Dijet Transverse Thrust cross sections at D

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Dijet Transverse Thrust cross sections at DØ

• Veronica Sorin
• University of Buenos Aires

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Outline

• Introduction
• Overview
• The KT algorithm
• Definition of the observable

• Dijet Transverse Thrust cross section
• Systematic uncertainties
• Comparison with theory
• Conclusions

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Theoretical Introduction
Quantum Chromodynamics describe the interaction
between quarks and gluons, which carry color
charge, conventionally called blue, red and
green.
Fundamental Vertices
Main QCD characteristics
Confinement quarks and gluons cannot be seen as
isolated particles, partons (q and g) are bound
together into hadrons.
Asymptotic freedom as the energy of the
interaction increases, the strength of the
coupling get smaller, allowing the aplication of
perturbative techniques (pQCD).
Jet Physics
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Jet Physics
At the final state of an hadronic collision, QCD
predicts the appareance of highly collimated
sprays of particles, which are called Jets .
At the DØ experiment using the Fermilab
Laboratory Tevatron collider, we study pp
collisions at a c.m. energy of 1.8 TeV. The bunch
crossing occurs every 3.5 µs. By identifying
these jets, experimental measurements can be
compared with pQCD predictions.
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Panoramic view of the Fermilab Laboratory
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Event Shapes

• Event shapes have been extensively studied at
  ee- and ep experiments to
• study spatial distribution of hadronic final
  states
• test perturbative QCD predictions
• extract a precise value of ?s
• recently to test QCD developments like
  resummation calculations and non-perturbative
  corrections

Resummations needed at small values of the
shape variable where fixed-order perturbative
calculations are expected to fail.
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Thrust
Jet production rate as2 is LO
as3 is NLO Thrust (T ? 1)
as3 is LO
as4 is NLO
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T in hadron colliders
Busy environment underlying event, pile-up,
multiple interactions and noise
We have derived a correction to eliminate on
average the energy contributions from sources
other than the hard interaction itself.
particles jets

The pp c.m system is not the parton-parton c.m.
Thrust is not invariant under z boosts
Transverse Thrust
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The DØ Calorimeters

• Transverse segmentation (towers)

• Liquid argon active medium and uranium absorber

Dh x D? 0.1 x 0.1
sE / E 15 /ÖE for electrons sE / E 45
/ÖE for pions

• Hermetic with full coverage

h lt 4.2 l int gt 7.2 (total)
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Jet Algorithms

• Parton jet q and g (before hadronization)
• Particle jet final state particles (after
  hadronization)
• Calorimeter jet measured object (after
  calorimeter shower)

Iterative
Fixed cone of radius R
Overlapping cones arbitrary criteria to resolve
ambiguities Sensitivity to soft
radiation Requires ad-hoc parameter for the theory
Recombination Distance parameter D
Infrared and collinear safe Same
algorithm in theory and experiment
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RunI DØ Analyses using the KT algorithm

• Subjet Multiplicity of Gluon and Quark Jets
• Phys. Rev. D 65, 052008 (2002)
• The Inclusive Jet Cross Section
• Phys. Lett. B 525, 211 (2002)

• Dijet Transverse Thrust Cross Sections
• paper in preparation

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KT Algorithm at DØ (RunI)
(Ellis-Soper PRD 48 3160)
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KT Algorithm at DØ (RunI)
For each particle or pair of particles
Beam
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Jet Momentum Scale Correction

• Offset (O) Ur noise, pileup, multiple
  interactions, underlying event (ue)
• Response (Rjet) Pmeas / Ptrue
• (using transverse momentum balance in g-jet
  events)

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Offset Correction
O UE N
Ur noise, pileup, multiple interactions
Underlying Event

The offset contribution is obtained as the
momentum difference between jets.
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Offset Correction
O UE N
Luminosity dependent (L in cm-2 s-1)
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Rjet Correction
Monte Carlo Closure
Rjet a b ln(Pjet) c ln2(Pjet)
Pmeas / Pptcl
D1 (KT jets)
Pptcl (GeV)
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Dijet TransverseThrust

• Sum done over jets
• Jets have been reconstructed with the KT
  algorithm with D1

Jet Momentum scale correction does not eliminate
low energy jets ( high probability to originate
100 from background) ? distort the shape of the
physical distributions
Only the two leading jets will be used to
calculate Thrust
Observable selected to reduce detector effects
and maximize the signal in a hadron collider.
The spatial configuration of the two leading jets
inherits the information of the other jets in the
event
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Effects of noise and luminosity on TT
Selection of the observable
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Selection of the observable
The event energy scale
Look for a variable correlated with Q2 and with
low sensitivity to noise
HT3 (scalar sum of the transverse momentum of
the three leading jets)
HT3 vs HT
Noise jets
ET3 spectrum
Data
HT at parton level measure of Q2
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Brief Recapitulation

• Measurement of cross section as a
  function of HT3
• Using jets for which we have derived a
  correction that eliminates on average the
  contributions not related with the hard
  interaction.
• Test quality of QCD predictions
• Study significance of resummation calculations

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Coming up now.

• Observed Dijet Transverse Thrust Cross Sections
• Systematic Uncertainties
• Momentum Scale Correction
• Energy and Angular resolutions
• Unfolding
• Final results and Comparison with Theory
• Conclusions

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Dijet Transverse Thrust cross section
KT algorithm (parameter D 1)
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HT3 scalar sum of the transverse momentum of
the three leading jets. ( use 3rd jet only when
?3 lt 3)
It is presented in four HT3 ranges
HT3 distributions
Four single jet triggers are used for different
HT3 ranges where they are fully efficient.
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Observed cross sections
Distributions still distorted due to finite
detector resolutions
Theoretical Predictions Jetrad QCD event
generator O(?s3). NLOJET NLO 3 jets
generator O(?s4).
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Momentum scale correction
Uncertainty on the Jet Momentum calibration
propagates to the thrust via two mechanisms
errors between 10-25
T value changes
Low energy jets
2-5 uncertainty to take into account
reconstruction efficiencies and contamination.
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Energy Resolutions
Fractional Resolution
Affects T via two mechanisms
T value changes
? Effect smaller than 5
Event migration between HT3 ranges
Deconvolution
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? Resolutions
Important effect in the limit T ? 1
1-Tsme
MC
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Unfolding
Smear MC at particle level by energy and angular
resolutions
Correction factor extracted from MC as
generated / smeared
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Correction factors
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DØ preliminary
DØ preliminary
DØ preliminary
DØ preliminary
CTEQ4HJ, µF µR PTmax/2
Only statistical errors are shown.
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DØ preliminary
DØ preliminary
DØ preliminary
DØ preliminary
Only statistical errors are shown.
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Sources of systematic uncertainties
(2nd Bin)
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Sources of systematic uncertainties
(2nd Bin)
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Thrust range 10-4-10-1.2 Thrust range 10-4-10-1.2
HT3 ?2
160-260 95.08
260-360 81.68
360-430 62.15
430-700 27.69
Strong point to point correlations in the
uncertainty
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DØ preliminary
DØ preliminary
Thrust range 10-4-10-1.2 Thrust range 10-4-10-1.2
HT3 ?2
160-260 28.86
260-360 8.25
360-430 3.89
430-700 4.54
430ltHT3lt700
DØ preliminary
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Conclusions