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Study of tt-Photon Events with the CMS-detector Bad Honnef 27. August 2007

Study of tt-Photon Events with the CMS-detector Bad Honnef 27. August 2007. Thomas Hermanns III. Physikalisches Institut B. tt-Photon Events at the LHC. Cross section for tt-pair production at s= 14 TeV: 830 pb (NLO) About 1 tt-pair per second (L=10 33 cm -2 s -1 )

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Study of tt-Photon Events with the CMS-detector Bad Honnef 27. August 2007

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  1. Study of tt-Photon Events with the CMS-detector Bad Honnef27. August 2007 Thomas HermannsIII. Physikalisches Institut B

  2. tt-Photon Events at the LHC • Cross section for tt-pair production at s= 14 TeV: 830 pb (NLO) • About 1 tt-pair per second (L=1033cm-2s-1) • Consider top-decays only via Vtb1 • Consecutive decays of the W-bosons • W+/-: electron or muon channel • W-/+: two (light) quarks • “Semileptonic or lepton+jets channel” • Branching fraction: 29.6% • High statistics appropriate for rare events in the realm of top-physics Thomas Hermanns Bad Honnef, 27. September 2007 III. Physikalisches Institut B

  3. Motivation of the Analysis • Determination of photon spectra at the CMS-experiment • Separation of photons radiated off top-quarks • Distinction of various QED-coupling scenarios • Lorentz-invariant vertex parameterisation • SM prediction at Born level U. Baur, A. Juste, L.H. Orr, D. Rainwater„Probing electroweak top quark couplings at hadron colliders“Physical Review D 71, 054013 (2005) Thomas Hermanns Bad Honnef, 27. September 2007 III. Physikalisches Institut B

  4. The Feynman-Diagrams • Three classes for the hard process • g + g  t + t + Photon (8 diagrams) • q + q  t + t + Photon (8 diagrams for q=u,d) • q + q + Photon t + t (8 diagrams for q=u,d) Thomas Hermanns Bad Honnef, 27. September 2007 III. Physikalisches Institut B

  5. The Dataset • Cross section for 2  3 process (TopRex generator) • E,min> 5 GeV: 16.8pb • E,min> 100 GeV: 0.2pb • Compare to: 830pb for top-quark-pair production (NLO) • Indistinguishable processes for photons radiated off incoming quarks and top-quarks in the case of annihilation • Study of a Pythia tt-dataset, to get photons radiated off top-quarks (final state radiation) Thomas Hermanns Bad Honnef, 27. September 2007 III. Physikalisches Institut B

  6. Photons in Signal Events • Signal event • Semileptonically (electron and muon) decaying tt-pair • Photon radiated off a top-quark (top photon) • CMS tt-inclusive dataset • Total number of events: 3,900,000 events • About 3,800 potential signal events (generator filter) • Less than 1,000 events remaining after preselection cuts •  Need for a private dataset • Filtering tt-semileptonic signal events on generator level • E >10 GeV • ||< 2.5 in events • Cross-section: 0.1 pb • 19,950 signal events Thomas Hermanns Bad Honnef, 27. September 2007 III. Physikalisches Institut B

  7. The Event Selection • Selection criteria applied on reconstructed objects • 1 electron or muon candidate • pT>20 GeV/c • Isolated in tracker and calorimeter • 2 jets candidates from b-quarks • Iterative cone algorithm (R=0.5) • pT>20 GeV/c • 2 jets candidates from light quarks • Iterative cone algorithm (R=0.5) • pT>20 GeV/c • 1 photon candidate • E >20 GeV/c • Isolated in tracker and calorimeter Thomas Hermanns Bad Honnef, 27. September 2007 III. Physikalisches Institut B

  8. Photon Classes • Try to match of every preselected photon candidates to a MC-photon • Distance in (,)-plane: (Rgen-reco) < 0.05 • Deviation of energy |Egen-reco| < 0.1 Egen • Signal Photons • Match of reconstructed photon candidate to generator top-photon • Background Photons • Match of reconstructed photon candidate to any generator photon but the top-photon • Fake Photons • No match of reconstructed photon candidate to any generator photon Thomas Hermanns Bad Honnef, 27. September 2007 III. Physikalisches Institut B

  9. Calorimeter Energy Deposits (Signal Events) • Hadronic over electromagnetic energy ratio • Energy deposited in HCAL behind ECAL-supercluster (R=0.3 around line through supercluster midpoint) • Implement cut at R=0.2 • Remove large tails of mainly fake photon candidates E(had)= E(i) with i  R-cone Signal Photons Fake Photons Bkg. Photons HCAL Super- cluster ECAL Thomas Hermanns Bad Honnef, 27. September 2007 III. Physikalisches Institut B

  10. Shower Shape Variables(Signal Events) • Spread of electromagnetic energy in calorimeter cells • Energy of clusters (squares and rectangles) • Compare ratio of different shape variables • Cut on E(3x3) divided by E(5x5) • Lowest overlap between signal and background/fake distributions (normalized) ECAL cells Signal Photons Fake Photons Bkg. Photons Thomas Hermanns Bad Honnef, 27. September 2007 III. Physikalisches Institut B

