NEW INTERACTIONS AND THE STANDARD MODELS

We discuss the general features of a new force that could be induced b y the exchanges of a spin-1 particle, in extensions of the standard mo del 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 unificati on between electroweak and strong interactions, and (ii) the presence or absence of a supersymmetry between bosons and fermions (since super symmetric theories require two electroweak Higgs doublets instead of o ne, and naturally allow for the gauging of extra U(1) symmetries). Wit h 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 i nto 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 th e (additive) 'fifth-force charge' Q(5) = xB + yL + zQ(el); within gran d unification, B and L only appear through their difference B - L, so that for neutral matter Q(5) would effectively be proportional to the number of neutrons. The corresponding force (which does not act on str angeness) is in general 'composition-dependent'. It could superpose it s effects onto those of gravitation, leading to apparent violations of the equivalence principle, or of-Newton's 1/r(2) law. Furthermore, th e axial part in the U-current would lead to new spin-dependent forces, which may well be significantly stranger than in the axion case. The intensities and ranges of such possible new forces are essentially unk nown, but may be related to the symmetry-breaking scale. For a given s cale the intensity of the (spin-independent) new force varies like 1/l ambda(2), where lambda is the range. If the new current has an axial p art the U-boson could be directly produced in particle physics experim ents. Quarkonium decays constrain the symmetry-breaking scale, restric ting the possible intensities and ranges of a spin-1-induced 'fifth-fo rce'.