Jet Reconstruction Algorithms

1
Jet Reconstruction Algorithms

• Eric Vazquez
• PHENIX- Journal Club

2
Outline

• Properties of Jets
• Algorithms
• Cone vs KT
• D0 data

3
Properties of Jets

• We want event properties that satisfy the
  following properties
• Well-defined and easy to measure
• Well-defined and easy to calculate order by order
  in pertubation theory from partonic final-state.
• Have a close correspondence with the distribution
  of the final-state quarks and gluons that we are
  really interested in.

4
Cone Algorithm

• Very simple to define
• Direction that maximizes the energy flowing
  through a cone drawn around it.

5
Cone Algorithm

• Advantages
• Energy corrections are easy to make due to its
  purely geometrical properties.
• Disadvantages
• Problems arise when cones overlap.
• Different definitions will give rise to different
  number of jets. (i.e. parton-level vs hadron
  level)

6
KT Algorithm

• Begin with a list of protojets and define
• If di is min then remove from list and add to
  list of jets else merge protojets from dij

7
KT Algorithm
8
KT Algorithm

• Things to consider
• Only the last protojets to be added to the list
  of jets are of interest.
• It is crucial that the cross section be infrared
  safe (i.e. no long distance effects)

9
KT Algorithm

• Advantages
• No overlap region to worry about.
• Less sensitive to pertubations from soft
  particles which results in smaller hadronization
  and detector corrections.
• Still similar to cone algorithm except for the
  fact that radius is adjusted on an event-by-event
  basis.

10
Comparing Algorithms

• Determine how close the kT jet come with respect
  to the matching cone-jet axis

11
Comparing Algorithms

• Differences arise by how the two algorithms deal
  with hadronization effects.

12
Analyzing internal Jet Structure

• In order to resolve internal jet structure, D is
  set to 0.5 in order to try and resolve any
  subjets.
• The ratio of gluon-quark multiplicity is
  determined and then compared to Monte Carlo
  Simulation.

13
Analyzing internal Jet Structure

• Define ratio as
• Multiplicities

14
Gluon-jet fraction
15
Results