  An external force dynamically drives an isolated mean-field Hamiltonian
system to a long-lasting quasistationary state, whose lifetime increases with
population of the system. For second order phase transitions in quasistationary
states, two non-classical critical exponents have been reported individually by
using a linear and a nonlinear response theories in a toy model. We provide a
simple way to compute the critical exponents all at once, which is an analog of
the Landau theory. The present theory extends universality class of the
non-classical exponents to spatially periodic one-dimensional systems, and
shows that the exponents satisfy a classical scaling relation inevitably by
using a key scaling of momentum.
