The status of the Higgs search at the upgraded D0 detector is discussed.Comment: Proceedings submitted for PASCOS03, Mumbai, India, Jan 3-8, 200

Snyder, Scott S.

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arXiv

The D0 experiment is recording physics quality data. Both the detector and the accelerator performance are continually improving. We are studying issues such as the b{bar b} mass resolution, b-jet tagging efficiency, missing E{sub T} resolution, and backgrounds to Higgs processes. We look forward to seeing exciting results. The status of the Higgs search at the upgraded D0 detector is discussed

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Fermilab FERMILAB-Conf-03/058-E  April 2003Prospects for Higgs Search at DScott S. Snyder (for the D Collaboration)Brookhaven National Laboratory, Upton, NY, USAApril 2, 2003AbstractThe status of the Higgs search at the upgraded D detector is dis-cussed.1 IntroductionA major goal of experimental high-energy physics in the next decade isto characterize the Higgs sector of the Standard Model. The minimalversion of the Standard Model requires a single scalar particle, thoughmany extensions predict additional particles. Fits to the global set ofelectroweak data provide an indirect upper limit on the mass of a scalarHiggs boson of mH < 195GeV=c2, with a preference for low masses, withthe minimum at mH = 81GeV=c2 [1, 2]. Direct searches at LEP haveruled out a scalar Higgs boson mH < 114:4GeV=c2. These results providea strong motivation to search for Higgs bosons at the Tevatron. Thisarticle summarizes the state of the Higgs searches at D as of summer2002.2 The Run II D DetectorBoth the Tevatron collider and the collider experiments have been signif-icantly upgraded since the last run. The Tevatron was augmented with anew injector, the collision energy was raised from 1:8 to 1:96 TeV, and thebunch spacing decreased from 3:6 s to 396 ns. The goal for the currentcollider run is to accumulate 10{15 fb 1 per experiment. By the end of2002, the peak collider luminosity was over 31031 cm 2 s 1, beating therecord from Run I. As of that date, D had recorded about 50 pb 1 ofdata, at a peak rate of about 1 pb 1 per day. These rates are expected toincrease with further collider improvements.Since Run I, the central tracker of the D experiment has been com-pletely replaced. It now consists of a silicon vertex detector with four su-perlayers surrounded by a 16-layer scintillating ber tracker, using 835 mdiameter bers, the whole immersed in a 2T solenoidal magnetic eld.Preshower detectors have also been added in front of the calorimeters.The calorimeters themselves are the same as Run I, except that some of1the electronics has been replaced to handle the reduced bunch spacing.The muon system has also been signicantly upgraded to improve cover-age and to allow better triggering in high luminosity environments. Thetrigger and data acquisition systems were also replaced. As of the endof 2002, all detector components were operating well, except some triggersystems which remain be commissioned.3 Higgs Production at the TevatronA comprehensive study of the potential for discovering Higgs at the Teva-tron was carried out by the Higgs Working Group (HWG) [3] based ona parameterized detector simulation that assumed averaged properties ofthe two detectors. Unlike at the LHC, here there is no single channelin which a signicant result can be expected. Rather, we must combineresults from all decay modes, and also combine the data from the two ex-periments. A 5 discovery for mH = 120GeV=c2 would then be expectedto require about 15 fb 1 of data.The gg! H mode has the largest cross section (about 1 pb for mH =120GeV=c2). However, for a light Higgs mH < 140GeV=c2, the dominantdecay mode is H ! bb, rendering this mode hopeless against the largebackgrounds of bb production from other sources. In this mass range,one must look for Higgs bosons produced in association with a W or Zboson, the latter then decaying leptonically (reducing the available crosssection by about a factor of 30). For higher mass Higgs bosons, the processgg! H !WW may be feasible. Modes involving Higgs bosons producedwith a heavy quark pair (bb or tt) have spectacular nal states, but verysmall cross sections (< 10 fb). However, some extensions to the StandardModel predict enhancements to the Hbb and H !  