Probing extra Yukawa couplings by precision measurements of Higgs boson properties

If one removes any ad hoc symmetry assumptions, the general two-Higgs-doublet model should have additional Yukawa interactions independent from fermion mass generation, in general involving flavor-changing neutral Higgs couplings. These extra couplings can affect the discovered Higgs boson $h$ through fermion loop contributions. We calculate the renormalized $hZZ$ coupling at the one-loop level and evaluate the dependence on heavy Higgs boson mass and extra Yukawa coupling ${\rho }_{tt}$. Precision measurements at future colliders can explore the parameter space, and can give stronger bounds on ${\rho }_{tt}$ than the current bound from flavor experiments. As a side result, we find that if ${\rho }_{tt}\cos \gamma <0$, where $\cos \gamma$ is the exotic Higgs component of $h$, the ${\rho }_{tt}$-induced top loop contribution cancels against bosonic loop contributions, and one may have alignment without decoupling, namely $\sin (-\gamma )\simeq 1$, but exotic scalar bosons could have masses of several hundred GeV.

Figure 1

One-loop diagrams contributing to the renormalized $hZZ$ vertex.

Figure 2

${\mathrm{\Delta }}_{\mathrm{loop}}$ vs $m_H$ in the alignment limit $0.99\le \sin (-\gamma )\le 1$, and under perturbativity and vacuum stability constraints. The red, green and blue regions in the left (right) panel correspond to ${\rho }_{tt}=0.5$, 1.0, 1.5 ($-0.5,-1.0,-1.5$), respectively.

Figure 3

${\mathrm{\Delta }}_{\mathrm{loop}}$ vs $m_H$ near alignment $0.985\le \sin (-\gamma )\le 0.995$ (solid band) and $0.95\le \sin (-\gamma )\le 0.96$ (dashed band), with settings the same as in Fig. 2.

Figure 4

$\mathrm{\Delta }{\kappa }_Z$ vs ${\rho }_{tt}$ for $m_H=500\mathrm{GeV}$ under perturbativity and vacuum stability constraints. For $\cos \gamma <0$ ($\cos \gamma >0$), solid (dashed) red, blue, green and orange bands are for $\sin (-\gamma )=0.995$, 0.99, 0.98 and 0.95, respectively.

Figure 5

Range of $\mathrm{\Delta }{\kappa }_Z$ for a given ${\rho }_{tt}$ value in the alignment limit, $0.98\le \sin (-\gamma )\le 1$, and under perturbativity and vacuum stability constraints. The blue (red) region is for $m_H=500$ (1000) GeV, with ${\rho }_{tt}$ outside of the dot-dashed (dotted) vertical lines excluded by $B_s$ mixing. In the right panel, which is a zoomed-in view for $\mathrm{\Delta }{\kappa }_Z\gtrsim 0$, a horizontal solid line shows a favorable central value of $\mathrm{\Delta }{\kappa }_Z$, with horizontal dashed (dot-dashed) lines indicating $2\sigma$ error bars at the ILC500 (CLIC350).