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International Centre for Theoretical Physics IC/92/193 a it** • INTERNATIONAL CENTRE FOR THEORETICAL PHYSICS MAJORON MODELS AND THE HIGGS SEARCH Anjan S. Joshipura and INTERNATIONAL Saurabh D. Rindant ATOMIC ENERGY AGENCY UNITED NATIONS EDUCATIONAL. SCIENTIFIC AND CULTURAL ORGANIZATION MIRAMARE'TRIESTE IC/92/193 The I, s mechanism is a key ingredient in the description of the standard model. International Atomic Energy Agency The preii -d Higgs boson is however not yet found. The latest [1] lower limit on the and mass of 1 -tandard model (SM) Higgs coming from the data at LEP in e+e" collisions is 52 Ge\ He LEP data has been applied [2] not only to restrict the SM Higgs mass but United Nations Educational Scientific and Cultural Organization also to o rain parameters of extensions of the SM. Two popular extensions [3] studied INTERNATIONAL CENTRE FOR THEORETICAL PHYSICS at length the minimal supersymmetric standard model (MSSM) and the general two Higgs do t models. These extensions contain more than one neutral Higgs boson which mi mong themselves. As a result of this mixing, the couplings of the neutral Higgs bo - in these models to the Z boson and fermions are different from those in the SM. I ce more freedom is available and the Higgs mass is less constrained in these MAJORON MODELS AND THE HIGGS SEARCH schemes. spite of this, the mass of any of the two neutral Higgs boson is not allowed to be les^ a about 44 GeVFl [2] in MSSM while in the general models with two Higgs doublets. could rule out a large range [2] in the values of the mixing angle as a function of the Hii mass. Anjan S. Joshipura" We w !o investigate here a possible bound on the Higgs mass in another popular extension ihe SM which is somewhat more economical and characteristically different International Centre for Theoretical Physics, Trieste, Italy from the • IT two extensions mentioned above. The class of models we consider is based on the mi ial SM extended by introducing an SU{2) x f/(l) singlet field 17, right-handed and neutrinos il a spontaneously broken global symmetry. This type of extension, with the global £/( quantum number identified with total lepton number, was first proposed [4] Saurabh D. Rindani as a novt I cans of generating neutrino mass. Since then, this type of models have been Theory Group, Physical Research Laboratory, popular i* generating neutrino masses and their decay to lighter neutrinos. The general feature of <?se models which is of primary interest here is the occurrence of a Goldstone Navarangpura, Ahmedabad 380 009, India. boson (t< generically called majoron) due to the spontaneous breaking of the £/(l) symmetry iggered by the vacuum expectation value of the singlet field r\. If the usual Higgs doi. t (p is chosen to be neutral under the global U(l), as we shall assume in the ABSTRACT following • Goldstone field is simply Im rj. Due to singlet nature of 7], the majoron couples O! (,0 right-handed neutrinos and strong constraints on possible interactions of If the SU( 2) x U( 1) model is extended by adding a complex singlet spin-zero field and the massl- ! >oson can be avoided in these models [4]. The majoron thus remains invisible as far as : fermionic and the gauge couplings are concerned. This does not happen in a spontaneously broken global f/( 1) symmetry, a Higgs scalar can decay into a pair of Goldstone the Higgs lor. Despite being singlet, the rj can couple strongly to the Higgs doublet (j> bosons. This decay could dominate over the conventional decay modes into fermion-antifermion through t (uartic coupling &{<tf<<£){rfy)- As long as 6 is not unnaturally small, the two pairs. If that happens, then the standard Higgs search through its ZZ*H coupling would not be massive 1! s in the theory have a significant coupling to the majoron and could decay able to detect the Higgs allowing it to be lighter than implied by the already existing limits from into a pa >{ them. As we will see, this decay could compete with or even dominate the LEP data on e* e~ annihilation. Detailed restrictions on the Higgs mass and other parameters of over the <i y of the Higgs to fermion-antifermion pairs. It is not possible to detect this the model are derived using the existing LEP data on the standard model Higgs search in the mass invisible <; iy mode directly. However, its presence could mask the standard Higgs signal region 10-50 GeV. by dilutii, , he branching ratio of the Higgs decay to 66. We wish to study this effect quantitat : v in this paper. Thou^ ; he leptonic sector of