LARGE MASS RIGIDITY FOR A LIQUID DROP MODEL IN 2D WITH KERNELS OF FINITE MOMENTS

Motivated by Gamow's liquid drop model in the large mass regime, we consider an isoperimetric problem in which the standard perimeter P(E) is replaced by P(E)-gamma P-epsilon(E), with 0 < gamma < 1 and P-epsilon a nonlocal energy such that P-epsilon(E)-+ P(E) as epsilon vanishes. We prove that unit area minimizers are disks for epsilon > 0 small enough. More precisely, we first show that in dimension 2, minimizers are necessarily convex, provided that epsilon is small enough. In turn, this implies that minimizers have nearly circular boundaries, that is, their boundary is a small Lipschitz perturbation of the circle. Then, using a Fuglede-type argument, we prove that (in arbitrary dimension n >= 2) the unit ball in R-n is the unique unit-volume minimizer of the problem among centered nearly spherical sets. As a consequence, up to translations, the unit disk is the unique minimizer. This isoperimetric problem is equivalent to a generalization of the liquid drop model for the atomic nucleus introduced by Gamow, where the nonlocal repulsive potential is given by a radial, sufficiently integrable kernel. In that formulation, our main result states that if the first moment of the kernel is smaller than an explicit threshold, there exists a critical mass m(0) such that for any m > m(0), the disk is the unique minimizer of area m up to translations. This is in sharp contrast with the usual case of Riesz kernels, where the problem does not admit minimizers above a critical mass.