# Presentation: 511853

## Plans for Jet Energy Corrections at CMS

**Robert Harris and Ia Iashvilli**
***Special thanks to***
**V. Buege, J. Cammin, D. Del Re, S. Esen, **
**D. Ferencek, K. Kousouris and A. Santocchia**** **
**CMS Physics Approval Talk**
**July 25, 2008**

## Outline

- Introduction
- Factorization of Correction
- Offset
- Relative: dependence
- Absolute: pT dependence
- EMF
- Flavor
- UE
- Parton
- Status and Workflow
- Review of the PAS
- Plans for JPT and PFlow (_Not for Approval_)
- Conclusions

- Robert Harris, Fermilab

## Introduction

- Here we present the plans for Jet Energy Corrections at CMS
- As described in the PAS JME-007-002
- The purpose of the PAS is to briefly describe the plans.
- To inform the collaboration and the wider HEP community.
- To illustrate the plans with a few plots for conferences.
- Here we only have time to present the big picture
- Our plans rely on many individual calibration methods.
- Each method is a separate analysis with many details.
- The details are discussed further in the documents
- PAS: JME-07-002
- Analysis Notes:  2007/055, 2008/003, 2008/031
- We request approval of this plan and the illustrative plots.

- Robert Harris, Fermilab

## Introduction to Corrections

- CaloJets are what we see in the  Calorimeter.
- GenJets are made from stable particles, in principle observable.
- Partons participate in hard scatter, but are not observable (colored).
- Correction goals (simplified):
- Correct CaloJets to have the same pT as GenJets on average.
- Provide corrections back to parton level quantities for optional use.
- Focus is on calorimeter-based jets
- Methodology is applicable to other jet reconstruction approaches.
- “Jet Plus Track” & “Particle Flow”.
- In this talk the studies are for Iterative Cone R=0.5.

- Robert Harris, Fermilab

## Factorization

**Plan:** the jet corrections will be factorized
- Correcting for each factor in a fixed sequence up to a level chosen by the user.
- Factorization facilitates the use of data-driven corrections
- Breaking the correction into pieces that are naturally measured in collider data:
**Offset**: pile-up and noise measured in zero-bias events.
**Relative**: jet response vs. relative to barrel found using dijet balance, etc.
**Absolute**: jet response vs. PT found in barrel using / Z + jets, top, etc.
- Allows data-driven corrections as they emerge to easily replace MC truth
- Provides a deeper understanding of jet corrections & their systematics.
- This was the experience of the Tevatron experiments.
- Each factor is a residual correction.
- Facilitates progress & standardization by not recreating the prior corrections.

- Robert Harris, Fermilab

## Offset Correction

**Offset Correction**

**Purpose:**
- Correct for energy from Noise and Pile-Up
**Plan:**
- Measure energy in jet area in zero-bias events.
- Offset we will subtract from the jet energy.

**Offset in Jet Area In Calorimeter**

**Pile-up**

**Noise**

**MC Estimates**
**Noise negligible**
- No offset energy above current cell and tower thresholds
- Scheme B CaloTowers & ET>0.5 GeV.
**Pile-up is small offset**
- 1 pile-up event gives
- <ET> ≈ 0.1-0.3 GeV
**Offset will increase if we lower thresholds**

**Offset Energy vs. ******

**Offset E****T**** vs. ******

- D. Ferencek

- D. Ferencek

- Robert Harris, Fermilab

## Relative Correction: Dependence

**Relative Correction: ********Dependence**

**Purpose:** Correct for relative response of jets vs. 
- Make response flat vs. 
- Equal to response in |jet |<1.3)
**Plan:**
- Correction from MC truth now.
-  Data-driven corrections when  collision data are available.

**Response vs. ********: Low p****T**

**Response vs ********: Higher p****T**

- K. Kousouris

- K. Kousouris

**MC estimates****:**
- Response r = most probable value (peak) of CaloJet pT / GenJet pT
- Large variations in response versus  that are pT dependent.
- Largest response (HF) will be reduced by HCAL calibrations before jet corrections. More on this later in talk.

- Robert Harris, Fermilab

## Data Driven Relative Correction from Dijet Balance

**Data Driven Relative Correction**** from Dijet Balance**

- Dijet balance uses pT conservation in data to measure relative response vs. 
- Relative response = r () / r (||<1.3)
- Technique from SPPS refined at Tevatron
- Select clean dijets for balance measurement
- Require JJ>2.5 and 3rd jet pT< 0.25 pTdijet
- Dijet balance and MC truth response agree
- To 1% in barrel, 2-3% in endcap, 5-10% in HF
- Plan to use with first data to measure correction for  dependence.

