Couplings of a light dilaton and violations of the equivalence principle

Experimental discovery of the dilaton would provide strong evidence for str ing theory. A light dilaton could show up in current tests of the inverse s quare law for gravity at sub-millimeter distances. In the large extra dimen sion scenario, Kaluza-Klein excitations of the dilaton can also contribute to the cooling of supernovae. In order to quantify these effects we compute the couplings of a low energy dilaton to matter. These predominantly arise from the fundamental dilaton coupling to the gluon field strength, which r eceives a sizable enhancement from QCD scaling. We show that detection of t he dilaton will give a direct measurement of the QCD coupling constant at t he string scale. Particular attention is paid to the size of equivalence pr inciple violating dilaton couplings.