  On the phase diagram of a system undergoing a continuous phase transition of
the second order, three lines, hyper-surfaces, convergent into the critical
point feature prominently: the ordered and disordered phases in the
thermodynamic limit, and a third line, extending into a domain of finite-size
systems, defined by the bifurcation of the distribution of the order parameter.
Unlike critical phenomena in the thermodynamic limit devoid of known
thermodynamic potential and described rather by the conformal symmetry, in
finite-size systems near the bifurcation line an explicit Hamiltonian for the
zero-mode of the order parameter is found. It bears the impress of the
universality class of the critical point in terms of the two critical
exponents: $\beta$ and $\nu$.
