1. Introduction Week of Jets FNAL, Aug. 24-28, 2009 Aug 23, 2009 Introduction To Jet Reconstruction In ATLAS Peter Loch University of Arizona Tucson, Arizona USA Peter Loch University of Arizona Tucson, Arizona USA

2. Introduction Week of Jets FNAL, Aug. 24-28, 2009 2 P. Loch U of Arizona Aug 23, 2009 My Road To Jets @ LHC Started with calorimeter construction for H1/HERA (1987-1989)… Started with calorimeter construction for H1/HERA (1987-1989)… The “hardware years”… The “hardware years”… …then Deep Inelastic Scattering with H1 (1989-1992,1993-1995) …then Deep Inelastic Scattering with H1 (1989-1992,1993-1995) Electron, hadron, and jet reconstruction and calibration in a non-compensating calorimeter Electron, hadron, and jet reconstruction and calibration in a non-compensating calorimeter Have always been attracted by complex response! Have always been attracted by complex response! Experimental sensitivities to jet properties Experimental sensitivities to jet properties We did not call it sub-structure then! We did not call it sub-structure then! Forward hadronic energy flow and jet production Forward hadronic energy flow and jet production E.g., DGLAP vs BFKL evolution at low Bjorken- x E.g., DGLAP vs BFKL evolution at low Bjorken- x …with a short stop at DZero/Tevatron (1992)… …with a short stop at DZero/Tevatron (1992)… Calorimeter response to minimum bias Calorimeter response to minimum bias Mostly detector oriented noise, alignment and beam position studies Mostly detector oriented noise, alignment and beam position studies …and at GEM/SSC (1992-1993)… …and at GEM/SSC (1992-1993)… Design and response evaluation of the forward calorimeter Design and response evaluation of the forward calorimeter Detector prototyping and test beams Detector prototyping and test beams …and a long tenure in ATLAS/LHC (since 1995) …and a long tenure in ATLAS/LHC (since 1995) Design and response evaluation of the forward calorimeter Design and response evaluation of the forward calorimeter Yes, it is the same design as for GEM Yes, it is the same design as for GEM Forward calorimeter detector construction management Forward calorimeter detector construction management My “Microsoft years” My “Microsoft years” Focus on preparation for physics at LHC (since 2002) Focus on preparation for physics at LHC (since 2002) Initial interest jet physics and QCD… Initial interest jet physics and QCD… Some leadership Some leadership Calorimeter software coordination Calorimeter software coordination Jet and missing Et reconstruction performance coordination Jet and missing Et reconstruction performance coordination

3. Introduction Week of Jets FNAL, Aug. 24-28, 2009 3 P. Loch U of Arizona Aug 23, 2009 Change of composition Change of composition Radiation and decay inside detector volume Radiation and decay inside detector volume “Randomization” of original particle content “Randomization” of original particle content Defocusing changes shape in lab frame Defocusing changes shape in lab frame Charged particles bend in solenoid field Charged particles bend in solenoid field Attenuation changes energy Attenuation changes energy Total loss of soft charged particles in magnetic field Total loss of soft charged particles in magnetic field Partial and total energy loss of charged and neutral particles in inactive upstream material Partial and total energy loss of charged and neutral particles in inactive upstream material Hadronic and electromagnetic cacades in calorimeters Hadronic and electromagnetic cacades in calorimeters Distribute energy spatially Distribute energy spatially Lateral particle shower overlap Lateral particle shower overlap 100 MeV 10 GeV 1 GeV Image of Jet in the Detector

