Implications of Higgs Discovery for the Strong CP Problem and Unification

TL;DR

This paper explores how a Z2 symmetry extension of the Standard Model, combined with specific Higgs sector modifications, can address the strong CP problem, achieve gauge coupling unification, and predict observable phenomena like neutron EDM and proton decay.

Contribution

It introduces a novel Z2 symmetry framework that links Higgs sector modifications to solutions for the strong CP problem and gauge unification, with detailed phenomenological predictions.

Findings

Neutron electric dipole moment around 10^{-27} e cm predicted.

Gauge coupling unification at ~10^{10} GeV consistent with Higgs mass.

Proton decay prospects improved with lowered unification scale.

Abstract

A $Z_2$ symmetry that extends the weak interaction, $SU(2)_L \rightarrow SU(2)_L \times SU(2)'$, and the Higgs sector, $H(2) \rightarrow H(2,1) + H'(1,2)$, yields a Standard Model quartic coupling that vanishes at scale $v' = <H'>~\gg~<H>$. Near $v'$, theories either have a "prime" sector, or possess "Left-Right" (LR) symmetry with $SU(2)' = SU(2)_R$. If the $Z_2$ symmetry incorporates spacetime parity, these theories can solve the strong CP problem. The LR theories have all quark and lepton masses arising from operators of dimension 5 or more, requiring Froggatt-Nielsen structures. Two-loop contributions to $\bar{\theta}$ are estimated and typically lead to a neutron electric dipole moment of order $10^{-27}$e cm that can be observed in future experiments. Minimal models, with gauge group $SU(3) \times SU(2)_L \times SU(2)_L \times U(1)_{B-L}$, have precise gauge coupling unification for $v' = 10^{10\pm1}$ GeV, successfully correlating gauge unification with the observed Higgs mass of $125$ GeV. With $SU(3) \times U(1)_{B-L}$ embedded in $SU(4)$, the central value of the unification scale is reduced from $10^{16-17}$ GeV to below $10^{16}$ GeV, improving the likelihood of proton decay discovery. Unified theories based on $SO(10) \times CP$ are constructed that have $H+H'$ in a ${\bf 16}$ or ${\bf 144}$ and generate higher-dimensional flavor operators, while maintaining perturbative gauge couplings.