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Measurement on the mass difference between top and anti-top quarks

Measurement on the mass difference between top and anti-top quarks. Vikash Chavda 1 , Un-Ki Yang 1 , Jahred Adelman 2 University of Manchester 1 , Yale University 2 ATLAS Top UK, August 15, 2012. Mass Difference?. Motivation Event selection How to reconstruct mass difference?

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Measurement on the mass difference between top and anti-top quarks

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  1. Measurement on the mass difference between top and anti-top quarks Vikash Chavda1, Un-Ki Yang1, Jahred Adelman2 University of Manchester1, Yale University2 ATLAS Top UK, August 15, 2012

  2. Mass Difference? • Motivation • Event selection • How to reconstruct mass difference? • Template fitting • Systematic studies • Final result

  3. Top Antitop Mass Difference • A mass difference between top and antitop quarks: • Implies CPT violation, thus constraint on CPT Violation raised by a new physics process • Currently, there have been measurements made on the ttbar mass difference by D0, CDF and CMS • Dm = mt-mtbar = 0.8 ± 1.8(stat.) ± 0.5(syst.) GeV (3.6fb-1) : event by event • Dm = -3.3 ± 1.4(stat.) ± 1.0(syst.) GeV (5.6fb-1) : event by event • Dm= -0.44 ± 0.46(stat.) ± 0.27(syst.) GeV (4.9fb-1): using diff. samples

  4. Event Selection • Semi-leptonic channel: follow the top group selection cuts and have full agreement in acceptance challenge • Cuts are: • Exactly one el with ET> 25 GeV and |η| < 2.47, with a crack veto (1.37 < |η| < 1.52), OR Exactly one mu with pT> 20 GeV and |η| < 2.5 with single lepton triggers • Remove events tagged as e-mu overlap • Require a primary vertex with Ntracks> 4 • MET > 35 GeV for the el channel, OR MET > 20 GeV for the mu channel • Mt(W) > 25 GeV for the el channel OR MET + Mt(W) > 60 GeV for the mu channel • ≥ 4 jets with ET> 25 GeV and |η| < 2.5 and |JVF|>0.75 • Events with loose bad jets with pT> 20 GeV are rejected • ≥ 2 jet tagged with MV1 weight > 0.60

  5. Signal MC Generation – ΔMtop ≠ 0 • Modification made to Pythia to allow generation of samples in the case that mt ≠ mtbar • Modifications were placed in package PythiaExo_i, and 15 Signal samples ranging with mass differences between -15GeV to 15GeV were produced centrally, in release 17

  6. How to get mass difference • Modify Kinematic fitter to allow different mass between top and anti-top quark • Use up to leading 5 jets to find best combination of 4 jets • Select the best comb. for • 3rd term: W mass constraint (leptonic & jet energies) • Last two terms: fit ΔM:  Δmreco = q(lep)*ΔM (=m(lep) - m(had)) • (mt+mtbar)/2 is constrained to be the PDG value • Jet and leptons energies allowed to be varied within their uncertainties

  7. Event Flow (Electron)

  8. Event Flow (Muon)

  9. Data-background Comparison Leptonic Bjet Jet Pt Light Jet Pt Missing Et Lepton Pt

  10. Reconstructed Top and Anti-top Top Mass Anti-top Mass

  11. Mass Difference • Fitted Mass Difference (Combined Electron and Muon Channels)

  12. Signal Template • Use a modified Pythia for signal samples (central and private production) • -15GeV to +15GeV (15 samples) • Fit a double Gaussian to the reconstructed mass difference from each of the MC signal sample (left) • Plot dependencies of variables of Gaussian (mean, sigma) as a function of input mass difference: see templates in backup slides • A good linear dependence: 2nd Gaussian (purple)

