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Jet Calibration in ATLAS

This presentation at the ATLAS UK Meeting in January 2008 covers various aspects of jet calibration, including hadronic calibrations, validation of energy scales, in situ calibrations, and more. The talk emphasizes the extensive work conducted by UK researchers in jet performance studies, focusing on detectors used in jet reconstruction and the calibration processes involved. Speaker Mark Hodgkinson details the complexities of hadronic showers, the need for calibration at both hadronic and particle jet scales, and the importance of energy corrections for accurate jet measurements. The talk showcases innovative calibration methods such as local hadron calibration and energy flow techniques, aimed at improving the precision and resolution of jet energy measurements.

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Jet Calibration in ATLAS

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  1. Jet Calibration in ATLAS Mark Hodgkinson ATLAS UK Meeting , IPPP January 2008

  2. Contents • Introduction to detectors used in Jet reconstruction • Hadronic calibrations for jets • Validation of hadronic energy scale • In Situ Calibrations • Other contributions from UK • Conclusions Far more work goes on than I am showing in this talk - as far as I know all UK jet performance work is in this talk… So this talk is biased to topics the UK is involved in…

  3. Introduction Jets • We measure jets kinematics via a spray of hadrons after the hadronisation process • Ideally associate all hadrons to the correct parton uniquely • Calorimeters used by default for jet finding • Trackers also being used for wider variety of tasks in jet group nowadays Charged and neutral components seen in calorimeter • Charged component seen in tracker Jet Finders • ATLAS uses Cone 0.4 and 0.7 • Also use Kt D = 0.4 and D = 0.6

  4. Calorimeter input to Jet Finding CaloTowers: • Towers are fixed grid of 0.1 x 0.1 in eta and phi CaloTopoClusters: • Define any CaloCells with |E| > as seed cells • Add any neighbouring cells (3D) with |E| > to seed • Repeat with new neighbours, until no neighbours pass • Noise suppression built in • Search for local maxima to decide if cluster needs splitting

  5. Why We Need a Calibration Hadronic Scale • Hadronic Showers complex: • Visible electromagnetic energy (electrons, photons, 0 decays) ~50% • Visible energy from ionisation ~25% • Invisible energy from nuclear interactions (excitation, break up) ~25% • Escaped energy (e.g. neutrinos) ~2% • Also account for visible energy in dead material A Gupta (Chicago) Particle Jet Scale • Jet response varies over the detector - e.g. in crack regions many particles cannot be detected • Charged particles with low pT bent out of cone in calorimeter Parton Scale • Energy not included in reconstructed jet, that does come from the hadronisation • Energy from underlying event included in reconstructed jet M Hodgkinson (Sheffield)

  6. Jet Calibrations H1 Style (BNL) 0.4 <  < 0.5 • 2 minimisation : • Where • Truth reference is generator level truth jet Local Hadron Calibration (Oxford,Stockholm,MPI Munich) Plots by C Issever, K Lohwasser (Oxford) and E Bergeaas (Stockholm) • Reference energy is true single pion energy (weights also tested on test beam data!) • Takes us to hadronic scale, not particle jet scale • Need additional corrections (magnetic field etc) on top - not done yet • Classify clusters as EM or Hadronic • Hadronic clusters get weights applied (12.0.6), dead material corrections (12.0.6) and out of cluster (not in 12.0.6) corrections 0.4 <  < 0.5

  7. M Hodgkinson, R Duxfield, D Tovey (Sheffield) Energy Flow with eflowRec • Charged particle response from single pion Monte Carlo • In eflowRec remove energy due to charged pion showers on cell by cell basis via ordering principle • Remaining clusters undergo local hadron calibration  = 0 single ± TDR: Tracking: pT/pT 0.036%pT1.3% Calo: E/E  50%/E3% • Cone 0.4, J1-J3 dijets, || < 1.8 • Improves linearity, comparable width • Improved width of jet energy resolution by discarding tracks matched to showers split into multiple clusters • At 6 GeV 60% of pions have < 80% energy in leading cluster - work in progress to deal with this • Also plan to derive H1 style weights Local Hadron TopoCluster Energy Flow H1 TopoCluster Pt = 17-140 GeV • Hadronic W mass resolution improved due to better linearity and angular resolution • Used sample 5200 (semileptonic top) • 4 Highest pt jets (H1 topocluster) pT > 50 GeV • Refined missing ET > 40 GeV Local Hadron Calo Only EFlow /E = 3.54 ± 0.49 /E = 5.02 ± 0.61

