet signal processes from MCFM
The ALPGEN is the Monte Carlo (MC) event generator for signal samples.
PYTHIA and are used for background processes and cross check.
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21. Object and Event Selection
These selections follow the standard W/Z inclusive measurement selections and for MC are applied at particle level to quantities to define the phase space region which is called fiducial cross section
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22. Object and Event Selection
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23. Object and Event Selection
W selection in 𝝁 channel : (a) leading jet 𝑝 𝑇 for the W channel (b) transverse mass 𝑚 𝑇
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24. Object and Event Selection
Z selection in 𝝁 channel : (a) leading jet 𝑝 𝑇 for the Z channel (b) di-lepton invariant mass 𝑚 𝜇𝜇
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25. Electroweak Background
Background Estimation
Electroweak Background
𝑁 𝑒𝑤𝑘 𝑗 and 𝑁 𝑠𝑖𝑔𝑛𝑎𝑙 are both obtained from simulated MC samples (ALPGEN)
No reliance on the absolute luminosity in data
Same generator model and detector simulation lead to the reduction of associated uncertainties in the background fraction
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26. Electroweak Background
Systematics on Background Estimation
Electroweak Background
For each source of systematics, probe the relative variations to the nominal ratio value :
Small systematics yield even in very conservative way
𝑝 𝑇 and 𝜂 resolution : take the difference between selecting lepton at reconstructed level and generator level
𝐸 𝑇 𝑚𝑖𝑠𝑠 correction : take the difference between with correcting energy brought by 𝜇 and without
Model uncertainty : take the difference between different generators, ALPGEN for the nominal and PYTHIA for comparison
Pile up effects : take the difference between the estimation from pile-up MC samples and no pile-up samples
1− 𝑓 𝑒𝑤𝑘, 𝑤 1− 𝑓 𝑒𝑤𝑘,𝑧
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27. QCD Background on W channel : Matrix Method
Background Estimation
QCD Background on W channel : Matrix Method
𝑓 𝑄𝐶𝐷 = 𝑁 𝑄𝐶𝐷 𝑁 𝑑𝑎𝑡𝑎
𝑁 𝑖𝑠𝑜 : event passed full W selection
𝑁 𝑙𝑜𝑜𝑠𝑒 : event passed W selection without selection
𝑁 𝑛𝑜𝑛𝑄𝐶𝐷 : the W signal and the other non-QCD background processes in 𝑁 𝑙𝑜𝑜𝑠𝑒
𝑁 𝑄𝐶𝐷 : the QCD background event in 𝑁 𝑙𝑜𝑜𝑠𝑒
𝜖 𝑛𝑜𝑛𝑄𝐶𝐷 𝑖𝑠𝑜 : the isolation efficiency of 𝑁 𝑛𝑜𝑛𝑄𝐶𝐷
𝜖 𝑄𝐶𝐷 : the isolation efficiency of 𝑁 𝑄𝐶𝐷
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28. QCD Background on W channel
Background Estimation
QCD Background on W channel
Assuming that signal process and other electroweak processes have the same 𝜇 isolation efficiencies
Estimate 𝜖 𝑛𝑜𝑛𝑄𝐶𝐷 𝑖𝑠𝑜 from 𝑍→𝜇𝜇 samples using Tag-and-Probe technique
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29. QCD Background on W channel
Background Estimation
QCD Background on W channel
Estimate 𝜖 𝑄𝐶𝐷 𝑖𝑠𝑜 from 𝜇 triggered control sample which is dominated by di-jet event.
Need to subtract the event coming from electroweak background
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30. QCD Background on Z channel
Background Estimation
QCD Background on Z channel
Obtain the scale factor from non-isolated 𝜇 pair events of MC and Data
Apply the scale factor to the isolated MC events as the QCD background of Z
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31. Correction of Rjets
𝑁 𝑙, 𝑉 = 𝑁 𝑑𝑎𝑡𝑎 ∗ 1− 𝑓 𝑄𝐶𝐷 ∗(1− 𝑓 𝑒𝑤𝑘 ) 𝝐 𝒕𝒓𝒊𝒈 𝒍 ∗ 𝜖 𝑙 ∗ 𝐶 𝑉 𝑙
Trigger Efficiency
Estimate the trigger efficiency using Tag-and-Probe technique
The trigger efficiency of 𝜇 is a function of 𝜂 and 𝑝 𝑇 of 𝜇
The trigger efficiency are calculated from MC but corrected by a scale factor derived from data.