4. Introduction Week of Jets FNAL, Aug. 24-28, 2009 4 P. Loch U of Arizona Aug 23, 2009 Change of composition Change of composition Radiation and decay inside detector volume Radiation and decay inside detector volume “Randomization” of original particle content “Randomization” of original particle content Defocusing changes shape in lab frame Defocusing changes shape in lab frame Charged particles bend in solenoid field Charged particles bend in solenoid field Attenuation changes energy Attenuation changes energy Total loss of soft charged particles in magnetic field Total loss of soft charged particles in magnetic field Partial and total energy loss of charged and neutral particles in inactive upstream material Partial and total energy loss of charged and neutral particles in inactive upstream material Hadronic and electromagnetic cacades in calorimeters Hadronic and electromagnetic cacades in calorimeters Distribute energy spatially Distribute energy spatially Lateral particle shower overlap Lateral particle shower overlap 100 MeV 10 GeV 1 GeV Image of Jet in the Detector

5. Introduction Week of Jets FNAL, Aug. 24-28, 2009 5 P. Loch U of Arizona Aug 23, 2009 Change of composition Change of composition Radiation and decay inside detector volume Radiation and decay inside detector volume “Randomization” of original particle content “Randomization” of original particle content Defocusing changes shape in lab frame Defocusing changes shape in lab frame Charged particles bend in solenoid field Charged particles bend in solenoid field Attenuation changes energy Attenuation changes energy Total loss of soft charged particles in magnetic field Total loss of soft charged particles in magnetic field Partial and total energy loss of charged and neutral particles in inactive upstream material Partial and total energy loss of charged and neutral particles in inactive upstream material Hadronic and electromagnetic cacades in calorimeters Hadronic and electromagnetic cacades in calorimeters Distribute energy spatially Distribute energy spatially Lateral particle shower overlap Lateral particle shower overlap 100 MeV 10 GeV 1 GeV Image of Jet in the Detector

6. Introduction Week of Jets FNAL, Aug. 24-28, 2009 6 P. Loch U of Arizona Aug 23, 2009 Change of composition Change of composition Radiation and decay inside detector volume Radiation and decay inside detector volume “Randomization” of original particle content “Randomization” of original particle content Defocusing changes shape in lab frame Defocusing changes shape in lab frame Charged particles bend in solenoid field Charged particles bend in solenoid field Attenuation changes energy Attenuation changes energy Total loss of soft charged particles in magnetic field Total loss of soft charged particles in magnetic field Partial and total energy loss of charged and neutral particles in inactive upstream material Partial and total energy loss of charged and neutral particles in inactive upstream material Hadronic and electromagnetic cacades in calorimeters Hadronic and electromagnetic cacades in calorimeters Distribute energy spatially Distribute energy spatially Lateral particle shower overlap Lateral particle shower overlap 100 MeV 10 GeV 1 GeV Image of Jet in the Detector

7. Introduction Week of Jets FNAL, Aug. 24-28, 2009 7 P. Loch U of Arizona Aug 23, 2009 Change of composition Change of composition Radiation and decay inside detector volume Radiation and decay inside detector volume “Randomization” of original particle content “Randomization” of original particle content Defocusing changes shape in lab frame Defocusing changes shape in lab frame Charged particles bend in solenoid field Charged particles bend in solenoid field Attenuation changes energy Attenuation changes energy Total loss of soft charged particles in magnetic field Total loss of soft charged particles in magnetic field Partial and total energy loss of charged and neutral particles in inactive upstream material Partial and total energy loss of charged and neutral particles in inactive upstream material Hadronic and electromagnetic cacades in calorimeters Hadronic and electromagnetic cacades in calorimeters Distribute energy spatially Distribute energy spatially Lateral particle shower overlap Lateral particle shower overlap 100 MeV 10 GeV 1 GeV Image of Jet in the Detector

