Talk presented at the XXIX Rencontres de Moriond, Electroweak Interactions and Unified Theories, March 12-19, 1994, MeribelTalk presented at the XXIX Rencontres de Moriond, Electroweak Interactions and Unified Theories, March 12-19, 1994, MeribelTalk presented at the XXIX Rencontres de Moriond, Electroweak Interactions and Unified Theories, March 12-19, 1994, Meribe

De Campos, F.

Lopez-Fernandez, A.

Romao, J.C.

Valle, J.W.F.

arXiv

Talk presented at the XXIX Rencontres de Moriond, Electroweak Interactions and Unified Theories, March 12-19, 1994, MeribelTalk presented at the XXIX Rencontres de Moriond, Electroweak Interactions and Unified Theories, March 12-19, 1994, MeribelTalk presented at the XXIX Rencontres de Moriond, Electroweak Interactions and Unified Theories, March 12-19, 1994, MeribelTalk presented at the XXIX Rencontres de Moriond, Electroweak Interactions and Unified Theories, March 12-19, 1994, Meribe

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arXiv:hep-ph/9405382v1  25 May 1994FTUV/94-28IFIC/94-26UPDATED LIMITS ON VISIBLY AND INVISIBLY DECAYING HIGGSBOSONS FROM LEPF. de Campos ∗ and J. W. F. Valle †Instituto de Fisica Corpuscular - IFIC/CSICDept. de F´isica Teo`rica, Universitat de Vale`ncia46100 Burjassot, Vale`ncia, SPAINA. Lopez-Fernandez ‡PPE Division, CERNCH-1211 Geneve 23, SwitzerlandandJ. C. Roma˜o §Departamento de F´ısica, Instituto Superior Te´cnicoAv. Rovisco Pais, 1 - 1096 Lisboa Codex, PORTUGALTalk presented at the XXIX Rencontres de Moriond,Electroweak Interactions and Unified Theories,March 12-19, 1994, Meribel - Savoy - FRANCEThe problem of mass generation remains one of the main puzzles in particle physics to-day. In the standard model all masses arise as a result of the spontaneous breaking ofSU(2)⊗U(1) the gauge symmetry. This implies the existence of an elementary Higgs bosonnot yet found. Recently the LEP experiments on e+e− collisions around the Z peak haveplaced important restrictions on the Higgs boson mass mHSM >∼ 60GeV.There are many reasons to think that there may exist additional Higgs bosons innature. One such extension of the minimal standard model is provided by supersymmetryand the desire to tackle the hierarchy problem [1]. Another reason is neutrino physics.Indeed, there are many extensions of the minimal standard model which induce neutrinomasses either at the tree level or radiatively through an enlargement in the Higgs sector[2].In many of these extensions one has the possibility that the Higgs boson may decayinto invisible particles, such as H → χχ where χ is the lightest neutralino in supersym-metry, possible when 2mχ < MH .∗Bitnet CAMPOSC@vm.ci.uv.es - Decnet 16444::CAMPOSC†Bitnet VALLE@vm.ci.uv.es - Decnet 16444::VALLE‡Bitnet ALFON@CERNVM§Bitnet ROMAO@PTIFM1Amongst the extensions of the standard model which have been suggested to gener-ate neutrino masses, the majoron models are particularly interesting and have been widelydiscussed [2]. The majoron is a Goldstone boson associated with the spontaneous breakingof the lepton number. Astrophysical arguments based from stellar cooling rates constrainits couplings to the charged fermions [3], while the LEP measurements of the invisible Zwidth restrict the majoron couplings to the gauge bosons in an important way. In partic-ular, models where the majoron is not a singlet [4] under the SU(2)⊗U(1) symmetry arenow excluded [5].There is, however, a wide class of models [6], motivated by neutrino physics, whichare characterized by the spontaneous violation of a global U(1) lepton number symmetryby an SU(2)⊗U(1) singlet vacuum expectation value 〈σ〉 [7]. These models may naturallyexplain the neutrino masses required by astrophysical and cosmological observations [8].Another example is provided by supersymmetric extensions of the standard modelwhere R parity is spontaneously violated [9].In all these extensions of the minimal standard model the global U(1) lepton numbersymmetry is spontaneously violated close to the electroweak scale. Such a low scale forthe lepton number violation is preferred