Experimental aspects of top quark physics Lecture

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Experimental aspects of top quark physics
Lecture 2

• Regina Demina
• University of Rochester
• Topical Seminar on Frontier of Particle Physics
• Beijing, China
• 08/15/05

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Outline

• Invariant mass
• Template method to measure top mass
• Matrix element method
• Jet energy scale calibration on W-boson
• Combined result
• Constraint on Higgs mass
• Control questions

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Invariant mass

• Top quark decays so fast there is no time to put
  it on a bathroom scale
• We measure its mass through energy and momentum
  of its products
• t?bW, W?qq
• E(t)E(b)E(q)E(q)
• P(t)P(b)p(q)p(q)
• M2(t) E2(t)-p2(t)
• M, E, p in GeV

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Challenges of Mtop Measurement
LeptonJets Channel

• Leading 4 jets combinations
• 12 possible jet-parton assignments
• 6 with 1 b-tag (b-tag helps)
• 2 with 2 b-tags
• Poor jet energy scale and resolution
• Hard to find the correct combination

Observed Final state Complicated final state
to reconstruct Mtop
Good b-tagging and jet energy scale and
resolution and good algorithm to reconstruct Mtop
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Template method

• c2 mass fitter
• Finds top mass that fits event best
• One number per event
• Additional selection cut on resulting c2

Data
Wbb MC
Massfitter
tt MC
Signals/background templates
Datasets
Data
Likelihoodfit
Result
Likelihood fit Best
signal bkgd templates to fit datawith
constraint on background normalization
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Mass Fitter (event by event)

• Try all jet-parton assignments with kinematic
  constraints, but assign b-tagged jets
  to b-partons
• Select the rec. mass Mt from the choice of lowest
  c2
• Badly reconstructed Mt (c2 gt 9 ) is removed

Top mass isfree parameter
All jets are allowed to be float according to
their resolutions to satisfy that
M(W)M(W-)80.4 GeV, M(t)M(t)
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Templates for different number of tags
More correct combination with b-tag
Mt(GeV/c2)
Mt(GeV/c2)
Mt(GeV/c2)
Mt(GeV/c2)
Bkgd is large in the 0-tag
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Signal templates for different masses

• Samples Herwig with
• Mtop 130 to 230 GeV
• Get analytical functions
• (2 Gaussian gamma)
• of reconstructed mass, Mt
• as a function of true mass, Mtop
• Fit parameters linear depend.
• on Mtop

Smooth PDFs (Mt true Mtop)
Mt(GeV/c2)
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Result on Mtop
Comb. Log Likelihood
Expected error
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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
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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

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Matrix Element Method
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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

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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

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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

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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)

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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

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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

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Calibration on Full MC
leptonjets
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Mt169.54.4 GeV/c2 JES1.0340.034
calibrated
calibrated
DØ RunII Preliminary
expected 36.4
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Systematics summary
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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

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Gluon radiation

• Extra jets from initial/final state gluons
• 80 of the time, leading 4 jets correspond to 4
  partons (qqbb)
• Final effect on top mass 0.34 GeV/c2

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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

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Summary of DØ Mt measurements
DØ Run II preliminary

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

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

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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.

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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

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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.

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High statistics (LHC) approach
In 100fb-1 about 1000 signal events is expected
No jes systematics !!!