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Top Quark Mass Measurement

Top Quark Mass Measurement. at the Tevatron. João Guimarães da Costa Harvard University For the CDF and D0 Collaborations. Why Measure the Top Quark Mass?. CDF/D0 2 fb -1 goal. t. W. W. b. The Top Quark Mass is a fundamental parameter of the Standard Model

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Top Quark Mass Measurement

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  1. Top Quark Mass Measurement at the Tevatron João Guimarães da Costa Harvard University For the CDF and D0 Collaborations ICHEP - Top Mass Measurement at the Tevatron

  2. Why Measure the Top Quark Mass? CDF/D0 2 fb-1goal t W W b • The Top Quark Mass is a fundamental parameter of the Standard Model • Only fermion with mass near electroweak scale • Correlated to other SM parameters via electroweak corrections • Precise measurement provides stringent SM test • Constrains the mass of the Higgs Boson H ? W W W W W ? MW MT2 MW ln MH New Physics mtop < 3 GeV

  3. Top Mass Reconstruction l b-jet W+ ν t Constraints X PT balance mlν=mW mjj=mW mt1=mt2 t W- b-jet Lepton + Jets • 4 jets => 12 jet-parton combinations x 2 neutrino Pz solutions = 24 solutions • Use b-tagging to reduce permutations: • 1 b-tag: 12 solutions • 2 b-tags: 4 solutions Dilepton • Two neutrinos => unconstrained system jet jet Kinematical fit B-Tagging

  4. Experimental Signatures and Measurements Run I New New New New New New New

  5. Data sample of ~125 pb-1 collected by D0 in 1994 - 1996 (Run I) Lepton + jets sample (no b-tagging requirement) - PRD 58 (1998), 052001 Matrix Element (“ME”) analysis technique using maximal event information New D0 Measurement from Run I PDFs Probability density for each event LO theoretical differential cross section Transfer function: the probability for a measured variable x to arise from a parton level variables y (energy resolution, etc…) Lepton momentum, jet angles, etc… Sum over all possible parton states Sum over all 12 permutations of jets and neutrino solutions Background process ME are explicitly included in the likelihood Dalitz, R. H. & Goldstein, G. R., Proc. R. Soc. Lond. A 445, 2803 (1999) K. Kondo, J.Phys. Soc.57, 4126 (1988) (Dynamical Likelihood Method)

  6. D0 Run I - Top Mass Analysis Using ME Method • Top Mass determined using maximum likelihood 91 candidate tt events 77 with exactly 4 jets selected 22 passing cut on background probability (Pbkg < 10-11) Expected statistical error pseudo-experiments Nature429, 638-642 (2004) Expected 5.4 GeV Observed 3.6 GeV Jet energy scale syst: 3.3 GeV/c2 Mtop = 180.1 ± 3.6 (stat) ± 3.9 (sys) GeV/c2 Comparable precision to all previous measurements combined

  7. New Top Mass World Average (Run I) mtop= 178.0 ± 4.3 GeV/c2 mH = 114 GeV mH < 260 GeV @ 95% C.L. mtop= 174.3 ± 5.1 GeV/c2 mH = 96 GeV mH < 219 GeV @ 95% C.L. +69 +69 -45 -45 With new world average, MT • New world average: mtop = 178.0 ± 4.3 GeV/c2 Significant change of Higgs mass value favored by electroweak fits mH = 19% mtop = 2%

  8. Run II performance Record luminosity on July 26, 2004 10.3 x 1031 cm-2 sec-1 • CDF and D0 detectors performing well • Tevatron with increasing performance • Run II Luminosity • Both experiments have recorded about 0.45 fb-1 (~4 x Run I dataset) • In this talk: • Data up to Fall 2003 • Luminosity < 200 pb-1 Integrated Luminosity

  9. CDF Run II – Dynamic Likelihood Method PDFs LO ttbar Matrix Element • Lepton + jets channel • 1 e, mu with pT > 20 GeV/c • Exactly 4 jets with ET > 15 GeV • ET > 20 GeV •  1 b-tag • Likelihood defined for each event: • Two summations over • Jet-Parton Assignments (It ) • Neutrino Solutions (Is ) • Transfer Function w (x,y) • (Eparton-Ejet)/Eparton • Parametrized as function of ET and  • Computed separately for b and light quark jets To obtain top mass, maximize i L(i)(Mtop)

  10. 22 tt candidate events selected 19% background fraction (mapping function) CDF Run II - DLM Results +4.5 mtop = 177.8 (stat) ± 6.2 (sys) GeV/c2 -5.0

