RR photons

TL;DR

This paper explores how Ramond-Ramond U(1) gauge symmetries in string compactifications can mix with hypercharge, revealing potential phenomenological effects such as corrections to gauge coupling unification.

Contribution

It provides geometric criteria for RR U(1) mixing with D-brane U(1)s, especially via St"uckelberg couplings involving torsional cycles, and discusses phenomenological implications.

Findings

RR U(1) mixing can occur via kinetic or mass mixing.

Torsional cycles lead to discrete gauge symmetries and Aharonov-Bohm effects.

Mass mixing may affect hypercharge unification in GUT models.

Abstract

Type II string compactifications to 4d generically contain massless Ramond-Ramond U(1) gauge symmetries. However there is no massless matter charged under these U(1)'s, which makes a priori difficult to measure any physical consequences of their existence. There is however a window of opportunity if these RR U(1)'s mix with the hypercharge $U(1)_Y$ (hence with the photon). In this paper we study in detail different avenues by which $U(1)_{RR}$ bosons may mix with D-brane U(1)'s. We concentrate on Type IIA orientifolds and their M-theory lift, and provide geometric criteria for the existence of such mixing, which may occur either via standard kinetic mixing or via the mass terms induced by St\"uckelberg couplings. The latter case is particularly interesting, and appears whenever D-branes wrap torsional $p$-cycles in the compactification manifold. We also show that in the presence of torsional cycles discrete gauge symmetries and Aharanov-Bohm strings and particles appear in the 4d effective action, and that type IIA St\"uckelberg couplings can be understood in terms of torsional (co)homology in M-theory. We provide examples of Type IIA Calabi-Yau orientifolds in which the required torsional cycles exist and kinetic mixing induced by mass mixing is present. We discuss some phenomenological consequences of our findings. In particular, we find that mass mixing may induce corrections relevant for hypercharge gauge coupling unification in F-theory SU(5) GUT's.