Typical entanglement entropy in systems with particle-number conservation

We calculate the typical bipartite entanglement entropy (SA)N in systems containing indistinguishable particles of any kind as a function of the total particle number N, the volume V, and the subsystem fraction f=VA/V, where VA is the volume of the subsystem. We expand our result as a power series (SA)N=afV+bV+c+o(1), and find that c is universal (i.e., independent of the system type), while a and b can be obtained from a generating function characterizing the local Hilbert space dimension. We illustrate the generality of our findings by studying a wide range of different systems, e.g., bosons, fermions, spins, and mixtures thereof. We provide evidence that our analytical results describe the entanglement entropy of highly excited eigenstates of quantum-chaotic spin and boson systems, which is distinct from that of integrable counterparts.