  11. CDF Run II – Template Method CDF Run II Preliminary with b-tags • Lepton+Jets Event Selection • Similar to DLM analysis • 4th jet with ET > 8 GeV •  1 b-tag • Reconstruct invariant top mass in each event • Compute 2as follows: • Use kinematic constraints • Minimize with mtopas a free parameter • Histogram reconstructed mass with smallest 2 • Build templates from MC for • Signal process with different mtop (Herwig) • Background processes (Alpgen, Herwig, Pythia)

  12. CDF Run II – Template Results with b-tags +7.1 Mtop = 174.9 (stat) ± 6.5 (sys) GeV/c2 -7.7 • 28 tt candidate events • 6.8 ±1.2 estimated background • Measured top mass: • Use a likelihood fit • Compare reconstructed mass distribution in data to signal and background templates Jet energy scale syst: 6.3 GeV/c2

  13. Lepton + jets events without b-tagging requirement ET4th jet > 21 GeV 39 events selected Similar template method Now: 24 possible solutions CDF Run II – Template Method +10.5 +6.0 Mtop = 179.1 (stat) ± 8.4 (sys) GeV/c2 Mtop = 176.7 (stat) ± 7.1 (sys) GeV/c2 -9.5 -5.4 With 0 b-tags Jet energy scale sys: 8.3 GeV/c2 Combined Result

  14. CDF Run II – Multivariate Method Multivariate Templates Scalar Sum of the Four Leading Jets PT (GeV/c2) Reconstructed Top Mass (GeV/c2) • Another lepton+jets template analysis •  1 b-tag jet • Treats jet energy scale differently • Adjustable jet energy scale calibrated in the reconstruction of the W  qq’ decay • Improves mass resolution • Three types of signal templates ( 4 leading jets) • Good permutations (correct partons) • Bad permutations (wrong partons) • Incorrect jets (wrong jets) • Uses kinematic variables to determine probability that best 2 results from correct jet-parton assignment • Weight signal templates accordingly • Increases signal/background separation by including kinematic information in the top mass reconstruction • pT of 4 leading jets Signal templates Backgrounds

  15. 33 tt candidate events Fitted background fraction CDF Run II – Multivariate Results +6.4 Mtop = 179.6 (stat) ± 6.8 (sys) GeV/c2 -6.3 Fb = 0.34 ± 0.14 This method will be more powerful with the future larger data sample

  16. Run II D0 – Template Method +10.2 Mtop = 170.0 ± 6.5 (stat) (sys) GeV/c2 -6.5 • Lepton + jets channel • 1 e, mu with pT > 20 GeV/c •  4 jets with ET > 15 GeV, || < 2.5 • No b-tag requirement • Large missing energy • > 20 GeV (electrons) • > 17 GeV (muons) • Method similar to CDF • Low bias discriminant (DLB) using topological variables. • Apply DLB > 0.4 • After this requirement: • 87 tt candidate events • Background fraction: 56% cut 160 pb-1 W+Jets MC tt 170 GeV + Bkg

  17. Run II D0 – Ideogram Method • Method similar to one used by DELPHI for W mass • Event by event likelihood • Kinematic fit similar to the template method • Takes into account the 24 jet+neutrino solutions from kinematic fit

  18. Run II D0 – Ideogram Method Results • Lepton + Jets sample • Similar selection as for template method • No cut on low bias discriminant • 191 tt candidate events • Background fraction: 68% Mtop = 177.5 ± 5.8 (stat) ± 7.2 (sys) GeV/c2

  19. Dilepton Sample Event Selection 2 e, with pT > 20 GeV  2 jets with ET > 15 GeV ET > 25 GeV Under-constrained system due to two  Introduce the constraint: Pztt = Pzt + Pzt = 0 to kinematically solve the system ( = 180 GeV/c) CDF Run II – Dilepton Channel (template method) +17.2 Mtop = 176.5 (stat) ± 6.9 (sys) GeV/c2 -16.0 12 tt candidate events 2.7 ± 0.7 background events Jet energy scale syst: 6.5 GeV/c2

  20. Measurements Summary • Many new top massmeasurements available: • D0 Run I matrix element technique • More precise Run I world average • Several CDF Run IIpreliminaryresults • DLM: most precise measurement from run II • First preliminary D0 Run II measurements • Tevatron performing very well • Large samples becoming available • Measurements will soon be limited by systematic uncertainties • Reduction of the jet-energy scale systematic uncertainty underway Precision measurements coming soon! Goal: mtop 2-3 GeV

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