  8. Check of Hadronic Scale M Hodgkinson QCD Di-jets • Use tracking to check if hadronic scale is correct in calorimeter • Minimum Bias events cover 400 MeV -> 10 GeV • Tau decays (W,Z) cover 10 GeV -> 140 GeV • Overlapping showers a problem just like in energy flow for jets • Have to suppress backgrounds (QCD di-jets and tau decays to neutral particles) • Need isolated charged hadrons - isolation criteria means taus cannot be used -> Try for QCD J. Lu, D. Gingrich (Alberta): QCD • Can select 7000 tracks from QCD jets in first year (i.e. use shower shape cuts, but not QCD rejection) • Not enough CSC (full sim) data to investigate… • Cannot use Atlfast due to calorimeter cuts used to select isolated charged hadrons QCD J1 Minimum Bias • Use loose cuts and subtract background • Background E/P measured from control sample pT=10 GeV N Davidson E Barberio (Melbourne)

  9. In Situ Calibration Paul Hodgson (Sheffield) QCD di-jets • Jet energy scale is not uniform in h • Di-jet (Sheffield, Heidelberg and Argonne) samples are used Di-Jet Balancing 0.4 <  < 0.6 is reference region Paul Hodgson • Cut on  to ensure jets back to back (suppress ISR/FSR effects) • pt balance = pt_probe/pt_ref • Relative Jet Energy Scale can be determined to < 1% with 3 pb-1 data (assuming nominal jet prescales, J70 prescale is 5, do not change) • Plan to try technique in FDR P Hodgson Other Methods •  + Jet, Z + Jet (M Hurwitz - Chicago) • W mass template method - J Schwindling (Saclay)

  10. In Situ Calibration at High Pt Jet Balancing • Di-jet,  + jet cross-sections are high pt is low • Balance high pt jet with a number of calibrated low pt jets • Glasgow, Tokyo, Heidelberg, Alberta • 1% error on JES, if low pt jets well calibrated • Systematic bias found by D.Clements, C. Buttar • Partly due to unclustered soft activity - mitigated with larger cone sizes, lower jet seed pT Truth Reco F Ruehr (Heidelberg) D Clements, C Buttar Pythia standalone Cone 0.7 280->1120 GeV Pt

  11. UK Software JetPerformance • K Lohwasser (Oxford), V Giangiobbe (INFN Pisa) • Should be used to make standard jet performance plots Y-Splitter • J Butterworth, A Davison, P Sherwood (UCL) • Y scale is splitting scale into subjets • Useful for finding e.g. hjgh pT W->qq Mid-Point Jet Algorithm • S Thompson and C Cheplakov (Glasgow) • Provides infrared safety • Performance as good as standard algorithms • Implemented in ATLAS software

  12. Conclusions • ATLAS pursuing a range of approaches to calibrating jet energy: - Initial calibration (H1 style , Local Hadron style + jet corrections, energy flow) - Validation of hadronic calibrations with E/P methods - Jet balancing (di-jet) to get uniform  response -  + jet (Z + jet), W->qq also useful - Then we can bootstrap calibration up to the very high pt jets - jet balancing favoured • UK is providing useful software for use in Jets work (JetPerformance, Y Splitter, Mid Point algorithm , eflowRec)

  13. Documents • Local Hadron Calibration Performance - E Bergeaas, C Issever, K Jon-And, K Lohwasser, B King, D Milstead: ATL-COM-LARG-2007-010 • Energy Flow for Jets - M Hodgkinson, D Tovey, R Duxfield: ATL-COM-PHYS-2007-082 • Jet Fragmentation and E/P work - M Hodgkinson, J Lu, D Gingrich, N Davidson, E Barberio: https://twiki.cern.ch/bin/view/Atlas/JetEoPCSCNote • DiJets - various talks in JetRec phone meetings • Gamma + Jet - https://twiki.cern.ch/bin/view/Sandbox/MartinaHurwitzSandbox • High Pt JES - again see talks in JetRec phone meetings • JetPerformance - talk in 28th Nov JetRec phone conference • Ysplitter - J Butterworth, A Davison, E Ozcan, P Sherwood: ATL-PHYS-INT-2007-015 • Mid Point Algorithm in ATLAS - A Cheplakov,S Thompson: ATL-PHYS-PUB-2007-007 • General very detailed introduction to all these issues by P Loch and M Lefevre at: https://twiki.cern.ch/twiki/bin/view/Atlas/IntroductionToHadronicCalibration

  14. BACKUPExtra Information

  15. Minimum BiasN.Davidson, E.Barberio (Melbourne) • Apply loose cuts and subtract background • Use control sample of late showering charged hadrons (MinBias) • Can define EMoutercone which only contains energy from background • Solve set of linear equations EM Had Outer ± Outer • Recover signal distribution • Also check mean is consistent within errors - yes, but need more statistics to get to 1% precision • Still needs more work to reduce signal contamination in control region (3%->1%) • However this idea looks promising - if it works can be used potentially for higher energy tracks in QCD jets pT=10 GeV

  16. In Situ Calibration Methods • Photon is well measured (EM scale) - - use balance of  + Jet to get to jet energy scale Hadronic W Decays Template Method J Schwindling (Saclay) • Generate template histograms of mjj using different sigma and mean for jet resolution • Find which one fits best to data • Read of Jet Energy Scale from template • Stable wrt event selection, combinatoric background shifts scale 1%

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