Fabbro, B

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Searches for Higgs bosons at LEPBernard FabbroDSM/DAPNIA/SPP CE Saclay 91191 Gif-sur-Yvette CEDEXALEPH CollaborationAbstract. Searches for Higgs bosons at LEP are summarized. No Higgs boson has been observed yet.For the Standard model Higgs boson, the lower bound on the mass is 113:3 GeV=c2. In the MinimalSupersymmetric Standard Model, for representative scans of the SUSY parameters, the limits on the CP-even and the CP-odd neutral Higgs bosons masses are both 90:5 GeV=c2. For the charged Higgs bosonspredicted by two-doublet extensions of the Standard Model, a lower bound of 77:5 GeV=c2is obtained.Limits on Higgs bosons predicted by more exotic extensions of the Standard Model are also given.INTRODUCTIONThe existence of a Higgs boson is an essential prediction of the Standard Model (SM) which is the simplestmodel, and historically the rst one, involving the Higgs mechanism. However, since no Higgs boson has so farbeen observed, many extensions of the SM, with more or less `exotic' Higgs sectors, have been developed. Inthese extensions, one or several higgses are expected. Some of them could be relatively light, as in the MinimalSupersymmetric Standard Model (MSSM) where the lightest higgs should have a mass less than 130 GeV=c2.Therefore, searches for higgses are very active at LEP since it has a mass reach of about 115 GeV=c2. In thisreview, a description of these searches is given. In section 1, we start with a presentation of the main modelstested at LEP. In section 2, the Higgs production processes at LEP energies are briey described, then anexample of analysis for the selection of events is presented as well as a statistical method designed to detecta possible discovery and to calculate the lower limits on the masses of the higgses. The results are nallypresented in section 3.I THEORETICAL MODELS TESTED AT LEPSM and beyond. In the SM, the Higgs sector is described by one doublet of complex scalar elds (four realcomponents). At the electroweak symmetry breaking, three components give mass to the electroweak gaugebosons Wand Z. The remaining component gives rise to one physical state: the SM Higgs boson h. Thereare mainly two kinds of extensions of the SM: 1) models with one doublet, as in the SM, but with anomalouscoupling of the Higgs boson to the gauge bosons and 2) the Two Higgs Doublet Models (2HDM), including theMSSM. More exotic extensions of the SM also exist [1] but will not be considered here since no direct searchfor the corresponding higgses has been recently performed at LEP.Anomalous couplings in one doublet models. In the SM, the Higgs boson couples to photons onlythrough loops of charged particles. The branching ratio is small [BR(h!  )  10 3for mh 90 GeV=c2].It can be increased, in one doublet models, by anomalous couplings to gauge bosons. These couplings aredescribed by operators of strength fi=2where  is the scale of the interaction and the fiare operators givingrise to dierent anomalous couplings such as h, hZ, hZZ, hWW [2].2HDM. In these models [1], two doublets of complex scalars H1, H2correspond to eight real components.Three components give rise to the massive electroweak gauge bosons and the remaining ve components resultin ve physical states: two neutral CP-even states h,H also called the neutral scalars, one neutral CP-odd stateA also called the neutral pseudo-scalar and two charged higgses H. Six parameters are needed to describe theHiggs sector: the masses of the particles mh, mH, mA, mH , the mixing angle  for the two neutral scalarsand the parameter tan  which is the ratio of the Vacuum Expectation Values of the two doublets. The 2HDMare classied in two types: the type I models in which only one doublet has couplings with fermions and typeII models in which down-type fermions have couplings with one doublet and up-type fermions have couplingswith the other doublet. The MSSM is a 2HDM of type II.MSSM. In the supersymmetric models [3] the Higgs sector is more constrained than in generic 2HDM's. InMSSM, only two parameters are needed: usually one uses mAand tan  from which one can express the otherparameters. For the masses, we have at the tree level:8><>>:mH;h2=12mA2+ mZ2q(mA2+mZ2)2  4mA2mZ2cos22mH2= mA2+ mW2(1)The rst relation leads to the condition: mh< j cos 2jmZwhich means that the lightest scalar should havebeen already discovered at LEP! However, radiative corrections are not negligible. The largest contributionscome