Top quark mass

1
Top quark mass

• For DØ collaboration
• Regina Demina
• University of Rochester
• Wine and Cheese seminar at FNAL, 07/22/05

2
Outline

• Introduction
• Top quark mass measurement in Run II
• Matrix element method description
• In situ jet energy scale calibration on hadronic
  W-mass
• Sample composition
• Result
• Systematics
• Tevatron combined top mass
• Top quark production
• Update on cross section in ljets channel
• Search for resonance production

3
Top Quark Mass Motivation

• Fundamental parameter of the Standard Model.
• Important ingredient for EW precision analyses at
  the quantum level
• which were initially used to indirectly
  determine mt.
• After the top quark discovery, use precision
  measurements of MW and mt to constrain MH.

4
Top production
At vs1.96 TeV top is produced in pairs via
quark-antiquark annihilation 85 of the time,
gluon fusion accounts for 15 of ttbar production
5
Top Lifetime and Decay

• Since the top lifetime
  ?top 1/ M3top10
  -24 sec
• ?qcd ?-1 10 -23 sec
• BR(t?Wb) ???????
• Both Ws decay via W? l?
• final state l??l?bb -
• DILEPTON
• One W decays via W?l?
• final state l??qq bb - LEPTONJETS
• Both Ws decay via W?qq
• final state qq?qq bb
• ALL HADRONIC

the top quark does not hadronize. It decays as a
free quark!
Lepton provides a good trigger, all jets are tough

6
Top ID in leptonjets channel

• 2 b-jets
• Lepton electron or muon
• Neutrino (from energy imbalance)
• 2 qs transform to jets of particles
• Note that these two jets come from a decay of a
  particle with well measured mass W-boson
  built-in thermometer for jet energies

7
DØ detector

• Electrons are identified as clusters of energy in
  EM section of the calorimeter with tracks
  pointing to them
• Muons are identified as particles passing through
  entire detector volume and leaving track stubs in
  muon chambers. Track in the central tracking
  system (siliconSciFi) is matched to track in
  muon system
• Jets are reconstructed as clusters of energy in
  calorimeter using cone algorithm DRlt0.5

8
Top mass using matrix element method in Run I

• Method developed by DØ (F. Canelli, J. Estrada,
  G. Gutierrez) in Run I

Single most precise measurement of top mass in
Run I Mt 180.13.6(stat) 4.0(syst) GeV/c2
Systematic error dominated by JES 3.3
GeV/c2 With more statistics it is possible to
use additional constraint on JES based on
hadronic W mass in top events in situ
calibration
9
Matrix element method

• Goal measure top quark mass
• Observables measured momenta of jets and leptons
  
• Question for an observed set of kinematic
  variables x what is the most probable top mass
• Method start with an observed set of events of
  given kinematics and find maximum of the
  likelihood, which provides the best measurement
  of top quark mass
• Our sample is a mixture of signal and background

10
Matrix Element Method
11
Transfer functions (parton?jet)

• Partons (quarks produced as a result of hard
  collision) realize themselves as jets seen by
  detectors
• Due to strong interaction partons turn into
  parton jets
• Each quark hardonizes into particles (mostly p
  and Ks)
• Energy of these particles is absorbed by
  calorimeter
• Clustered into calorimeter jet using cone
  algorithm
• Jet energy is not exactly equal to parton energy
• Particles can get out of cone
• Some energy due to underlying event (and detector
  noise) can get added
• Detector response has its resolution
• Transfer functions W(x,y) are used to relate
  parton energy y to observed jet energy x

12
h Dependence of JES

• h dependence of JES is derived on gjet data, but
  the overall scale is allowed to move to optimize
  MW

13
JES in Matrix Element

• All jets are corrected by standard DØ Jet energy
  scale (pT, h)
• Overall JES is a free parameter in the fit it
  is constrained in situ by mass of W decaying
  hadronically
• JES enters into transfer functions

