THE VACUUM-STRUCTURE OF LIGHT-FRONT PHI(1-THEORY(1)(4))

We discuss the vacuum structure of phi(4)-theory in 1 + 1 dimensions q uantised on the light-front x(+) = 0. To this end, one has to solve a non-linear, operator-valued constraint equation. It expresses that mod e of the field operator having longitudinal light-front momentum equal to zero, as a function of all the other modes in the theory. We analy se whether this zero mode can lead to a nonvanishing vacuum expectatio n value of the field phi and thus to spontaneous symmetry breaking. In perturbation theory, we get no symmetry breaking. If we solve the con straint, however, non-perturbatively, within a mean-field type Fock an satz, the situation changes: while the vacuum state itself remains tri vial, we find a non-vanishing vacuum expectation value above a critica l coupling. Exactly the same result is obtained within a light-front T amm-Dancoff approximation, if the renormalisation is done in the corre ct way.