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Jet Reconstruction in Athena
Atlas Calorimeter Energy Calibration Workshop
Ringberg Castle
23/07/02Ambreesh Gupta, University of Chicago
Outline : Introduction
Athena : The Atlas Software Framework
Jet Reconstruction
Tau Reconstruction
Energy Flow
Summary
People Involved
Martine, Monika, Peter, Ed, Srini, Tom, Hong, Jim,Jon, Frank, Frank…
The group of people involved in jet reconstruction in the atlas software framework -
People interested can join the JetRec bi-weekly telephone meeting at 5 pm CERN time on Wednesdays.
p p
q
q
Initial Parton in Hard Scattering
Reconstructed Jet in Calorimeter
ISR/FSRFragmentation
Underlying Event
e/Dead materialMagnetic field
Electronic NoisePile Up
EventProperties
DetectorEffects
LHC Designs = 14 TeV L = 10-34 cm-2 sec-1
Collision Every 25ns 23 Interaction/Collision
The challenge is to identify reconstructed jet with the initial parton : Nontrivial
Goal of HadronCalorimetry: . Resolution 50%/E 3%. . Jet Scale 1%.
The Steps of Jet Measurement
Jet Measurement can be broadly divided in three steps
Jet Reconstruction
Energy Calibration
Flavor Identification
Energy deposit in the calorimeter is clustered by Jet reconstruction algorithms, e.g Cone and Kt.
Energy of a Jet is calibrated for non-compensation, dead material, magnetic field, etc.
b-tag, -tag, etc.
Athena : The Atlas Software Framework
TransientEvent
Data Store
TransientDetector
andCalibrationData Store
AlgorithmA
AlgorithmB
AlgorithmC
Framework Philosophy:Data Classes are Stable,Algorithms Change. User Can Plug in New Algorithms.
Framework Provides Services: Execute, Monitor and Output From Algorithms.
Jet Reconstruction Package
CalCell
CaloCluster
Tracks
CaloTower
Many Kind of Input Data
MC Truth
Many Kind ofAlgorithms
Cone
Kt Cluster
…..ProtoJet
Energy Flow
Abstract The Inputs For Algs
ProtoJet Class:. Can be created for different sub system.. Homogenous.
Jet Reconstruction Package
Cone
Kt Cluster
…..
Jet
Jet ReconstructionAlgorithms
Jet Class:. Jets are composed of ProtoJet. Provide Mechanism to deal with Overlapped Jets.. Provide mechanism to deal with Recombination Schemes.
Energy CorrectionAlgorithms
Sampling BasedCorrection
H1 Style Correction
Jet
Working with data abstractions is great but the information lost is required by correction Algorithms…..
Navigating Composite ObjectsProblem Statement : Given a Jet made of ProtoJet – How do I know what kind of CaloCell (LAr,Tile ,etc.) are they made of ?
Jet
PJet
PJet
PJet
TowerTower
Cell CellCell
JetToken
PJetToken
TowerToken
CellToken
Record structural pattern in Tokens. Access them through special processor classes.
Available Tools and Packages
• There are many ‘Packages’ that help in Jet Reconstruction. The
three listed below are the typical names associated with, jet, tau
and energy flow reconstruction -
. JetRec
. tauRec
. eflowRec
JetRec
Jet Algorithms
Jet algorithms are employed to map final states, both in QCD pert.theory and in the data, onto jets. The motivating idea is that thesejets are surrogates for the underlying energetic parton.
Cone jetKT jet
Clustering - hadrons, Calorimeter Cells,Towers etc., for nearness… Nearness in angle => Cone Algorithm.
Nearness in relative transverse momentum => Kt algorithm.
Historically hadron collider use cone algorithms : easier calibration
Recombination Scheme - The momentum addition rule of particles in a jet.
Cone Algorithm
Cluster particles within a radius R = 2 2. Cone iterateduntil a stable Et weighted cone is achieved.
Possible to produce overlapped cones – Needs a Split-Merge step.
Various version of cone algorithm dealing with issues of speed and theoretical uncertainty.
Implementation of a seedless coneand split-merge algorithm in JetRec.
Configurable through jobOption file.
Chrono services provided by Athena time profile algorithms. Average reconstruction time for 1 GeV jet ~ 0.7 sec. 1/E GeV-2
/E
Kt AlgorithmCluster “particles” in order of increasing relative transverse momentum. Requires a method to terminate clustering.
The algorithm is O(n3) => Pre-Cluster particles.
No overlapping Jets. Theoretically well behaved by design.
Implementation of one Pre-Clusterand kT algorithm in JetRec.
Reco time for 1000 GeV jet . Apply an Et cut of 100 MeV => 200 input ProtoJet. Average time ~1 sec. . Not Et cut => 800 input ProtoJet. Average time ~ 1 min. 1/E GeV-2
/E
Energy Correction AlgorithmsVarious method of energy correction used by Atlas in studies –Parametric minimization, in-situ, e/h.
