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INVISIBLE HIGGS DECAYS and NEUTRINO PHYSICS Anjan S. Joshipura1 J. W. F. Valle Y ABSTRACT

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INVISIBLE HIGGS DECAYS and NEUTRINO PHYSICS Anjan S. Joshipura1 J. W. F. Valle Y ABSTRACT CERN-TH.6652/92 FTUV/92-35 IFIC/92-34 INVISIBLE HIGGS DECAYS AND NEUTRINO PHYSICS 1 Anjan S. Joshipura Theoretical Physics Division, CERN CH-1211 Geneve 23, Switzerland and y J. W. F. Valle Instituto de Fsica Corpuscular - IFIC/CSIC Dept. de Fsica Teorica, Universitat de Valencia 46100 Burjassot, Valencia, SPAIN ABSTRACT A wide class of neutrino physics-motivated mo dels are characterized by the sp onta- neous violation of a global U (1) lepton numb er symmetry at or b elow the electroweak < scale byan SU (2) U (1) singlet vacuum exp ectation value h i O (1) TeV. In all these mo dels the main Higgs decaychannel is likely to b e "invisible", e.g. h ! JJ, where J denotes the asso ciated weakly interacting pseudoscalar Goldstone b oson - the ma joron. This leads to events with large missing energy that could b e observable at LEP and a ect the Higgs mass b ounds obtained, as well as lead to novel ways to search for Higgs b osons at high energy sup ercolliders such as the LHC/SSC. CERN-TH.6652/92 Septemb er 1992 1 Permanent address: Theory Group, Physical Research Lab., Ahmedabad, India Bitnet JOSHIPUR@CERNVM y Bitnet VALLE@EVALUN11 - Decnet 16444::VALLE 1 Intro duction One of the main puzzles in particle physics to day is the problem of mass generation. It is b elieved that the masses of the fermions as well as that of gauge b osons arise as a result of the sp ontaneous breaking of the gauge symmetry. The key ingredient for this scenario, namely the Higgs b oson [1], has not yet b een found. It is only recently, with the LEP exp eriments, that one has seriously started constraining the relevant parameters, including the Higgs b oson mass [2]. The limits on the Higgs mass are, however, rather mo del dep endent. The present limit on the standard mo del Higgs + coming from the data on e e collisions at LEP is 60 GeV. An extension of the minimal standard mo del is desirable for many reasons. One is the question of neutrino masses. Indeed neutrino masses vanish in the mini- mal standard mo del and almost all attempts to induce them require an enlargement in the Higgs sector of the theory [3]. Among these, mo dels known as ma joron mo dels are particularly interesting and have b een extensively studied [3]. The ma joron is a Goldstone b oson asso ciated with the sp ontaneous breaking of the lepton numb er. In the mo dels we shall consider it has very tiny couplings to the charged fermions as well as to the gauge b osons. As a consequence, the ma joron remains invisible. The ma joron can however have signi cant couplings to Higgs b osons even if its other couplings are suppressed. This could have imp ortant implications for Higgs physics. In particular, the normal doublet Higgs b oson could decayinvisibly as h ! J + J; (1) where J denotes the Goldstone b oson {the ma joron { asso ciated with sp ontaneously broken lepton numb er symmetry. The p ossibility of a Higgs b oson decaying invisibly was raised by Shro ck and Suzuki and reconsidered by Li, Liu and Wolfenstein [4] in the context of the triplet ma joron mo del [5]. This typ e of mo dels are now excluded since they lead to an invisible Z width in con ict with LEP observations [6]. Despite this, the p ossibilityofinvisible Higgs decay still remains op en and exp erimentally very amusing [7]. A concrete example [8] was recently provided in the context of sup ersymmetric SU (2) U (1) mo dels where the R parityisspontaneously violated at (or b elow) the electroweak scale [9]. The lightest Higgs b oson h decays in this mo del through ma joron emission. Unfortunately, its pro duction rates are likely to b e small in this case, esp ecially in the low mass region. While this completely avoids 1 the existing LEP1 limits, it is not so useful for the exp erimental detection of the new e ects at LEP1 (prosp ects of observing such decays are b etter at higher energies). The ab ovetyp e of suppression in the pro duction of the low mass Higgs b oson need not o ccur in all mo dels. An example where such suppression can b e absentis provided [10 ] by the seesaw ma joron mo del [11 ], provided the scale of lepton number < O (1) TeV. This mo del, with suchvacuum exp ectation value violation ob eys h i (VEV), mayhaveinteresting