Studies of the motion and decay of axion walls bounded by strings - art. no. 023505.

We discuss the appearance at the QCD phase transition, and the subsequent d ecay, of axion walls bounded by strings in N = 1 axion models. We argue on intuitive grounds that the main decay mechanism is into barely relativistic axions. We present numerical simulations of the decay process. In these si mulations, the decay happens immediately, in a time scale of order the ligh t travel time, and the average energy of the radiated axions is [omega(a)] similar or equal to 7 m(a) for upsilon(a)/m(a) similar or equal to 500. [om ega(a)] is found to increase approximately linearly with ln(upsilon(a)/m(a) ). Extrapolation of this behavior yields [omega(a)] similar to 60m(a) in ax ion models of interest. We find that the contribution to the cosmological e nergy density of axions from wall decay is of the same order of magnitude a s that from vacuum realignment, with however large uncertainties. The veloc ity dispersion of axions from wall decay is found to be larger, by a factor 10(3) or so, than that of axions from vacuum realignment and string decay. We discuss the implications of this for the formation and evolution of axi on miniclusters and for the direct detection of axion dark matter on Earth. Finally we discuss the cosmology of axion models with N > 1 in which the d omain wall problem is solved by introducing a small U-pQ(1) breaking intera ction. We find that in this case the walls decay into gravitational waves. [S0556-2821(98)06324-3].