Jukka Nysten ACAT03 1

PHOTON RECONSTRUCTION IN CMS

Jukka Nysten

Helsinki Institute of Physics

for the CMS Collaboration

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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