1. The Top Mass Measurement Dilepton final state @ ATLAS: 7 TeV, 4.7 fb-1
AEPSHEP 2014
Robyn Lucas on behalf of Group A
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AEPSHEP 2014: Group A

2. Outline Introduction: why top? ATLAS Selecting Events
Backgrounds? What backgrounds?
mLB and template fits
Systematic uncertainties
Results
Comparison with CMS
Conclusions
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3. Introduction
Top quark first proposed in Koyabayashi & Maskawa in 1973: took 22 years to find (CDF & D0 at the Tevatron)
The heaviest elementary particle; mtop ~ mass of gold atom!
Largest coupling to the Higgs boson… some role in EW symmetry breaking?
Free parameter of the SM: test its self consistency…
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4. Why Top? value of top mass arXiv 1205.6497 15/11/2014
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5. Why Top?
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6. Top decays Here, study top mass through ttbar production 15/11/2014
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7. Atlas Detector Muon Spectrometer Inner Tracker Solenoid
Calorimeter Liquid Argon
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8. Data & MC
Data = TeV using the ATLAS detector: 2011 data (~10 primary vertices per bunch crossing)
MC samples used for background estimations:
NNLO
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9. Event Selection Jets Leptons
At least one lepton match with single lepton triggers
MC: leptons matched with generator level leptons (W,Z,t decay)
Jets
B-tag: MV1, 70% efficiency
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10. Background Estimation
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11. Control Plots
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12. Gives reduced sensitivity to systematic uncertainties; 77% efficient
mLB & Template fit
The 2 neutrinos in signal make it very difficult to constrain top mass: use mLB estimator
“the lowest average mass of the two lepton+b-jet permutations to measure mtop”
Gives reduced sensitivity to systematic uncertainties; 77% efficient
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13. Systematic Uncertainties
~80% of the total systematic uncertainty comes from JES and b-JES
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14. Jet Energy Scale
Expressed with 21 parameters (one of which b-jet scale), each varied by ±1σ. Effect on top mass is calculated by quadratic sum of 20 components
+ b-jet energy scale, accounting for remaining differences between jets originating from light quarks & b quarks
= 80%
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15. mt = 173.09 ± 0.64 (stat) ± 1.50 (syst) GeV
Results
Total uncertainty similar with ATLAS mtop measurements in the l+jets channels
eμ, e+e- & μ+μ- channels are used and lower lepton pT thresholds: decrease in stat uncertainty wrt to previous measurement
Use of mLB estimator reduces systematics – don’t require full event reco: ISR, FSR, QCD radiation & energy scales uncertainties reduced
Extract top quark mass with template method: better stat. precision than using the mean of distribution
The fitted PDFs for signal and background. The inset shows the −2 ln L profile as a function of the fitted top quark mass.
mt = ± 0.64 (stat) ± 1.50 (syst) GeV
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16. And CMS?
CMS also measured ttbar in dilepton final 7 TeV…
Agree within uncertainties: same syst error, slightly lower stat…
cf. ATLAS
Looser event selection so higher stats
Exclusive 3 categories ee, em, mm &
2 b-tag bins
Top mass reconstructed using an analytical matrix weighting technique:
Use top mass parameter to fully constrain ttbar system and create a likelihood which compare to simulation for each mt
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17. Summary and Conclusion
Top quark mass, with W mass, important for limiting the Higgs mass: precision test of SM
The mLB estimator allows sensitive measurement without fully reconstructing top quarks: lower systematics & analysis method improves stats
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18. Conclusion Excellent precision test of the SM
Top mass now constrained better than the W!
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19. BACK UP
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20. ATLAS Pixel Detector
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21. Which top?
Top mass not an “observable” per se: inferred from its effect of kinematic observables
Cannot be well-defined at LO
Pole mass corresponds to physical intuition of a stable particle; top quark inherently unstable
The “pole” in the top quark propagator
It cannot be determined better than lambda_QCD
At hadron colliders, measure the “MC mass”: ~1 GeV different
Under debate!
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22. TDR 1999
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33. Systematic Uncertainties: Details
Method Calibration – The mass top measured by applying the method on the signal MC should be same as the true mass value used for generating the sample.
Signal MC Generator – Larger of the following two is quoted
Comparison of mass measured from signal MC generated with different event generators PWHEG). Hadronization is done with Herwig in both the cases.
Effect on mass measurement by moving renormalization and factorization scales up by a factor 2 or down by a factor of ½.
