Higgs Doublets, Split Multiplets and Heterotic SU(3)_C x SU(2)_L x   U(1)_Y Spectra
  A methodology for computing the massless spectrum of heterotic vacua with Wilson lines is presented. This is applied to a specific class of vacua with holomorphic SU(5)-bundles over torus-fibered Calabi-Yau threefolds with fundamental group Z_2. These vacua lead to low energy theories with the standard model gauge group SU(3)_C x SU(2)_L x U(1)_Yand three families of quark/leptons. The massless spectrum is computed, including the multiplicity of Higgs doublets. 
Ron Donagi, Yang-Hui He, Burt Ovrut, Rene Reinbacher