physical implications including neutrinos with masses very near their present exp erimental limits [12 ]. However, this is not the most natural choice for the lepton numb er violation scale if neutrino masses are very tiny O (1) eV. The masses of the light neutrinos are given by 2 m D m ; (2) M R where m = h i and M hi. Here hi is the VEV that breaks the SU (2) U (1) D R symmetry while h i breaks the global lepton numb er symmetry. Barring unnaturally small Yukawa couplings , the smallness of neutrino masses follows only if h i O(1) TeV. Typical mo dels asso ciate h i to a large mass scale at which some higher symmetry such as left-right, Peccei-Quinn or grand-uni ed symmetries get realized. As we shall discuss the ma joron-Higgs coupling is suppressed in this case. In this pap er, we note that there exists a wide class of interesting mo dels for neutrino masses in which lepton numb er breaking is driven by an isosinglet VEV (as required by the LEP constraints), but in which the asso ciated scale ob eys < h i O (1) TeV. The distinguishing feature of these new mo dels is that, unlike 1 seesaw mo dels, where m /hi ,inany of the present mo dels m ! 0as hi!0. As a result a lowvalue of h i is required in order to obtain a small neutrino mass either at the tree level or radiatively. We discuss the invisible decay of the Higgs b osons in this typ e of mo dels. In contrast with the two situations discussed ab ove, neither the invisible decay nor the pro duction of the Higgs b osons need to b e suppressed in these mo dels. Moreover, this feature p ersists even when the lepton numb er symmetry is broken at a scale much smaller than the weak scale. The latter would lead to the p ossibility of enhanced ma joron-neutrino couplings. These could, in turn, haveinteresting implications in neutrinoless double b eta decayaswell as astrophysics [13 ]. In addition, in all cases these mo dels can lead to interesting physical e ects such as large rates for zen events at LEP, and avour{violating muon and tau decays with large branching ratios. The 2 former would b e asso ciated to single neutral heavy lepton pro duction and the latter to neutral heavy lepton exchange in higher order weak pro cesses. The corresp onding rates can b e large enough to b e exp erimentally measurable. [14]. In the next section, we discuss the main features of various mo dels of neutrino masses with the lepton numb er broken at a relatively low scale. The third section contains details of the Higgs p otentials and the ma joron couplings to the Higgs b osons. The corresp onding decay and pro duction rates are studied in section 4. The last section contains a discussion of some of the phenomenological implications. The technical details related to very low-scale breaking of the lepton numb er symmetry are given in the app endix. 2 Mo dels Wenow consider several SU (2) U (1) mo dels that have b een suggested in neutrino physics in order to generate naturally small neutrino masses, either as a result of z radiative corrections or at the tree level . In all these mo dels lepton number is a < symmetry of the Lagrangian. This is sp ontaneously broken by h i O (1) TeV, thus generating a ma joron given by J =Im: (3) In most resp ects, these mo dels all share the existence of a massless isosinglet pseu- doscalar ma joron, very much the same as the original one in ref. [11 ]. As a result, all astrophysical constraints [15] related to stellar co oling by ma joron emission can easily b e ob eyed. However, there is an imp ortant di erence. In the seesaw ma joron mo del, the smallness of neutrino masses is linked to the lepton numb er violation at a very high mass scale, i.e. m ! 0as hib ecomes large. In all the present mo dels m ! 0 as h i!0. The remarkable fact that no mass scale is intro duced ab ove the weak scale in any of these mo dels is crucial in ensuring the imp ortance of the invisible Higgs decay (1) relative to the standard mo des suchas h ! ff.Aswe will see, the invisibly decaying Higgs b oson signature may p ersist even in the limit where z For simplicitywe assume that all of the Yukawa coupling constants are real in all that follows. 3 h iO (1) TeV. This maybeinteresting from the p oint of view of pro cesses such as neutrinoless double b eta decay with ma joron emission [13]. 2.1 Singlet ma joron in "-mo del" This is an SU (2) U (1) variant [16] of the "seesaw" mo del with the heavy Dirac lepton suggested in ref.
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