Higgs data does not rule out a sequential fourth generation with an extended scalar sector

Contrary to common perception, we show that the current Higgs data does not eliminate the possibility of a sequential fourth generation that get their masses through the same Higgs mechanism as the first three generations. The inability to fix the sign of the bottom-quark Yukawa coupling from the available data plays a crucial role in accommodating a chiral fourth generation which is consistent with the bounds on the Higgs signal strengths. We show that effects of such a fourth generation can remain completely hidden not only in the production of the Higgs boson through gluon fusion but also to its subsequent decay to $\gamma \gamma$ and $Z\gamma$. This, however, is feasible only if the scalar sector of the standard model is extended. We also provide a practical example illustrating how our general prescription can be embedded in a realistic model.

Figure 1

The gray shaded region, based on the benchmark given in Eq. (11), is allowed at 95% CL from the current data for Higgs signal strengths [3], and $\mathrm{\Delta }S$, $\mathrm{\Delta }T$ for the benchmark of Eq. (11). The dashed and dotted lines represent the contours for $\cos (\beta -\alpha )=2/\tan \beta$ and $\cos (\beta -\alpha )=\sin 2\beta$ respectively.

Figure 2

Variation of the NP contributions to the $S$ and $T$ parameters with $\tan \beta$ assuming the validity of Eq. (8). This plot has been obtained for the benchmark values of Eq. (11).