since, in these models, mν → 0 as 〈σ〉 → 0. As aresult, a relatively low value of 〈σ〉 is required in order to obtain naturally small neutrinomasses [6].In any model with a spontaneous violation of a global U(1) symmetry around theweak scale (or below) the corresponding Goldstone boson has significant couplings to theHiggs bosons, even if its other couplings are suppressed. This implies that the Higgs bosoncan decay with a substantial branching ratio into the invisible mode [6, 10, 11] h→ J + Jwhere J denotes the majoron.Such an invisible Higgs decay would lead to events with large missing energy thatcould be observable at LEP and affect the corresponding Higgs mass bounds. In orderto do this we determine the Higgs boson production and visible and invisible decay rates,which involves three independent parameters: the Higgs boson mass MH , its couplingstrength to the Z, normalized by that of the standard model, we call this factor ǫ2, andthe invisible Higgs boson decay branching ratio.We have used the same method as described in [16], in order to deduce the regionsin the parameter space of the model that can be ruled out already. The procedure was thefollowing: In each case we have allowed the Higgs boson to decay into invisible channelswith an arbitrary branching ratio between 0 and 1, and kept the weakest limit. In orderto do this we have first considered the two extreme limits where the Higgs boson decays100% visibly or invisibly; we obtained the absolute bound on the coupling ZZh (ǫ2) as afunction of mH . The analysis was made considering the two differents signals given in theHiggs boson decay: the invisible case and the visible one.2For the invisible case, namely: Z → HZ∗,H →invisible, Z → qq¯, we used directlythe results presented by the Aleph Collaboration in reference [18], in which they set alimit on the maximum allowed coupling of the Higgs to the Z0 as a function of its mass.For the visible case: Z → HZ∗, H → qq¯, Z → νν or ll where we directly applied thelimits from the standard searches, using results from all experiments [17, 18, 19, 20]. Inthese case the background events were considered where they existed. For all values of theHiggs mass, we found that the weakest limit was obtained in the case of 100% standarddecays, since in this channel it is not possible to tag the events in which the Z0 decayshadronically due to the large background from normal processes.As an illustration we show in Figure 1 the exclusion contours in the plane ǫ2 vs.BR(H → visible) for the particular choice for the Higgs mass MH = 50 GeV. The twocurves corresponding to the searches for visible and invisible decays are combined to givethe final bound; values of ǫ2 above 0.1 are ruled out independently of the value of BR(H →visible). The solid line in Figure 2 shows the region in the ǫ2 vs. MH that can be excludedby the present LEP analyses, independent of the mode of Higgs decay, visible or invisible.An analysis on invisible higgs bosons can also be found in [12].We have also estimated the additional range of parameters that can be covered byLEPII. We assumed that the total luminosity collected will be 500 pb−1, and give theresults for two values of the centre-of-mass energy: 175 GeV and 190 GeV. Our resultson the visible decays of the Higgs are based on the study of efficiencies and backgroundsin the search for the Standard Model Higgs described in reference [13]. For the invisibledecays of the Higgs we considered only the channel HZ with Z → e+e− or Z → µ+µ−,giving a signature of two leptons plus missing transverse momentum. The requirementthat the invariant mass of the two leptons must be close to the Z mass can kill most of thebackground from WW and γγ events; the background from ZZ events with one of the Zdecaying to neutrinos is small and the measurement of the mass recoiling against the twoleptons allows to further reduce it, at least for MH not too close to MZ . Hadronic decaysof the Z were not considered, since the background from WW and Weν events is verylarge, and b-tagging is much less useful than in the search