Yukawa Couplings at Infinite Distance and Swampland Towers in Chiral Theories

TL;DR

This paper investigates how Yukawa couplings vanish in certain quantum gravity limits, revealing infinite towers of light particles, decompactification, and implications for Standard Model small couplings.

Contribution

It demonstrates the behavior of Yukawa couplings approaching zero in type IIA orientifolds, linking to tower decompactification and the emergence of kinetic terms in quantum gravity.

Findings

Yukawa couplings scale as 1/u^r in large complex structure limits.

Infinite towers of charged particles become massless as Yukawa couplings vanish.

Results support the magnetic WGC and have implications for Standard Model phenomenology.

Abstract

We study limits of vanishing Yukawa couplings of 4d chiral matter fields in Quantum Gravity, using as a laboratory type IIA orientifolds with D6-branes. In these theories chiral fermions arise at brane intersections, where an infinite tower of charged particles dubbed gonions are localised. We show that in the limit $Y\rightarrow 0$ some of these towers become asymptotically massless, while at the same time the kinetic term of some chiral fields becomes singular and at least two extra dimensions decompactify. For limits parametrised by a large complex structure saxion $u$, Yukawa couplings have a behaviour of the form $Y \, \sim \, {1}/{u^r}$, with $r$ some positive rational number. Moreover, in this limit some of the gauge couplings associated to the Yukawa vanish. The lightest gonion scales are of order $m_{\rm gon} \sim g^s M_{\rm P}$ with $s>1$, verifying the magnetic WGC with room to spare and with no need of its tower/sublattice versions. We also show how this behaviour can be understood in the context of the emergence of kinetic terms in Quantum Gravity. All these results may be very relevant for phenomenology, given the fact that some of the Yukawa couplings in the Standard Model are very small.