Jet Reconstruction Algorithms Eric Vazquez PHENIX- Journal Club

Outline Properties of Jets Algorithms Cone vs K T D0 data

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.

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

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…)

K T Algorithm Begin with a list of protojets and define: If d i is min then remove from list and add to list of jets else merge protojets from d ij :

K T 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)

K T 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.

Comparing Algorithms Determine how close the k T jet come with respect to the matching cone-jet axis

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

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.

Analyzing internal Jet Structure Define ratio as: Multiplicities:

Gluon-jet fraction

Results