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Title: String phenomenology in the era of the LHC
Author: Ashfaque, Johar Muhammad
ISNI:       0000 0004 6422 795X
Awarding Body: University of Liverpool
Current Institution: University of Liverpool
Date of Award: 2017
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The heterotic string-derived models in the free fermionic formulation give rise to some of the most realistic string models to date, which possess N = 1 space-time supersymmetry (SUSY). The characteristics of the required spectrum are well motivated in heterotic string constructions that allow for a light Z0. Anomaly cancellation of the U(1)Z0 symmetry requires the existence of the Standard Model (SM) singlet and vector-like states in the vicinity of the U(1)Z0 breaking scale. We show that the agreement with the gauge coupling data at one-loop is identical to the case of the Minimal Supersymmetric Standard Model (MSSM), owing to cancellations between the additional states. It is also shown that effects arising from heavy thresholds may push the supersymmetric spectrum beyond the reach of the LHC, while maintaining the agreement with the gauge coupling data. On the other hand, lack of evidence for SUSY at the LHC has led to the recent interest in non-supersymmetric heterotic string vacua. We explore what may be learned in this context from the quasi{realistic free fermionic models. We show that constructions with a low number of families give rise to proliferation of a priori tachyon producing sectors, compared to the non{realistic examples which typically may contain only one such sector, followed by a concrete example of a non{supersymmetric, non{tachyonic, heterotic string vacuum where we compare the structure of its massless spectrum to the corresponding supersymmetric vacuum. While in some sectors SUSY is broken explicitly, i.e. the bosonic and fermionic sectors produce massless and massive states, other sectors, and in particular those leading to the chiral families, continue to exhibit Fermi-Bose degeneracy. In these sectors the massless spectrum, as compared to the supersymmetric cases, will only differ in some local or global U(1) charges. Our example model contains an anomalous U(1) symmetry, the cancellation mechanism for which generates a tadpole diagram at one loop{order in string perturbation theory. We entertain the possibility of the cancellation of this tadpole diagram against the corresponding diagram generated at one{loop by the non{vanishing vacuum energy and that in this respect the supersymmetric and non{supersymmetric vacua should be regarded on equal footing.
Supervisor: Faraggi, A. Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral