Phenomenological Constraints on FourDimensional Superstrings.
Abstract
String theory has been proposed as the ultimate unified theory which provides a finite theory of quantum gravity. Although only a few consistent string theories are known to exist, a large number of solutions have been constructed. At the current stage of development one cannot decide from first principles which solution should be realized in the physical world. This thesis describes an analysis of the problem which leads to a drastic reduction in the number of acceptable solutions, and an investigation of the only model known to satisfy our constraints. We impose two constraints any acceptable model should obey: (1) the model must have spacetime supersymmetry; and (2) it must contain chiral fermions. The effective theory at energies below the Planck (string) scale should be a Grand Unified Theory (GUT). Unified theories are built on two basic assumptions: (1) the unified gauge symmetry breaks to the standard model gauge symmetry at a suitable highenergy scale; and (2) the model should remain perturbative at all energy scales. Symmetry breaking is achieved in most models through the standard Higgs mechanism by letting scalar fields in the adjoint representation of the gauge group acquire vacuum expectation values. It is a result of our calculations that either of the constraints alluded to above eliminates all adjoint scalars from the physical spectrum. The flipped SU(5) times U(1) model is the only known phenomenologically acceptable model which does not use adjoint scalars for its symmetry breaking. We have translated the second GUT restriction into a treelevel unitarity condition, and devised a method to systematically derive unitarity bounds on any unified model. We have applied this method to the flipped SU(5) times U(1) model and found bounds on several model parameters. As a consequence, we obtain upper bounds on the top quark mass of m_{t} <=q 200 GeV, and on the lightest Higgs mass of M_{H}_sp{1} {0} <=q 155 GeV, which guarantee that these particles will be accessible in future experiments if the flipped SU(5) times U(1) model is correct.
 Publication:

Ph.D. Thesis
 Pub Date:
 1989
 Bibcode:
 1989PhDT.......140L
 Keywords:

 Physics: Elementary Particles and High Energy