Probing an additional bottom Yukawa coupling via $bg\to bA\to bZH$ signature

The recent discovery of the bottom quark Yukawa coupling ($hbb$) of the 125 GeV scalar motivates one to search for extra bottom Yukawa coupling that may exist in nature. The two Higgs doublet model without a discrete $Z_2$ symmetry allows the possibility of additional bottom Yukawa coupling ${\rho }_{bb}$. We show that ${\rho }_{bb}$ can be searched directly at the LHC via the $bg\to bA\to bZH$ and $gg\to b\overline{b}A\to b\overline{b}ZH$ processes, where $A$ and $H$ are the $CP$-odd and $CP$-even scalars, respectively. We find that the $bg\to bA\to bZH$ process could be discovered with $\sim 300{\mathrm{fb}}^{-1}$ integrated luminosity if $m_A\sim 300\mathrm{GeV}$, while the latter process may emerge in the high luminosity LHC (HL-LHC) run. A discovery might touch upon the parameter space required for the electroweak baryogenesis.

Figure 1

The constraints on ${\rho }_{bb}$ from Higgs signal strength measurements of ATLAS (red shaded region), Higgs signal strength measurements of CMS (green shaded region), $\mathcal{B}(B\to X_s\gamma )$ (purple shaded region) and $\mathrm{\Delta }{\mathcal{A}}_{\mathrm{CP}}$ (red dotted line). The figures are generated with $c_{\beta -\alpha }=0.05$ and ${\rho }_{tt}=0.1$, for $m_{H^{\pm }}=350\mathrm{GeV}$ (left panel) and 450 GeV (right panel). See text for further details.

Figure 2

The scanned points plotted in the $|{\eta }_3+{\eta }_4-{\eta }_5|$ vs $m_A$ (left) and ${\mu }_{22}^2/v^2$ vs $m_A$ (right) plane.

Figure 3

The normalized $m_{\ell \ell }$ (left) and $m_{\ell \ell bb}$ (right) distributions of the three BPs together with different backgrounds for the $bZH$ (upper panel) and $bbZH$ (lower panel) processes.