The CDF and D0 experiments at the Tevatron are currently the only capable of searching for the Standard Model Higgs boson. This article describes their most sensitive searches in the expected Higgs mass range, focusing on advanced methods used to extract the maximal sensitivity from the data. CDF presents newly updated results for H {yields} W{sup +}W{sup -} and Zh {yields} l{sup +}l{sup -}b{bar b}. D0 presents two new searches for WH {yields} lvb{bar b}. These new analyses use the same 1 fb{sup -1} dataset as previous searches, but with improved techniques resulting in markedly improved sensitivity

Kilminster, Ben

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Searches for the Standard Model Higgs at the TevatronB.Kilminster on behalf of the CDF and DØ CollaborationsDepartment of Physics, Ohio State University, 1040 Physics Research Building, Columbus, Ohio43210-1117The CDF and DØ experiments at the Tevatron are currently the only capable of searchingfor the Standard Model Higgs boson. This article describes their most sensitive searches inthe expected Higgs mass range, focusing on advanced methods used to extract the maximalsensitivity from the data. CDF presents newly updated results for H →W+W− and ZH →l+l−bb¯. DØ presents two new searches for WH → lνbb¯. These new analyses use the same1 fb−1 dataset as previous searches, but with improved techniques resulting in markedlyimproved sensitivity.1 IntroductionThe Higgs boson is the last remaining Standard Model particle to be discovered, and the oneresponsible for generating the W and Z gauge boson masses. Direct searches at the LEPexperiments have excluded a Higgs boson with mass less than 114.4 GeV/c2 at 95% ConfidenceLevel (CL) in the production mode e+e− → ZH 1. Experimental measurements of the top quarkand W boson masses provide the strongest indirect constraints on mH . Considering the newestCDF and DØ combined top mass measurement of mt = 170.9 ± 1.8 GeV/c2, and the newestCDF W mass measurement of mW = 80.398 ± 0.025 GeVc2, in addition to other precisionelectroweak other observables from LEP and SLD, the Higgs boson mass is expected to be lessthan 144 GeV/c2 at 95% CL 2.The Tevatron at Fermilab provides 1.96 TeV center-of-mass energy from proton-antiprotoncollisions in the two multi-purpose detectors, CDF and DØ . Gluon fusion is the highest cross-section process for producing a Higgs boson, but because of high backgrounds at lower masses,this production process is sensitive to Higgs mainly for mH > 135 GeV/c2, where BR(H →W+W−) starts at 68 % and increases up to 90% for mH = 160 GeV/c2. For H → W+W− inthis mass range, the most sensitive final state topology is two charged leptons with large missingFERMILAB-CONF-07-197-Etransvere energy ( 6ET). For 114 < mH < 135 GeV/c2, quark annhilation into an offshell W orZ boson, which then emits a Higgs boson, provides the best opportunity for discovery. At thismass range, the Higgs decays predominantly H → bb¯. CDF and DØ have previously done 1fb−1 searches for H →W+W− → l+νl−ν¯, WH → lνbb¯, ZH → l+l−bb¯, and ZH → νν¯bb¯, wherel = e, µ. Because of the small expected Higgs signals, to maximize search sensitivity over theallowed Higgs mass range, it is necessary to combine all searches from both the CDF and DØexperiments, as well as improve analysis techniques.This proceeding outlines updates to searches in several of these channels, focusing mainly onimprovements in analysis techniques. New CDF searches for ZH → l+l−bb¯ and H →W+W− →l+ν¯l−ν are presented for the first time, as well as two new DØ searches in the WH → lνbb¯channel.2 H →W+W− → l+νl−ν¯CDF presents a new search in the H → W+W− → l+νl−ν¯ channel. One improvement inthis analysis is the increasing of geometric lepton acceptance by defining new, less stringentlepton types for regions of the detector without complete instrumentation, such as leptons notidentifiable as electrons or muons since they are not fiducial to calorimeters or muon chambers.Such lepton types were successfully used in CDFs observaton of WZ production 3. The numberof