Four Generations and Higgs Physics

TL;DR

The paper investigates whether a sequential fourth generation of chiral fermions can be compatible with current experimental constraints and how such a generation would reshape Higgs physics. By systematically mapping the allowed masses and mixings that minimize oblique corrections, it finds regions where $m_H$ can lie up to $315$ GeV (68% CL) or even $750$ GeV (95% CL) while remaining consistent with electroweak data, and it shows that Higgs production via gluon fusion is substantially enhanced and certain decays or channels are modified. ItAlso identifies new signals such as Higgs decays to ν4 ν4 or same-sign dileptons when Majorana masses are present and demonstrates that Higgs pair production can be boosted, enabling measurements of the self-coupling $\lambda_{HHH}$. The study also analyzes vacuum stability and triviality constraints, finding that the presence of a fourth generation typically limits the cutoff scale to around the TeV–few-TeV range unless new physics intervenes, with a preferred Higgs mass near 300 GeV for maximal perturbativity. Overall, if a fourth generation exists, the LHC should probe it rapidly through enhanced Higgs production and direct production of heavy fermions, while complementary new-physics scenarios might be necessary to stabilize the Higgs potential.

Abstract

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% CL. 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 we 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.

Figures (7)

• Figure 1: Contours of constant $\Delta S_q$ (diagonal, blue) and $\Delta T_q$ (horizontal, red) 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 $m_{u_4,d_4} > 258$ GeV excludes the shaded (yellow) region.
• Figure 2: The 68% and 95% CL constraints on the $(S,T)$ parameters obtained by the LEP Electroweak Working Group Alcaraz:2006mxhowwegotthis. The shift in $(S,T)$ resulting from increasing the Higgs mass is shown in red. The shifts in $\Delta S$ and $\Delta T$ from a fourth generation with several of the parameter sets given in Table \ref{['tab:params']} are shown in blue.
• Figure 3: Branching ratio of the Higgs with fourth--generation effects assuming $m_{\nu} = 100$ GeV and $m_{\ell} = 155$ GeV. The loop effects to $H \rightarrow gg$ and $H \rightarrow \gamma\gamma$ 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 studies by CMS are included. The significances have naively been scaled to the modified production rates and branching rations 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 $m_H=200$ GeV