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1 Measurement of the Mass of the Top Quark in Dilepton Channels at DØ Jeff Temple University of Arizona for the DØ collaboration DPF 2006.

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Presentation on theme: "1 Measurement of the Mass of the Top Quark in Dilepton Channels at DØ Jeff Temple University of Arizona for the DØ collaboration DPF 2006."— Presentation transcript:

1 1 Measurement of the Mass of the Top Quark in Dilepton Channels at DØ Jeff Temple University of Arizona for the DØ collaboration DPF 2006

2 2 370 pb -1 835 pb -1 tt dilepton decays Event selection Reconstructing top mass – Matrix weighting – Neutrino weighting Combined dilepton result for 370 pb -1 sample Result in e  channel for 835 pb -1 sample Overview

3 3 Fermilab Accelerators 5 accelerators Collide protons and antiprotons at √s=1.96 TeV Collisions at CDF, DØ DØ CDF

4 4 DØ Detector Silicon, Fiber Trackers within 2 T solenoid LAr calorimeter 3 layers of muon scintillators and wire chambers – Toroid between 1 st and 2 nd layers

5 5 W+W+ W-W- b b Dilepton Decay Channel 2 high-p T jets 2 high-p T leptons Significant missing transverse energy (MET) from neutrinos t t pp e+e+ -- e 

6 6 Cons: Small branching fraction Neutrinos not measured directly! hadronic e+jets  +jets  +X ee+  +e  < 5% ! Dilepton Channel Pros and Cons Pros: Small background yields – Z ee,  – Z  – WW, WZ – Instrumental Fakes Few Jet Combinations

7 7 Dilepton Event Selection 2 leptons – p T >15 GeV 2 jets – p T >20 GeV Channel-specific cuts (“b-tagging”: require one jet to be identified with a secondary vertex) ee channel Reject 80 GeV<M ee <100 GeV if M ee 40 GeV if M ee >100 GeV, MET>35 GeV Sphericity > 0.15  channel Z fitter MET > 35 GeV increase MET requirement based on MET-  angle e  channel MET > 25 GeV Energy of jets + leading lepton >140 GeV Cut on electron shower shape

8 8 Additional Selection: Lepton+Track Select one lepton + one isolated central track Increases dilepton acceptance Veto events satisfying ee, e ,  selection Lepton + Track selection 1 lepton – p T > 15 GeV 1 isolated track – p T > 15 GeV 2 jets – p T >20 GeV – at least one b-tagged jet MET> 15-35 GeV

9 9 Dilepton Events in Data ~370 pb -1 of data Slightly different selection criteria for matrix weighting, neutrino weighting

10 10 Reconstructing Top Mass 18 independent kinematic variables 12 measured directly (jets, charged leptons) Also measure MET x, MET y 2 constraints from m W+, m W- 1 constraint from m t =m t t t W+W+ W-W- pp b b e+e+ -- e  Insufficient constraints to determine m t !

11 11 Assigning Event Weight Assume m t Assign weight W(m t ) to mass assumption based on agreement with observables – matrix weighting – neutrino weighting Repeat for many values of m t Use weights to determine mass Sample event weight distribution for Monte Carlo 175 GeV top

12 12 Matrix Weighting (MWT) f(x) = parton distribution function P(E|m t )=probability that lepton has energy E in rest frame of top quark: Weight based on consistency of observed lepton energy with m t hypothesis R. H. Dalitz and G.R. Goldstein, Phys. Rev. D 45, 1531 (1992)

13 13 Neutrino Weighting ( WT) Ignore MET x, MET y Assume m t,    Repeat for many rapidity assumptions Weight based on agreement between assumed total neutrino momentum and observed MET

14 14 Generating Event Templates Extract mass information from event weights MWT uses peak weight Collection of event weight info is a template Form templates for signal, backgrounds – Monte Carlo for different values of m t – Monte Carlo/Data for backgrounds nWT uses entire (rebinned) distribution

15 15 Determine m t with maximum likelihood fit Use ensemble tests to calibrate fit minimum Likelihood Fit MWT ensemble tests WT ensemble tests Slope=1.00 Offset=1.9 GeV Slope=0.99 Offset=1.7 GeV

16 16 Results from 370 pb -1 Data Set open circles: WT – dilepton – lepton + track closed circles: MWT – dilepton b-tagged no b-tagged Combined measurement:

17 17 Estimating Systematics Largest uncertainty: Jet Energy Scale (JES) Create ensembles with jet energies varied by their uncertainties, and compare to original templates Similar approach for other uncertainties Combined JES uncertainty: 4.3 GeV

18 18 Systematic Uncertainties

19 19 Combined Result for 370 pb -1 m t =178.1±8.3 GeV – Submitted to PRL – hep-ex/0609056 Consistent with previous measurements Significant improvement on Run I dilepton uncertainty

20 20 Expected Yields Event Selection for 835 pb -1 MWT, WT analyses for e  channel only e  event selection 1 muon, p T >15 GeV 1 electron, p T > 15 GeV 2 jets, p T > 15 GeV Energy of jets + leading lepton > 120 GeV OBSERVED: 28

21 21 MWT Measurement m t (GeV) -log (likelihood) Event Weight Peaks vs. Top Mass DØ Run II preliminary

22 22 Revised WT Event Weights 3 approaches – Coarse binning same as previous WT analysis 6 bins, not 10 – peak value à la MWT Modified fitting procedure – Mean + RMS

23 23 WT Ensemble Tests 6 binspeak valuemean+RMS Each method shows good agreement between input, output top mass DØ Run II preliminary Slope=0.99 Offset=-0.6 GeV Slope=0.99 Offset=0.2 GeV Slope=0.95 Offset=-0.4 GeV

24 24 WT Measurements 6 bins: m t = 173.6 ± 6.7 (stat.) +5.1 -4.0 (syst.) GeV peak: m t = 165.7 ± 9.7 (stat.) +4.4 -4.7 (syst.) GeV Mean+RMS: m t = 171.6 ± 7.9 (stat.) +5.1 -4.0 (syst.) GeV 6 binspeak valuemean+RMS DØ Run II preliminary

25 25 Results for 835 pb -1 New results consistent with one another and world average top mass Uncertainties in e  channel comparable to combined uncertainty for 370 pb -1

26 26 Conclusions Combined 370 pb -1 Run II dilepton measurement improves upon Run I result 835 pb -1 result in e  channel provides further reduction in uncertainty – ee,  analyses proceeding – WT converging on preferred mass extraction method Approaching point where measurement is limited by systematics!

27 27 Backup Slides Run II luminosity Sample event weights Individual mass measurements Pull Distributions

28 28 DØ Run II Integrated Luminosity Run II started in April, 2002 Data collection efficiency > 85% 370 pb -1 collected by fall 2004 835 pb -1 collected by fall 2005 (compare to 110 pb -1 for Run I) 370 pb -1 835 pb -1

29 29 Sample Event Weights – WT 370 pb -1 Correct assumption of m t,    leads to non-zero weight Incorrect assumptions can also produce valid solutions!

30 30 Detector Smearing – WT 370 pb -1 Object momenta not measured perfectly Repeat weight calculations, smearing over detector resolutions ~150 smears “stabilizes” weight distribution

31 31 Individual Channel Results – WT 370 pb -1

32 32 WT dilepton WT dilepton & lepton+track MWT dilepton Pull Distributions – 370 pb -1

33 33 Ensemble Checks – 835 pb -1 WT Peak Calibration MWT Pull Widths WT 6-bin Expected Error


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