The String Theory Photoverse

TL;DR

This work argues that string theory generically predicts a photoverse of massless hidden photons arising from abundant abelian gauge factors in compactifications. The authors show that, because these superhidden photons lack renormalizable couplings to light SM currents, their leading interactions with the Standard Model are dimension-6 dipole operators generated through a detailed 8d→6d→4d reduction of D7-brane fermions and bulk RR-field profiles. They derive the dipole couplings for both brane and RR photons, estimate the suppression scale $\Lambda$ for these operators, and translate a broad set of astrophysical, cosmological, and laboratory bounds into lower bounds on the string scale $M_s$, with strong sensitivity in large-volume/anisotropic compactifications. The results imply that current data already probe significant portions of the string-scale parameter space and can exceed collider bounds in some scenarios, particularly when moduli stabilization (e.g., LVS) and SUSY breaking are taken into account. The paper also sketches the possibility of a photinoverse via hidden-photino mixing with MSSM neutralinos, motivating further exploration of cosmological and collider implications. Overall, dipole-mediated interactions offer a robust, model-dependent avenue to test generic features of the string theory landscape through experimental and observational data.

Abstract

String theory compactifications come with numerous $U(1)$ factors, implying the presence of many hidden photons in the low-energy EFT. One may call this the ``string photoverse''. We argue that, generically, these hidden photons are massless and do not couple to any light dark current such that, naively, kinetic mixing with the Standard Model is unobservable. The leading interactions of these ``superhidden'' photons are then dimension-6 dipole operators which couple them to quarks or leptons and the Higgs field. This induces magnetic and electric dipole moments with respect to both the superhidden photons as well as, through kinetic mixing, to the Standard Model photon. We derive these couplings by dimensionally reducing the fermionic action of 7-branes realizing the Standard Model: In the first step to 6d theories on intersection curves and then, in the presence of fluxes, to our 4d chiral EFT. We analyze how experiments and observations can employ this effect to place lower bounds on the string scale, which is relevant for compactifications with very large volumes. Finally, we briefly discuss how supersymmetry implies the presence of relatively light photinos and hence an accompanying ``photinoverse'', which may be observed via renormalizable mixing effects.

Figures (5)

• Figure 1: Illustration of our geometric setup. We consider type IIB orientifolds with D3/D7-branes. The SM is realized on intersecting D7-branes on a 4-cycle with volume $\tau_{\rm SM}$. An isolated spacetime-filling D3-brane sits somewhere in the internal space. The D3-brane worldvolume field strength sources $H_{(3)},\,F_{(3)}$ profiles which in turn couple to matter living on the SM D7s. Additional 4d $U(1)$ gauge bosons arise from the dimensional reduction of $C_{(4)}$ on three-cycles (depicted in green).
• Figure 2: Local geometry of the realization of the Pati--Salam model. A $U(4)$ stack of branes and two $U(2)$ stacks intersect at right angles. The SM fermions live on the matter curves at the intersection of the $U(4)$ stack and the two $U(2)$ stacks while the Higgs live on the intersection of the two $U(2)$.
• Figure 3: Feynman diagram calculating the contribution to the dipole operator $X_{\mu\nu}\overline e H_d\sigma^{\mu\nu}\ell$ that arises from integrating out massive KK modes of the lepton field $\ell$.
• Figure 4: Example of a process mediated by SUSY partners, provided the existence of the $\overline{\tilde{H}_u}\gamma^{\mu\nu}\tilde{H}_d X_{\mu\nu}$ operator. $\tilde{H}_u$ and $\tilde{H}_d$ are the superpartners of $H_u$ and $H_d$ while $\tilde{d}/\tilde{Q}$ denote the down/doublet squarks. The arrows in the diagram are drawn according to the two-component formalism often used in SUSY phenomenology.
• Figure 5: Geometric picture of our setup in the LVS scenario. The cycles $\tau_{\rm b}$ and $\tau_{\rm s}$ schematically represent respectively the large and small cycles in a swiss-cheese realization of the LVS. Stacks of D7-branes wrap relatively small cycles, collectively denoted $\tau_{\rm SM}$, and the SM is realized on their intersection. An isolated D3-brane sits somewhere in the internal space.