Massless spectra of three generation U(N) heterotic string vacua

We provide the methods to compute the complete massless spectra of a class of recently introduced supersymmetric E-8 x E-8 heterotic string models which invoke vector bundles with U(N) structure group on simply connected Calabi-Yau manifolds and which yield flipped SU (5) and MSSM string vacua of potential phenomenological interest. We apply Leray spectral sequences in order to derive the localisation of the cohomology groups H-i (X, V-a circle times V-b), H-i (X, boolean AND(2) V) and H-i( X, (SV)-V-2) for vector bundles defined via Fourier-Mukai transforms on elliptically fibered Calabi-Yau manifolds. By the method of bundle extensions we define a stabl U(4) vector bundle leading to the first flipped SU(5) model with just three generations, i.e. without any vector-like matter. Along the way, we propose the notion of lambda-stability for heterotic bundles.