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PHOTON RECONSTRUCTION IN CMS Jukka Nysten Helsinki Institute of Physics for the CMS Collaboration

Learn about photon selection, reconstruction, and primary vertex finding in the CMS experiment, aiding in Higgs boson detection. Discover techniques for handling converted and unconverted photons.

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PHOTON RECONSTRUCTION IN CMS Jukka Nysten Helsinki Institute of Physics for the CMS Collaboration

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  1. PHOTON RECONSTRUCTION IN CMS Jukka Nysten Helsinki Institute of Physics for the CMS Collaboration ACAT03

  2. Outline • Accelerator and experiment • Online event selection: level 1 and high level trigger (HLT) • H gg channel • Photon conversions • Offline analysis: handling of unconverted and converted photons ACAT03

  3. r z Equipment • LHC • Proton proton collider • Collision energy 14 TeV • Luminosity • Bunch crossings every 25 ns • 20 interactions in each bunch crossing • CMS • General purpose experiment • Silicon tracker: inner pixels, silicon microstrips • Crystal electromagnetic calorimeter: lead tungstate crystals (22mm x 22mm) • Simulations • Event generation: Pythia • Detector simulation: CMSIM (Geant3), OSCAR (Geant4) • Reconstruction and electronics simulation: ORCA ACAT03

  4. Level-1 ECAL reconstruction Threshold cut Level-2 Level-2.5 Pixel matching Level-3 Electrons Track reconstruction E/p, matching (Dh) cut Photons Threshold cut Isolation Triggers: Level-1 & HLT L1:40MHz  100kHz HLT:100kHz  100Hz • Bring event rate down • Select signal events with high efficiency • Reject background events with many orders of magnitude • Hierarchical structure Level -1:hardware HLT: software HLT ACAT03

  5. HLT output rates ACAT03

  6. H  gg signal extraction • Promising signal for Higgs boson mass region m < 150 GeV/c • Large background • Reducible background (neutral pions from jets) • Irreducible background (prompt photons) • An excellent energy resolution and the knowledge of the primary vertex is required 2 H ACAT03

  7. Photons in the tracker • Large amount of material in the tracker volume • Photon can convert into an electron positron pair in the tracker material with high probability • Probability of a photon converting: • Energy deposit in ECAL is spread because the 4T magnetic field affects the charged particles  degradation of energy resolution • 70% of H  gg events have at least one converted photon ACAT03

  8. Offline photon reconstruction • Energy measurement • Conversion handling • Primary vertex finding • prejection 0 ACAT03

  9. Energy measurement ECAL clustering algorithms: • Fixed window: for unconverted photons • Hybrid: barrel • Island: endcaps ACAT03

  10. Energy reconstruction for converted photons Separation of e+ e- pair on ECAL Use different algorithms for energy reconstruction depending on the conversion pattern. Converted photons have worse energy resolution than unconverted ones Photon energy resolution ECAL noise terms unconverted converted ACAT03

  11. Track finding for conversions • Use the Kalman filter for track reconstruction • Track inward in the tracker barrel • Form the initial guess from the super-cluster and the hit on the outermost tracker layer Conversion tracking should provide: • conversion position • z coordinate for primary vertex • means to do p rejection 0 ACAT03

  12. r z Z coordinate of the primary vertex • Track extrapolated to the beam line in the (r, z) plane • Data: photons with no pile up, tracker barrel region • Stereo layers essential for position resolution (r=20,30,60,70 cm) r < 20 cm 20 cm< r < 65 cm r > 65 cm ACAT03

  13. Primary vertex • Procedure for finding the primary vertex with charged tracks: • Reconstruct charged particle tracks in the event • Reconstruct vertices • Choose the Higgs production vertex • Uses the knowledge that Higgs production events are harder than pileup events. • At high luminosity choosing the right vertex is quite difficult. • Primary vertex location from a converted photon might help. ACAT03

  14. Summary • Precise photon reconstruction is required to discover the Higgs boson in the two photon channel. • Photon selection is done in level-1 and High level triggers. • Unconverted and converted photons require separate algorithms. • Reconstructing conversions can provide information about the z coordinate of the primary vertex, conversion radius and help in p rejection. 0 ACAT03

  15. References • The Electromagnetic Calorimeter Project Technical Design Report CERN/LHCC 97-33 CMS TDR 4 15 December 1997 • CMS Technical Proposal CERN/LHVV 94-38 (1994) • The TriDAS Project Technical Design Report, volume 2. Data Aquisition and High-Level Trigger CERN/LHCC 02-26 CMS TDR 6 15December 2002 • E. Meschi, T.Monteiro, C.Seez, P.Vikas, CMS Note 2001/034 ACAT03

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