Searches for Higgs Bosons at CDF

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Searches for Higgs Bosons at CDF. Thomas Wright University of Michigan SLAC Experimental Seminar February 13, 2007. Run 2 at the Tevatron. Worlds highest-energy collider Record luminosity 278E30 cm -2 s -1 Will have 2 fb -1 on tape in ~1 week! Integrating 40-45 pb -1 /week. - PowerPoint PPT Presentation


<ul><li><p>Searches for Higgs Bosons at CDFThomas WrightUniversity of Michigan</p><p>SLAC Experimental SeminarFebruary 13, 2007</p></li><li><p>Run 2 at the TevatronWorlds highest-energy colliderRecord luminosity 278E30 cm-2s-1Will have 2 fb-1 on tape in ~1 week!Integrating 40-45 pb-1/weekResults shown here use data samples of ~1 fb-1Look for updates on expanded samples this summer</p></li><li><p>The CDF II Detector</p></li><li>The CDF II DetectorAzimuthally symmetric barrel geometryCentral detectors cover ||</li><li><p>Particle IdentificationCharged leptons identified by characteristic energy deposition patternsPresence of neutrinos is inferred from energy imbalance missing energyBecause net pz of the scattering partons is not known, mostly work in the transverse plane (i.e pT, ET, missing-ET)</p><p>B-jet identification uses the silicon tracker8 layers, 704 ladders, 722432 channelsTotal sensor area = 6 m2SVX II 5 double-sided layers (r + rz)L00 r only mounted directly to beampipe (R = 1.4 cm)</p></li><li><p>The CDF II Trigger SystemInteraction rate very high, but most not interestingLimited bandwidth to mass storage must be choosyLevel1 systemSynchronous no deadtimeSingle CAL towers (photons and jets), COT tracks (with pair correlations), track-tower matches (electrons and taus), muons, missing energyLevel2 systemAsynchronous - ~5% deadtimeAll Level1 objects, plus CAL clusters (jets) and silicon trackingLevel2 accept triggers full detector readout (few % deadtime)Level3 runs a version of the offline reconstruction final rate reduction before writing to tapeAlways tuning the system to accommodate higher luminosity2.5 MHz crossing rate (396 ns)Output 20-30 kHzSynchronousLatency 25-30 sOutput ~700 HzOutput 70-90 HzReadout latency ~650 s</p></li><li><p>The Higgs Boson of the Standard Model Electroweak symmetry can be broken using the Higgs mechanism4 new scalar fieldsThree give mass to the Ws and ZOther manifested as a single scalar the Higgs boson</p><p>If there is such a particle, precision electroweak measurements favor a low massLEP2 searches exclude mH &lt; 114.4 GeV/c2 @ 95% CLSM fit requires mH &lt; 166 GeV/c2 @ 95% CL (199 if including LEP2 direct searches) as of last yearNew CDF W mass (most precise single measurement) moves best fit to 80+36-26 GeV/c2 (was 85+39-28)</p></li><li><p>Higgs Production and DecayIdeally, use gg H bb, WWBut, QCD bb background too highFor low mH, use WH+ZH, H bb (associated production)At high mH the WW decay mode opens up can use gg H production(pb-1)</p></li><li><p>The H WW* l l ChannelLargest BR for mH &gt; 135 GeV/c2</p><p>Uses gg H productionNow including VBF (6-10% extra cross section)</p><p>Event selectionTwo isolated leptons with pT &gt; 20 and 10 GeV/cOpposite chargeMissing-ET &gt; mH/4If missing-ET aligned with lepton, &gt; 50 GeVmll &gt; (mH/2)-5 GeV/c2pT,1+pT,2+missing-ET &lt; mHJet veto</p><p>Including W BRs, acceptance is 0.3-0.7% depending on mHW l BR not included</p></li><li><p>H WW* BackgroundsPredominantly WWAlso Drell-Yan and other diboson channels, and from fake leptons</p><p>Not possible to reconstruct Higgs mass due to multiple neutrinosCan exploit scalar nature of HiggsLeptons from H WW* are closer in </p><p>Treat each bin of as a separate counting experiment</p></li><li><p>H WW* Cross Section LimitsSensitivity about 6x SM level at 160 GeVFor 6 fb-1 and combined with D, within factor 2 of SM Get more events WZ search almost doubled acceptance by adding new lepton typesGet more out of events More than just Df Multivariate (NN) Matrix elementsexcluded at 95% C.L.