- K. Kousouris

- K. Kousouris

- Robert Harris, Fermilab

## Absolute Correction: pT Dependence

**Absolute Correction: p****T**** Dependence**

**Purpose:** Flatten the absolute jet response of calorimeter vs. pT
- Corrects energy of jet back to the particle level in control region (||<1.3)
**Plan:**
- Corrections based on MC truth will be used until data driven corrections are avaliable from collision data.
**MC estimates:**
- QCD jet events require large corrections for low calorimeter response.

- K. Kousouris

- K. Kousouris

**Absolute Jet Response vs. p****T**

**Absolute Jet Correction vs. p****T**

- Robert Harris, Fermilab

## Data Driven Absolute Correction from + Jet pT Balance

**Data Driven Absolute Correction**** from ********+ Jet p****T**** Balance**

**Plan:** measure absolute jet correction from pT conservation in / Z + jet process.
- Standard  / Z reconstruction will be used
- Currently we select clean events with  >  – 0.2, extra jet pT < 0.1 PT (0.2 PTZ).
-  + jet pT balance MC estimates
- Will be able to measure absolute jet correction with  pT < 600 GeV for 100 pb-1.
- Photon isolation is used to reduce rate of QCD jets faking a photon.
- Z() + jet pT balance MC estimates
- Will be able to measure absolute jet correction with  pT < 400 GeV for 100 pb-1.

****** ****+ jet balance**

**Z + jet balance**

****** ****& Background Rate**

- V. Buege

- D. Del Re

- D. Del Re

- Robert Harris, Fermilab

## Steps to Measure Absolute Correction

**Steps to Measure Absolute Correction**

**Plan**
- Measure nominal correction (pT,Z / pTJET) for  and Z. Also top.
- Transform to jet correction at the particle jet level for QCD dijet mix of quarks and gluons using the MC
- Example study shown here for Z + jet
- Combine converted corrections together based on errors
- Extrapolate to higher jet pt with the QCD MC correction curve

-  / Z + jet pT balance (also top events discussed in PAS TOP-07-004)
- Measures jet corrections to the parton level for special mix of quarks / gluons.
- Doesn’t measure jet corrections over the complete pT range needed.
- Need to transform corrections to the particle jet level for a “generic” jet
- Need to extrapolate to higher jet pT we need for QCD jet analysis.

- V. Buege

- Robert Harris, Fermilab

## Optional EMF Dependence Correction

**Optional EMF Dependence Correction**

**Purpose:** improve resolution by correcting for variation in jet pT with EM fraction
- EMF dependent corrections improve jet and MET resolution.
- See PAS JME-07-001  for details on how it improves MET resolution.
**Plan**
- Provide optional correction.   Available now from MC truth.

******P****T**** vs. EMF in P****T**** Bins**

**Jet Resolution**

- S. Esen

- S. Esen

- This plot
- replaces
- the
- existing
- plot in
- the PAS:
- one curve
- removed
- at JetMET
- Request

- Robert Harris, Fermilab

## Optional Jet Flavor Correction

**Optional Jet Flavor Correction**

**Purpose**: Corrects for flavor variations in response of CaloJets to GenJets
- Gluon, c quark, and b quark jets all have lower response than light quark jets
**Plan**
- Provide MC truth corrections for flavor from QCD dijet sample. Available now.
- CMS will also need to develop analysis & process specific corrections
- Corrections for  tagged objects: e.g. b-jets from a specific b-tagger.

**Flavor Variation of Jet Response**

**Flavor Fraction for QCD Dijets**

- J. Cammin

- J. Cammin

- Robert Harris, Fermilab

## Optional Underlying Event Correction

**Purpose:** Removes luminosity independent energy in jet from UE.
- Small effect, comparable to offset correction.
-  <ET> ≈ 0.1- 0.3 GeV for one Min-bias event.
**Plan:**
- Measure in low luminosity data and MC using same techniques as for pile-up.
- No effort in this area so far.

