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Jet Calibration in ATLAS

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

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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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