Search for a Standard Model Higgs Boson in the Diphoton F inal S tate at the CDF Detector

1. Search for a Standard Model Higgs Boson in the Diphoton Final State at the CDF Detector Karen Bland [ E-mail: kbland@fnal.gov ] Department of Physics, Baylor University, Waco, TX 76798, USA for the CDF Collaboration We present the results of a search for a standard model Higgs boson in the h gg decay channel at the CDF experiment using 5.4 fb-1 of integrated luminosity from proton-antiproton collisions at the Fermilab Tevatron. Searches for a lower-mass Higgs typically rely on the h  bb decay channel due to the large branching fraction; however, the diphoton decay mode is complementary in that its backgrounds are significantly different, and the identification efficiency and energy resolution for photons are typically much better than that for b-quark jets. Event Selection Motivation Collider Detector at Fermilab (CDF) • Discovery of the Higgs boson would provide evidence for a mechanism responsible for electroweak symmetry breaking • The mass of this Higgs particle is unknown • Evidence for this particle would be one of the greatest discoveries in particle physics! • Use standard CDF photon ID • Select 2 central photons with Mgg > 30 GeV/c2 • Data: • Use diphoton triggers • ~5.4 fb-1 of integrated luminosity • Signal MC: • Generated using PYTHIA • 100–150 GeV/c2 in 10 GeV/c2 intervals • Scale factors derived from Z e+e–studies Hadronic Calorimeter Electromagetic Calorimeter Data-Driven Background Model • CEM (Central EM calorimeter) • Calorimeters measure the energy of particles produced in collisions. Photons and electrons deposit their energy in the electromagnetic (EM) calorimeter. • One wedge of CEM is shown in the upper right photo. 24 total wedges cover azimuthal angle. Adjacent energy deposits in a single wedge are called an EM cluster. • CEM provides coverage in the central region of the detector, |η| < 1.1. • CES (Central EM shower maximum detector) • Located inside the CEM, the CES refines the position measurement of the EM cluster Higgs Production at the Tevatron • Composition • Real SM photons via QCD interactions • 1 or 2 jets faking a photon • (example Feynman diagrams at right) The most important processes for Higgs production at hadron colliders are gluon fusion, associative production, and vector boson fusion. The dominant production process is gluon fusion. • Background Model • Fit to sideband region of the Mgg distribution • Exclude 12 GeV/c2 window around signal test mass • Interpolate fit into signal region Gluon Fusion:σ ≈ 1000 fb* Photon Identification Detector profile consistent with a direct photon(a photon originating from the interaction point): • Compact and isolated EM cluster • No track (no electric charge) • Not in a jet (no color charge) Associated Production:σ ≈ 225 fb* (Higgs test mass of 120 GeV/c2 shown.) No resonance in the data observed over the background, so we set limits on hgg production Electron (background) Systematic Uncertainty • Signal • Acceptance and efficiency (in table) • Cross section: • σggH (12%) • σVH ( 5%) • σVBF (10%) • Luminosity: 6% • Background • 4% rate uncertainty • Obtained from studies allowing normalization of fit to vary in the signal region Vector Boson Fusion: σ ≈ 70 fb* Photon (signal) Direct photon (signal) – – *s for √s = 1.96 TeV pp collisions for Mh = 120 GeV/c2 Jet(background) Higgs Decay Modes Indirect photons (background) Advantages of Using Photons Limits on Higgs Production 12 GeV/c2 signal window for each test mass used to set 95% confidence level upper limits on σBr relative to SM prediction Great mass resolution • s/Mgg about 4 x smaller than best jet algorithms • Small signal mass resolution reduces search to a bump hunt! • We search for a narrow peak over a smooth background • Use sideband fits to estimate background • Low mass search: focus on 100–150 GeV/c2 • Br(hgg) < 0.0025: smaller branching ratio than other channels in this mass range • Overall σ ~1300 fb: larger overall production cross section than other channels (gluon fusion production mode not included for other channels because of higher backgrounds) • Signal Expectation: • ≈ 16 events produced with 5.4 fb-1 of data • ≈ 2 events after acceptances and efficiencies • Large signal acceptance • ≈ 13% overall signal acceptance • Will double with forward photons added • Expected and observed limits in good agreement • Most sensitive for range 110–130 GeV/c2 at about 20 x SM expectation The CEM response to photons is calibrated using electrons from Z  e+e– decays.  This process ensures small uncertainties on ID efficiencies, data-MC scale factor, and energy scale