8. Introduction Week of Jets FNAL, Aug. 24-28, 2009 8 P. Loch U of Arizona Aug 23, 2009 Jet Reconstruction Task Jet Reconstruction Challenges physics reaction of interest (interaction or parton level) added particles from underlying event added particles from in-time (same trigger) pile-up event jet reconstruction algorithm efficiency added particles from underlying event added particles from in-time (same trigger) pile-up event jet reconstruction algorithm efficiency longitudinal energy leakage detector signal inefficiencies (dead channels, HV…) noise from (off- and in-time) pile-up electronic noise calo signal definition (noise suppression etc.) dead material losses (front, cracks, transitions…) detector response characteristics (e/h ≠ 1) jet reconstruction algorithm efficiency lost soft tracks due to magnetic field longitudinal energy leakage detector signal inefficiencies (dead channels, HV…) noise from (off- and in-time) pile-up electronic noise calo signal definition (noise suppression etc.) dead material losses (front, cracks, transitions…) detector response characteristics (e/h ≠ 1) jet reconstruction algorithm efficiency lost soft tracks due to magnetic field Experiment (“Nature”)

9. Introduction Week of Jets FNAL, Aug. 24-28, 2009 9 P. Loch U of Arizona Aug 23, 2009 ATLAS Jet Reconstruction Move to better motivated jet algorithms Move to better motivated jet algorithms ATLAS uses FastJet and SISCone libraries ATLAS uses FastJet and SISCone libraries Common implementation theory/CMS/ATLAS Common implementation theory/CMS/ATLAS Narrow (0.4) AntiKt jets preferred for initial physics Narrow (0.4) AntiKt jets preferred for initial physics Most regularly shaped Most regularly shaped More straight forward handle on extra energy from pile-up (?) More straight forward handle on extra energy from pile-up (?) Legacy (ATLAS) seeded fixed cone still available Legacy (ATLAS) seeded fixed cone still available Usefulness basically limited to LO final states Usefulness basically limited to LO final states Fast algorithm ok for trigger and event filter Fast algorithm ok for trigger and event filter Factorized reconstruction Factorized reconstruction Attempt to use calorimeter signal definitions to address most detector specific issues Attempt to use calorimeter signal definitions to address most detector specific issues Use towers and clusters Use towers and clusters Calibrations and corrections determined independently Calibrations and corrections determined independently Distinguish carefully between relative and absolute corrections by least interdependency Distinguish carefully between relative and absolute corrections by least interdependency Jet energy scale evolution Limit to factorization introduced by calorimeter Limit to factorization introduced by calorimeter E.g. detector couples response and fragmentation – corrections often only possible on average More details on Thursday! More details on Thursday!

10. Introduction Week of Jets FNAL, Aug. 24-28, 2009 10 P. Loch U of Arizona Aug 23, 2009 First Data Jet Calibration Priorities (plots from CERN-OPEN-2008-020) Need flat jet response rather quickly Need flat jet response rather quickly Basis for first physics analyses Basis for first physics analyses E.g., inclusive jet cross-section E.g., inclusive jet cross-section Systematics can dominate total error already at 10-200 pb -1 Systematics can dominate total error already at 10-200 pb -1 Depending on final state, kinematic range considered, fiducial cuts,… Depending on final state, kinematic range considered, fiducial cuts,… Requires understanding of hadronic response Requires understanding of hadronic response Minimum bias Minimum bias Single charged hadron response of calorimeter Single charged hadron response of calorimeter Pile-up with tracks and clusters Pile-up with tracks and clusters Di-jets Di-jets Underlying event with tracks and clusters Underlying event with tracks and clusters Direction dependent response corrections Direction dependent response corrections Prompt photons Prompt photons Absolute scale by balancing, missing Et projection fraction, etc. Absolute scale by balancing, missing Et projection fraction, etc. Initial focus on simple approaches Initial focus on simple approaches Least number of algorithms Least number of algorithms Least numerical complexity Least numerical complexity Realistic systematic error evaluation Realistic systematic error evaluation Conservative ok for first data! Conservative ok for first data! But also try new or alternative approaches! But also try new or alternative approaches!