14
Normalization
ejets
µjets
15
Signal Integration

• Set of observables momenta of jets and leptons
  x
• Integrate over unknown
• Kinematic variables of initial (q1,q2) and final
  state partons (y 6 x3 p) 20 variables
• Integral contains 15 (14) d-functions for
  e(m)jets
• total energy-momentum conservation 4
• angles are considered to be measured perfectly
  2x4 jet 2 lepton
• Electron momentum is also considered perfectly
  measured, not true for muon momentum 1(0)
• 5(6) dimensional integration is carried out by
  Vegas
• The correspondence between parton level variables
  and jets is established by transfer functions
  W(x,y) derived on MC
• for light jets (from hadronic W decay)
• for b-jets with b-hadron decaying semi-muonically
• for other b-jets
• Approximations
• LO matrix element
• qq?tt process only (no gluon fusion 15)

16
Background integration

• Wjets is the dominant background process
• Kinematics of Wjets is used as a representation
  for overall background (admixture of multijet
  background is a source of systematic uncertainty)
• Contribution of a large number of diagrams makes
  analytical calculation prohibitively complex
• Use Vecbos
• Evaluate MEwjjjj in N points selected according
  to the transfer functions over phase space
• Pbkg- average over points

17
Sample composition

• Leptonjets sample
• Isolated e (PTgt20GeV/c, hlt1.1)
• Isolated m (PTgt20GeV/c, hlt2.0)
• Missing ETgt20 GeV
• Exactly four jets PTgt20GeV/c, hlt2.5 (jet
  energies corrected to particle level)
• Use low-bias discriminant to fit sample
  composition
• Used for ensemble testing and normalization of
  the background probability.
• Final fraction of ttbar events is fit together
  with mass

18
Calibration on Full MC
leptonjets
19
Mt169.54.4 GeV/c2 JES1.0340.034
calibrated
calibrated
DØ RunII Preliminary
expected 36.4
20
Systematics summary
21
B-jet energy scale

• Relative data/MC b/light jet energy scale ratio
  
• fragmentation -0.71 GeV/c2
• ? different amounts of p0, different p momentum
  spectrum
• ? fragmentation uncertainties lead to
  uncertainty in b/light JES ratio
• compare MC samples with different fragmentation
  models
• Peterson fragmentation with eb0.00191
• Bowler fragmentation with rt0.69
• calorimeter response 0.85 -0.75 GeV/c2
• uncertainties in the h/e response ratio
• charged hadron energy fraction of b jets gt
  that of light jets
• ? corresponding uncertainty in the b/light JES
  ratio
• Difference in pT spectrum of b-jets and jets from
  W-decay 0.7 GeV/c2

22
Gluon radiation

• The effect is reduced by
• Requiring four and only four jets in the final
  state
• High PT cut on jets
• Yet in 20 of the events there is at least one
  jet that is not matched (DR(parton-jet)lt0.5) to
  top decay products
• These events are interpreted as background by ME
  method
• We study this systematic by examining ALPGEN ttj
  sample and varying its relative fraction between
  0 and 30 (verified on our data by examining the
  fraction of events with the 5th jet)
• Final effect on top mass 0.34 GeV/c2

23
Signal/Background Modeling

• QCD background -0.67 GeV/c2
• Rederive calibration including QCD events from
  data (lepton anti-isolation)
• (note sample statistics limited) can be reduced
  in the future

• Wjets modeling -0.32 GeV/c2
• study effect of a different factorization scale
  for Wjets events
• (ltpT,jgt2 instead of mW2 SpT,j2)
• PDF uncertainty -0.07 GeV/c2
• CTEQ6M provides systematic variations of the
  PDFs
• reweight ensembles to compare CTEQ6M with its
  systematic variations
• (by default the measurement uses CTEQ5L
  throughout
• use a LO matrix element, and for
  consistency with simulation)

