TeV Strings and Collider Probes of Large Extra Dimensions

TL;DR

This paper argues that in the ADD framework with large extra dimensions, TeV-scale string Regge excitations of Standard Model states dominate collider phenomenology over traditional KK graviton effects. Using a simple open-string toy model on a D3-brane, it derives a universal string form factor ${\cal S}(s,t)$ that multiplies SM amplitudes, yielding leading corrections of order $M_S^{-4}$ and predicting a spectrum of SR resonances at masses $M_n=\sqrt{n} M_S$. It then analyzes concrete processes (e.g., $e^+e^- \to \gamma\gamma$, Bhabha scattering, $e^+e^- \to \gamma G$, and $\gamma\gamma$ scattering) to show SR effects dominate and provide distinctive collider signatures, while KK-graviton effects are subleading. The paper translates these results into experimental constraints, finding a robust lower bound $M_S \gtrsim 1$ TeV and corresponding $M \gtrsim 1.6$ TeV, with the LHC expected to probe several TeV scales, making stringy collider phenomenology a central test of TeV-scale gravity models.

Abstract

Arkani-Hamed, Dimopoulos, and Dvali have proposed that the fundamental gravitational scale is close to 1 TeV, and that the observed weakness of gravity at long distances is explained by the presence of large extra compact dimensions. If this scenario is realized in a string theory of quantum gravity, the string excited states of Standard Model particles will also have TeV masses. These states will be visible to experiment and in fact provide the first signatures of the presence of a low quantum gravity scale. Their presence also affects the more familiar signatures due to real and virtual graviton emission. We study the effects of these states in a simple string model.

Figures (11)

• Figure 1: Schematic diagrams contributing to scattering amplitudes in a string generalization of the Standard Model: (a) tree-level 2-body scattering; (b) graviton emission; (c) loop-level 2-body scattering.
• Figure 2: Nonzero 4-point ordered tree amplitudes of Yang-Mills theory. Wavy lines represent gauge bosons; straight lines represent fermions. The sign for each line is the helicity, directed inward.
• Figure 3: Schematic illustration and world-sheet diagram of the scattering process involving four open strings on a D-brane.
• Figure 4: Factorization of the open-string amplitude to produce a pole in $(k_1 + k_2)^2$ and a three-gluon vertex.
• Figure 5: Comparison of data on Bhabha scattering at 183 GeV with models of corrections to the Standard Model from higher-dimension operators. The plot shows the fractional deviation from the Standard Model, $(d \sigma/d\cos\theta / d \sigma/d\cos\theta/|_{\hbox{\scriptsize SM}} -1)$ versus $\cos\theta$. The four curves represent: solid, string model with $M_S = 410$ GeV; dotted, KK exchange with $M_H = 830$ GeV; dashed, VV contact interactions with $\Lambda = 8800$ GeV; dot-dash, AA contact interactions with $\Lambda = 6700$ GeV.