Suyong Choi (SKKU) SUSY Standard Model Higgs Searches at DØ Suyong Choi SKKU, Korea for DØ Collaboration

Suyong Choi (SKKU) SUSY 2008 Suyong Choi (SKKU) PASCOS 2006, Columbus, Ohio 2 Higgs in the Standard Model 1.Mass of elementary particles – coupling to massive particles stronger 2.Electro-weak symmetry breaking Higgs in the Standard Model 1.Mass of elementary particles – coupling to massive particles stronger 2.Electro-weak symmetry breaking Particles of the Standard Model

Suyong Choi (SKKU) SUSY Limits on M H Current limits –Direct searches at LEP M H > % CL –Fits to electroweak data M H < % CL –M H <190 GeV if direct search result included  Light Higgs favored Tevatron –Direct Searches : rule out or find evidence –Precision m t and M W measurements M H = GeV

Suyong Choi (SKKU) SUSY SM Higgs Production at the Tevatron Though Higgs production copious, not all channels are accessible gg→H –Useful for M H >140 GeV –H→WW→ll –Background: WW qq  W/Z+H –M H <140 GeV –WH→l bb –ZH→llbb, bb –Background: W+bb, Z+bb, top Sensitivity studies have shown that all channels must be studied, CDF + D0 combination is essential pb

Suyong Choi (SKKU) SUSY 2008 Tevatron Collider Status Excellent performance –Steady increase in instantaneous luminosity –>85% data collecting efficiency 5 Results presented today are based on 1.0 ~ 2.3 fb -1 of data 4.3 fb fb -1

Suyong Choi (SKKU) SUSY 2008 W Associated Production WH → bb Sensitive for M H <140 GeV –Large  x Br 6 b jets Two b-jets form a resonance

Suyong Choi (SKKU) SUSY WH Search Single and double b-tagged jet samples are analyzed separately and optimized 193 observed 204  31 expected 2.3 WH expected Expected signal x 10

Suyong Choi (SKKU) SUSY 2008 Neural Network Selection Variables for ANN –p T of two jets –Opening angle of jets –Dijet system p T and mass –p T (lepton +  ) Observation in agreement with background only hypothesis

Suyong Choi (SKKU) SUSY 2008 WH Search Results No excess observed  Set limits on cross section x Br Limits obtained by fitting the NN output –ST and DT treated as independent channels Systematics 9 Expected limits SourcesError Luminosity6% Trigger3~10% Jet energy scale2~6% B-tagging4~6% Background x- section 16~20%

Suyong Choi (SKKU) SUSY 2008 WH Search Results

Suyong Choi (SKKU) SUSY

Suyong Choi (SKKU) SUSY Z Associated Production ZH → + - bb Clean Small cross section x Br –M H <140 GeV b jets

Suyong Choi (SKKU) SUSY 2008 Neural Network Analysis No significant excess Set limits on  x Br –NN output distributions –Systematic errors and correlations considered Systematics –Background error : 28% –Signal eff. error : 8% SourcesError Jet energy scale1~7% B-tagging7%

Suyong Choi (SKKU) SUSY 2008 Higgs Limits from ZH → + - bb Analysis

Suyong Choi (SKKU) SUSY ZH → bb Advantage of large branching fraction of Z→ –MET + 2 jets – cannot reconstruct Z explicitly –Large multijet background –Recovers leptonic decays of WH and ZH, where leptons were not reconstructed explicitly

Suyong Choi (SKKU) SUSY 2008 Missing E T + jets Data: 2.1 fb -1 MET > 50 GeV Jets –2 or 3 jets, p T >20 GeV –2 leading jets should not be back-to-back W+jets and multijets dominant –Multijet background due to mismeasured jet E T signal x 500

Suyong Choi (SKKU) SUSY 2008 Multivariate Analysis Boosted decision tree result using 26 variables after b-tagging

Suyong Choi (SKKU) SUSY 2008 ZH → bb Search Result Systematic uncertainties Limits –best limit in W/Z+H SourcesError Luminosity6.1% Trigger5.5% Jet energy scale1~7% B-tagging6% Background x- section 6~16% Heavy flavor fractions 50%

Suyong Choi (SKKU) SUSY H → WW → + - H → WW → + - Important for m H >140 GeV –Final state: 2 leptons + MET –Cannot reconstruct M H Data: 2.3 fb -1 –1.1 (IIa data) + 1.2(IIb data) –ee, e ,  Selection –2 oppositely charged leptons –Large MET –Di-lepton mass –min( M T (e), M T (  ) ) –  –H T  reduce Z, W+jets, tt-bar Analysis optimized for each M H Signal 1.1fb -1

Suyong Choi (SKKU) SUSY 2008 NN Analysis PreselectionAfter final selection

Suyong Choi (SKKU) SUSY H → WW Factor 2.4 away from SM For M H =160 GeV Systematic uncertainty Combine distributions from different channels –Statistical uncertainty + Correlated systematics SourcesError Trigger5% Jet energy scale5~10% Muon momentum resolution (  ) 11% Muon ID efficiency-5% +8% WW x-section4%

Suyong Choi (SKKU) SUSY 2008 Other SM Higgs Searches WH  WWW –3 lepton final state –Recovers sensitivity M H ~ 140 GeV H   –Not a discovery channel at the Tevatron –Analysis with less model dependence 22

Suyong Choi (SKKU) SUSY 2008 Combined DØ SM Higgs Results Correlations of systematic errors taken into account

Suyong Choi (SKKU) SUSY 2008 Combined CDF and DØ Results 24 Factor 1.1 away!