11. Introduction Week of Jets FNAL, Aug. 24-28, 2009 11 P. Loch U of Arizona Aug 23, 2009 Initial Jet Calibration Scheme optional data driven MC

12. Introduction Week of Jets FNAL, Aug. 24-28, 2009 12 P. Loch U of Arizona Aug 23, 2009 Beyond First Physics Initial tasks focus on jet energy scale (JES) errors Initial tasks focus on jet energy scale (JES) errors Binds most of the manpower today Binds most of the manpower today Rather wide spectrum of signal and calibration choices Rather wide spectrum of signal and calibration choices But did not want to make hard decisions based on MC alone! But did not want to make hard decisions based on MC alone! Future activity more focused on jet features Future activity more focused on jet features Jet-by-jet corrections to reduced fluctuations of JES Jet-by-jet corrections to reduced fluctuations of JES Attempt to unfold residual fragmentation dependencies Attempt to unfold residual fragmentation dependencies Useful observables include jet shapes Useful observables include jet shapes Jet-by-jet pile-up suppression Jet-by-jet pile-up suppression Qualitative change from initial run scenarios Qualitative change from initial run scenarios Pile-up signal history (= noise in ATLAS) depends on instantaneous luminosity and bunch crossing frequency Large source of signal fluctuations affects all jet final states Large source of signal fluctuations affects all jet final states Experimental resolution for jet masses and substructure Experimental resolution for jet masses and substructure Discovery tool for heavy particle decays Discovery tool for heavy particle decays E.g., fully hadronic decay of boosted top (“3 in 1”) Experimentally challenging but not impossible Experimentally challenging but not impossible (more on Wednesday & Thursday)

13. Introduction Week of Jets FNAL, Aug. 24-28, 2009 13 P. Loch U of Arizona Aug 23, 2009 Multiple interactions between partons in other protons in the same bunch crossing Multiple interactions between partons in other protons in the same bunch crossing Consequence of high rate (luminosity) and high proton-proton total cross- section (~75 mb) Consequence of high rate (luminosity) and high proton-proton total cross- section (~75 mb) Statistically independent of hard scattering Statistically independent of hard scattering Similar models used for soft physics as in underlying event Similar models used for soft physics as in underlying event Signal history in calorimeter increases noise Signal history in calorimeter increases noise Signal 10-20 times slower (ATLAS) than bunch crossing rate (25 ns) Signal 10-20 times slower (ATLAS) than bunch crossing rate (25 ns) Noise has coherent character Noise has coherent character Cell signals linked through past shower developments Cell signals linked through past shower developments E t ~ 58 GeV E t ~ 81 GeV without pile-up Pile-Up Prog.Part.Nucl.Phys. 60:484-551,2008

14. Introduction Week of Jets FNAL, Aug. 24-28, 2009 14 P. Loch U of Arizona Aug 23, 2009 Multiple interactions between partons in other protons in the same bunch crossing Multiple interactions between partons in other protons in the same bunch crossing Consequence of high rate (luminosity) and high proton-proton total cross- section (~75 mb) Consequence of high rate (luminosity) and high proton-proton total cross- section (~75 mb) Statistically independent of hard scattering Statistically independent of hard scattering Similar models used for soft physics as in underlying event Similar models used for soft physics as in underlying event Signal history in calorimeter increases noise Signal history in calorimeter increases noise Signal 10-20 times slower (ATLAS) than bunch crossing rate (25 ns) Signal 10-20 times slower (ATLAS) than bunch crossing rate (25 ns) Noise has coherent character Noise has coherent character Cell signals linked through past shower developments Cell signals linked through past shower developments E t ~ 58 GeV E t ~ 81 GeV with design luminosity pile-up Pile-Up Prog.Part.Nucl.Phys. 60:484-551,2008

