Theta in new QCD-like sectors

TL;DR

This paper explores new collider signatures from QCD-like hypercolor sectors, focusing on diboson resonances sensitive to a vacuum angle, and discusses their implications for dark matter and the necessity of an axion in QCD.

Contribution

It introduces the study of $CP$-violating hypermeson decays as collider signatures and analyzes their discovery potential at current and future colliders, linking them to dark matter models.

Findings

Paired diboson resonances can originate from hypermeson decays.

$CP$-violating hypermeson decays are sensitive to the hypercolor vacuum angle.

Observability of these signatures implies the existence of an axion in QCD.

Abstract

New QCD-like `hypercolor' sectors can generate a broad class of new signatures at hadron colliders and furnish a variety of dark matter candidates. Paired diboson resonances are a particularly important collider signature, arising both from $CP$-conserving vector hypermeson decays of the form $\widetilde\rho \rightarrow\widetilde\pi\widetilde\pi\rightarrow 4V$ and from $CP$-violating pseudoscalar hypermeson decays of the form $\widetilde\eta\rightarrow \widetilde\pi\widetilde\pi\rightarrow 4V$. The latter are sensitive to the vacuum angle $\widetilde \theta$ in the hypercolor sector. We study single- and paired-diboson resonance signatures in final states involving gluons and photons at the LHC and a future 100 TeV $pp$ collider, illustrating the discovery potential at both colliders in simple benchmark models. We also describe some of the theoretical and cosmological consequences of $\widetilde \theta$. If $CP$-violating hypermeson decays are observable at hadron colliders, ordinary QCD must have an axion. Such scenarios also provide a natural setting for a dark pion component of dark matter, with its relic abundance set by $CP$-violating annihilations. If the new vacuum angle is relaxed to zero by a dark axion, the relic density can instead be a mixture of axions and dark axions. Overproduction of dark axions is most easily avoided if the universe underwent a period of early matter domination.