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DØ Search for the Higgs Boson in Multijet Events Alex Melnitchouk for the DØ Collaboration DepartmentofPhysicsandAstronomy, 6 TheUniversityofMississippi, 0 108LewisHall,P.O.Box1848 0 University,MS38677-1848,USA. 2 n Abstract. WepresenttwosearchesfortheHiggsbosonin√s=1.96TeVpp¯ collisionsusingdata a taken with the DØ detector during Run II of the Fermilab Tevatron collider. The first study is a J searchforneutralHiggsbosonsproducedinassociationwithbottomquarks.Thecrosssectionsfor 4 theseprocessesareenhancedinmanyextensionsofthestandardmodel(SM),suchasinitsminimal 2 supersymmetricextensionat large tanb . The results of our analysisagree with expectationsfrom theSM,andweuseourmeasurementstosetupperlimitsontheproductionofneutralHiggsbosons 1 v inthemassrangeof90to150GeV. 1 ThesecondstudyisasearchforthestandardmodelHiggsbosonproducedinassociationwiththe 4 Zboson.Westudythepp¯ ZH n n¯bb¯ channel,whichisoneofthemostsensitivewaystosearch → → 0 forlightHiggsbosonsattheTevatron.Weselectmultijeteventswithlargeimbalanceintransverse 1 momentum and two b-tagged jets. Then we search for a peak in invariant mass distribution of 0 two b-tagged jets. After subtracting the backgrounds, we set the 95% C.L. upper limits on the 6 s (pp¯ ZH n n¯) BR(H bb¯)forHiggsmassesbetween105and135GeV. 0 → → × → / Keywords: Higgsboson,StandardModel,Supersymmetry x PACS: 14.80.Bn,14.80.Cp e - p e INTRODUCTION h : v The Higgs boson, which is hypothesized to be responsible for electroweak symmetry i X breaking,istheonlyparticleintheSMthathasnotbeendirectlyobserved.Higgsboson r alsoappearsintheextensionsoftheSM,suchassupersymmetry(SUSY).Intheminimal a supersymmetric extension of the SM (MSSM) [1], there are two Higgs fields and five physicalHiggsbosons,threeofwhichareneutral.Tevatronexperimentscanbesensitive tobothSM HiggsbosonandMSSM neutral Higgsbosons. In both searches presented in this paper [2, 3], final states contain b-jets. Higgs signal would appear as a "bump" in the di-b-jet invariant distribution. Therefore, good understandingofthecalorimeterresponseandb-taggingarethemainingredientsofboth analyses. STANDARD MODEL HIGGS SEARCH IN MULTIJET EVENTS We use data collected by the DØ detector [4] between March 2003 and June 2004, corresponding to an integrated luminosity of about 261 pb 1. We select events with − two acoplanar b-jets with transverse energy above 20 GeV. We require that missing transverseenergy in the event is above25 GeV. In addition, we use kinematicvariables basedon vectorsumoftransversemomentaofjetsand tracks. We distinguish between two types of background, such as physics background and instrumental background. Physics backgrounds are mainly due to misidentified b-jets in Z+jets and W+jets processes with final state neutrinos that escape the detector. Instrumental backgrounds are multijet events in which energy of jets is mismeasured. Physics backgrounds are estimated using using PYTHIA, COMPHEP, and ALPGEN Monte Carlo event generators. Monte Carlo samples were processed through the DØ detector simulation and reconstruction software. Instrumental background is estimated fromdata. No excess of events is observed. We derive upper limits on s (pp¯ ZH) BR(H bb¯) for four Higgs mass points in the range between 105 and 135 G→eV. The×results a→re plottedin Figure 1. SUPERSYMMETRIC HIGGS SEARCH IN MULTIJET EVENTS Werequirethreejetsabove15GeV atthehighesttriggerlevel.Intheofflinewerequire at least three b-tagged jets. Both the p thresholds and pseudorapidity range selections T are optimized for Higgs mass and number of required jets. Signal rates and kinematics arenormalizedto NLOcalculations. MajorsourceofbackgroundisSMmultijetproduction.Therearetwomaincategories ofmultijetbackground.Onecontainsgenuineheavy-flavorjets,whiletheotherhasonly light-quark or gluon jets that are mistakenly tagged as b-quark jets, or correspond to gluonsthat branch intonearly collinearbb¯ pairs. Background shape is determined from double b-tagged data by applying the tag rate function to non-b-tagged jets. Then the backgroundisnormalizedto thedataoutsidethesignalregion. No excess of events is observed and upper limits on neutral Higgs production cross- section are derived. The limit is shown as a function of Higgs mass and tanb in Figure 2. ACKNOWLEDGMENTS WethankthestaffsatFermilabandcollaboratinginstitutions,andacknowledgesupport from the DOE and NSF (USA); CEA and CNRS/IN2P3 (France); FASI, Rosatom and RFBR (Russia); CAPES, CNPq, FAPERJ, FAPESP and FUNDUNESP (Brazil); DAE andDST(India);Colciencias(Colombia);CONACyT(Mexico);KRFandKOSEF(Ko- rea); CONICET and UBACyT (Argentina); FOM (The Netherlands); PPARC (United Kingdom); MSMT (Czech Republic); CRC Program, CFI, NSERC and WestGrid Project (Canada); BMBF and DFG (Germany); SFI (Ireland); Research Corporation, AlexandervonHumboldtFoundation,and theMarieCurieProgram. REFERENCES 1. H.P.Niles,Phys.Rept.110,1(1984);H.E.HaberandG.L.Kane,Phys.Rept.117,75(1985). 103 b) DØ Run II Preliminary p ( ZH fi n n bb, L =261pb-1 ) b 102 int b 95% C.L. upper limit fiH 10 Measured ( B Expected · H) 1 Z Standard Model fi p10-1 p ( s10-2 110 120 130 140 Higgs Mass (GeV) FIGURE1. 95%C.L.upperlimitsonthecrosssectionforZHproductiontimesthebranchingfraction forH bb¯ → 110000 MSSM Higgs bosons DØ P bbf (fi bb), f = h, H, A 8800 E L t a 6600 d bbnn e aa d tt4400 u No mixing l c x Max. mixing 2200 E 8800 110000 112200 114400 mm ((GGeeVV)) AA FIGURE 2. The 95%C.L. upperlimit on tanb as a functionof m for two scenariosof MSSM, “no A mixing"and"maximalmixing".AlsoshownarethelimitsobtainedbytheLEPexperimentsforthesame twoscenariosoftheMSSM[5] 2. http://www-d0.fnal.gov/Run2Physics/WWW/results/prelim/HIGGS/H10/H10.pdf 3. “SearchforneutralsupersymmetricHiggsbosoninmultijeteventsat√s=1.96TeV’,V.M.Abazov etal.,Phys.Rev.Lett.95,151801(2005);hep-ex/0504018,Fermilab-Pub-05/058-E. 4. B.Abachietal.,(DØCollaboration),Nucl.Instr.andMethodsA338185(1994). 5. TheLEPWorkingGroupforHiggsBosonSearches,LHWG-Note2004-01.