ALICE HLT Jet Studies



Content:

A. Status/Updates...

1. Abstract/Physics motivation

2. Goals of this study

3. Jet definitions

4. Models

5. Jet algorithms

6. ALICE simulations

7. Results

7.1 Results from standalone tests of the different jet-finders

7.1.1 Cone Algorithms
7.1.2 kT Algorithms
7.1.3 Charged Jet Algorithm
7.1.4 Hijing Background Studies
7.1.5 Trigger Algorithm studies

7.2 Combined results

7.2.1 Trigger Efficiciencies and Background Rates

7.3 Results taken for HLT CDR report

7.3.1 Parts of the Review Talk given at Cern
7.3.2 Review Page at Cern

8. Summary



A. Status/Updates...

As usual, preliminary and incomplete!

01/15/02: Added CDR review talk - see section 7.3.1
Some slides shown at the Panel Review.

01/09/02: Added CDR section - see section 7.3
Results and parts of the corresponding comments that went into the CDR.

11/15/02: New results on HIJING background - see section 7.1.4
First estimate of trigger background rate as function of nParticles,PtThreshold and Cone size.

11/14/02: New results on HIJING background - see section 7.1.4
Preliminary results on number of charged particles/cone in hijing events (trigger background rate).

11/13/02: Updates results from the cone jet finder - see section 7.1.1
Has now study of trigger efficiency with the proposed trigger algorithm for different trigger algorithm parameters.

11/07/02: New section on HIJING background studies - see section 7.1.4

11/05/02: First results from the cone jet finder - see section 7.1.1

10/31/02: First results from the charged jet finder
- see section 7.1.3

10/30/02: Let's start again - divided web page into different ones...
Lot's of work in the meantime, but webpage not updated...

09/22/02: First results from HIJING / updated PYTHIA results - new section 7
Still w/o merging/joining and cone finder only... Added figures on protojet properties for PYTHIA. First results on central HIJING events, same figures as for PYTHIA events. Figures are now links to eps versions.

09/19/02: First description of models - new section 4, details still missing...

09/18/02: Include comments from Constantin - new section 1, lots of changes in the other sections...

09/05/02: First version of cone finder implemented - first results on Pythia simulation in section 7 :-)
So far w/o merging/joining, still no description of algorithm in section 5, no description of Pythia parameters...
Updated comments on TPC & HLT simulations...
Complete ALICE simulations are coming - maybe these are the ones to use (makes proposal in section 6.3 unnecessary)

08/29/02: First version of this page:
So far only section 6 (description of ALICE simulation enviroment), I'm currently working on implementing/testing the different jet finders (short description in section 5). For the moment I'm working only with PYTHIA simulations, as soon as I get reasonable results, I show them in section 7 and complete the model section on PYTHIA (4.1) and the description of the algorithms...


1. Abstract/Physics motivation

The goal of jet spectroscopy in heavy ion collisions is the measurement of the radiative energy loss of the leading partons traversing QCD matter e.g. jet attenuation. The standard approach would be to compare high Et inclusive jet production cross section in heavy ion collisions (Pb+Pb) and elementary hadron collisions (p+p), so that one gets the jet suppression factor by applying QCD scaling.

The design of the ALICE DAQ is such, that one can record about 10 central Pb+Pb events of the TPC. The TPC delivers up to 200 good events (pileup free + DDL bandwidth saturates) per second at design luminosity.

Mangano et al. predicts from QCD a jet production rate of around one jet with Et in about 2000 such events where Et>100 GeV. Jet attenuation could lower the rate by a factor 2-3, which means, that we can roughly expect one >100GeV jet in 4000 to 6000 such events.

The standard DAQ will therefore roughly record a few 1000 of such jets per ALICE year. An online jet trigger at a rate of 200Hz could improve the statistics by a factor of ~20.


2. Goals of this study

Preliminary studies of jet-production in heavy ion collisions at sqrt(s_NN)=5.5TeV for the ALICE-High Level Trigger are investigated in order to:
- study different jet finders (jet definitions),
- test if they are appliable as trigger algorithm

The overal goal is to found a profund base for more careful studies of HLT jet triggers.


3. Jet definition

- Since the scattered partons carry nonzero colour charge and the observed hadrons do not, there can be no unique assosciation of a jet of hadrons with a single initial quark or gluon.
"Jets are ambiguous"
or
"The jet definition depends on the used algorithm"

- In hadron-hadron collisions producing a high-pT jet, only a small part of the final state hadrons is associated with the jet. The other particles present in the event can be thought of as associated with "beam jets", e.g. soft interactions of the remaining partons in the incident hadron which are in first order uncorrelated with the hard process (the jet). And this is even worse for heavy ion collisions, since there a more "beam jets"...



8. Summary

From the present level of jet physics simulation, to be able to present an optimized HLT cone jet finder trigger for inclusive jets of E_T > 100 GeV that delivers 75-80 % efficiency with a background suppression selectivity of 1-2 %. The expected CPU time budget is surely below 10% of the effort required for cluster finding and tracking.


Thorsten Kollegger, kollegge AT (no spam) ikf DOT uni-frankfurt DOT de
Constantin Loizides, loizides AT (no spam) ikf DOT uni-frankfurt DOT de

IKF - University of Frankfurt
Last updated: 01/17/2003