Quantum Gravity and Extra Dimensions at High-Energy Colliders

TL;DR

The paper demonstrates that if gravity propagates in delta large extra dimensions with a TeV-scale MD, a tower of KK gravitons yields distinctive collider signals, notably missing energy in association with photons or jets. By building a 4D effective theory, deriving Feynman rules, and computing both real-emission and virtual-exchange processes, it quantifies the discovery reach across e+e-, muon, LEP2, Tevatron, and LHC experiments. Graviton production through jet+MET and gamma+MET channels can probe MD in the multi-TeV to tens-of-TeV range depending on delta, while a dimension-8 operator 𝒯 from virtual graviton exchange provides complementary tests, especially in γγ final states. The work also discusses perturbativity limits and UV sensitivity, highlighting how different colliders offer complementary avenues to test TeV-scale gravity and extra dimensions.

Abstract

Recently it has been pointed out that the characteristic quantum-gravity scale could be as low as the weak scale in theories with gravity propagating in higher dimensions. The observed smallness of Newton's constant is a consequence of the large compactified volume of the extra dimensions. We investigate the consequences of this supposition for high-energy collider experiments. We do this by first compactifying the higher dimensional theory and constructing a 3+1-dimensional low-energy effective field theory of the graviton Kaluza-Klein excitations and their interactions with ordinary matter. We then consider graviton production processes, and select photon plus missing energy and jet plus missing energy signatures for careful study. We find that both a 1 TeV e+e- collider and the CERN LHC will be able to reliably and perturbatively probe the fundamental gravity scale up to several TeV, with the precise value depending on the number of extra dimensions. Similarly, searches at LEP2 and the Tevatron are able to probe this scale up to approximately 1 TeV. We also discuss virtual graviton exchange, which induces local dimension-eight operators associated with the square of the energy-momentum tensor. We estimate the size of such operators and study their effects on two fermions to two photons observables.

Figures (10)

• Figure 1: Total $\gamma + {\rm nothing}$ cross-section at an $e^+e^-$ collider for $\sqrt{s}=1\hbox{\rm,TeV}$ with $E_{T,\gamma}>E_{T,\gamma}^{\rm min}$. The dash-dotted line represents the background, and the solid lines represent the signal for various numbers of extra dimensions and $M_D=1.5\hbox{\rm,TeV}$. To eliminate the background contribution from $\gamma Z\rightarrow \gamma \bar{\nu}\nu$ we have required $E_\gamma < 450\hbox{\rm,GeV}$ for both the signal and the background. The dashed line is the Standard Model background subtracted signal from a representative dimension-6 operator.
• Figure 2: Total $e^+e^-\rightarrow \gamma + {\rm nothing}$ cross-section at a $1\hbox{\rm,TeV}$ centre-of-mass energy $e^+e^-$ collider. The signal from graviton production is presented as solid lines for various numbers of extra dimension ($\delta =2,3,4,5$). The Standard Model background for unpolarized beams is given by the upper dash-dotted line, and the background with $90\%$ polarization is given by the lower dash-dotted line. The signal and background are computed with the requirement $E_\gamma < 450\hbox{\rm,GeV}$ in order to eliminate the $\gamma Z\rightarrow \gamma\bar{\nu}\nu$ contribution to the background. The dashed line is the Standard Model background subtracted signal from a representative dimension-6 operator.
• Figure 3: The total ${\rm jet}+{\rm nothing}$ cross-section at the LHC integrated for all $E_{T,{\rm jet}}> E_{T,{\rm jet}}^{\rm min}$ with the requirement that $|\eta_{\rm jet}|< 3.0$. The Standard Model background is the dash-dotted line, and the signal is plotted as solid and dashed lines for fixed $M_D=5\hbox{\rm,TeV}$ with $\delta=2$ and $4$ extra dimensions. The a ( b) lines are constructed by integrating the cross-section over $\hat{s} < M_D^2$ (all $\hat{s}$).
• Figure 4: The total ${\rm jet}+{\rm nothing}$ cross-section versus $M_D$ at the LHC integrated for all $E_{T,{\rm jet}}> 1\hbox{\rm,TeV}$ with the requirement that $|\eta_{\rm jet}|< 3.0$. The Standard Model background is the dash-dotted line, and the signal is plotted as solid and dashed lines for $\delta=2$ and $4$ extra dimensions. The a ( b) lines are constructed by integrating the cross-section over $\hat{s} < M_D^2$ (all $\hat{s}$).
• Figure 5: The total ${\gamma}+{\rm nothing}$ cross-section at the LHC integrated for all $E_{T,\gamma}> E_{T,\gamma}^{\rm min}$ with the requirement that $|\eta_\gamma| < 2.5$. The Standard Model background is the dash-dotted line, and the signal is plotted as solid and dashed lines for fixed $M_D=2\hbox{\rm,TeV}$ with $\delta=2$ and $4$ extra dimensions. The a ( b) lines are constructed by integrating the cross-section over $\hat{s} < M_D^2$ (all $\hat{s}$).