  We study the nature of the \nu=5/2 quantum Hall state in wide quantum wells
under the mixing of electronic subbands and Landau levels. We introduce a
general method to analyze the Moore-Read Pfaffian state and its particle-hole
conjugate, the anti-Pfaffian, under periodic boundary conditions in a
"quartered" Brillouin zone scheme containing both even and odd numbers of
electrons. We examine the rotational quantum numbers on the torus, and show
spontaneous breaking of the particle-hole symmetry can be observed in
finite-size systems. In the presence of electronic-subband and Landau-level
mixing the particle-hole symmetry is broken in such a way that the
anti-Pfaffian is unambiguously favored, and becomes more robust in the vicinity
of a transition to the compressible phase, in agreement with recent
experiments.
