QCD at LHC with ATLAS

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QCD at LHC with ATLAS

• Arthur M. Moraes
• University of Sheffield, UK
• (on behalf of the ATLAS collaboration)

APS April 2003 Meeting Philadelphia, PA
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Outline

• LHC and ATLAS.
• Precision tests measurements in unexplored
  kinematic region.
• Jet physics.
• Parton luminosities and p.d.f.s ( high-Q2
  processes at LHC parton-parton collider ).
• Direct photon production ( fg(x), background to H
  ? ??, parton dynamics ).
• Measurement of the aS at very large scales.
• Background processes multi-parton interaction,
  minimum-bias and the underlying event.
• Conclusion.

APS April 5, 2003
QCD physics at ATLAS
A. M. Moraes
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LHC (Large Hadron Collider)
p-p collisions at vs 14TeV
bunch crossing every 25 ns (40 MHz)

• low-luminosity L 2 x 1033cm-2s-1
• (L 20 fb-1/year)

• high-luminosity L 1034cm-2s-1
• (L 100 fb-1/year)

large statistics small statistical error!
-
Production cross section and dynamics are largely
controlled by QCD.
-
Mass reach up to 5 TeV
Test QCD predictions and perform precision
measurements.
APS April 5, 2003
QCD physics at ATLAS
A. M. Moraes
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ATLAS A Toroidal LHC AparatuS
7,000 tons
Multi-purpose detector coverage up to ?
5 design to operate at L 1034cm-2s-1
Inner Detector (tracker) Si pixel strip
detectors TRT 2 T magnetic field coverage up
to ?lt 2.5.
22m

• Calorimetry
• highly granular LAr EM calorimeter
• ( ? lt 3.2)
• hadron calorimeter scintillator tile
• ( ? lt 4.9).

44m
Jet energy scale precision of 1 ( W ? jj Z (
ll) jets)

• Muon Spectrometer
• air-core toroid system
• ( ? lt 2.7).

Absolute luminosity precision 5 ( machine,
optical theorem, rate of known processes)
Most of the QCD related measurements are expected
to be performed during the low-luminosity
stage.
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LHC Parton Kinematics

• Essentially all physics at LHC are connected to
  the interactions of quarks and gluons (small
  large transferred momentum).

• This requires a solid understanding of QCD.

• Accurate measurements of SM cross sections at
  the LHC will further constrain the pdfs.

• The kinematic acceptance of the LHC detectors
  allows a large range of x and Q2 to be probed
  ( LHC coverage y lt 5 ).

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Jet physics
L 30 fb-1

• Test of pQCD in an energy regime never probed!

• The measurement of di-jets and their properties
  (ET and ?1,2) can be used to constrain p.d.f.s.

? 0 lt ? lt 1 ? 1 lt ? lt 2 2 lt ? lt 3

• Inclusive jet cross section aS(MZ) measurement
  with 10 accuracy.

( can be reduced by using the 3-jet to 2-jet
production )
ds/dET nb/GeV

• Multi-jet production is important for several
  physics studies
• tt production with hadronic final states
• Higgs production in association with tt and bb
• Search for R-parity violating SUSY (8 12 jets).

-
-
-
ET Jet GeV

• Systematic errors
• jet algorithm,
• calorimeter response (jet energy scale),
• jet trigger efficiency,
• luminosity (dominant uncertainty 5 -10 ),
• the underlying event.

Q2 GeV2
At the LHC the statistical uncertainties on the
jet cross-section will be small.
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Measuring parton luminosities and p.d.f.s

• For high Q2 processes LHC should be considered
  as a parton-parton collider instead of a p-p
  collider.

Uncertainties in p-p luminosity (5) and
p.d.f.s (5) will limit measurement
uncertainties to 5 (at best).

• Using only relative cross section measurements,
  might lead eventually to accuracies of 1.

-
(u,d)
qq
-
g
s, c, b
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Direct photon production
pT? gt 40 GeV

• Understanding photon production
• Higgs signals (H???) background
• prompt-photon can be used to study the
  underlying parton dynamics
• gluon density in the proton, fg(x)

( requires good knowledge of as)
Production mechanism
qg??q
dominant (QCD Compton scattering)
-
qq??g
Background mainly related to fragmentation (
non-perturbative QCD)
Isolation cut reduces background from
fragmentation (p0)
( cone isolation)
ATLAS high granularity calorimeters ( ? lt 3.2
) allow good background rejection.
Low luminosity run the photon efficiency is more
than 80 ( LAr calorimeter ).
?
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Determination of as scale dependence

• Verification of the running of aS check of QCD
  at the smallest distance scales yet uncovered
• aS 0.118 at 100 GeV
• aS 0.082 at 4 TeV

• However, measurements of aS(MZ) will not be able
  to compete with precision measurements from ee-
  and DIS (gluon distribution).

• Differential cross-section for inclusive jet
  production (NLO )

ECM 14 TeV, -1.5 lt ?jet lt 1.5

• A and B are calculated at NLO with input
  p.d.f.s.

• Systematic uncertainties
• p.d.f. set ( 3),
• parametrization of A and B,
• renormalization and factorization scale ( 7).

• Fitting this expression to the measured
  inclusive cross-section gives for each ET bin a
  value of aS(ET).

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Multiple parton interactions (MPI)

• AFS, UA2 and more recently (and crucially!) CDF,
  have measured double parton interactions.

b
seff 14.5 1.7 mb

• seff has a geometrical origin
• parton correlation on the transverse space
• it is energy and cut-off independent.

• sD decreases as pT ? 8 and grows as pT ? 0.

4-jet production
2 ? 4 v (2 ? 2)2

• sD increases faster with s as compared to sS.

Multiple parton collisions are enhanced at the
LHC!
2 ? 4
(2 ? 2)2
-

• Source of background
• WHX? (l?) bbX,
• Zbb? (l?) bbX,
• W jets, Wb jets and Wbb jets,
• tt ? llbb,
• final states with many jets pTmin 20 30 GeV.

-
-
-
-
-
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Minimum-bias and the underlying event
Minimum bias events
Underlying event in charged jet evolution (CDF
style analysis)
Experimental definition depends on the
experiment trigger! Minimum bias is usually
associated to non-single diffractive events
(NSD), e.g. ISR, UA5, E735, CDF.
It is not only minimum bias event!
In a hard scattering process, the underlying
event has a hard component (initial final-state
radiation and particles from the outgoing hard
scattered partons) and a soft component
(beam-beam remnants).
Df f - fljet
sn.dif 55 - 65mb
stot 102 - 118 mb
(PYTHIA)
(PHOJET)
(PYTHIA)
(PHOJET)
Best described with MPI models!
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Conclusions

• LHC will probe QCD to unexplored kinematic
  limits
• Jet studies (test of pQCD, constrain p.d.f.s,
  physics studies)
• Luminosity uncertainties can be reduced by
  measurements of relative luminosities high-Q2
  and wide x-range
• Prompt-photon production will lead to improved
  knowledge of background levels (H???), fg(x) and
  parton dynamics
• as at high-energy scales (test of the running of
  as)
• Multiple parton scattering source of background
  and/or new physics channels
• Minimum-bias and the underlying event improved
  understanding of events dominated by soft
  processes.