photon reconstruction in cms jukka nysten helsinki institute of physics for the cms collaboration
DESCRIPTION
PHOTON RECONSTRUCTION IN CMS Jukka Nysten Helsinki Institute of Physics for the CMS Collaboration. Outline. Accelerator and experiment Online event selection: level 1 and high level trigger (HLT) H gg channel Photon conversions - PowerPoint PPT PresentationTRANSCRIPT
Jukka Nysten ACAT03 1
PHOTON RECONSTRUCTION IN CMS
Jukka Nysten
Helsinki Institute of Physics
for the CMS Collaboration
Jukka Nysten ACAT03 2
Outline
• Accelerator and experiment
• Online event selection: level 1 and high level trigger (HLT)
• H channel
• Photon conversions
• Offline analysis: handling of unconverted and converted photons
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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
123433 1010 scm
r
z
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Triggers: Level-1 & HLT• Bring event rate down
– Select signal events with high efficiency
– Reject background events with many orders of magnitude
• Hierarchical structureLevel -1:hardware
HLT: software
Level-2
Level-3
Level-1
Level-2.5
PhotonsThreshold cut
Isolation
ElectronsTrack reconstruction
E/p, matching () cut
ECAL reconstructionThreshold cut
Pixel matching
HLT
L1:40MHz 100kHz
HLT:100kHz 100Hz
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HLT output rates
1233102 scm
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H 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
2H
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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
oxx
ePconv97
1
• 70% of H events have at least one converted photon
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Offline photon reconstruction• Energy measurement
• Conversion handling
• Primary vertex finding
• rejection0
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Energy measurementECAL clustering
algorithms:• Fixed window: for
unconverted photons• Hybrid: barrel• Island: endcaps
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Energy reconstruction for converted photons
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
Separation of e+ e- pair on ECAL
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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 rejection0
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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
cGeVpT 35r
z
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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.
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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 rejection.
0
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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