ON THE INSTANTON CONTRIBUTIONS TO THE MASSES AND COUPLINGS OF E(6) SINGLETS

We consider the gauge neutral matter in the low-energy effective actio n for string theory compact compactification on a Calabi-Yau manifold with (2,2) world-sheet supersymmetry. At the classical level these sta tes (the 1's of E(6)) correspond to the cohomology group H-1(M, End T) . We examine the first order contribution of instantons to the mass ma trix of these particles. In principle, these corrections depend on the Kahler parameters t(i) through factors of the form e(2 pi iti) and al so depend on the complex structure parameters, For simplicity we consi der in greatest detail the quintic threefold P-4[5]. It follows on gen eral grounds that the total mass is often, and perhaps always, zero. T he contribution of individual instantons is however non-zero and the c ontribution of a given instanton may develop poles associated with ins tantons coalescing for certain values of the complex structure. This c an happen when the underlying Calabi-Yau manifold is smooth. Hence the se poles must cancel between the coalescing instantons in order that t he superpotential be finite. We examine also the Yukawa couplings invo lving neutral matter 1(3) and neutral and charged fields 27 . <(27)ove r bar> . 1, which have been little investigated even though they are o f phenomenological interest, We study the general conditions under whi ch these couplings vanish classically. We also calculate the first-ord er world-sheet instanton correction to these couplings and argue that these also vanish.