New interactions and the standard models

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Abstract
We discuss the general features of a new force that could be induced by the exchanges of a spin-1 particle, in extensions of the standard model of electroweak and strong interactions. The possible couplings of such a particle (the U-boson) can be restricted using gauge invariance, in connection with: (i) the presence or absence of a grand unification between electroweak and strong interactions, and (ii) the presence or absence of a supersymmetry between bosons and fermions (since supersymmetric theories require two electroweak Higgs doublets instead of one, and naturally allow for the gauging of extra U(1) symmetries). With only one Higgs doublet any extra U(1) symmetry generator should act as a linear combination of baryonic and leptonic numbers with the weak hypercharge, Y. With two Higgs doublets - as in supersymmetric theories - it may also involve an axial symmetry generator. In both cases it is blind to quark generation. After mixing effects with the Z are taken into account, we get the current to which the U-boson should couple. In general, it involves a vector part, as well as an axial part if there is more than one Higgs doublet. The vector part is associated with the (additive) `fifth-force charge' ; within grand unification, B and L only appear through their difference B-L, so that for neutral matter would effectively be proportional to the number of neutrons. The corresponding force (which does not act on strangeness) is in general `composition-dependent'. It could superpose its effects onto those of gravitation, leading to apparent violations of the equivalence principle, or of Newton's law. Furthermore, the axial part in the U-current would lead to new spin-dependent forces, which may well be significantly stronger than in the axion case. The intensities and ranges of such possible new forces are essentially unknown, but may be related to the symmetry-breaking scale. For a given scale the intensity of the (spin-independent) new force varies like , where is the range. If the new current has an axial part the U-boson could be directly produced in particle physics experiments. Quarkonium decays constrain the symmetry-breaking scale, restricting the possible intensities and ranges of a spin-1-induced `fifth-force'.