modes.4 D StatusThe D experiment has repeated the analyses of the HWG, using the fullD detector simulation. The conclusions are roughly the same. Work isin progress on applying more sophisticated analysis techniques, such asneural networks, to the problem.For the H ! bb channels, identifying b jets is crucial, both by usingdisplaced vertices and by looking for soft leptons from b-quark decay.Monte Carlo (MC) simulations show that displaced vertex tagging canhave eÆciencies as high as 60% for high-transverse energy (ET ) jets, witha fake rate from light quarks on the order of a few percent. The taggingperformance depends on the track impact parameter resolution, whichhas been measured in current data to be 20 m, already close to MCexpectations. The work of determining the alignment of the trackingcomponents is still in progress, so this will improve.For the W (! e=)H(! bb) channels, one looks for a nal statecontaining exactly one high-pT lepton, large missing ET (E/T ), and twob jets. The major physics background to these channels is Wbb; othersignicant backgrounds include tt and WZ. An example of the results2for the channel is shown in gure 1. Here, we require ET (e) > 20GeV,E/T> 20GeV, two tagged b-jets with ET > 15GeV, and for the third jet(if any), ET (j3) < 25GeV. This last requirement rejects tt background.The resulting S=pB of 0.20 for the electron channel is comparable to theexpectations from the HWG, but should be improved signicantly withthe use of more sophisticated, multivariate analysis methods. (GeV)jjm40 60 80 100 120 140 160 180fb/6 Gev0123456 10)× (b bν e→HW backgroundb + WbttFigure 1: mbb distribution from theWH ! ebbMC analysis, using full detectorsimulation. Plotted are the contributions from the principal backgrounds (Wbband tt) and the signal expectation for mH = 120GeV=c2 (scaled up by a factorof 10).For Z(! ee=)H(! bb), one requires two oppositely charged isolatedleptons with a mass consistent with a Z boson and two b jets. A signif-icance comparable to the WH channels is achievable though the overallevent rate is lower. The bb channel is also powerful since the branchingfraction of a Z boson to neutrinos is three times larger than that for anysingle lepton avor. For this channel, one requires the b jets plus largeE/T.The H ! WW  ! `` channels are important for higher massHiggses, mH > 130GeV=c2, where the decay H !WW is signicant. Im-portant backgrounds include WW , tt, W=Z+jets, and QCD. In our pre-liminary analysis, we require two electrons with ET > 20GeV, m(ee) <78GeV=c2, E/T> 20GeV, and no jets with ET > 20GeV. In addition,since theW boson pair comes from the decay of a spin-0 particle, spin cor-relation variables are useful, like the azimuthal angle between the leptons((``)) (see gure 2).5 SummaryThe D experiment is recording physics quality data. Both the detectorand the accelerator performance are continually improving. We are study-ing issues such as the bb mass resolution, b-jet tagging eÆciency, missingET resolution, and backgrounds to Higgs processes. We look forwards toseeing exciting results!3eeΦ ∆0 0.5 1 1.5 2 2.5 3Φ ∆dN/d10-310-210-1110102DO Run 2 PreliminaryDataMulti-jetγZ/W+jetsWWγW+tteeΦ ∆0 0.5 1 1.5 2 2.5 3Φ ∆dN/d10-310-210-1110102DO Run 2 PreliminaryDataMulti-jetγZ/=160GeV) x 50HHiggs (MWWγW+W+jetsFigure 2: Results from the H ! WW  ! ee analysis, plotting the (ee)distribution for the data and the principal backgrounds. The left plot is afterthe electron ET cuts only, the right plot is after all section cuts. The right plotalso shows the signal expected for mH = 160GeV=c2 (scaled up by a factor of50 to make it more visible).References[1] The LEP Working Group for Higgs Boson Searches, LHWGNote/2002-01.[2] The LEP Electroweak Working Group,http://hepewwg.web.cern.ch/LEPEWWG/[3] M. Carena, J. S. Conway, H. E. Haber, J. D. Hobbs, et al., hep-ph/0010338 (2000).4

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