High-PT Jet Physics at CDF

1
High-PT Jet Physics at CDF
Olga Norniella
IFAE-Barcelona
(on behalf of the CDF Collaboration)
The high-PT jet physics program at Tevatron Run
II addresses fundamental questions on QCD and
provides the ground for future discoveries of new
physics at Tevatron and the LHC.
Probing Quark Substructure, PDFs Jet
Fragmentation
Differential Jet shapes
Inclusive Jet Production Cross Section
Highest mass dijet events so far

• The measurement of the inclusive jet cross
  section in Run II probes distances of 10-12 cm
  and thus it is sensitive to a possible quark
  substructure. In addition, comparisons with fixed
  order pQCD calculations provide a test of the
  standard model predictions including a fine
  determination of the strong coupling constant.
  Combined with similar measurements for forward
  jets, this measurement provides the best
  constrains to the knowledge of the gluon density
  in the proton at high x.

ET 666 GeV ?0.43
ET 633 GeV ?-0.19
Calorimeter Lego Plot

• Precise measurements of the internal jet
  structure constitute a stringent test of the 
  modeling for multigluon emission and the
  transition from partons to jets of hadrons in the
  final state, as built in parton shower programs.
  In addition, comparison with NLO pQCD
  calculations provides an alternative and very
  competitive mean of measuring the strong coupling
  and its running with jet hardness.

CDF central tracking chamber and Calorimeter.
(r-? view)
Measured inclusive jet cross section for central
jets (0.1lt ?jetlt0.7) compared to NLO pQCD
predictions using CTEQ6.1 PDF's. The gray band
indicates the total systematic error on the
measurement dominated by a 3 uncertainty on the
calorimeter energy scale. A reasonable agreement
is observed within errors.
Measured uncorrected differential jet shape,
?(r), for central jets and jet transverse energy
in the region 30 GeV lt ETjet lt 135 GeV.
Measurements are performed using both calorimeter
(CAL) towers and tracks from the central outer
tracker (COT) and compared to PYTHIA predictions.
Good description of the measured shapes is
observed in all regions.
Weak Boson Jets Physics
W N Jets ET spectra
W N Jets Production

• The study of high-pTjets in combination with
  weak bosons is one of the pillars of the QCD
  program at Tevatron. These processes constitute
  major backgrounds for Higgs and top quarks
  physics both at Tevatron and at the LHC. Studies
  for different jet multiplicities address
  fundamental questions on QCD physics like, for
  example, the combination of fixed order pQCD
  terms and parton showers (to model multigluon
  emissions), leading to a proper description of
  the jet dynamics in the final state.

Measured ET distribution of the less energetic
jet in W Njets events compared to LO
predictions with different renormalization
scales. The less energetic jet is particularly
sensitive to the interplay between LO
calculations and the modeling of parton showers.
The gray band indicates the total systematic
error  dominated by the uncertainty on the
calorimeter energy scale.
Measured inclusive W gt Njets cross section
compared to LO predictions from ALPGEN. The
magenta band indicates the uncertainty coming
from the selection of renormalization and
factorization scale in the calculations.
Example of Feynman diagram for W 1 jet
High-PT photon Physics
Diphoton Cross Section
g heavy quark production

• The study of processes involving high-PT photons
  in the final state constitute a unique test of
  pQCD predictions and provides a particularly
  clean way of understanding the emission of gluons
  in the initial state and intrinsic-kT effects. A
  high statistics sample of ?heavy quarks events
  will provide direct constrains to the heavy quark
  densities within the proton. Finally, a deep
  knowledge of the shape and magnitude of these
  processes is crucial in searches for new physics
  with the presence of hard photons in the final
  state.

Measured differential cross section for g b
quark production as a function of photon Et,
compared to LO predictions from PYTHIA. The heavy
quarks are selected by finding a secondary vertex
inside the jet cone. The  relative contribution
from charm, bottom and light quarks is estimated
from the data itself, using the shape of the
measured invariant mass of the tracks pointing to
the vertex.
Example of Feynman diagram for diphoton
production
Example of Feynman diagram for ?b production
Measured cross section as a function of the
diphoton invariant mass, M??,  in diphoton
production, compared to NLO predictions. A good
agreement is observed.
Underlying Event studies
?PT Density in Transverse region
Transverse region in dijet events

• In hadron-hadron collisions, final state jets
  from the hard interaction are affected by soft
  gluon contributions coming from initial- and
  final-state radiation, interactions between
  proton/antiproton remnants and secondary
  parton-parton semi-hard interactions.

Min-Bias 0.24 GeV/c per unit h-f
In dijet events, the ?-? space is divided in
forward region (??1 lt60º) along the direction
of the leading jet, away region (??1 gt120º)
where second leading jet is found,  and
transverse region (60ºlt??1 lt120º) dominated by
the underlying event. In order to remove 
contributions from additional jets in the
transverse region clean back-to-back
configurations (??12 gt150º, ETjet2/ ETjet1 gt0.8)
are selected,  which allows a direct measurement
of the underlying event, highly decoupled from
the hard interaction.
Averaged ?PT density of charged particles in the
transverse region as a function of the transverse
energy of the leading jet compared to PYTHIA and
HERWIG MC predictions. For back-to-back
configurations, the measured activity is
approximately independent of the hardness of the
primary interaction, and thus directly reflects
the underlying event contributions. PYTHIA (tune
A), which includes a physical model for soft
gluon interactions, describes the data.

• Such contributions must be removed before a
  proper comparison with pQCD calculation can be
  carried out. In addition,  a good knowledge of
  the hadronic final state activity in the
  non-perturbative regime is crucial to obtain an
  accurate estimation of background processes in
  searches for new physics both at Tevatron and the
  LHC.