- Robert Harris, Fermilab

## Optional Parton Level Correction

**Optional Parton Level Correction**

**Purpose:** Correct jet back to parton from hard scatter.
- Replaces energy from the parton not included in jet (final state radiation, etc.)
- Depends on jet algorithm, process, parton definition, radiation / fragmentation.
**Plan**
- Provide MC truth corrections for light quarks, c, b, g and dijet process mixture.
- Available now for the partons from the QCD dijet process.
- CMS will also need to develop analysis & process specific corrections.

- A. Santocchia

- A. Santocchia

- Robert Harris, Fermilab

## Status of Factorized Corrections

- Offset
- No correction needed for current thresholds for events w/o pileup.
- Small correction per pile-up event with current thresholds.
- Relative:  dependence
- Correction available from MC truth.
- Data-driven corrections available from MC using dijet balance.
- Dedicated dijet balance trigger proposed.
- Absolute: pT dependence
- Correction available from MC truth.
- Data driven corrections available from MC using / Z + jet balance.
- Optional EMF, Flavor, and Parton.
- Corrections available from MC truth.
- Parton correction contains UE correction which is not available independently.

- Robert Harris, Fermilab

## Jet Energy Correction Startup Plans

- HCAL Tower In-Situ Calibrations
-  symmetry tower response calibration in MinBias samples
- Tower to tower energy calibrations using isolated tracks for || < 2.5
-   dependent tower response corrections with dijets for || > 2
- Jet Energy Correction Workflow
- MC truth jet energy correction delivered with latest simulation
- CMS plans to use un-corrected jets in HLT trigger
- However, CMS will reduce scale of HF using tower calibration from step C
- HCAL tower analysis begins to get calibrations for reprocessing
- 1st data driven jet corrections begins in parallel with tower analysis
- Deliver offset,  and pT dependence corrections before the re-processing
- Data-driven & MC truth jet corrections re-derived after re-processing both data and MC with new calorimeter calibrations
- Repeat from step 4 as necessary.  Repeat from step 3 less frequently

- Robert Harris, Fermilab

## Review of PAS

- CMS PAS JME-07-002
- Analysis Review Committee
- Peter Schleper (chair)
- Frank Chlebana
- Boaz Klima
- Four drafts of PAS
- June 5
- June 26
- June 30
- July 2
- Comments from ARC & conveners
- Received  on 1st draft.
- Addressed in 2nd and 3rd drafts.
- No outstanding comments / issues.
- PAS moves to approval stage.

- Robert Harris, Fermilab

## Plans for JPT & pFlow: Jet Response  Not for Approval

- On June 17 JetMET held a mini-workshop on Jet + Track and Particle Flow
- Studies of +jet balance and jet corrections shown below were begun after that meeting.
- Jet response from JPT and PFLOW closer to unity
- Because they use track momentum as well as calorimeter energy
- Calibration of JPT and PFLOW jets are part of the CMS plan
- Studies of data-driven corrections using +jet pT balance have begun
- Development of Monte Carlo Truth corrections are in progress
- Plan to re-use JEC machinery, developed for CaloJets, wherever possible.

**Jet Corrections**

**Jet Response**

**Calo**

**PFlow**

**JPT**

**PFlow**

**Calo**

- A. Nikitenko et al

- D. Del Re, July 15

- K. Kousouris, July 18

- Robert Harris, Fermilab

## Plans for JPT & pFlow: Jet Resolution  Not for Approval

- JPT and PFlow jets have better pT resolution than Calo-only jets at low pT.
- A proper comparison of jet resolutions should involve calo jets corrected for energy response
- An equal footing comparison would require all resolutions to be produced from fully calibrated jets
- Calo-only jet energy resolutions are not as bad, however, & the JPT and pFlow improvement not as dramatic as could be interpreted from plots based on uncorrected calojets

- M. Zielinski, Z. Qi

- Robert Harris, Fermilab

## Conclusions

- We’ve presented a plan for jet energy corrections at CMS
- With plots to describe the correction techniques publicly
- Jet corrections at CMS are factorized
- Required corrections: offset, relative (), absolute (pT)
- Optional corrections: EMF, flavor, UE, parton
- Corrections available now from MC truth
- Jet corrections at CMS will be data-driven
- Data-driven techniques agree with MC truth and await real data
- Data-driven corrections tested successfully in CSA08 exercise
- CMS will replace MC truth with data-driven corrections as necessary
- We request CMS approval of this plan and supporting plots.
- Plans for JPT and PFlow are not for approval at this time.

- Robert Harris, Fermilab