Four generations and Higgs physics

In the light of the LHC, we revisit the implications of a fourth generation of chiral matter. We identify a specific ensemble of particle masses and mixings that are in agreement with all current experimental bounds as well as minimize the contributions to electroweak precision observables. Higgs masses between 115–315 (115–750) GeV are allowed by electroweak precision data at the 68% and 95% C.L. Within this parameter space, there are dramatic effects on Higgs phenomenology: production rates are enhanced, weak-boson-fusion channels are suppressed, angular distributions are modified, and Higgs pairs can be observed. We also identify exotic signals, such as Higgs decay to same-sign dileptons. Finally, we estimate the upper bound on the cutoff scale from vacuum stability and triviality.

Figure 1

Contours of constant $\Delta S_q$ (diagonal blue lines) and $\Delta T_q$ (horizontal red lines) for the fourth-generation quarks. The plot is symmetric with respect to $m_{u_4}-m_{d_4}\leftrightarrow m_{d_4}-m_{u_4}$, since $\Delta T_q$ is positive definite. The Tevatron bound $m_{u_4,d_4}>258\mathrm{GeV}$ excludes the shaded (yellow) region.

Figure 2

The 68% and 95% C.L. constraints on the $(S,T)$ parameters obtained by the LEP Electroweak Working Group [34, 35]. The shift in $(S,T)$ resulting from increasing the Higgs mass is shown in red (solid line). The shifts in $\Delta S$ and $\Delta T$ from a fourth generation with several of the parameter sets given in Table  are shown in blue (arrow lines).

Figure 3

Branching ratio of the Higgs with fourth-generation effects assuming $m_{\nu }=100\mathrm{GeV}$ and $m_{\ell }=155\mathrm{GeV}$. The loop effects to $H\to gg$ and $H\to \gamma \gamma$ are largely insensitive to the fourth-generation quark masses. For the fourth-generation masses we follow the reference point (b).

Figure 4

Scaled LHC discovery contours for the fourth-generation model. All channels studied by CMS are included. The significances have naively been scaled to the modified production rates and branching ratios using the fourth-generation parameters of reference point (b).

Figure 5

Angular distribution of vector-boson fusion channel assuming reference point (b) with its Higgs mass $m_H=200\mathrm{GeV}$.

Figure 6

Invariant mass distribution for Higgs pair production at the LHC. We show the standard model and fourth-generation curves in the reference point (b). For the dashed line the Higgs self-coupling is set to zero.

Figure 7

The maximum scale at which new physics enters into the Higgs potential to avoid either a too short-lived vacuum or to avoid a Landau pole in $\lambda$. These two constraints are qualitatively distinct: metastability can be restored by weakly coupled physics below a TeV scale, whereas the Landau pole signals a strongly interacting Higgs sector. The dashed curve reproduces the SM triviality bound.