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32. Correction of Rjets
Trigger Efficiency W Z
Trigger efficiency of 𝜇 is independent of jet 𝑝 𝑇 , slightly decrease in high 𝑝 𝑇 jet bin, because of greater recoil results in more central events while the RPCs (barrel) has lower trigger efficiencies than TGCs (endcap)
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33. Vector Boson Acceptance
Correction of Rjets
Vector Boson Acceptance
Not easy to separate detector acceptance from detector efficiency due to large extrapolation distances and inhomogeneous efficiency
Following systematics studied:
Signal Model Uncertainty
Uncertainty due to pile-up
Muon momentum scale
Muon momentum resolution
𝐸 𝑇 𝑚𝑖𝑠𝑠 Uncertainty due to Jet Energy Scale(JES) or Jet Energy Resolution (JER)
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34. Correction of Rjets 𝑅 𝑗𝑒𝑡𝑠 = 𝑁 𝑙, 𝑊 𝑁 𝑙, 𝑍 × 𝐶 𝑗𝑒𝑡 𝑙
𝑅 𝑗𝑒𝑡𝑠 = 𝑁 𝑙, 𝑊 𝑁 𝑙, 𝑍 × 𝐶 𝑗𝑒𝑡 𝑙
The correction removes the effect of the detector on the jets
The difference between PYTHIA and Alpgen cancels to a high degree on taking the ratio
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35. Systematics Uncertainty
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36. Relative Uncertainty of Rjets
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37. Vector Boson Acceptance – Signal Model Uncertainty
Correction of Rjets
Vector Boson Acceptance – Signal Model Uncertainty W Z
Most of the kinematic range, the difference is less than 3%, above the jet threshold of 140 GeV the difference may be inadequately determined due to a lack of available statistics.
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38. Vector Boson Acceptance – Pile Up
Correction of Rjets
Vector Boson Acceptance – Pile Up W Z
Comparison between the Alpgen samples with and without pile up simulated shows smaller than 2% impact for the W (Z)
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39. Vector Boson Acceptance – Muon Momentum Scale and Resolution
Correction of Rjets
Vector Boson Acceptance – Muon Momentum Scale and Resolution
Scale
Resolution
Left : when muon scaled by the uncertainty in the muon momentum scale measurement.
Right : when varied with a Gaussian of the width to match the Z-peak
Both : less than 0.5% on the ratio for all jet 𝑝 𝑇 thresholds
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40. Vector Boson Acceptance - 𝐸 𝑇 𝑚𝑖𝑠𝑠
Correction of Rjets
Vector Boson Acceptance - 𝐸 𝑇 𝑚𝑖𝑠𝑠
JES
JER
Standard: no smearing
Conservative: Naive extrapolation of smearing from the region of applicability
Over-conservative: Conservative 20% flat smearing
Less than 2% variation shift in the correction for scaling over most of the kinematic range
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41. Jet 𝑝 𝑇 Spectrum Correction
Correction of Rjets
Jet 𝑝 𝑇 Spectrum Correction Z W
Difference in the jet migration for W and Z because of the vector boson selection prior to the jet selection
There is a difference between PYTHIA and ALPGEN of the order of 2%
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42. Results and Conclusion
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43. Results – Fiducial Phase Space
Statistical and systematic errors on 𝑅 𝑗𝑒𝑡𝑠 added in quadrature
Theory uncertainty includes contributions from PDFs and renormalization and factorization scales
The results in muon channel agree with the MCFM prediction (corrected to particle level) within the respective errors
From 140 GeV , the statistics in both data and MC are poor
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44. Results – Fiducial Phase Space
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45. Results – Full Phase Space
To correct the fiducial measurement to full phase space, the ratio between the acceptances at fiducial and full phase space is estimated at generator level
Data and theory agree well
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46. Results – Combined Result
Results are extrapolated to common space to allow direct combination in full phase space or |η|<2.5
The combined results illustrate the data has been well modeled by theory
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47. Conclusion
This is the first measurement which shows the W/Z cross section ratio with 1 jet as function of jet 𝑝 𝑇 at LHC
The results are consistent with theoretical predictions, statistically limited over most high 𝑝 𝑇 range.
The same measurement with 2011 collision data at higher luminosity is ongoing
From August 2011 to 2012, I contributed to the search for excited lepton with ATLAS detector, and the result has been published (Phys. Rev. D85 (2012) )
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48. List of Publication
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49. A measurement of the ratio of the W + 1 jet to Z + 1 jet cross sections with ATLAS, ATL-COM-PHYS
A measurement of the ratio of the W and Z cross sections with exactly one associated jet in pp collisions at 𝒔 = 7 TeV with ATLAS, Phys.Lett.B 708 (2012) , CERN-PH-EP
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50. Search for excited leptons in proton–proton collisions at 𝒔 = 7 TeV with the ATLAS detectors, ATL-COM-PHYS
Search for excited leptons in proton-proton collisions at 𝒔 = 7 TeV with the ATLAS detector, Phys. Rev. D85 (2012) , CERN-PH-EP
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51. Backup Slides
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52. Systematics Uncertainty
PDF Uncertainty
Theoretical systematics due to PDF uncertainties. As expected, these are much smaller when looking at the ratio
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53. Systematics Uncertainty
Scale Uncertainty
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54. Systematics Uncertainty
Strong Coupling
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55. Systematics Uncertainty
K-Factor
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56. Jet Related Uncertainty
Systematics Uncertainty
Jet Related Uncertainty
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