Suyong Choi (SKKU) SUSY 2008 Prospects for M H <140 GeV We achieved 1.7 factor improvement in sensitivity since 2005 –not including gains due to lumi We expect additional x2 gain in sensitivity –Optimized b-tagging with inner silicon Layer 0 –semileptonic b-tags –dijet mass resolution –lepton efficiencies –refined multivariate analyses 25

Suyong Choi (SKKU) SUSY 2008 Prospects for M H >140 GeV We achieved 1.7 factor improvement in sensitivity since 2005 (not including gains due to lumi) We expect additional x1.4 gain in sensitivity –lepton efficiencies –multivariate analyses 26

Suyong Choi (SKKU) SUSY 2008 Expected Higgs Sensitivity in 2009/2010 Assuming 2 experiments

Suyong Choi (SKKU) SUSY 2008 Summary We searched for Standard Model Higgs boson in all the sensitive channels using the D Ø data Results in agreement with expected backgrounds In 2008, we may be able to exclude new M H range beyond that of LEP –CDF+D0 results combined Many improvements expected to raise sensitivity in a broad range of M H - most exciting years to come!

Suyong Choi (SKKU) SUSY Standard Model Higgs Searches WH ZH H→WW

Suyong Choi (SKKU) SUSY The DØ Detector Tracking –Precision silicon vertex detector –Scintillating fiber tracker –2T B-field Calorimetry –Liquid Argon-Uranium –|  |<4 –Excellent linearity and resolution Muon detector –Low punch throughs –1.8T toroidal B-field Trying to exploiting full capabilities

Suyong Choi (SKKU) SUSY Search for WH → bb Data: 1.7 fb -1 with e and  Event preselection –lepton: p T >15 GeV –Missing E T :E T >20 GeV –2 Jets: p T >20 GeV |  |<2.5 Veto on –Additional high p T track –4 th jet Background is well understood

Suyong Choi (SKKU) SUSY  s=1.96 TeV Circumference 6 km The only place to directly look for Higgs and supersymmetric particles until LHC

Suyong Choi (SKKU) SUSY b-jet Tagging D0 developed sophisticated Neural Network based algorithm –Lifetime of a b hadron is quite long (a few mm) –Superb efficiency –Samples and performance derived from data –Fakes are due to finite resolutions of the tracking detector Detector view –Decay length –Impact parameter –Measurement errors Primary vertex Secondary vertices

Suyong Choi (SKKU) SUSY 2008 Neural Network Analysis After final selection Neural network used to maximize sensitivity –p T of leptons –m –  –MET –angles between MET and leptons –minimum transverse mass 34

Suyong Choi (SKKU) SUSY Search for H → WW → + - Search for H → WW → + - Important channel for m H >140 GeV Final state: 2 leptons + MET –Explicit mass cannot be reconstructed –Signal / background separation by exploiting event topology differences WW decays from a spin 0 particle  leptons prefer to decay in the same direction

Suyong Choi (SKKU) SUSY 2008 Combined SM Higgs Results Use as inputs the discriminant outputs from each analysis

Suyong Choi (SKKU) SUSY 2008 Kinematic Distributions of b- jets

Suyong Choi (SKKU) SUSY 2008 b-tagging in ZH → + - bb Use both single and double-tagged events –S/B are different Single tag –  b =45%,  j =0.5% Electron Channel Muon Channel Observed7387 Expected 57   38 Higgs ( m H =115GeV)

Suyong Choi (SKKU) SUSY 2008 b-tagging in ZH → + - bb Double tag –  b =72%,  j =6% per jet Electron Channel Muon Channel Observed2453 Expected28  8 46  13 Z + jets (udsgc) 1019 Z + bb Tt-bar Higgs (m H =115GeV)

Suyong Choi (SKKU) SUSY 2008 Neural Network Analysis 9 kinematic variables used 10k signal events and 100k background events NN architecture optimized to yield best significance

Suyong Choi (SKKU) SUSY 2008 MET + b-jets Asymmetric b-tagging requirements on the two hadronic jets –maximizes sensitivity events439 events

Suyong Choi (SKKU) SUSY Standard Model Higgs Decays Higgs prefers to decay to massive particle kinematically allowed bb for M H < 140 GeV WW for M H > 140 GeV

Suyong Choi (SKKU) SUSY 2008 ZH → + - bb Selection Data: 1.1 fb -1 Dileptons –p T >15 (10) GeV for e (  ) –In well-instrumented region of the detector and isolated –70 GeV < M < 110 GeV Jets –p T >15 GeV and |  |<2.5 –b-tagging After dilepton+jets selection before b-tag