Constraints on extra-dimensional theories from virtual-graviton exchange

TL;DR

This paper develops an effective-field-theory framework for extra-dimensional gravity with Standard Model fields confined to a 3-brane, introducing a cutoff Lambda and a fundamental gravity scale M_D to compare real-graviton emission with virtual-graviton exchange. It identifies a key dimension-6 operator, Upsilon, generated by graviton loops, and a dimension-8 tree-level tau operator, examining their regulatorDependence and experimental bounds. Using modified NDA and explicit regulators, the authors analyze contributions from graviton loops and graviton-plus-gauge loops, finding that LEP2 bounds on Upsilon provide some of the strongest constraints on weak-scale extra-dimensional gravity, often outperforming electroweak precision tests and (g−2)_μ. The work highlights the UV sensitivity of loop-induced coefficients and suggests that a UV completion or new states at scale Lambda are essential for robust predictions, while also noting potential collider signatures from both graviton emission and associated contact interactions.

Abstract

We study the effective interactions induced by loops of extra-dimensional gravitons and show the special role of a specific dimension-6 four-fermion operator, product of two flavour-universal axial currents. By introducing an ultraviolet cut-off, we compare the present constraints on low-scale quantum gravity from various processes involving real-graviton emission and virtual-graviton exchange. The LEP2 limits on dimension-6 four-fermion interactions give one of the strongest constraint on the theory, in particular excluding the case of strongly-interacting gravity at the weak scale.

Figures (2)

• Figure 1: Fig. \ref{['fig:treeloop']} a: tree-level graviton exchange generating the dimension-8 operator $\tau$. Fig. \ref{['fig:treeloop']} b: one-loop graviton exchange generating the dimension-6 operator $\Upsilon$. Fig. \ref{['fig:treeloop']} c: exchange of gravitons and vector bosons at one loop generating dimension-6 operators that affect electroweak precision data.
• Figure 2: $\sigma (gg\to h)\times {\rm BR}(h\to \gamma \gamma)$ in units of its SM value, in presence of the new operators $-(\pi/\Lambda^2)H^\dagger H F_{\mu\nu}F^{\mu\nu}$ and $-(\pi/\Lambda^2)H^\dagger H G^a_{\mu\nu}G^{a\mu\nu}$.