Permutation-Symmetric Multicritical Points in Random Antiferromagnetic Spin Chains

We present a general theory of a class of multicritical points in the phase diagrams of random antiferromagnetic spin chains. We show that low-energy properties of these points are almost completely determined by a permutation symmetry of the effective theory not shared by the microscopic Hamiltonian. One case provides an analytic theory of the quantum critical point in the random spin-[Formula presented] chain, studied in a recent work by Refael, Kehrein, and Fisher.