Electrons and Photons in CMS and their role in searches for New Physics

1. 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

2. 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

3. 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

4. Inside the LHC Tunnel Circular tunnel 27 Km in circumference Buried around 50 and 175 meters underground A total of ~ 5000 superconducting magnets

5. 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

6. 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

7. Inner Vacuum Tank Magnet 3.8 Tesla

8. Muon EndCap (RPC) Hadronic Calorimeter EndCap

9. Tracker EndCap Tracker Inner Barrel

10. Physics Motivation Standard Model ( t, b, QCD, EWK… ) Supersummetry Lepton Quark Gauge Boson Slepton Squark Gaugino Dark Matter Higgs Extra Dimensions Dark Matter

11. 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

12. SubModule (10 crystals) Module (40 / 50 SubModules) APD (Barrel) VPT (EndCap) EndCap SuperCrystal(25 Crystals) Barrel SuperModule (4 Modules / 1700 crystals)

13. Completed EndCap Dee Completed Barrel

14. 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

15. 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.

16. 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

17. 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

18. EM Objects / Resolutions Testbeam  = 850 m 50 GeV electrons Reconstructed I.P (mm) Reco I.P – True I.P (mm) Beam Energy (GeV)

19. CMS Trigger Overview Crossing Rate: 40 MHz Event Size: ~ 1 MB 100 KHz 100 Hz

20. Electron / Photon Triggers Subsystems for L1 & HLTelectron / photon Triggers γ e+

21. 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

22. 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)

23. 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

24. 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.

25. 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.

26. 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.