  13. Background Template • Various backgrounds added together (Single Top, W+Jets,QCD) • Stop from MC, W+jets from the data-driven method • QCD from the data-driven method event estimated from JetElectronQCDModel • https://twiki.cern.ch/twiki/bin/viewauth/AtlasProtected/JetElectronQCDModel • Non-ttbar background (8.2%) QCD Single Top W+Jets

  14. Likelihood • Use an extended maximum likelihood fit with 3 fit parameters • Expected number of signal events (ns) • Expected number of background events (nb) • Fitted mass difference ( ) • The fit machinery and parameterizations are tested using PE’s • The fit with a Gaussian bkgd constraint is also checked out. • PE setup: • Draw events from Δmreco histograms • 2000 PEs • Fit using the LH, take the mean on the fitted mass difference from the 2000 PE results

  15. Sensitivity • Expected stat. error in 2-tag for 4.7/fb based on PEs with Dm=0 GeV sample (left) • Expected statistical error (2-tag) plotted as a function of the mass difference (right) Exp. statistical error on Dm: =0.6 to 0.7 GeV Exp. error for different Dm Exp. error dist. for Dm=0 GeV

  16. Systematic • Systematic uncertainties applied to both the top and antitop equally, is expected to have a small contribution • Jet Energy Scale, Lepton ID, B Jet Energy Scale • But systematics which are related to an asymmetry between top and antitop can be large • General procedure • Apply uncertainty to key variables in analysis (vary by ± 1σ ) • Re run full analysis • Get the distribution • Run through 2000 PE’s to get ΔM • The shift with respect to a nominal sample is taken as the systematic Δmreco

  17. Systematic Definitions In addition, the effect from non-zero Δm in single-top production: 20 MeV for Δm = 2.5 GeV (check, not syst. Item)

  18. Check on Parameterisation • In the mean of the narrow Gaussian, see large fluctuation in the sub 1 GeV range • Due to low statistics signal samples • Uncertainty on the slope: 25 MeV effect on ΔM • Question raised whether the fitter has a sensitivity for a precision measurement at the sub-GeV level

  19. Precision at the Sub GeV? • Test parameterisation sensitivity for a precision measurement at the sub-GeV level Strategy: • Check the parameterization at at Δm =0 GeV using high-statistics Pythia ISR/FSR Avg sample • Use high statistics (15M) SM ttbar MC@NLO sample and reweight them to our current signal samples • Use high statistics new Pythia samples (0, 1, -1 GeV)

  20. Sub GeV sensitivity - ISR/FSR • Use mixture of the ISR More/Less sample to create an ISR Average sample • Given that the ISR/FSR systematic is small, ideal sample to test • ISR More ,ISR Less ,average sample • All fitted values in our standard double gaussian fit to the templates agree with our parameterised function at ΔM=0 • The PE result is Δm= 97 MeV ( probably due to the ISR/FSR syst. effect plus fast vs fullsim)

  21. Sub GeV Sensitivity: Reweighting • A second method we used was to reweight MC@NLO sample to ±1 GeV, ±0.6 GeV, ±0.3 GeV, and to the Signal Pythia 0 GeV sample to obtain high stats signal samples • The mass of the top and antitop were distributed independently during generation • Reweight top and anti-top mass distributions separately at truth level: for 1 GeV case: reweight top mass to 173 GeV, and antitop mass to 172 Gave, keeping the average mass remains 172.5 GeV Pythia 0 GeV sample MC@NLO sample

  22. PE results • The values in the previous tables are fairly consistent with the input mass values within 50 MeV with an offset of 175 MeV • Offset mostly due to difference between Pythia and MC@NLO (NLO, Generator, Parton Shower) plus a possible bias from fitter • 3 high statistic samples will help in the understanding of the offset • Measurement from fit will be calibrated to MC@NLO sample • 175 MeV will be subtracted from fitted value as final measurement with additional calibration uncertainty of 50 MeV