the majoron model has been widely studied, not much MIRAMARE-TRIESTE attention , been paid to the Higgs sector of the model. The possibility of a Higgs decaying • 1 pair of Goldstone bosons was pointed out in [5]. The Higgs sector of the August 1992 majoron 1 lei was also discussed recently [6] by Jungman and Luty who identified the scale of t1 -inglet vacuum expectation value with the weak scale. There is no strong reason U, mtify these two scales, and we have kept < rj > arbitrary and analysed constrain' n it from the existing LEP data. In far1 .nee < rj > sets the scale of neutrino masses one may be able to restrict it Permanent address: Physical Research Laboratory, Navarangpura, Ahmedabad, India. only in t) >ntext of a some specific model. Otherwise it is arbitrary to a large extent. T r If no bare mass term is presentFi, < r\ > sets the scale of the right handed neutrino following coupling of H, S to the majoron J: mass matrix M- The masses of the light left-handed neutrinos are then generated in the seesaw model by the matrix Ttt£>M~1mp where mrj describes the Dirac mass term for the neutrino. Both mrj and M are arbitrary in the SU(2) x U(l) theory and the observed g. n limit on the neutrino masses cannot be used to determine the < T) >. However, if the model is embedded in some grand unified theory like the 50(10) model, then < ij > gets The above equation gives rise to the decay of H and S to two majoron states. The restricted. In some models [8], < rj > sets the scale of B — L symmetry breaking which corresponding decay width is given, for example in case of H, by is restricted from other considerations. Moreover, the neutrino mass matrices mp may get related [8] to the corresponding matrices for the charge-1 (u) quarks or the charged T{H -. JJ) = (6) 32TT ^eptons (/) in these models. If this happens then mv w 7^>- The observed limit on the neutrino masses can then be used in to set a limit on < T) >, e.g., for mo fa rnf, mUt < 1 The decay rate for H -* 66 remains essentially the same as in SM [3] except for modifica- eV implies < fj >> 250C?eV if the relevant Yukawa couplings are not much bigger than 1. tion introduced by the mixing, eq.(3}. But if one does not insist on grand unification then there are really no strong constraints on < Tj >. T(H - 66) = cos2 9. We shall therefore keep < t; > arbitrary and subject the model to the available LEP data and derive the constraints on the Higgs mass as well as on < IJ >. To do this, we The search strategy for the Higgs depends upon its mass and upon decay characteristics first discuss the parameters and the majoron couplings in the model. relevant for this mass. Accordingly [3], the Higgs with My < SOGeV can be looked for Irrespective of the nature of the neutrino masses, the Higgs sector of the majoron-type at LEPI or LEPII. Heavier Higgs can be searched at hadron colliders. If its maiis is model can be described by the following scalar potential less than twice the mass of the W, one has to rely upon its rare decays like 77, while heavier Higgs can be found through its WW and ZZ decay modes. All these searches can become difficult once the Higgs decay to majorons becomes possible. We shall mainly ) (1) concentrate on the already existing data on the search at LEPI and comment upon the other possibilities. Terms like r/2 are omitted above in view of the imposed (7(1) invariance under which we The SM Higgs is searched for [3] at LEP through the coupling HZZ". Both H and require r? to transform nontrivally and tj>t0 be trivial. Let TJ = -%• -f "**$', 4>° = 7k~^*Rj£l' S have this coupling in the majoron model due to mixing eq.(3). A priori, both of them where we have set < JJ >= ^ and < ^° >= -75. The above potential then leads to a could be lighter than Z and could be copiously produced in e+e~ collisions at the Z physical massless Goldstone boson namely the majoron J and two massive neutral scalars resonance. For simplicity of analysis, we shall assume that only one of them is lighter denoted by H and S respectively. These are given in terms of the original fields as follows than the Z and investigate the consequences of such an assumption. For definiteness, we take H to be lighter than Z. The complementary case of the lighter S can be obtained H = cos 6(J>R - sin from above by the replacement 9 —* 6 + ff/2. S = s (2) For a Higgs whose mass lies above the threshold for 66 production, the standard signal is two jets accompanied by missing energy or charged lepton pairs coming from the decay J = 1 of the virtual Z.
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