# Presentation: 758600

## Jets at CMSFedor Ratnikov, University of Maryland

- MIT, August 1, 2008

## Introduction

- I am in CMS JetMET POG since 2005
- Since beginning of the CMSSW era
- Exclusively responsible for design and implementation of jet reconstruction chain, as well as for design and implementation of Jets Energy Corrections machinery
- Involved into and support most ongoing Jet activities
- Jet contact for the CMS RECO group

- Jet reconstruction at CMS.    F.Ratnikov

- August 1, 2008

## Credits

- CMS Jet/MET DPG is a big team
- This presentation includes results obtained by different people
- A.Anastassov, A.Bhatti, V.Buege, J.Cammin, F.Chlebana, R.Harris, S.Essen, O.Kodolova, K.Kousouris, A.Nikitenko, A.Oehler,  D. del Re, G.Salam, M.Zielinski

- Jet reconstruction at CMS.    F.Ratnikov

- August 1, 2008

## Outline

- Why jets are tricky
- Jet reconstruction algorithms
- General requirements
- Algorithms used at CMS
- Performance
- Jet flavors: CaloJets GenJets, PFJets
- Jet reconstruction software design
- Resolution, efficiency
- Jet Energy Corrections
- Generic design
- Supported corrections
- Summary

- Jet reconstruction at CMS.    F.Ratnikov

- August 1, 2008

## Basic Reconstructed Physics Objects

- partons

- particles

- tracker

- calorimeter

- muon detector

- 

- 

- 

- 

- e,

- e,

- e

- q,g

- 0,,*,...

- ±,,,e,

- 

- Jet reconstruction at CMS.    F.Ratnikov

- August 1, 2008

## Requirements to Jet Algorithms

- Procedure must be stable with respect to reasonable variations of the energy pattern

- Jet reconstruction at CMS.    F.Ratnikov

- August 1, 2008

## Jet Algorithms

- Iterative Cone
- Seeded
- Single pass - fast
- IR2 unsafe, collinear unsafe
- CDF Midpoint Cone
- Seeded
- With split/merge passes
- IR3 unsafe, collinear unsafe

- Jet reconstruction at CMS.    F.Ratnikov

- August 1, 2008

## Jet Algorithms (cont)

- kT
- Seedless
- N3N·ln(N) - fast
- IR, collinear safe
- Seedless Infrared Safe Cone (SISCone)
- Seedless
- N4N2·ln(N) - Reasonably fast
- IR, collinear safe
- CMS routinely uses
- IC R=0.5
- SISC R=0.5, R=0.7
- kT D=0.4, D=0.6

- Jet reconstruction at CMS.    F.Ratnikov

- August 1, 2008

## Midpoint Cone vs SIS Cone

- CMS switched to using SISCone instead of   Midpoint Cone recently

- Jet reconstruction at CMS.    F.Ratnikov

- August 1, 2008

## Comparing Jet Properties

- IC 0.5

- IC 0.5

- KT 0.4

- KT 0.4

- SISC 0.5

- SISC 0.5

- Jet reconstruction efficiency

- Jet energy resolution

- Jet reconstruction at CMS.    F.Ratnikov

- August 1, 2008

## Algorithms Speed

- SIS Cone R=0.5
- Midpoint Cone R=0.5
- Iterative Cone R=0.5
- KT D=0.4

- # of inputs

- CPU time (ms)

- Jet reconstruction at CMS.    F.Ratnikov

- August 1, 2008

## CaloJets

- Baseline for Jet reconstruction at CMS
- Input is CaloTower
- Mostly follow HCAL granularity
- Contains few HCAL cells and many (25 in the barrel) ECAL cells

- CaloJets include information about electromagnetic fraction, energies deposited in different subdetectors etc.
- Produce CaloJets for every approved algorithm
- Part of “RECO” sequence
- People are free to run own jet algorithm on CaloTowers
- Necessary information is available in all standard datasets

- Jet reconstruction at CMS.    F.Ratnikov

- August 1, 2008

## GenJets

- Clustering energies of MC level particles
- Include “invisible” particles?
- Muons
- Neutrinos
- SUSY, if any
- Current approach
- Include all “invisible” except ones from prompt decays of gauge bosons
- Produce GenJets for every approved algorithm
- Part of “post SIM” sequence
- People are free to make own selection of MC particles and run jet algorithm on them
- Necessary information is available in all standard datasets

- Jet reconstruction at CMS.    F.Ratnikov

- August 1, 2008

## PFJets

- +  +0 0

- PFCandidate combines information from various detectors to make the best combined estimation of particle properties
- PFJet is made from PFCandidates and contains information about contributions of every particle class:
- Electromagnetic/hadronic
- Charged/neutral

- Jet reconstruction at CMS.    F.Ratnikov

- August 1, 2008

## Jet Reconstruction at CMS

- Factorize algorithms and jet flavors
- Algorithms treat inputs as set of 4-momenta
- Flavor specific pre/postprocessing are shared by all algorithms

- inputs

- Generic Candidates
- (Lorentz Vectors)

- Jet reconstruction
- Algorithm

- protojet

- flavor specific
- Jets

- event

- flavor specific

- algorithm specific

- Jet reconstruction at CMS.    F.Ratnikov

- August 1, 2008

## HI Specifics

- HI events are huge on particle level
- Not a big deal for CaloJets
- Not more than ~5K fired CaloTowers anyway
- Big deal for GenJets
- One event - 35K particles
- SISCone killer (remember: N2LnN algorithm)
- 40 min of CPU time
- 30 Gbytes of memory
- In most cases just crashes the reconstruction job
- kT survives: takes ~20 sec of CPU time