  11. Isolation in Silicon Tracker (Signal Events) • Veto against tracks in the vicinity of the photon direction • Number of tracks in a cone around photon candidate • Varying cone size as well as ratio of track momentum and photon energy • No tracks within R=0.2 with pTrack > 0.1 EPhoton Signal Photons Fake Photons Thomas Hermanns Bad Honnef, 27. September 2007 III. Physikalisches Institut B

  12. Numerical Results of the Photon Identification • 10,022 out of 19,950 events fulfil minimum tt-photon requirements • Results of the photon identification • S/B improved by a factor of about 26 • less than 25% of all signal photons lost Thomas Hermanns Bad Honnef, 27. September 2007 III. Physikalisches Institut B

  13. Photon Spectrum (Signal Events) • Energy spectrum of signal, background and fake photons • Spectrum of signal photons slightly harder • Increase photon energy cut from 20 GeV to a higher value Signal Photons Fake Photons Bkg. Photons Signal Photons Fake Photons Bkg. Photons Thomas Hermanns Bad Honnef, 27. September 2007 III. Physikalisches Institut B

  14. Background to tt-Events • Events with a similar decay structure • W/Z+Jets • Vector-Boson pair production • W+Photon • tt-dileptonic • ... • Signal-Background Separation Strategy • Reject background events via an appropriate tt-event solution(using Top Quark Analysis Framework) • Remove photons as efficient as background and fake photons in signal event • Incorporation of background events recently started • First Overview for WZ- and Z+4Jet Events Thomas Hermanns Bad Honnef, 27. September 2007 III. Physikalisches Institut B

  15. Number of Photons Signal Events WZ-Events Z+4 Jets-Events After Preselection Thomas Hermanns Bad Honnef, 27. September 2007 III. Physikalisches Institut B

  16. Numerical Results • Demand for a high energetic photon essential already at preselection step • WZ: 1,446 out of 190,000 events • Z+4Jets: 3,735 out of 21,402 event • Rejection of background and fake photons comparable to signal events Thomas Hermanns Bad Honnef, 27. September 2007 III. Physikalisches Institut B

  17. Conclusions • Signal Photon Identification using Calorimeter Isolation and Tracker Isolation • Robust criteria to reject background and fake photons • Comparable efficiencies for various input dataset • tt-background demanding a proper tt-reconstruction • Preselection already gives a reasonable separation • Using TQAF should reduce that component further • tt-Photon analysis should ... • consider angular relations between photon and objects of the tt-decay(probably correlated to tracker isolation) • respect kinematic constraints if the tt-photon decay chain • Compare current results with TopRex-Dataset • More realistic description of physics Thomas Hermanns Bad Honnef, 27. September 2007 III. Physikalisches Institut B

  18. Thomas Hermanns Bad Honnef, 27. September 2007 III. Physikalisches Institut B

  19. Efficiency  Purity Thomas Hermanns Bad Honnef, 27. September 2007 III. Physikalisches Institut B

  20. TQAF • Top Quark Analysis Framework (TQAF) • effort of the CMS-top-group to establish a common framework for top-quark analysis • integration of CMS-standard tools (electron-ID, kinematic fit, ...) • analysis code used and debugged by many people • benefit from work one has to do but it was already done in the past • Three-Layered-Structure • production of top-objects independent of final state and analysis goal • lepton identification, calibration of jets, ... • Building of event solutions assuming a certain event hypothesis • combing jets to build a W-Boson, top-quark, ... • Actual analysis • direct access to objects according to a certain event solution • criterion to select best solution: MC-matching solution Thomas Hermanns Bad Honnef, 27. September 2007 III. Physikalisches Institut B

  21. Photon radiated off elektron/muon x-axis: R(photon - elektron/muon) y-axis: R(photon - top-Quark) R-Cuts(Generator Particles) • Photon radiated off top-quark • x-axis: R(photon - top-quark) • y-axis: R(photon - elektron/muon) Thomas Hermanns Bad Honnef, 27. September 2007 III. Physikalisches Institut B

  22. Identification of top-Photons Energy (reco./gen.) Pseudorapidity (reco./gen.) -Angel (reco./gen.) Thomas Hermanns Bad Honnef, 27. September 2007 III. Physikalisches Institut B

  23. Efficiency of Signal Photon Identification • High efficiency in identifying the signal photons • Tight cuts on energy and distance in (,)-plane Thomas Hermanns Bad Honnef, 27. September 2007 III. Physikalisches Institut B

  24. Identifikation der top-Photonen • Kriterien zur Identifikation • E(reco-gen) • (reco-gen) • (reco-gen) Energie der rekonstruierte Photonen Energie der generierten top-Photonen Thomas Hermanns Bad Honnef, 27. September 2007 III. Physikalisches Institut B

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