A Novel String Derived Z' With Stable Proton, Light-Neutrinos and R-parity violation

TL;DR

The paper identifies a gauged, anomaly-free U(1) symmetry in a class of left-right symmetric free-fermionic heterotic-string models that can suppress proton-decay mediating operators while preserving Yukawa couplings and a neutrino seesaw mechanism. It introduces a string-inspired effective Z' model with the combination $U(1)_{\zeta'}=(1/5)U(1)_{Z'}-U(1)_\zeta$, and demonstrates how proton stability can be ensured even with lepton-number violation and R-parity violation. The authors outline the neutrino mass generation through an extended seesaw and provide estimates for the breaking scale $\Lambda_{\zeta'}$, showing that a TeV-scale Z' could be accessible depending on the vevs of certain fields. These results offer a concrete, testable link between string-derived gauge structures and low-energy proton stability with distinct collider phenomenology and potential dark-matter implications.

Abstract

The Standard Model indicates the realization of grand unified structures in nature, and can only be viewed as an effective theory below a higher energy cutoff. While the renormalizable Standard Model forbids proton decay mediating operators due to accidental global symmetries, many extensions of the Standard Model introduce such dimension four, five and six operators. Furthermore, quantum gravity effects are expected to induce proton instability, indicating that the higher energy cutoff scale must be above 10^{16}GeV. Quasi-realistic heterotic string models provide the arena to explore how perturbative quantum gravity affects the particle physics phenomenology. An appealing explanation for the proton longevity is provided by the existence of an Abelian gauge symmetry that suppresses the proton decay mediating operators. Additionally, such a low-scale U(1) symmetry should: allow the suppression of the left-handed neutrino masses by a seesaw mechanism; allow fermion Yukawa couplings to the electroweak Higgs doublets; be anomaly free; be family universal. These requirements render the existence of such U(1) symmetries in quasi-realistic heterotic string models highly non-trivial. We demonstrate the existence of a U(1) symmetry that satisfies all of the above requirements in a class of left-right symmetric heterotic string models in the free fermionic formulation. The existence of the extra Z' in the energy range accessible to future experiments is motivated by the requirement of adequate suppression of proton decay mediation. We further show that while the extra U(1) forbids dimension four baryon number violating operators it allows dimension four lepton number violating operators and R-parity violation.