Proton Stability and Light $Z^\prime$ Inspired by String Derived Models

TL;DR

This paper explores how string-derived models can naturally incorporate light Z' gauge bosons that help suppress proton decay operators, addressing proton stability within a quantum gravity framework.

Contribution

It constructs string-inspired models with anomaly-free U(1) symmetries that suppress dangerous proton decay operators while remaining phenomenologically viable.

Findings

Identifies anomaly-free U(1) symmetries in string models

Augments Standard Model spectrum with additional states

Discusses phenomenological implications of light Z' bosons

Abstract

Proton stability is one of the most perplexing puzzles in particle physics. While the renormalizable Standard Model forbids proton decay mediating operators due to accidental global symmetries, many of its extensions introduce such dimension four, five and six operators. Furthermore, it is, in general, expected that quantum gravity only respects local gauge, or discreet, symmetries. String theory provides the arena to study particle physics in a consistent framework of perturbative quantum gravity. An appealing proposition, in this context, is that the dangerous operators are suppressed by an Abelian gauge symmetry, which is broken near the TeV scale. A viable U(1) symmetry should also be anomaly free, be family universal, and allow the generation of fermion masses via the Higgs mechanism. We discuss such U(1) symmetries that arise in quasi--realistic free fermionic heterotic--string derived models. Ensuring that the U(1) symmetry is anomaly free at the low scale requires that the Standard Model spectrum is augmented by additional states that are compatible with the charge assignments in the string models. We construct such string--inspired models and discuss some of their phenomenological implications.