</p></li><li><p>Higgs Production and DecayIdeally, use gg H bb, WWBut, QCD bb background too highFor low mH, use WH+ZH, H bb (associated production)At high mH the WW decay mode opens up can use gg H production(pb-1)</p></li><li><p>B-Jet Identification (b-tagging)B-hadrons are long-lived search for displaced verticesConstruct event-by-event primary within beamspot (10-32 m)Fit displaced tracks and cut on Lxy significance ( ~ 200 m)Calibrate performance from data (low-pT lepton samples)Tag this jetb-fraction ~80%measure tag efficiency in data and MC</p></li><li><p>Fake B-Tags (mistags)Fake tags are (almost) symmetric in displacement LxyRate of tags with Lxy 0Lxy &lt; 0</p></li><li><p>The WH lbb ChannelEvent selectionIsolated e or with pT&gt;20 GeV/cMissing-ET &gt; 20 GeVExactly two jets with ET&gt;15 GeVAt least one b-tagged jet</p><p>Acceptance is 1.8-2.1%</p><p>Backgrounds includeNon-W events (fake lepton, fake missing-ET, b decays)W + mistagged jetsW + heavy flavor jetsDiboson production (WW, WZ, ZZ)Z Top quark production (including single top)This channel uses a neural net filter on the b-tags to reject half of the background (~10% signal loss)</p></li><li><p>WH BackgroundsUse W+1-jet bin to test W+HF bkgdTop pair cross sectionMeasured from theW+3,4-jets eventsSM Higgs would be about two events</p></li><li><p>WH Dijet MassAt least one jet b-tagged with NNBoth jets b-tagged</p></li><li><p>WH Cross Section Limitsexcluded at 95% C.L.</p></li><li><p>The ZH bb ChannelDistinctive final state of b-jets recoiling against missing-ET</p><p>Event selectionMissing-ET &gt; 75 GeVLepton vetoExactly two jets with ET &gt; 60 and 20 GeVMissing-ET not aligned with either jetAcceptance is 0.5-0.8% (for ZH)About half the events would be WH with lost lepton</p><p>Backgrounds includeQCD with fake missing-ETQCD bb productionW/Z + jetsTop productionDiboson production</p></li><li><p>ZH Dijet Mass</p></li><li><p>ZH Cross Section Limits</p></li><li><p>The ZH llbb ChannelLook for ZH also in Z decays to charged leptons (e or m)Lose in BR, gain in background Z+jets and top pairsInstead of dijet mass, enhance S/B using neural networks</p></li><li><p>ZH Cross Section Limits</p></li><li><p>Combined Limits (Relative to SM)Not including new WW*Factor of ~8 away from SM prediction at 115 GeVExpect factor 2-3 from more luminosity Another factor of 2 from combination with DAs with WW*, need to improve the analyses in order to reach SM sensitivity</p></li><li><p>Multivariate B-TaggerMultivariate b-tag algorithm in the pipelineMuch like the one used by SLD</p><p>Plots are for simulation only, performance characterization on data is in progressUp to 30% higher b-tag efficiency compared to the NN tagger already used in the WH search channelMore double-tagged events Better S/B Better dijet mass resolution</p></li><li><p>Double-Tagging in WHSimilar limits for exclusive 1-tag and 2-tag samples20% improvement over inclusive 1-tag result</p></li><li><p>The W/Z+H qqbb ChannelWas competitive with the leptonic W channel in Run I (but a little lucky)</p><p>70% of W/Z decays are into hadrons</p><p>Base selection is four jets with two b-taggedSearch in tagged dijet massCan use the four-jet trigger designed for all-hadronic top pairs also working on a new dedicated trigger</p><p>Background dominated by QCD multijet production (with real tags)Data-driven estimates in progress</p></li><li><p>Higgs in the MSSMMinimum of five scalars: h, H, A, H+, H-Separate couplings for up-type and down-type fermions</p><p>Properties of the Higgs sector largely determined by mZ and by two other parameters:mA : mass of pseudoscalartanb : ratio of down-type to up-type couplings</p><p>If tanb is small, then h looks a lot like a standard model Higgs</p><p>If tanb is largeProduction via b quarks can be greatly enhanced (factor ~tan2)Decays to bb (~90%) and (~10%) dominate</p><p>LEP-II searches have excluded mA</p></li><li><p>MSSM Higgs MassesAt high tanb, the A becomes nearly degenerate with h or HCan search for single resonance, double the cross sectionThe other neutral Higgs is like SM Higgs (no production enhancement)</p></li><li><p>Scenario DependenceHiggs properties are largely, but not completely, determined by mA and tanbLoop corrections introduce dependence on other SUSY parametersM. Carena et al., Eur.Phys.J. C45 (2006) 797-814 (hep-ph/0511023) Db is a function of the other SUSY parameters and depends on the benchmark scenario Db ~ mtanb (sign of m critical)</p><p>For tanb = 50, m = -200mhmax: Db = -0.21no-mixing: Db = -0.11</p><p>Need both channels to get the full story</p></li><li><p>The gg+bb ChannelHigh cross section and unique final state (not QCD)</p><p>Best signature is one decay into e or and the other hadronically (46% BR)Now also e+m channel (+6%)</p><p>Event selectionOne e or with pT &gt; 10 GeV/cOne hadronic with pT &gt; 15 GeV/c, mass &lt; 1.8 GeV/c2 (including p0s)Or, e+m with pT &gt; 6 GeV/cOpposite chargeMissing-ET not