Chiral perturbation theory, finite-size effects, and the three-dimensional XY model.

s We study finite size effects of the d=3 XY model in terms of the chiral perturbation theory. We calculate by Monte Carlo simulations physical quantities which are, to order of (1/L), uniquely determined only by two low energy constants. They are the magnetization and the helicity modulus (or the Goldstone boson decay constant) in infinite volume. We also pay a special attention to the region of the validity of the two possible expansions in the theory. * e-mail:[email protected] ** e-mail:[email protected]