Electroweak Symmetry Breaking and Large Extra Dimensions

TL;DR

This work analyzes electroweak symmetry breaking in theories with large extra dimensions, where the fundamental gravity scale $\Lambda$ can lie near the TeV range. Using a general effective field theory with dimension-6 operators, the authors show that precision electroweak constraints on the Higgs mass can be relaxed or even removed for $4\,\mathrm{TeV} \lesssim \Lambda \lesssim 11\,\mathrm{TeV}$, depending on operator signs and flavor structure. They further study two operators, ${\cal O}_G$ and ${\cal O}_{\gamma}$, which modify Higgs production via gluon fusion and Higgs decay to $\gamma\gamma$, predicting potentially observable excesses at the Tevatron Run II and the LHC, especially if constructive interference occurs. The results imply no universal preference for a light Higgs in these scenarios and suggest distinctive gravitational signatures in Higgs channels that could signal extra dimensions, with collider reach extending up to $\Lambda \sim 10\text{ TeV}$ for favorable cases.

Abstract

If spacetime contains large compact extra dimensions, the fundamental mass scale of nature, $Lambda$, may be close to the weak scale, allowing gravitational physics to significantly modify electroweak symmetry breaking. Operators of the form $(1/Lambda^2) |phi^* D phi|^2$ and $(1/Lambda^2) phi^* W B phi$, where $W$ and $B$ are the SU(2) and U(1) field strengths and $phi$ is the Higgs field, remove the precision electroweak bound on the Higgs boson mass for values of $Lambda$ in a wide range: $4 TeV < Lambda < 11 TeV$. Within this framework, there is no preference between a light Higgs boson, a heavy Higgs boson, or a non-linearly realized SU(2)xU(1) symmetry beneath $Lambda$. If there is a Higgs doublet, then operators of the form $(1/Lambda^2) phi^* phi (G^2, F^2)$, where $G$ and $F$ are the QCD and electromagnetic field strengths, modify the production of the Higgs boson by gluon-gluon fusion, and the decay of the Higgs boson to 2 photons, respectively. At Run II of the Tevatron collider, a 2-photon signal for extra dimensions will be discovered if $Lambda$ is below 2.5 (1) TeV for a Higgs boson of mass 100 (300) GeV. Furthermore, such a signal would point to gravitational physics, rather than to new conventional gauge theories at $Lambda$. The discovery potential of the LHC depends sensitively on whether the gravitational amplitudes interfere constructively or destructively with the standard model amplitudes, and ranges from $Lambda$ = 3 - 10 (2 - 4) TeV for a light (heavy) Higgs boson.

Figures (3)

• Figure 1: Precision electroweak limits on the Higgs mass as a function of the scale of new physics. For this figure, $\Lambda_{BW}$ and $\Lambda_{\Phi}$ are chosen equal, while the signs $f_{BW}$ and $f_\Phi$ are chosen to maximize the allowed region. Hatched regions are disallowed at 95%, while the dashed line borders the region allowed in the SM alone.
• Figure 2: $5\sigma$ discovery reaches for $pp,p\overline{p}\to h\to\gamma\gamma$ in current and future colliders. Only the ${\cal O}_\gamma$ operator has been included. In (a), signs are chosen to maximize the signal, while they are chosen to minimize the signal in (b).
• Figure 3: $5\sigma$ discovery reaches for $pp,p\overline{p}\to h\to\gamma\gamma$ in current and future colliders. Both ${\cal O}_G$ and ${\cal O}_\gamma$ have been included, with $\Lambda_\gamma=\Lambda_G\equiv\Lambda$. In (a), signs are chosen to maximize the signal, while they are chosen to minimize the signal in (b).