Bounds on compositeness from neutrinoless double beta decay.

Assuming the existence of a heavy Majorana neutral particle arising from a composite model scenario we discuss the constraints imposed by present experimental limits of half-life neutrinoless double beta decay (0νββ) measurements on the coupling of the heavy composite neutrinos to the gauge bosons. For neutrino masses MN = 1 TeV we obtain a rather weak lower bound on the compositeness scale: Λ ≥ 0.23 TeV. Heavy neutral Majorana particles with masses in the TeV region are predicted in various theoretical models, such as superstring-inspired E6 grand unification [1] or left-right symmetric models [2]. In addition the possibility of a fourth generation with a heavy neutral lepton, that could be of Majorana type, is not yet ruled out [3, 4]. In this paper we discuss the possibility that a heavy Majorana neutrino might arise from a composite model of the ordinary fermions [5]. Composite models, which describe quarks and leptons as bound states of still more fundamental particles, generally called preons, have been developed as alternatives to overcome some of the theoretical problems of the standard model [6]. Although no completely consistent dynamical composite theory has been found to date, various models have been proposed, and one common, (inevitable), prediction of these models is the existence of excited states of the known quarks and leptons, much in the same way as the hydrogen atom has a series of higher energy levels above the ground state. The masses of the excited particles should not be much lower than the compositeness scale Λ, which is expected to be at least of the order of a TeV according to experimental constraints. For example the search for four-fermion contact interactions gives Λ(eell) > 0.9− 4.7 TeV depending on the chirality of the coupling and on the lepton flavour [7, 8]. We expect therefore the heavy fermion masses to be of the order of a few hundred GeV. The CDF experiment has excluded excited quarks in the mass range 90−570 GeV from γ+ jet and W + jet final states [12]. Phenomenological implications of heavy fermions have been discussed in the literature [10, 17] using weak isospin (IW ) and hypercharge (Y ) conservation. Assuming that such states are grouped in SU(2)× U(1) multiplets, since light fermions have IW = 0, 1/2 and electroweak gauge bosons have IW = 0, 1, to lowest order in perturbation theory, only multiplets with IW ≤ 3/2 can be excited. Also, since none of the gauge fields carry hypercharge, a given excited multiplet can couple only to a light 1 multiplet with the same Y . In addition, current conservation forces the coupling of the heavy fermions to gauge bosons to be of the magnetic moment type. We will only consider here the excited multiplet with IW = 1/2 Y = −1 E = (