SO(10) heterotic M-theory vacua
  The SO(10) embedding of the Standard Model spectrum is supported by evidence for neutrino masses. This thesis adapts the available formalism to study a class of heterotic M-theory vacua with SO(10) grand unification group. Compactification to four dimensions with N = 1 supersymmetry is achieved on a torus fibered Calabi-Yau 3-fold Z = X / tau_{X} with first homotopy group pi_{1}(Z) = Z_{2}. Here X is an elliptically fibered Calabi-Yau 3-fold which admits two global sections and tau_{X} is a freely acting involution on X. The vacua in this class have net number of three generations of chiral fermions in the observable sector and may contain M5-branes in the bulk space which wrap holomorphic curves in Z. Vacua with nonvanishing and vanishing instanton charges in the observable sector are considered. The latter case corresponds to potentially viable matter Yukawa couplings. Since pi_{1}(Z) = Z_{2}, the grand unification group can be broken with Z_{2} Wilson lines.   Realistic free-fermionic models preserve the SO(10) embedding of the Standard Model spectrum. These models have a stage in their construction which corresponds to Z_{2} x Z_{2} orbifold compactification of the weakly coupled 10-dimensional heterotic string. This correspondence identifies associated Calabi-Yau 3-folds which possess the structure of the above Z and X. This, in turn, allows the above formalism to be used to study heterotic M-theory vacua associated with realistic free-fermionic models. It is argued how the top quark Yukawa coupling in these models can be reproduced in the heterotic M-theory limit. 
Richard S. Garavuso