from top and stop loops, because of the heavy mass of the top (mt' 175 GeV=c2). Finally, includingcomplete calculations of the radiative corrections, the MSSM upper bound for mhis found to be: mh<130 GeV=c2. This leaves a window to escape detection at LEP, which mass reach is ' 115 GeV=c2.Invisible Higgs. In supersymmetry with R-parity conserved, the Higgs boson can decay into stable non-interacting particles (Lightest Supersymmetric Particle (LSP)) [4].Fermiophobic Higgs. In 2HDM of type I, the direct coupling to fermions ghffis proportional tocos= sin . Then it can be suppressed by tuning  to2. The physical states with no coupling to fermions arecalled Fermiophobic Higgses [5].II HIGGS PRODUCTION, EVENT SELECTION AND ANALYSISA Higgs production at LEPThe SM Higgs boson production process is the Higgsstrahlung process : e+e !Z! hZ. The Higgsdecays predominantly into bb and the main nal states are: bbqq, bb and bbl+l (l = e; ;  ).For the 2HDM neutral higgses there are two processes: the Higgsstrahlung and the associated production:e+e !Z! hA. In the MSSM, the cross-sections of the two processes are complementary: the Higssstrahlung cross-section is proportional to sin2(   ) while the associated production cross-section is propor-tional to cos2(   ). The h and the A decay predominantly into fermion pairs for the masses that LEP issensitive to. An additional decay is possible: h ! AA when mh> 2mA.The 2HDM charged higgses are produced via the process of annihilation of a Zor a : e+e !Z; !H+H . The nal states and experimental signatures will be described in detail in B.The invisible higgs is produced via the Higgsstrahlung process. The cross-section can be expressed asinv.= 2SM(e+e !hZ) [0 < 2< 1] where  is a model dependent constant. The Higgs decays in twoLSP. The experimental signature consists in two acoplanar leptons or quark jet and missing energy.The fermiophobic higgs is produced via the processes: e+e !hA, e+e !hA ! hhZ, e+e !hZ ande+e !hA ! AAA. The possible nal states are: A, bb, , qq,  , qq and bbbbbb.For the anomalous couplings in one doublet models, the production processes are: e+e !h,e+e !hZ=, e+e !hZ, e+e ! (ZZh)e+e and e+e ! (WWh). The possible nal states are: ,bb, qq, e+e and . Dierent kinds of limits have been measured at LEP: limit on fi=2(DELPHI),limit on the cross-section for the process e+e !h !  (L3).B Event selection: an example for charged higgsesThe methods of search for the higgses in the dierent channels are relatively similar. We give here an examplefor the case of the charged higgses. In 2HDM dierent from the MSSM, mH can be lower than mW. Thecharged higgs production cross-section is independent of model and has values in the range 0:12   0:27 pb 1for masses between 70 and 80 GeV=c2and for the 1999 LEP energies (192; 196; 200;202GeV=c2). For anintegrated luminosity of 230 pb 1, which corresponds to the average luminosity by experiment at LEP in 1999,and for mH ' 75 GeV=c2, one expects  50 H+H pairs.At LEP energies, the dominant decay modes are H ! sc and H !  . Then the search is performedassuming BR(H ! sc) + BR(H !  ) = 1, which leads to three nal states topologies:H+H ! cssc : four hadronic jetsH+H ! cs : two hadronic jets, one tau jet, missing energy carried by one neutrinoH+H !  : two tau jets, missing energy carried by two neutrinoswhere hadronic jets (tau jets) are jets with high (low) multiplicities.Dierent processes contibute to the background to Higgs signal, according to the nal state. For instance, inthe case of the  nal state, the main background processes are the following: 1) WW where each of thetwoW 's emits a  and a neutrino, 2) ZZ where one Z decays in two  and the other Z decays in two neutrinos,3)  where an incident electron emits a photon which is lost in the beam pipe, simulating the missing energy,and 4)  where the two incident electrons emit two photons exchanging a neutrino and giving rise to thetwo  's (multiperipheral diagram) and where the two outcoming electrons are lost in the beam pipe.An example of distribution of reconstructed charged Higgs mass is given in Fig.1. The expected signal wouldappear as a small excess in the data. For a typical value of mh= 75 GeV=c2, the signal would produce 72:0events for the cssc nal state and 40:6 events for the cs nal state while the numbers of expected (observed)events for these two nal states are respectively 2245:5(2208) and 471:6(488) [6].0100200300400500600700Events / 3 GeV √s– = 192 - 202 GeVLEP (ADLO) data