Setup of example algorithm to do energy correction - weights taken from earlier studies (Lefevre & Santoni, Martine)- iterative procedure to approximate the true energy for parameters- use Navigation packages.
The correction improves both linearity and resolution, but not in exact match with earlier studies.
New studies with H1 style calibration … see later
/E
1/E GeV-2
tauRec
Tau Reconstruction
Identify Cluster
Associate Tracksto Cluster
Calibrate Cluster
Tau ID cuts
A Sliding Window algorithm with window 0.5x0.5
Three highest pT tracks with pT > 2 GeVAnd within R < 0.4
1) H1 style calibration 2) Hadronic calibration
Variables to distinguish hadronic tau decays from QCD jets. 20 fold reduction in bkg. with 50% acceptance for SUSY signal.
Tau’s have unique importance in SUSY searches.
Reconstruction Steps :
Hadronic Calibration (Used in Physics TDR)
EM EM,3 Tile,1 Tile HECE = 1.03E + 1.00 E E + 1.17E + 1.35E
Weights derived from single pion
Corrects the overall energy scale but does not give optimal resolution.
H1 Type Calibration
Makes use of the fact that hadronic showers are more localized than EM showers.
Derive weights in bins of Et for each layer - EM3, Tile and HECNo weights for EM1, EM2.
Minimize function :
With constraint :
Samples of single pion with pT = 10, 20, 40, 80, 160 GeV wereused.
Calibrated ET/PT () vs H1 weights)
Pion resolution with H1 weights much better than fixed weights.
/E = 38.56%/E 3.56%
Applying the H1 weights to the reconstructed tau events gave significant improvement in both the average response and the resolution.
Extending H1 style Tau Calibration to Jets.
Jet35 sample was chosen for this study. 35 GeV dijet sample with :
. pT ( hard.scat. ) > 35 GeV. . Electron rich trigger.
To compare to MC, same jet reconstruction algorithm applied to ProtoJets generated from Truth in ATLFAST.
Calibration weights re-derived for this sample. Since sample is dominated by low Et jets, f(ET) chosen to be – f(ET) = ET
2
Comparing Et/Et,MC
kt Jet : Mean close to 1. Increases with Jet Et cut. Resolution curves show tails in gaussian fit. Fits fairly well with double gaussian.
cone Jet : Mean close to 1. Similar to above, increases with jet Et cut. Tails, substantially reduced compared to above.
ET/ET(MC) after calibration ( kT jet )
Fitted H1 weights for Jets
Bin W(EM2) W(EM3) W(Tile) W(HEC)
0=1 1.203 2.000 3.999 3.123
2 1.645 1.393 2.230 1.163
3 1.423 1.500 1.894 0.950
4 1.137 1.165 1.355 0.922
5 1.043 1.079 1.223 1.000
6 0.984 1.451 1.135 1.078
7 0.964 1.128 1.104 1.202
8=9 1.041 1.206 1.052 1.241
Cryo term = 0.471* (EEM3 * ETile)Gap weight = 0.915
ET Bins = 1/32, 1/16, 1/8, … 16 GeV
Investigating the Tails
Possible effect of miss measurement of jet energy due to magnetic field studiedby taking in to account of tracks that sweep in(out) of the jet.
Cleaner Jet sample ( without electron rich trigger ) for several ET
bins being requested for further studies on the tails.
No noticeable change.
eflowRec
Energy Flow Concept
Introduced first by LEP experiments led to significant improvement in jet energy resolution.
The idea is simple but challenge to realize – requires building the particle ID associated with the track. This starts running in to difficulties in high track multiplicity environment and coarse calorimeter granularity.
Basic Idea : The well measured particle momentum substitutes random fluctuation of energy in the calorimeter => better resolution.
Energy Flow Package
ECAL HCAL
neutral
neutral
charged
eflowRec is a first attempt to combine calorimeter, tracking andPID information to improve energy resolution for jet and ET
Miss
Tracks and cluster matched in and ( neutral particle ) and using helix parameter of tracks ( charged particles ) to form topologically connected eflow objects.
Subtract expected energy deposit in EM and Had cluster.
Loop over the remaining EM clusters and subtract expected energy in HCAL clusters
Loop over tracks
Estimated based on the particleID hypothesis.
Algorithm flow :
Plans : eflowRec
Combined muon algorithm as input.
Brem recovery and conversion finding.
PID internal to eflowRec.
3D cluster reconstruction.
Limitation at present due the availability of full PID packages.
Various choices to be made:Need MC studies to constraint.
scalar sum pT missing pT
x-comp. missing pTy-comp. of missing pT
Summary
• The jet reconstruction setup in Athena is fairly mature.
• The important things that should be on the priority are – documentation and a setup to test and validate released code in a simple way.
• New studies in calibration studies driving the software setup
to produce the right tools.
• Progress in tau reconstruction and H1 style calibration.
• First implementations of an Energy Flow package available in Athena.
Please send comment/suggestions to the jets-combined mailing list
or join the bi-weekly phone meetings for further details.