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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.
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We interpret the recent discovery of a 125 GeV Higgs-like state in the context of a two Higgs doublets model with a heavy 4th sequential generation of fermions, in which one Higgs doublet couples only to the 4th generation fermions, while the second doublet couples to the lighter fermions of the 1st-3rd families. This model is designed to accommodate the apparent heaviness of the 4th 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 4th generation fermions. We find that the lightest Higgs, h, is a good candidate for the recently discovered 125 GeV spin-zero particle, when tanbeta ~ O(1), for typical 4th generation fermion masses of M_{4G} = 400 -600 GeV, and with a large t - t mixing in the right-handed quarks sector. This, in turn, leads to BR(t -> t h) ~ O(1), which drastically changes the t 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 SM) in the pp -> h -> tau tau channel and a reduced VV -> h -> gamma gamma and pp -> V -> Vh -> Vbb ones. Finally, the heavier CP-even Higgs is excluded by the current data up to m_H ~ 500 GeV, while the pseudoscalar state, A, can be as light as 130 GeV. These heavier Higgs states and the expected deviations from the SM in some of the Higgs production channels can be further excluded or discovered with more data.
We study $mu - e$ conversion with sequential four generations. A large mass for the fourth generation neutrino can enhance the conversion rate by orders of magnitude. We compare constraints obtained from $mu - e$ conversion using experimental bounds on various nuclei with those from $mu to e gamma$ and $mu to ebar e e$. We find that the current bound from $mu - e$ conversion with Au puts the most stringent constraint in this model. The relevant flavor changing parameter $lambda_{mu e} = V^*_{mu 4}V_{e4}^{}$ is constrained to be less than $1.6times 10^{-5}$ for the fourth generation neutrino mass larger than 100 GeV. Implications for future $mu -e$ conversion, $mu to egamma$ and $mu to ebar e e$ experiments are discussed.
To include the quark sector, the $A_{5}equiv I$ (icosahedron) four generation lepton model is extended to a binary icosahedral symmetry $I$ flavor model. We find the masses of fermions, including the heavy sectors, can be accommodated. At leading order the CKM matrix is the identity and the PMNS matrix, resulting from same set of vacua, corresponds to tribimaximal mixings.
The implications of the discovery of a scalar Higgs boson at the LHC with a mass of approximately 125 GeV are summarised in the context of the Standard Model of particle physics with its unique scalar boson and of its most celebrated new physics extension, the minimal supersymmetric Standard Model or MSSM, in which the Higgs sector is extended to contain three neutral and two charged scalar bosons. Discussed are the implications from the measured mass, the production and decay rates of the observed particle and, in the MSSM, from the constraints in the search for the heavier Higgs states. The perspectives for Higgs and new physics searches at the next LHC upgrades as well as at future hadron and lepton colliders are then briefly summarized.
I review the theoretical aspects of the physics of Higgs bosons, focusing on the elements that are relevant for the production and detection at present hadron colliders. After briefly summarizing the basics of electroweak symmetry breaking in the Standard Model, I discuss Higgs production at the LHC and at the Tevatron, with some focus on the main production mechanism, the gluon-gluon fusion process, and summarize the main Higgs decay modes and the experimental detection channels. I then briefly survey the case of the minimal supersymmetric extension of the Standard Model. In a last section, I review the prospects for determining the fundamental properties of the Higgs particles once they have been experimentally observed.
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