combinedbackgroundH+H- Signal(mH = 75 GeV)a)H+H- → qq–qq–0204060801001201400 20 40 60 80 100 120Reconstructed Mass m  [GeV]Events / 3 GeV √s– = 192 - 202 GeVLEP (ADLO) data combinedbackgroundH+H- Signal(mH = 75 GeV)b)H+H- → τντqq–FIGURE 1. Distribution of reconstructed charged Higgs mass combining the data of the four LEP experiments(ADLO) at energies from 192 to 202 GeV=c2. Upper part: H+H ! cssc channel; lower part: H+H ! cs channel where BR(H+! +) = 0:5 is assumed. Figure reprinted from Ref. [6].C Statistical method for discovery and limit on the massA statistical method is performed to analyse the numbers of observed events and to extract from them quanti-tative informations in terms of discovery or mass limit. For each mass hypothesis mh, a global test-statistic Xmhis constructed to classify the experimental result between background-like and (signal+background)-like situa-tions. The value of Xmhis calculated for the data (XObsmh) and for a large number of Monte-Carlo experiments(XMCmh). Two basic `condence levels' are then calculated: CLb(mh) (probability that XMCmhis more b-like thanXObsmh) and CLs+b(mh) (probability that XMCmhis less (s+b)-like than XObsmh). Finally, two quantities are usedto express the results: 1   CLb(mh) (indicator for a possible signal) and CLs(mh) = CLs+b(mh)=CLb(mh)(indicator for lower bound on mh). In abscence of signal the quantity 1   CLb(mh) is close to 0:5. A Higgsboson with true mass m0would produce a drop in this quantity for mh' m0, the 5 (3) discovery corre-sponding to the value 5:710 7(2:710 3). The lowest value of mhwhich yields CLs(mh) = 0:05 denes the95%CL lower limit (observed limit). The average value < CLs(mh) >bobtained from simulated `backgroundonly' experiments indicates the expected range of sensitivity for exclusion (expected limit) [6].III RESULTSThe statistical method described above was used to search for the dierent higgses: SM Higgs boson, MSSMneutral higgses, 2HDM charged higgses, invisible higgs, fermiophobic higgs. So far, no direct evidence for theexistence of these Higgs bosons has been found. The limits, corresponding to a combination of the four LEPexperiments, are listed in table 1. For the MSSM, excluded domains in tan are also given.MSSM benchmark scans. For the MSSM, the results are presented for a constrained MSSM with sevenparameters: MSUSY= 1TeV, M2= 200GeV, m~g= 800GeV,  =  200GeV, mA= 1TeV, 0:4 < tan  < 50and Xt= A    cot . The parameter M1is derived from M2using the relation M1=53M2tan2W, whereWis the weak mixing angle. Since the main correction to mhstems from the t  ~t sector, the limit on mhis sensitive to the stop mixing parameter Xt. Two scenarios are considered: the maximal mhscan scenario(Xt= 2MSUSY) and the no mixing scenario (Xt= 0). The maximalmhscan scenario is designed to maximizethe largest value of mhallowed by the model at each value of tan  [6].TABLE 1. Limits on the masses of the higgses for dierent models and ex-clusion in tan  for MSSM. LEP combination, 00/07/20.Observed limit Expected limith (SM) 113.3 GeV=c2113.4 GeV=c2h (MSSM) max mhscan 90.5 GeV=c292.2 GeV=c2no mixing 90.5 GeV=c292.4 GeV=c2A (MSSM) max mhscan 90.5 GeV=c292.8 GeV=c2no mixing 90.5 GeV=c292.9 GeV=c2Excluded domains max mhscan 0.5  tan   2.3 0.5  tan   2.3in tan  (MSSM) no mixing 0.4  tan   7.7 0.4  tan   8.6H(2HDM) 77.5 GeV=c278.8 GeV=c2Invisible Higgs 107.7 GeV=c2109.9 GeV=c2Fermiophobic Higgs 106.4 GeV=c2105.6 GeV=c2CONCLUSIONA large eort has been performed at LEP to search for many kinds of Higgs bosons with highly sophisticateddata analysis techniques.REFERENCES1. J.F Gunion, H.E. Haber, G. Kane, S.Dawson, The Higgs Hunter's Guide. Addison-Wesley(1990).2. K. Hagiwara, R. Szalapski and D. Zeppenfeld, Phys. Lett. B318(1993)155-1623. S.P. Martin, hep-ph/9709356; K.A. Olive, hep-ph/99113074. For a review, see: M. Carena and P.M. Zerwas (Conveners) et al. in \ Higgs Physics at LEP2", Ed: G. Altarelli, T.Sjostrand, F.Zwirner, CERN-96-01(1996)3515. A.G. Akeroyd, Phys. Lett. B363(1996)89 ; H. Haber et al. Nucl. Phys. B161(1979)4936. ALEPH, DELPHI, L3 and OPAL Collaborations, The LEP working group for Higgs bosons searches, Searches forHiggs bosons: Preliminary combined results using LEP data collected at energies up to 202GeV . ALEPH 2000-028CONF2000-023, DELPHI 2000-050 CONF365, L3 Note 2525, OPAL Technical note TN646. \XXXVth Rencontresde Moriond", Les Arcs, France, March 11-25, 2000.

Searches for Higgs bosons at LEP

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