  23. Syst. Errors on DM CMS Syst. 0.04 ± 0.08 0.04 ± 0.06 0.08 ± 0.03 0.1 ± 0.1 0.09 0.01 0.11 ± 0.14 0.27

  24. Dm Result • Final measurement is presented • Mass difference obtained by fitting to 4.7 pb-1 of 2011 data Final result including the subtraction of the 175 MeV offset

  25. Cross checks on the results • Results with default fitting • Results with a Gaussian bkgd constraint(30%) • Electron vs muon channel: consistent • 200 MeV difference is expected based on the Pes • Electron • Muon

  26. Conclusions and Plans • The top anti-top mass difference analysis presented: • The current sensitivity on the ΔM; • Stat: 610 MeV • Syst: 192 MeV • Achieve great sensitivity • Paper written • One cross check left before ATLAS circulation

  27. Backup Slides

  28. Performance Plots • Using MC@NLO ttbar sample, show performance of the Kinematic fitter on the mass difference and hadronic top mass • See improvement in distribution when using fitted energy of jets and leptons (black curve) • Further improvement is seen when applying cut (pink curve) • The cut value 10 was tuned to achieve the best sensitivity

  29. Mass Difference? • Why? CPT violation • What events? tTbar in semi-leptonics • How to reconstruct mass difference? Kinematic fitter • Template fitting • What systematics? • Final result

  30. Data and MC Samples Data • High-pt lepton (e/mu) trigger sample in 2011, 4.7pb-1 , periods B-M • (the standard single lepton sample the Top group used) • MC Samples • ttbar events (dominant): • SM MC (Δm=0): MC@NLO • signal MC (Δm≠0):??? • Backgrounds Single top (Wt, s chan) : MC@NLO • Single top, tchan : AcerMC+Pythia • W/Z+jets: ALPGEN+Herwig • Diboson: MC@NLO • QCD : data driven method

  31. Motivation top quark • Top quark has largest mass of all the fundamental particles • Only fermion with unsurpressed coupling to electroweak symmetry breaking sector ne nm nt e-m t u d s c b . . . . • CP non conservation and T violation seen in neutral kaon system, and the test of CPT violation have been explored in neutrino sector • Top most precisely measured quark, (assuming top mass equal anti-top mass), but the test of the CPT violation has been just started. • With High-statistics LHC data, a great opportunity to search for it

  32. Mass Difference • Reconstruct hadronic and leptonic top masses in semi-leptonic decay channel • Take the mass difference between top and anti-top quarks • If positive lepton in final state, the mass difference will be top (leptonic) – anti-top (hadronic) • Apply template fitting

  33. Motivation • The CPT Theorem (Combination of Charge, Parity and Time reversal) states: • Any local theory, which is invariant under Lorentz Transformations and defined by a Hermitian Hamiltonian is said to converse CPT • CPT Conservation for particles and antiparticles implies: • Equal masses • Equal lifetimes • Any mass difference between a particle and its antiparticle is unambiguous evidence of CPT Violation • CPT is a fundamental piece of QFT, on which particle physics is based

  34. Top Antitop Mass Difference • A mass difference between top and antitop quarks • Implies a CPT violation, thus constraint on CPT Violation raised by a new physics process • There have been measurements made on the ttbar mass difference by D0, CDF and CMS • ΔMtop =Mt-Mtbar = 0.8 ± 1.8(stat.) ± 0.5(syst.) GeV (3.6fb-1) • ΔMtop= -3.3 ± 1.4(stat.) ± 1.0(syst.) GeV (5.6fb-1) • ΔMtop= -0.44 ± 0.46(stat.) ± 0.27(syst.) GeV (4.9fb-1) • Analysis done in Semi-leptonic channel with 2 b-tag events ( 4.7 fb-1 in Rel 17) • Note: ATL-COM-PHYS-2012-658

