125 GeV Higgs state in the context of four generations with two Higgs doublets

We interpret the recent discovery of a 125 GeV Higgs-like state in the context of a two-Higgs-doublet model with a heavy fourth sequential generation of fermions, in which one Higgs doublet couples only to the fourth-generation fermions, while the second doublet couples to the lighter fermions of the first three families. This model is designed to accommodate the apparent heaviness of the fourth-generation fermions and to effectively address the low-energy phenomenology of a dynamical electroweak-symmetry-breaking scenario. The physical Higgs states of the model are, therefore, viewed as composites primarily of the fourth-generation fermions. We find that the lightest Higgs, $h$, is a good candidate for the recently discovered 125 GeV spin-zero particle, when $\tan \beta \sim \mathcal{O}(1)$, for typical fourth-generation fermion masses of $M_{4\mathrm{G}}=400–600\mathrm{GeV}$, and with a large $t\mathrm{\text{−}}{t}^{\prime }$ mixing in the right-handed quark sector. This, in turn, leads to $\mathrm{BR}(t^{\prime }\to th)\sim \mathcal{O}(1)$, which drastically changes the $t^{\prime }$ decay pattern. We also find that, based on the current Higgs data, this two-Higgs-doublet model generically predicts an enhanced production rate (compared to the Standard Model) in the $pp\to h\to \tau \tau$ channel, and reduced rates in the $VV\to h\to \gamma \gamma$ and $p\overline{p}/pp\to V\to hV\to Vbb$ channels. Finally, the heavier $CP$-even Higgs is excluded by the current data up to $m_H\sim 500\mathrm{GeV}$, while the pseudoscalar state, $A$, can be as light as 130 GeV. These heavier Higgs states and the expected deviations from the Standard Model din some of the Higgs production channels can be further excluded or discovered with more data.

Figure 1

$\Gamma (h\to \gamma \gamma )$ and $\Gamma (h\to gg)$ as a function of $\alpha$, $\tan \beta$ and ${ϵ}_t$, for some representative values of these parameters (as indicated in the plots) and with $M_{4\mathrm{G}}\equiv m_{t^{\prime }}=m_{b^{\prime }}=m_{l_4}=m_{{\nu }_4}=400\mathrm{GeV}$.

Figure 2

The relevant branching ratios of $h$ in the 4G2HDM, as a function of $\alpha$, with $m_h=125\mathrm{GeV}$, $M_{4\mathrm{G}}=400\mathrm{GeV}$, ${ϵ}_t=0.5$, and $\tan \beta =1$.

Figure 3

${\chi }^2$ (left plot) and $p$ values (right plot) as a function of $\tan \beta$ for the lightest 4G2HDM $CP$-even scalar $h$, with $m_h=125\mathrm{GeV}$, ${ϵ}_t=0.1$ and 0.5, and $M_{4\mathrm{G}}\equiv m_{t^{\prime }}=m_{b^{\prime }}=m_{l_4}=m_{{\nu }_4}=400$ and 600 GeV. The value of the Higgs mixing angle $\alpha$ is the one which minimizes ${\chi }^2$ for each value of $\tan \beta$. The SM best fit is shown by the horizontal dashed line; the dash-dotted line in the right plot corresponds to $p=0.05$ and serves as a reference line.

Figure 4

Same as Fig. 3, where here we minimize with respect to both ${ϵ}_t$ and $\alpha$ for each value of $\tan \beta$. Also shown are the ${\chi }^2$ and $p$ values for a 125 GeV Higgs in the SM and in the type-II 2HDM with a fourth-generation of fermions (denoted by 2HDMII).

Figure 5

A comparison between the $p$ values for the compatibility of the 125 GeV $h$ of the 4G2HDM with the Higgs search results, with (dashed lines) and without (solid lines) imposing the constraints from EWPD as given in Refs. 34, 52. The dash-dotted line corresponds to $p=0.05$. Here also, the parameters $\alpha$ and ${ϵ}_t$ are chosen by a minimization of ${\chi }^2$.

Figure 6

The individual pulls $\frac{(R_{XX}^{\mathrm{Model}}-R_{XX}^{\mathrm{Obs}})}{{\sigma }_{XX}}$ (upper plot), and the signal strengths $R_{XX}^{\mathrm{Model}}$ (lower plot), in the different channels, that correspond to the best-fitted 4G2HDM curve with $m_h=125\mathrm{GeV}$ and $M_{4\mathrm{G}}=400\mathrm{GeV}$, shown in Fig. 4.

Figure 7

The signal strength in the $H\to ZZ$ channel for the six best-fitted sets of values in Table . Also shown is the approximate observed CMS limit on the signal strength in the $ZZ$ channel; i.e., $R_{ZZ}^{\mathrm{Obs}}$ (see also the text).

Figure 8

The signal strengths in the $A\to \tau \tau$ channel for the six best-fitted sets of values in Table . Also shown is the approximate observed CMS limit on signal strengths in the $\tau \tau$ channel; i.e., $R_{\tau \tau }^{\mathrm{Obs}}$ (see also the text).

Figure 9

The signal strengths in the $A\to \gamma \gamma$ channel, for the 6 best-fitted sets of values in Table . Also shown is the approximate observed CMS limit on signal strengths in the $\gamma \gamma$ channel; i.e., $R_{\gamma \gamma }^{\mathrm{Obs}}$ (see also the text).