Higgs signal events expected in the data for mH = 160 Gev/c, increases from 2.5 to 4.0 eventswith this new selection, as compared to the previous CDF analysis with the same dataset. Thenew analysis also improves upon the technique for extracting the signal from the data. Theprevious search had performed a likelihood fit for the Higgs signal using the distribution of ∆Φbetween the two leptons which is sensitive to the angular correlations from WW produced bya scalar Higgs boson. The newest CDF measurement is done by constructing matrix elementprobabilties using the observed lepton four-vectors and 6ET for the processes H → WW , WW ,ZZ, W + γ, and W+parton. A likelihood ratio (LR) is formed for each event by dividing thesignal probability by the sum of the signal and background probabilities. LRs are constructedfor different processes specified as signal in order to validate background modeling. Figure 1shows the H → WW LR distribution for mH = 160 GeV/c2, which is used to search for anexcess consistent with Higgs signal for a range of masses. No significant excess is measured,and limits are set such that the observed (expected) upper limit is 5 (3.5) times larger than theStandard Model expected cross-section for the most sensitive Higgs mass of mH = 160 GeV/c2 4.3 ZH → l+l−bb¯CDF also presents a new result in the ZH → l+l−bb¯ channel. Previous results first presentedat ICHEP 2006 5 demonstrated the use of a two-dimensional neural network trained to separateZH → l+l−bb¯ from the dominant background of Z+ >= 2 jets production and tt¯→ WbWb→lνblνb. The new result uses the same dataset but makes several improvements which result inimproved sensitivity. One technique is in the identification of b hadrons from H → bb¯ usingsecondary vertex finders or “b-tagging” algorthms. The previous ZH analysis selected eventswith >= 1 “b-tag”, using tight requirements for the secondary vertex significance (40% efficient).Since S:B is 200:1 for events with one b-tag (40% efficient), but 50:1 for events with two looserb-tags (each 50% efficient), there is an improvement in Higgs sensitivity by fitting these classesof events separately. Another new technique is to improve the resolution of the H → bb¯ dijetmass distribution, which is one of the most important Neural Network inputs. Since the maincause of 6ET in ZH → l+l−bb¯ events is from jet energy mismeasurement, a correction is appliedwhich corrects the leading two jets independently according to their projection onto the 6ETdirection. The effect is to reduce the dijet mass resolution from 14% to 9% for events with twoWW, high S/B)→LR (H0 0.2 0.4 0.6 0.8 1Events / 0.04 020406080100 tt γWW+jetsDYdata(160)H m×10WWWZZZCDF Run II Preliminary -1L dt = 1.1 fb∫WW, high S/B)→LR (H0.4 0.5 0.6 0.7 0.8 0.9 1Events / 0.04 024681012)2 (GeV/cjjM50 100 150 200 25002468101214161820 -1Ldt = 0.97 - 1.02 fb∫CDF II Preliminary Data (Single Tag)Backgrounds w/ Alpgen)2 = 120 GeV/cH llbb X 50 (M→ZH  llbb X 50 (before MPDF)→ZH Figure 1: New CDF analyses. Left plot shows the H →WW discriminant, comparing data with Standard Modeland with a 10 times expected H →WW signal. Right, dijet mass improvement in the ZH analysis shown beforeand after the Met Projection Dijet Fitter (MPDF) is used to improve the resolution of the dijet mass distribution,which is then input into the 2D Neural Network discriminant used to fit the ZH signal.b-tags. With these two enhancements, the analysis improves its Higgs search sensitivity by afactor of two in terms of an effective luminosity increase as compared to the previous versionof the analysis with the same dataset. For mH = 115 GeV/c2, the observed (expected) upperlimits for σZH are 16 times that of the Standard Model6.4 WH → lνbb¯DØ presents two new searches in the WH → lνbb¯ channels. The first analysis improves overprevious analyses by using multiple muon triggers in order to retain 100% muon acceptancefor WH → µνbb¯. This results in 50% more signal than previous techniques. The b-taggingselection is optimized to separate events into one tight b-tag and two