  35. Previous Measurements Template method, semi-leptonic channel, B-tagged, PRL 106, 152001 (2011)

  36. PE results • PE results are consistent with 175 MeV in the range of ΔM= +1 to -1 GeV.: the offset: Pythia vs MC@NLO diff. (NLO, PS, Gen) plus a bias from the fit • Thus, 175 MeV offset (MC@NLO) will be applied to the measured value, • But what syst error? 50 MeV or any dependence on ΔM? • 3 high statistics samples (0,+1,-1 GeV) will help here

  37. Data and MC Samples Data • Using data recorded in 2011, 4.7pb-1 , periods B-M MC (AFII, FS, mc11b, mc11c) • Single top (Wt, s chan) -> MC@NLO+Herwig • Single top, tchan -> AcerMC+Pythia • Diboson (WW,WZ,ZZ) -> MC@NLO • Z+bb -> Alpgen+Herwig • Nominal ttbar -> MC@NLO • ISR More/Less -> AcerMC+Pythia • Parton Shower Model -> Powheg+Pythia(Herwig) Signal (FS, mc11c) • 15 samples -> Pythia

  38. Jet Energy Scale Uncertainty • Evaluated: • Vary jet momentum in analysis by ± 1σ • Result 0.043 GeV • ΔMreco distribution with JES ± σ (left), and ΔMreco distribution after the kinematic fit with JES ± σ (right)

  39. Asymmetry between top and anti-top • The identification (ID) of the top and anti-top quarks is based on the lepton charge • B/Bbar oscillation has no effect on the top ID • Mis-charge on the lepton? (muon: prob. < 0.0001, electron:~0.001) • Asymmetry in energy response between top and anti-top quarks • B-jet response: in principle, no different at the generator level, but a possible small difference at the detector level ( different k-P/k+P interaction rate in the calorimeter), but the simulation knows it. • W+/W- response: asymmetry in c/cbar, s/sbar, lepton: the effect will be further reduced due to the W mass constraint • Production asymmetry (t/tbar: different eta distributions due to qqbar process): PDF uncertainty is the smallest one.

  40. Asymmetry in JES for b/bbar, c/cbar,s/sbar From MC@NLO FS sample b/bbar asymmetry: 0.27+0.1% c/cbar asymmetry: <0.13+-0.1% s/sbar asymmetry: no asymm.

  41. b/bbar asymm.: • MC@NLO+Herwig FullSim and FastSim results for the b/bar asymmetry are consistent each other: different interaction rate of K- vs K+ on proton in calorimeter is not observed. • But the Powheg+Pythia samples are consistent with no asymmetry. • The half difference is taken as the systematic: 80 MeV • Possible asymm. (0.1%) in for c/cbar: 13 MeV

  42. Parton Shower Model Systematic • PS samples: Powheg interfaced with Pythia and Herwig • Dm: 0.192 GeV for Herwig, and -0.191 GeV for Pythia • A large difference, 0.40 GeV on Dm • Source of the difference • b/bbar-JES asymmetry • Powheg with Herwig (AF-II): 0.28% • Powheg with Pythia (AF-II): 0.09% • The difference 0.21% can only example 0.12 GeV on Dm, but not able to explain a whole difference

  43. Parton Shower Model • A new Powheg+Pythia sample generated (using Perugia 2011 tune) • Produced on the basis of top mass studies which found a big discrepancy with data due to the Pythia tune. • https://indico.cern.ch/getFile.py/access?contribId=2&resId=0&materialId=slides&confId=201594 • Using this sample, the PS systematic reduces dramatically from 400 -> 56 MeV! • Sample is currently being validated by many analysis to check viability for Top2012 deadlines

  44. Sanity Checks • No bias (Residuals are consistent with zero) • Pull widths are consistent with unity

  45. Sub GeV Sensitivity - ISR/FSR • ISR/FSR average sample agrees with all of the parameterized value at Δm=0 (plots and PE result) • The PE result is Δm= 97 MeV ( probably due to the ISR/FSR syst. effect plus fast vs fullsim)

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