recoiling against leptons (rejects W l)Acceptance is 1.2-2.1%</p><p>Backgrounds includeZ W l +jet fake hadQCD multijet (both fake)</p></li><li><p>Higgs DiscriminantBecause of multiple neutrinos cannot reconstruct tt invariant massIn cases where taus are not back-to-back can use missing-ET projectionsLow efficiencyInstead, use visible massMass of the visible parts of the two taus and the missing-ET</p></li><li><p>Sample FitSignal is normalized to 95% CL exclusion limit</p></li><li>MSSM Di-Tau LimitsBackground-only pseudoexperiments indicate </li><li><p>MSSM Interpretation</p></li><li><p>The gg bb bbbb Channelbb final state not unique enoughRequire one of the additional bs to have high pT and b-tag itResults are for a different cross section than the tt case (factor ~4)</p><p>Basic event selection is three b-tagged jetsSearch in dijet mass of two leading jets</p><p>Background expected to be ~100% QCD multijet productionAlmost all bbq, bbc, bbbData-driven estimates exist</p><p>At high tanb the Higgs can develop significant width fit templates are function of cross section</p><p>We expect CDF result on 1 fb-1 out within a few weeks</p><p>*editorial comment belongs to me, not DDawson, Jackson, Reina, Wackeroth hep-ph/0603112?*</p></li><li><p>SM Higgs at the LHCDiscovery prospects are excellent</p><p>Strategies for low-mass region are different more focused on backgrounds than cross section</p><p>Experience gained at the Tevatron will be very usefultt event reconstructiondijet mass reconstructionW/Z + jets background estimation techniquesb-tagging and ID at hadron colliders</p><p>Starting from a bump at the Tevatron gets us there that much faster!</p></li><li><p>Tevatron Prospects</p></li><li><p>SummaryCDF is searching for the Higgs in a variety of production and decay scenariosTools are in place to combine results from different channelsLots of effort going into adding new channels and improving the existing ones</p><p>MSSM Higgs searches looking quite interesting</p></li><li><p>Backup Material</p></li><li><p>The Standard ModelMatter is made out of fermions: quarks and leptons3 generationsForces are carried by Bosons:Electroweak: ,W,ZStrong: gluonsHiggs boson:Gives mass to particles</p><p>Much is still unknownIs the EW symmetry really broken by a Higgs? What kind of Higgs(es)?Are there any other particles? New gauge bosons (Z, W)? Extra generations of quarks? Extra dimensions? Superpartners?</p><p>Lets have a look!H</p></li><li><p>What about Neutrinos?Not nearly enough material to stop them in the detectorInstead, infer their presence by energy imbalancei.e. add up everything you see, then reverse itCommonly called missing-ET</p><p>Only get net neutrino momentum if &gt;1Only works in transverse plane</p></li><li><p>PDFs</p></li><li><p>Non-W Background to WH ChannelUse missing-ET and isolation ratio (assumed uncorrelated) in sidebands to extrapolate into signal regionIsolation ratio = (lepton pT)/(non-lepton energy in cone with - radius 0.4 around the lepton)</p></li><li><p>B-Tag Efficiency MeasurementLarge b-hadron mass gives a wide pT,rel distribution relative to non-b contributionsFit untagged and tagged jets with b and one of four non-b templates to get b-tag efficiencySpread of results using the four non-b used as a systematic error</p></li><li><p>Dijet Mass ResolutionRaw: what we use nowH1: track + CAL energy flowMTL: correct for soft leptonsHyperball: multivariate nearest-neighbor algorithm, pick the most likely true dijet mass </p></li><li><p>W + jets SimulationLots of activity in recent yearsWe use the ALPGEN generatorTree-level W + N partonsAlso W+c+Np, W+cc+Np, W+bb+NpHERWIG parton shower adds soft gluon radiationMonte Carlo prediction normalized to observed number of W+jetsFraction of events containing heavy quarks calibrated from datab-tag rates in data and ALPGEN multijet samplesScale ALPGEN prediction by 1.5 0.4</p></li><li><p>Untagged Control Sample</p></li><li><p>ZH bb BackgroundsQCD bb background normalization fixed in Control Region 1 extrapolate into othersOther backgrounds checked in Control Region 2Now search in the signal region</p></li><li><p>The CDF II Detector</p></li><li>Electrons and PhotonsElectron ID :Covers |h|</li><li>MuonsMuon ID :Coverage : |h|</li><li>TausTau ID :Narrow iso. clusterLow # tracks p0 identificationCoverage : |h|</li><li>Jets and b-TaggingJet ID :Cluster of CAL towersCoverage : |h|</li></ul>


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