ASPECTS OF LOW ENERGY EFFECTIVE STRING THEORY

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The property of minimal coupling is investigated in the low energy limit of string theory. Effective lagrangians for ge-ne-ric theories presenting electroweak symmetry breaking are studied, with particular emphasis on effective Higgs-gauge vector interactions. Higgs fields are represented by open string states localized at brane intersections and the four-dimensional theory is obtained from the ten-dimensional one compactifying on a six torus. The coefficient of the six-dimensional operator HHFF (where H is Higgs field and F the field strength) is computed to first order in the string loop expansion. This operator does not emerge at tree level, in accord with the hypothesis of minimal coupling. It does emerge at one loop, however, which shows the absence of hidden symmetries which protect it