15. Introduction Week of Jets FNAL, Aug. 24-28, 2009 15 P. Loch U of Arizona Aug 23, 2009 Multiple interactions between partons in other protons in the same bunch crossing Multiple interactions between partons in other protons in the same bunch crossing Consequence of high rate (luminosity) and high proton-proton total cross- section (~75 mb) Consequence of high rate (luminosity) and high proton-proton total cross- section (~75 mb) Statistically independent of hard scattering Statistically independent of hard scattering Similar models used for soft physics as in underlying event Similar models used for soft physics as in underlying event Signal history in calorimeter increases noise Signal history in calorimeter increases noise Signal 10-20 times slower (ATLAS) than bunch crossing rate (25 ns) Signal 10-20 times slower (ATLAS) than bunch crossing rate (25 ns) Noise has coherent character Noise has coherent character Cell signals linked through past shower developments Cell signals linked through past shower developments Pile-Up Prog.Part.Nucl.Phys. 60:484-551,2008

16. Introduction Week of Jets FNAL, Aug. 24-28, 2009 16 P. Loch U of Arizona Aug 23, 2009 ATLAS Status ATLAS is up and running ATLAS is up and running Used the LHC delay for continued sub-detector commissioning and repair Used the LHC delay for continued sub-detector commissioning and repair Cosmics for long term signal stability estimates and detector timing and alignment Cosmics for long term signal stability estimates and detector timing and alignment Understanding of calorimeter noise with random (“empty”) events Understanding of calorimeter noise with random (“empty”) events Millions of cosmics recorded Millions of cosmics recorded Test of computing chain Test of computing chain Re-reconstruction of cosmics trains ATLAS world wide computing Re-reconstruction of cosmics trains ATLAS world wide computing Some initial impressions for jets and missing Et Some initial impressions for jets and missing Et Noise contributions to missing Et reconstruction in random events Noise contributions to missing Et reconstruction in random events Calorimeter signal definition evaluation Calorimeter signal definition evaluation Jets in cosmics Jets in cosmics Helps develop in-time background suppression strategies Helps develop in-time background suppression strategies

17. Introduction Week of Jets FNAL, Aug. 24-28, 2009 17 P. Loch U of Arizona Aug 23, 2009 Cosmics In ATLAS Single cosmic muons traversing the experiment help understanding mostly timing and detector sensitivities

18. Introduction Week of Jets FNAL, Aug. 24-28, 2009 18 P. Loch U of Arizona Aug 23, 2009 Cosmic Showers In ATLAS Cosmic showers hitting the detector help understanding fake jet rates from this background in collision physics

19. Introduction Week of Jets FNAL, Aug. 24-28, 2009 19 P. Loch U of Arizona Aug 23, 2009 Understanding Cosmics

20. Introduction Week of Jets FNAL, Aug. 24-28, 2009 20 P. Loch U of Arizona Aug 23, 2009 Understanding Noise The scalar missing Et sum is a very powerful diagnostic variable in random events (stability with time, noise suppression, major detector faults…)

21. Introduction Week of Jets FNAL, Aug. 24-28, 2009 21 P. Loch U of Arizona Aug 23, 2009 Conclusion & Outlook ATLAS is ready for collisions ATLAS is ready for collisions The detector is operating since ~1 year The detector is operating since ~1 year Major faults detected in 2008 are repaired Major faults detected in 2008 are repaired Calorimeter noise is well understood Calorimeter noise is well understood Response to cosmics can be simulated reasonably well Response to cosmics can be simulated reasonably well First algorithms for suppression of cosmic background available First algorithms for suppression of cosmic background available New studies with overlaying simulated signal events with real cosmics underway New studies with overlaying simulated signal events with real cosmics underway More on ATLAS this week More on ATLAS this week Wednesday Wednesday Experimental aspects of jet reconstruction in new physics (informal) Experimental aspects of jet reconstruction in new physics (informal) Jet masses, shapes, topologies, missing Et in ATLAS,… Thursday Thursday Talk on “Jet Reconstruction in ATLAS” Talk on “Jet Reconstruction in ATLAS” More details on calorimeter signals, algorithms, performance expectations, jet shapes,…