for ZHSM with Z → νν¯, sincethe Zbb¯ branching ratio is much smaller than Hbb¯ in the standard model. The dashed anddotted curves on figure 2 show the exclusion contours in the ǫ2 vs. MH plane that can beexplored at LEPII, for the given centre-of-mass energies. Again, these contours are validirrespective of whether the Higgs decays visibly, as in the standard model, or invisibly.The possibility of invisible Higgs decay is also very interesting from the point of view of alinear e+e− collider at higher energy [15].AcknowledgementsThis work was supported by DGICYT and by Accion Integrada Hispano-Portuguesa,under grant numbers PB92-0084 and HP-50B. F. de Campos was supported by a fellowshipfrom the Brazilian government (CNPq). He thanks the organizers, especially C. A. Savoyfor the hospitality at Moriond.3Figure CaptionsFigure 1 shows the exclusion contours in the plane ǫ2 vs. BR(H → visible) for the particular choicemH = 50 GeV. The two curves corresponding to the searches for visible (curve A) and invisible (curve B)decays are combined to give the final bound, which holds irrespective of the value of BR(H → visible).4The solid curve shows the region in the ǫ2 vs. mH that can be excluded by the present LEP analyses.The dashed and dotted curves on figure 2 show the exclusion contours in the ǫ2 vs mH plane that can beexplored at LEPII, for the given centre-of-mass energies.References[1] H. P. Nilles, Phys. Rep. 110 (1984) 1; H. Haber, G. Kane, Phys. Rep. 117 (1985)75[2] J. W. F. Valle, Prog. Part. Nucl. Phys. 26, 91 (1991) and references therein.[3] J. E. Kim, Phys. Rep. C150 (1987) 1, and references therein.[4] G. Gelmini and M. Roncadelli, Phys. Lett. B99 (1981) 411. R. E. Schrock and M.Suzuki, Phys. Lett. B110 (1982) 250 L. F. Li, Y. Liu, L. Wolfenstein, Phys. Lett.B159 (1985) 45; See also D. Chang and W. Keung, Phys. Lett. B217 (1989) 238.[5] J. Steinberger, in Electroweak Physics Beyond the Standard Model, ed. J. W. F. Valleand J. Velasco (World Scientific, Singapore, 1992) p. 3.[6] A. Joshipura and J. W. F. Valle, Nucl. Phys. B397 (1993) 1055[7] Y. Chikashige, R. N. Mohapatra and R. D. Peccei, Phys. Lett. B98 (1980) 265.[8] J Peltoniemi, J. W. F. Valle, Phys. Lett. B304 (1993) 147; Nucl. Phys. B406 (1993)409[9] A Masiero, J W F Valle, Phys. Lett. B251 (1990) 273; P Nogueira, J C Romao, J WF Valle, Phys. Lett. B251 (1990) 142; J C Romao, N Rius, J W F Valle, Nucl. Phys.B363 (1991) 369; J C Romao, J W F Valle, Nucl. Phys. B381 (1992) 87; J C Romao,C A Santos, J W F Valle, Phys. Lett. B288 (1992) 311; M C Gonzalez-Garcia, J CRomao, J W F Valle, Nucl. Phys. B391 (1993) 100; G Giudice etal, Nucl. Phys.B396 (1993) 243; M Shiraishi, I Umemura, K Yamamoto, preprint NEAP-50, 1993[10] A. S. Joshipura, S. Rindani, Phys. Rev. Lett. 69 (1992) 3269; R. Barbieri, and L.Hall, Nucl. Phys. B364 (1991) 27. G. Jungman and M. Luty, Nucl. Phys. B361(1991) 24. E. D. Carlson and L. B. Hall, Phys. Rev. D40 (1989) 3187.[11] J. C. Romao, F. de Campos, and J. W. F. Valle, Phys. Lett. B292 (1992) 329.[12] D. P. Roy et al., Phys. Rev. D48 (1993) 4224.[13] P. Janot, LAL report 92-27.[14] J. W. F. Valle, Physics at New Accelerators: Looking Beyond the Standard Model,invited talk at XV Int. Conference on Neutrino Physics and Astrophysics, ed.A. Morales etal (North-Holland, 1992), CERN-TH.6626/92.[15] O. Eboli etal, workshop on e+e− collisions at 500 GeV, the physics potential, ed. P.Zerwas, p. 55; O. Eboli etal, Valencia FTUV/93-50, Nucl. Phys. B (1994) , in press[16] Lopez-Fernandez et al., Phys. Lett. B312 (1993) 240.[17] DELPHI Collab., Marseille Conference, 1993.[18] ALEPH Collab. Phys. Lett. B313 (1993) 312.[19] L3 Collab.,O. Adriani et al., Zeit. fur Physik C (1993) 355.[20] OPAL Collab., Marseille Conference, 1993.6This figure "fig1-1.png" is available in "png" format from:http://arxiv.org/ps/hep-ph/9405382v1This figure "fig2-1.png" is available in "png" format from:http://arxiv.org/ps/hep-ph/9405382v1

Updated limits on visibly and invisibly decaying Higgs bosons from LEP

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