24
Signal fraction

• Signal fraction 0.50 -0.17 GeV/c2
• Fitted top mass depends slightly
• on true signal fraction (if signal
• fraction is smaller than expected)
• gt Vary signal fraction within uncertainties
• from topological likelihood fit
• - Note ftop fit yields identical result
• with factor v2 smaller uncertainties

Cross check on data cut on log10(pbkg)lt-13
Ftop31?466 Mtop170.24.1 GeV/c2
25
Systematics summary
26
Result and cross checks

• Run II top quark mass based on leptonjets
  sample Mt169.5 4.4(statJES) 1.7-1.6 (syst)
  GeV/c2
• JES contribution to (statJES) 3.3 GeV/c2
• Break down by lepton flavor
• Mt(ejets)168.8 6.0(statJES) GeV/c2
• Mt(mjets)172.3 9.6(statJES)GeV/c2
• Cross check W-mass

27
Summary of DØ Mt measurements
DØ Run II preliminary

• Statistical uncertainties are partially
  correlated for all ljets Run II results

28
Projection for uncertainty on top quark mass

• Assumptions
• only leptonjets channel considered
• statistical uncertainty normalized at L318 pb-1
  to performance of current analyses.
• dominant JES systematic is handled ONLY via
  in-situ calibration making use of MW in ttbar
  events.
• remaining systematic uncertainties include
  b-JES, signal and background modeling, etc (fully
  correlated between experiments) Normalized to 1.7
  GeV at L318 pb-1.
• Since most of these systematic uncertainties are
  of theoretical nature, assume that we can use the
  large data sets to constrain some of the model
  parameters and ultimately reduce it to 1 GeV
  after 8 fb-1.

29
Combination of Tevatron results
JES is treated as a part of systematic
uncertainty, taken out of stat error
30
Combination

• Mt172.72.9 GeV/c2
• Stat uncertainty 1.7GeV/c2
• Syst uncertainty 2.4GeV/c2
• hep-ex/0507091
• Top quark Yukawa coupling to Higgs boson
• gtMtv2/vev0.9930.017

31
What does it do to Higgs?
68 CL
MW,GeV/c2
MH,GeV/c2
Mt,GeV/c2

• MH9145-32GeV/c2
• MHlt186 GeV/c2 _at_95CL

32
And now for something completely different...
33
ttbar cross section in ljets with b-tag
DØ RunII Preliminary, 363pb-1

• Isolated lepton
• pTgt20 GeV/c, helt1.1, hmlt2.0
• Missing ETgt20GeV
• Four or more jets
• pTgt15 GeV/c, hlt2.5
• s8.11.3-1.2(statsyst)0.5(lumi) pb

34
Cross section summary
DØ RunII Preliminary
Submitted for publication
Updates
35
ttbar resonances in ljets with b-tag

• Check ttbar invariant mass for possible resonance
  production

DØ RunII Preliminary, 363pb-1
sNNLO(tt)6.770.42

• Events are kinematically constrained
• mT175GeV/c2
• Leptonic and hadronic W masses

36
ttbar resonances in ljets with b-tag

• Limit M(Z)gt680 GeV/c2 with G1.2MZ at 95CL

DØ RunII Preliminary, 363pb-1

R. Harris, C. Hill, S. Parke hep-ph/9911288
Run I limit 560 GeV/c2
Run II limit 680 GeV/c2
37
Conclusion

• First DØ RunII top mass measurement in ljets
  channel to surpass Run I precision
• Mt169.5 4.4(statJES) 1.7-1.6 (syst) GeV/c2
• Developed method for in situ jet energy scale
  calibration using hadronic W-mass constraint
• Combined Tevatron top mass measurement reaches a
  precision of 1.7
• ttbar production cross sections updated for
  ljets channel
• Invariant mass of ttbar system probed for
  resonance production, exclusion limit for
  M(Z)gt680 GeV/c2 at 95CL

38
Backup slides
39
Parton Level Tests
Text
40
Ljets sample composition
41
Kinematics in ljets sample
DØ RunII Preliminary, 363pb-1