Theoretical motivations for Equivalence Principle tests Original Research Article

We discuss the theoretical motivations for new long-range forces induced by spin-1 or spin-0 particles. They would add their effects to those of gravity and lead, because of their composition-dependence, to apparent violations of the Equivalence Principle, possibly at a detectable level. The couplings of a new spin-1 gauge particle, the U boson, are obtained from gauge invariance, taking into account mixing effects with the Z boson. The corresponding charge is expressed as Q5 = xB + yL + zQel and, within grand-unification, is effectively proportional to the number of neutrons N. If the new force has a finite range λ, its effective intensity α is related to this range and to the extra-U(1) symmetry-breaking scale F, by a α ∼ 1(λ2F2). Quite surprisingly, particle physics experiments can provide some constraints on such a new force, even if it is extremely weak. In fact, an “equivalence theorem” shows that a very light spin-1 U boson does not in general decouple when its gauge coupling vanishes, but behaves like a quasi-massless pseudoscalar. Particle physics experiments then constrain F to be larger than the electroweak scale.