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Electrons and Photons in CMS and their role in searches for New Physics. Stilianos Kesisoglou. Summer School 2011 Institute of Nuclear Physics National Center for Scientific Research “Demokritos” Jul 04-15, 2011 - Athens, Greece. Presentation Overview. CERN / Large Hadron Collider (LHC)
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Electrons and Photons in CMSand their rolein searches for New Physics Stilianos Kesisoglou Summer School 2011 Institute of Nuclear Physics National Center for Scientific Research “Demokritos” Jul 04-15, 2011 - Athens, Greece
Presentation Overview • CERN / Large Hadron Collider (LHC) • Compact Muon Solenoid Detector (CMS) • Physics Motivation • Electromagnetic Calorimeter (ECAL) • Electrons and Photons (EM Objects) • Reconstruction • Energy & Position Resolution • Trigger • Important issues (Calibration, Dead Channels, Vertex, Conversions) • Searches for New Physics • Higgs • Supersymmetry (SUSY) • Extra Dimensions • Dark Matter
European Organization for Nuclear Research (CERN) CMS ALICE ATLAS Location: Swiss - France borders Years Active:1954 – today Collisions: Proton bunches Proton Speed: 99.999999% c Circumference: 27 Km Number of bunches:~ 3000 Beam Energy: 3.5 TeV LHC - 4.25 Km SPS LHCb
Inside the LHC Tunnel Circular tunnel 27 Km in circumference Buried around 50 and 175 meters underground A total of ~ 5000 superconducting magnets
Main Dipole Magnet Number of Dipoles: 1232 Field Strength: 8.4 Tesla (105 Earth Field) Operating Temperature: 1.9 Kelvin Operating Current: 11700 Amperes Length: 14.3 meters Weight: 35 Tons Cost: 0.5 M CHF
Compact Muon Solenoid (CMS) Coil + Inner Vacuum Tank EndCap Muon Chambers ECAL Crystals Silicon Tracker Barrel HCAL 3600 Scientists 183 Institutes 38 Countries Cessy, France 1998-2008 500 MCHF
Inner Vacuum Tank Magnet 3.8 Tesla
Muon EndCap (RPC) Hadronic Calorimeter EndCap
Tracker EndCap Tracker Inner Barrel
Physics Motivation Standard Model ( t, b, QCD, EWK… ) Supersummetry Lepton Quark Gauge Boson Slepton Squark Gaugino Dark Matter Higgs Extra Dimensions Dark Matter
Electromagnetic Crystal Calorimeter Truncated Pyramids: 3o off-pointing geometry Barrel EndCap 61200 Crystals 14648 Crystals ~ 22 x 23 x 230 mm3 ~ 30 x 30 x 220 mm3 25.8 X0 25.0 X0
SubModule (10 crystals) Module (40 / 50 SubModules) APD (Barrel) VPT (EndCap) EndCap SuperCrystal(25 Crystals) Barrel SuperModule (4 Modules / 1700 crystals)
Completed EndCap Dee Completed Barrel
EM Objects / Challenges • Excellent Energy & Position Resolution for em-objects • Efficient triggering for the em-object stream • Well calibrated ECAL • Dead Channels • Good knowledge of the Primary Vertex • Recover photon conversions • Reject pion conversions to photons
EM Objects / Reconstruction Tracker Material Budget • Solenoidal magnetic field • Electron & Positron trajectories are bend • Radiated energy spread in φ • Tracker Material • Electron energy loss (Bremsstrahlung) • Photon Conversion • Energy and position resolution is affected. • Photons. • Electrons & Positrons.
EM Objects / Reconstruction (position) PbWO4 Crystal Array γ / e Energy deposition in 5x5 crystal matrix • Energy weighted mean position of the crystals in the cluster • xi : Position of each crystal • wi : Weight for each crystal
EM Objects / Reconstruction (energy) Measured energy fraction Basic Cluster Super Cluster • Electrons • Energy is clustered by building cluster of clusters (supercluster) • Photons (unconverted) • Energy is contained in a 5x5array of crystals Barrel EndCap
EM Objects / Resolutions Testbeam = 850 m 50 GeV electrons Reconstructed I.P (mm) Reco I.P – True I.P (mm) Beam Energy (GeV)
CMS Trigger Overview Crossing Rate: 40 MHz Event Size: ~ 1 MB 100 KHz 100 Hz
Electron / Photon Triggers Subsystems for L1 & HLTelectron / photon Triggers γ e+
ECAL Calibration Transmission Variations • ECAL’s response variesfrom channel-to-channel • Barrel Crystals • Variation of scintillation light • RMS ~ 13% • EndCap Triodes • VPT signal yield • RMS ~25% • Equalize crystals response(inter-calibration) • Calibration: • Before Data taking • TestBeam, Cosmics • After Data taking • Isolated electrons
Dead ECAL Channels 5x5 Crystal Array Energy Ratio Before Correction After Correction Ratio = 1.00 ± 0.27(%) • Dead Channels affect: • Electron & Photon reconstruction • Degrade physics performance • Correction: • Correlate the energy content of dead channels with the neighboring functioning ones (inside the 5x5 crystal array)
Primary Vertex Selection Luminosity Pile-up 2 x 1033 cm-2 s-1 Low ~ 4 events Easy 1034 cm-2 s-1 High ~20 events Difficult Selection is based on multiplicity of charged tracks that balanceHiggs production
π0 - photon Discrimination π0 Rejection percentage • Solution: • Separate the two closely spaced photons from π0 decay. • Preshower usage in EndCaps • Limitations: • Lateral shower shape in crystals (Barrel) • Cluster separation in preshower (EndCaps) • Pions can decay into two closely spaced photons. • Reconstruction code might “reconstruct” these two photons as being one.
Photon Conversions Photons from H → γγ • Photons, in the presence ofa nucleus can convert toa pair of electron-positron. • Detector material affectsthe passage of photons(photon conversion). • Photon conversion affectsthe detection rate of allelectromagnetic objects. • Fake rate for photons. • Fake rates for electrons and positrons.
Role in Searches for New Physics • Higgs: • H → 2 photons • H → Z Z → 4 leptons • H → W W → 4 leptons + 2 neutrinos • H → 2 taus → leptons & jets • Supersymmetry: • On many models the next-to-lightest SUSY particles (NLSP’s) decay to final states involving electrons & photons. • Extra Dimensions and Dark Matter • Electrons & photons appear also in these theories.