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This presentation discusses the search for the Standard Model Higgs boson in the diphoton final state at CDF. It covers topics such as Higgs production and decay, photon ID selection, advantages of the diphoton search, analysis description, results, and summary.
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Search for a SM Higgs boson in the diphoton final state at CDF Karen Bland On behalf of the CDF Collaboration • Higgs production and decay • Photon ID Selection • Advantages of Diphoton Search • Analysis Description • Results • Summary √s = 1.96 TeV Pheno Symposium May 10, 2010 Madison, WI
hγγfinal state Higgs production Gluon Fusion:σ ≈ 1000 fb* • Low mass search: Focus on 100 – 150 GeV/c2 • Overall σ: ~1300 fb: larger overall cross section than other channels • Br(hγγ) < 0.0025: smaller branching ratio than other channels • Signal Expectation: ≈ 16 events produced with 5.4 fb-1 of data ≈ 2 events after acceptances and efficiencies included Associated Production:σ ≈ 225 fb* Vector Boson Fusion: σ ≈ 70 fb* *σ for √s = 1.96 TeV p-pbar collisions and Mh = 120 GeV/c2
The Central Electromagnetic (EM) Calorimeter Hadronic Calorimeter Electromagnetic Calorimeter • CEM (Central EM calorimeter) • Made of alternating sheets of scintillator and lead • Energy resolution: ~13.5%/√E + 2% • |η| < 1.1 • 24 wedges distributed in ϕ (cracks between wedges) • CES (central EM shower maximum detector) • Six radiation lengths into CEM • Refines position measurement of the EM cluster so can match to a track
Identifying a Photon Detector profile consistent with a prompt photon: • Compact and isolated EM cluster • No track (no electric charge) • Not in a jet (no color charge)
Centralγ Selection • FiducialEM cluster in well instrumented region of CES • Had/EMMajority of calorimeter energy in CEM (not hadronic) • Cal IsolationPhoton EM cluster is isolated • Lateral shower shape in CESComparison to test beam consistent with prompt photon (rather than those from neutral pions, for example) • Track Isolation: No high pT tracks leading to CES cluster and pT of tracks near cluster restricted • 2nd CES clusterEnergy of any 2nd CES cluster restricted to reject neutral mesons decaying to two photons
Advantages of Using PhotonsMass resolution and signal acceptance Large signal acceptance • ≈ 13% overall signal acceptance • Will double with inclusion of forward photons • Great mass resolution • σ/Mγγabout 4 x smaller than best jet algorithms • Can then just search for narrow resonance! • Sideband fits can be used to estimate background
Advantages of using PhotonsCalibration Process: Z e+e– • Study photon ID efficiencies • Compare data and MC to derive simulation correction factor • Check electromagnetic energy scale Use of Ze+e– process ensures small uncertainties on ID efficiencies, data-MC scale factor, and energy scale Diphoton event Dielectron event Can calibrate EM calorimeter response to photons using electrons…
Analysis Overview • Event Selection: • Use standard CDF photon ID • Select central 2 photons w/ Mγγ> 30 GeV/c2 • Data: • Diphoton triggers • ~5.4 fb-1 from between Feb. 2004 – July 2009 • Signal MC: • Generated using PYTHIA 6.2, CTEQ5 PDFs and the standard CDF UE tune-A • 100 – 150 GeV/c2 in 10 GeV/c2 intervals • Scale factors derived from Ze+e– studies • Small signal mass resolution reduces search to a bump hunt – we search for a narrow peak over a smooth background • Data-Driven Background Model: • Use fits to sideband of data to predict background shape in signal region • Search for Resonance • Search for resonance over background • If no sign of resonance, then set 95% CL limits on σ x Br
Data-Driven Background Model • Composition • Real SM photons via QCD interactions • 1 or 2 jets faking a photon • Background Model • Fit to sideband region of the Mγγ distribution • Excluding 12 GeV/c2 window around signal test mass • Interpolate fit into signal region • Fit in signal region used as background model Higgs test mass window of 120 GeV/c2 shown. Process repeated for each test mass.
Visually Search for Resonance • Use background model obtained from sideband fit • Interpolated fit used for data-fit residuals • Inspect residuals for signs of resonance • Repeated for each Higgs test mass • Results: No resonance observed, so then set limits hγγ production…
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
Limits on hγγ • 12 GeV/c2 signal region for each test mass used to set upper limits set on σ Br relative to SM prediction • Expected and observed limits in good agreement • Most sensitive for range 110 – 130 GeV/c2 Will contribute sensitivity to SM Higgs at Tevatron, particularly around 120 GeV/c2 • For Comparison… • Limits similar or better for current results from hττ (2.0 fb-1) • Comparable to channels like ZHllbb at 140 and 150 GeV/c2
Summary and Comments • Summary • Searched for SM hγγ using 5.4 fb-1 of data and data-driven background model • Expected 95% C.L. limits on (σ Br) / SM are about 20 x SM at 120 GeV/c2 • Plans • Forward photons to soon be added (will double acceptance) • Use results to contribute to Higgs combination, particularly in 110–130 GeV/c2 region • Potential Improvements in Future • Add photons that convert to e+e– pairs • Improve photon ID efficiency
The Collider Detector at Fermilab (CDF) A multipurpose detector that observes: • electrons • photons • jets • muons • neutrinos
Method used to set limits The likelihood as a function of cross section: • Nid, Nib, and Nis are the number of data, bkg, and sig events in the ith bin • A is acceptance • is efficiency • is luminosity • Ntots is the total number of signal events passing selection requirements The 95% confidence limit was obtained by finding the value of σ95 for which: