Dark Matter and Enhanced Higgs to Di-photon Rate from Vector-like Leptons

TL;DR

The paper explores a minimal SM extension by adding a vector-like lepton family that provides a dark matter candidate and can enhance the Higgs to di-photon decay via loop effects. It performs a comprehensive analysis of electroweak precision constraints, LEP limits, and vacuum stability to bound Yukawa couplings and mass splittings, identifying a co-annihilation regime with a near-degenerate lightest charged lepton that yields both correct relic density and sizeable di-photon enhancements. The authors compute dark matter relic density and direct-detection cross sections, and outline LHC signatures through Drell-Yan production and related channels, arguing that the scenario is testable in the near future. They also discuss the implications of gauge coupling running and the need for UV completion to ensure vacuum stability at higher scales.

Abstract

In this paper, we study an extension of the standard model with a vector-like generation of leptons. This model provides a viable dark matter candidate and a possibility to enhance the Higgs decay rate into a pair of photons. We evaluate constraints from electroweak precision tests and from vacuum stability, and find that the latter provide an upper limit on the lepton Yukawa couplings. A large enhancement of the Higgs di-photon rate can therefore only be obtained when the mass of the lightest charged lepton is close to the LEP limit. The relic density constraint suggests a co-annihilation scenario with a small mass difference between the lightest charged and neutral leptons, which also weakens the LEP limit on the lightest charged lepton mass and allows for larger Higgs di-photon decay rates. Cross sections for direct detection of the dark matter candidate are calculated, and prospects for detecting the new particles at the LHC are discussed briefly.

Figures (8)

• Figure 1: Decoupling of effects on electroweak precision observables. The green (grey) shaded region shows the allowed parameter space as function of the mass splitting parameters $\Delta m' = (Y_c' - Y_n') v$ and $\Delta m" = (Y_c" - Y_n")v$. Also shown are contours of constant $\Delta T$ (solid) and $\Delta S$ (dashed). On the left the vector-like masses are all vanishing, while the right plot is for $m_\ell = m_e = m_\nu = 400$ GeV.
• Figure 2: Decoupling of $S$ (red, solid) and $T$ (blue, dashed) with increasing vector masses $m_\ell = m_e$, for $Y_c'=Y_c" =0.8$, $Y_n' = Y_n"=0$ and $M' =M"=100$ GeV, as function of the lightest charged lepton mass $m_{e_1} = m_\ell - Y_c v$. Also shown is the ratio $R_{\gamma \gamma}$ (green, dotted) that is introduced in Sec. \ref{['sec:higgs']}.
• Figure 3: Contours of constant $R_{\gamma\gamma}$ (green, solid) for $Y_c'=Y_c"=0.8$ as a function of the explicit mass terms $m_\ell$ and $m_e$. The blue (grey) shaded region indicates a mass for the lightest charged lepton below $62.5$ GeV, while the blue, dashed contours indicate a charged lepton mass $m_{E_1}$ of 62.5, 100, 150, and 200 GeV.
• Figure 4: The ratio $R_{\gamma\gamma}$ for random values of $m_\ell \in [0,600]$ GeV, $m_e \in [0,600]$ GeV and $Y_c',\; Y_c" \in [0,0.5]$ (blue), $Y_c',\; Y_c" \in [0.5,1.0]$ (red), and $Y_c',\; Y_c" \in [1.0,1.5]$ (green). On the x-axis we show the mass of the lightest charged lepton.
• Figure 5: One loop running of the gauge couplings (solid lines) compared with SM running (dashed lines). From top to bottom $\alpha_1^{-1}$ (blue), $\alpha_2^{-1}$ (red) and $\alpha_3^{-1}$ (green) are shown, and both green curves lie on top of each other.