loose b-tags as is describedin Section 3. But in addition, making use of a neural network b-tagging algorithm which usesvariables in addition to the secondary vertex displacement to identify b-quarks, the b-taggingefficiency is increased to 50% for tight b-tags and 70% for loose b-tags, with misidentificationrates of 0.5% and 4.5%, respectively. The discriminant used to search for the Higgs signal is thedijet invariant mass (Figure 2), and the combined limit from both single and double b-taggedevents is expected (observed) to be less than 9 (10) times the Standard Model expectation 7.The second analysis makes use of a matrix element technique similar to the one describedin Section 2. This matrix element technique was developed originally in the context of the DØsingle-top analyses which established 3-σ evidence for single-top production 8. By fitting thematrix element discriminant for WH (Figure 2), DØ obtains expected (observed) limits of 9(13) times the Standard Model expectation 9. However, this analysis does not make use of theimproved muon acceptance and optimized b-tagging used in the first analysis. Incorporatingthese improvements into the matrix element approach is expected to yield 30% better expectedsensitivity.5 ConclusionsCDF and DØ are improving analysis techniques in order to make gains in Higgs sensitivitywhich scale much faster than increasing statistics alone. New results in H → WW , ZH, andWH rely on improved lepton acceptance and triggers, higher efficiency b-taggers, and dijet mass Dijet Mass (GeV)0 50 100 150 200 250 300Events020406080EventsDØ Preliminary-1L = 1.0 fb W + 2 jets /  2 b-tagsData W + jets QCD  tt bWbotherWH115 GeV (x10)EventsWH ME Discriminant0.8 0.85 0.9 0.95 101234567DataWH (m=115) x10tb+tqbttDibosonW+bb/ccW+jjMultijet-1DØ Run II Preliminary, L=0.9 fbFigure 2: New DØ analyses. Left, dijet mass distribution for WH dijet mass fit analysis, and right, discriminantfor WH matrix element analysis.resolution improvements. The newWH and H →WW results are better in their most sensitivemass ranges than the combined results of all CDF and DØ channels presented a year ago.AcknowledgmentsWe thank the Fermilab staff and the technical staffs of the participating institutions for theirvital contributions. This work was supported by the U.S. Department of Energy and NationalScience Foundation; the Italian Istituto Nazionale di Fisica Nucleare; the Ministry of Educa-tion, Culture, Sports, Science and Technology of Japan; the Natural Sciences and EngineeringResearch Council of Canada; the National Science Council of the Republic of China; the SwissNational Science Foundation; the A.P. Sloan Foundation; the Bundesministerium fu¨r Bildungund Forschung, Germany; the Korean Science and Engineering Foundation and the KoreanResearch Foundation; the Particle Physics and Astronomy Research Council and the Royal So-ciety, UK; the Institut National de Physique Nucleaire et Physique des Particules/CNRS; theRussian Foundation for Basic Research; the Comisio´n Interministerial de Ciencia y Tecnolog´ıa,Spain; the European Community’s Human Potential Programme under contract HPRN-CT-2002-00292; and the Academy of Finland.References1. R. Barate et al. [LEP Working Group for Higgs boson searches], Phys. Lett. B 565, 61(2003) [arXiv:hep-ex/0306033].2. LEP Electronweak Working Group, http://lepewwg.web.cern.ch/LEPEWWG, March2007 results.3. A. Abulencia et al. [CDF Collaboration], arXiv:hep-ex/0702027.4. CDF Collaboration. CDF public note 8774, 2007.5. B.Kilminster [CDF and DØ collaborations], ICHEP 2006 proceedings, hep-ex arXiv:hep-ex/0611001.6. CDF Collaboration. CDF public note 8742, 2007.7. D0 Collaboration. D0 public note 5350, 2007.8. V. M. Abazov et al. [D0 Collaboration], Phys. Rev. Lett. 98, 181802 (2007) [arXiv:hep-ex/0612052].9. D0 Collaboration. D0 public note 5365, 2007.

Searches for the standard model Higgs at the Tevatron

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Searches for the standard model Higgs at the Tevatron

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