- Jet reconstruction at CMS.    F.Ratnikov

- August 1, 2008

## Event By Event Pileup Subtraction

- FASTJET approach
- Applicable to IR safe algorithms only, any jet flavor
- Calculate jet area by ghost method
- Put extra very small ghost energies into nodes of - grid
- Assume pileup jet area proportional to pT
- Use low pT jets to derive PU energy density
- Estimate high pT jets PU contribution using PU energy density and jet area
- CMS approach
- Applicable to any algorithm, CaloJets only
- Run regular jet reconstruction
- Get average energy in CaloTowers outside reconstructed jets in every   ring
- Subtract that energy from CaloTowers contributing to the jet
- Re-calculate jet parameters using corrected energies of contributing towers

- Jet reconstruction at CMS.    F.Ratnikov

- August 1, 2008

## Slide 18

- Jet reconstruction at CMS.    F.Ratnikov

- August 1, 2008

## JetArea/PU Subtraction Costs 

- Jet reconstruction at CMS.    F.Ratnikov

- August 1, 2008

## Jet Energy Scale

- Jet corrections
- Generic: applied to all jets
- Optional: fine tuning for specific jet types/hypothesis
- Factorized (chained) corrections approach
- Different jet correctors are services in CMSSW
- Chained corrector is a service on top of individual correctors
- Use cases:
- Producers to apply corrections to all jets, produce new jet correction
- Use correction services directly from analysis modules
- Use simple corrections (not including extra event information) from FWLite/ROOT scripts

- Jet reconstruction at CMS.    F.Ratnikov

- August 1, 2008

## L1: Offset/Zero Suppression

- Offset in Jet Area In Calorimeter

**Pile-up**

**Noise**

- Jet reconstruction at CMS.    F.Ratnikov

- August 1, 2008

## L2: Relative to Barrel (, pT)

- No assumptions about  symmetry

- Jet reconstruction at CMS.    F.Ratnikov

- August 1, 2008

## Measuring Relative Scale: Dijet Balance

- Utilize pT conservation for dijet events
- Use dedicated calibration triggers to collect dijet events

- Before Corrections

- After Corrections

- Jet reconstruction at CMS.    F.Ratnikov

- August 1, 2008

## L3: Absolute Scale (pT)

- Jet reconstruction at CMS.    F.Ratnikov

- August 1, 2008

## Measuring Absolute Scale: /Z Balance

- Utilize pT conservation for -jet and Z()-jet events
- Rescale back from parton level to particle level using MC

- Precision
- -jet balance for 1 fb-1

- Statistics for 1 fb-1

- Jet reconstruction at CMS.    F.Ratnikov

- August 1, 2008

## L4: Electromagnetic Fraction

- Corrects uncompensated Calorimeter response

- PT vs. EMF in PT Bins

- PT vs. EMF in  Bins

- Jet reconstruction at CMS.    F.Ratnikov

- August 1, 2008

## L5: Flavor Corrections

- Scales jet energy to particle level
- Needs jet flavor hypothesis *a priori*

- Jet reconstruction at CMS.    F.Ratnikov

- August 1, 2008

## Track Corrections

- Reconstruct calorimeter jet
- Subtract expected response of
- “in-calo-cone” tracks from calo
-  jet ET and add track momentum
- Add momentum of “out-calo-cone”
-  tracks

- Jet reconstruction at CMS.    F.Ratnikov

- August 1, 2008

## Conclusions

- Jet reconstruction in CMS is mature
- Infrastructure is stable for years
- Bells and twisters still being added, further polishing
- Infinite process anyway
- Algorithms to be used in CMS analysis are established
- At least for the beginning of data taking
- Still flexible enough to accommodate special desires for special analysis
- Jet calibrations are ready for data taking
- MC based corrections for the very beginning of data taking
- Calibration data samples and procedures to extract corrections from data (10pb-1, 100pb-1 scenarios)

- Jet reconstruction at CMS.    F.Ratnikov

- August 1, 2008

## BACK UP SLIDES

## CMS Detector

- Jet reconstruction at CMS.    F.Ratnikov

- August 1, 2008

## Jet Energy Clusterization

- We are mostly interested to reconstruct parton level objects parameters basing on visible measurables
- Direct measurements for muons
- Corrections for bremsstrahlung / conversion for electrons and photons
- Multiple steps conversions for quarks/gluons:
- original partons
- hadronization into long living particles
- showering in EM and HAD calorimeters
- measured energies
- Original parton is seen as energy cluster in the calorimeter (and tracker), a.k.a. Jet
- Algorithms for energy clusters search
- a.k.a. Jet Reconstruction

- Jet reconstruction at CMS.    F.Ratnikov

- August 1, 2008

## FastJet Package

- Stand alone C++ package supported by G.Salam&Co
- Contains
- Fast implementation of kT algorithms family
- SISCone
- Ancient: MidPoint, JetClu, etc.
- Newer: anti-kt
- Provide bridges
- Between different experiments
- Between experiments and theory
- CMS uses FastJet for kT, SISCone

- Jet reconstruction at CMS.    F.Ratnikov

- August 1, 2008

## Jet Area

- May be well defined for IR safe algorithms only

- Jet reconstruction at CMS.    F.Ratnikov

- August 1, 2008