SYMMETRY-BREAKING AT ENHANCED SYMMETRY POINTS

The influence of world-sheet boundary condensates on the toroidal comp actification of bosonic string theories is considered, At the special points in the moduli space at which the closed-string theory possesses an enhanced unbroken G x G symmetry (where G is a semi-simple product of simply laced groups) a scalar boundary condensate parameterizes th e coset G x G/G. Fluctuations around this background define an open-st ring generalization of the corresponding chiral non-linear sigma model , Tree-level scattering amplitudes of on-shell massless states ('pions ') reduce to the amplitudes of the principal chiral model for the grou p G in the low-energy limit. Furthermore, the condition for the vanish ing of the renormalization group beta function at one loop results in the familiar equation of motion for that model. The quantum correction s to the open-string theory generate a mixing of open and closed strin gs so that the coset-space pions mix with the closed-string G x G gaug e fields, resulting in a Higgs-like breakdown of the symmetry to the d iagonal G group. The case of non-oriented strings is also discussed.