Proton stability in grand unified theories, in strings and in branes

String theory and the path to unification: A Review of recent developments

This is a pedagogical review article surveying the various approaches towards understanding gauge coupling unification within string theory. As is well known, one of the major problems confronting string phenomenology has been an apparent discrepancy between the scale of gauge coupling unification predicted within string theory, and the unification scale expected within the framework of the Minimal Supersymmetric Standard Model (MSSM). In this article, I provide an overview of the different approaches that have been taken in recent years towards reconciling these two scales, and outline some of the major recent developments in each. These approaches include 
1) string GUT models; 
2) higher affine levels and non-standard hypercharge normalizations; 
3) heavy string threshold corrections; 
4) light supersymmetric thresholds; 
5) effects from intermediate-scale gauge and matter structure beyond the MSSM; 
6) strings without supersymmetry; and 
7) strings at strong coupling.

String Theory and the Path to Unification: A Review of Recent Developments

Cosmic strings are predicted by many field-theory models, and may have been formed at a symmetry-breaking transition early in the history of the universe, such as that associated with grand unification. They could have important cosmological effects. Scenarios suggested by fundamental string theory or M-theory, in particular the popular idea of brane inflation, also strongly suggest the appearance of similar structures. Here we review the reasons for postulating the existence of cosmic strings or superstrings, the various possible ways in which they might be detected observationally and the special features that might discriminate between ordinary cosmic strings and superstrings.

https://royalsocietypublishing.org/doi/pdf/10.1098/rspa.2009.0591

Cosmic strings and superstrings

We discuss the non-perturbative renormalization group flow of Quantum Electrodynamics (QED) coupled to Quantum Einstein Gravity (QEG) and explore the possibilities for defining its continuum limit at a fixed point that would lead to a non-trivial, i.e. interacting field theory. We find two fixed points suitable for the Asymptotic Safety construction. In the first case, the fine-structure constant α vanishes at the fixed point and its infrared (“renormalized”) value is a free parameter not determined by the theory itself. In the second case, the fixed point value of α is non-zero, and its infrared value is a computable prediction of the theory.

QED coupled to QEG

http://cds.cern.ch/record/234848/files/ppe-91-233.pdf

Ugo Amaldo

Papers

Consistency checks of grand unified theories