Hadronization – Different in the mass measurement when hadronization of the POWHEG signal MC done with
PYTHIA with P2011C tune
HERWIG and JIMMY with ATLAS AUET2 tune
Underlying Event – Difference in mass measurement with two different tunes for POWHEG+PYTHIA : Perugia2011 and Perugia2011 MPIHI.
Color reconnection parameters are the same in the two tunes. Perugia2011 MPIHI has more semi-hard MPI than Perugia2011.
Color Reconnection – Difference in mass measurement with two different tunes for the POWHEG+PYTHIA sample : Perugia2011 and Perugia2011 noCR.
ISR/FSR – Difference in mass measurement by varying the PYTHIA showering parameters in the ACERMC+PYTHIA(P2011CTune) sample.
PDF – CT10 with sample used.
Half the quadratic sum of the differences if 26 pairs.
Signal sample also rewighted to the central PDF set for MSTW2008 or NNPDF23. These differences are smaller than the variations within CT10.
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34. Systematic Uncertainties: Details
Background – Background fraction has an uncertainty of about 28%. Total background contribution is shifted by ±1σ and the impact on the mass measurement in estimated.
Jet Energy Scale – A 21 parameterization with 21 uncorrelated parameters (20 + bJES) is used to express JES. The 20 parameters are varied by ±1σ and effect on top mass is calculated. The uncertainty on top mass is calculated by adding the 20 differences quadratically.
b-JES - This uncertainty is uncorrelated with the JES uncertainty and accounts for the remaining differences between jets originating from light quarks and those originating from b-quarks after the global JES has been determined.
b-tagging efficiency and mistag rate - The b-tagging scale factors account for differences in the efficiency and mistag rate of the b-tagging algorithm between data and MC.
Jet Energy Resolution - To assess the impact of this uncertainty, before performing the event selection, the energy of each reconstructed jet in the simulation is additionally smeared by a Gaussian func- tion such that the width of the resulting Gaussian distribution corresponds to the one including the uncertainty on the jet energy resolution [53]. The mass difference with respect to the unsmeared case is taken as the uncertainty.
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35. Systematic Uncertainties: Details
Jet Reconstruction Efficiency – The jet reconstruction efficiency for data and the Monte Carlo simulation are found to be in agreement with an accuracy of better than 2% [35]. To account for the remaining uncertainty, 2% of the jets are randomly removed from the events. The event selection and the fit are repeated on the changed sample. The effect on the measured top quark mass is negligible.
Missing Transverse Momentum - The impact of a possible miscalibration of the ETmiss is assessed by changing the energy scale and resolution of the soft calorimeter energy deposits within their un-certainties. These energy deposits, which are not included in the reconstructed jets and leptons, only contribute to ETmiss.
Pile-up –To investigate the uncertainty due to the presence of additional p-p interactions in the event, the fit is repeated in data and simulation at mtop = GeV as a function of the number of reconstructed vertices nvtx and the average number of interactions per bunch crossing <μ>. The effect on the measurement from data is reproduced by MC within statistical uncertainty. However, a possible residual effect on mtop is assessed by computing the sum of the differences of a linear interpolation of the fitted masses to the full sample in every nvtx and ⟨μ⟩ bin in simulation, weighted either with the relative frequency of observing a given nvtx and ⟨μ⟩ in data, or with the corresponding frequencies in simulation. The difference of the sums in data and simulation is taken as the uncertainty from this source.
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36. Systematic Uncertainties: Details
Electron and Muon Uncertainties - This category takes into account the uncertainties in the efficiency of the trigger, in the identification and reconstruction of electrons and muons, as well as residual uncertainties due to a possible miscalibration of the lepton energy scales. The number quoted is the quadratic sum of all the studied components and is dominated by the uncertainty on the lepton energy scales.
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37. Jet Energy Scale
21 parameter fit
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38. CMS Looser event selection:
1 or more b-tags (loose ID), MET > 40 GeV for same sign dileptons, wider Z mass window
Similarly syst. uncertainties dominated by JES, and b-tagging
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39. CMS
Likelihood using AMWT technique, in all exclusive bins & 3 channels:
@ 8 TeV:
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40. top quark mass data from CMS and TEVATRON
CMS data of Top quark
Average September 2012
± 0.38stat. ± 0.91syst. GeV
TEVATRON data of top quark
Average May 2013
± 0.51stat. ± 0.71syst. GeV

41. Finally data from ATLAS 2011 !!
± 0.64stat. ± 1.50syst. GeV
ATLAS data of Top quark