Graviscalars from higher-dimensional metrics and curvature-Higgs mixing

TL;DR

The paper analyzes scalar degrees of freedom arising from gravity in extra dimensions, focusing on graviscalars in ADD and the radion in RS. It shows that a non-minimal curvature coupling ξ enables Higgs–graviscalar mixing, producing an invisible Higgs width and altering Higgs phenomenology in ADD, where a dense KK spectrum acts as a decay channel. In the RS scenario, the radion mixes with the Higgs, leading to modified couplings and novel signatures, notably gluon-fusion production via trace anomaly and mass eigenstates whose decays depend sensitively on ξ and Λ_φ. The work demonstrates that collider experiments, especially the LHC, can probe the presence of non-factorizable geometries by measuring invisible widths and radion-induced signals across multi-TeV scales and varied Higgs-curvature mixing.

Abstract

We investigate the properties of scalar fields arising from gravity propagating in extra dimensions. In the scenario of large extra dimensions, proposed by Arkani-Hamed, Dimopoulos and Dvali, graviscalar Kaluza-Klein excitations are less important than the spin-2 counterparts in most processes. However, there are important exceptions. The Higgs boson can mix to these particles by coupling to the Ricci scalar. Because of the large number of states involved, this mixing leads, in practice, to a sizeable invisible width for the Higgs. In the Randall-Sundrum scenario, the only graviscalar is the radion. It can be produced copiously at hadron colliders by virtue of its enhanced coupling to two gluons through the trace anomaly of QCD. We study both the production and decay of the radion, and compare it to the Standard Model Higgs boson. Furthermore, we find that radion detectability depends crucially on the curvature-Higgs boson mixing parameter.

Figures (9)

• Figure 1: The contour of integration ${\cal C}$ (solid line) and ${\cal C}_{\bar{\epsilon}}$ (dashed line) in the $k^2-{\bar{k}}^2$ complex plane. It is assumed that $\Delta m_G^2\ll\bar{\epsilon}\ll\Gamma^2$, where $\Delta m_G^2$ is the splitting between graviton poles.
• Figure 2: Branching fraction of the Higgs boson to decay invisibly as a function of its mass, for $M_D=2\hbox{\rm,TeV}$ and $\xi =1$. The rapid decrease at $m_h\simeq 160\hbox{\rm,GeV}$ is due to the onset of $h\rightarrow WW$ on-shell decays.
• Figure 3: Upper bound on the sensitivity to $M_D$ from invisible width measurements of the Higgs boson. We have approximated the width measurement capability as the expected uncertainty in the SM Higgs width determination gunion talks at a muon collider (on-shell scan with $0.4\hbox{\rm, fb}^{-1}$) and $e^+e^-$ linear collider ($\sqrt{s}=500\hbox{\rm,GeV}$ with $200\hbox{\rm, fb}^{-1}$) given $\xi=1$ and various choices of the number of extra dimensions $\delta$.
• Figure 4: Differential signal cross-section of $e^+e^-\rightarrow ZH^{(\vec{n})}$ at LEP2 with $200\hbox{\rm,GeV}$ center of mass energy and for the parameter choices $M_D=1\hbox{\rm,TeV}$ and $\xi=0$. $M_{\rm miss}$ is the missing mass associated with escaping graviscalars.
• Figure 5: Branching fractions of $\varphi'$ as a function of its mass given $m_h=125\hbox{\rm,GeV}$, $\Lambda_\varphi =10\hbox{\rm,TeV}$ and $\xi=0$. The left and right panels are the same except a different range in radion mass is covered.