Four generations and Higgs physics

Topic

natural sciences

Frequently Asked Questions (FAQ)

What is a sequential fourth generation of chiral matter? A sequential fourth generation of chiral matter is a hypothetical extension of the Standard Model (SM) of particle physics. The SM currently has three generations of particles, each containing two quarks, one charged lepton, and one neutrino. Adding a fourth generation would mean introducing a new set of these particles with similar properties to the existing ones.
How does a fourth generation affect the oblique electroweak parameters, and why is it important? Oblique electroweak parameters are a set of quantities that describe the quantum corrections to the electroweak interactions. A fourth generation contributes to these parameters, potentially pushing their values outside the range allowed by experimental data. This tension with precision electroweak measurements has led some to believe that a fourth generation is ruled out, but it can be accommodated by careful adjustments to the fourth generation masses and mixings.
What is the significance of splitting the masses within a fourth-generation doublet? Splitting the masses of the up-type and down-type fermions within a fourth-generation doublet (e.g., making the fourth-generation up quark heavier than the fourth-generation down quark) can help reduce the contribution to the oblique parameter S. This reduction can bring the theory back into agreement with experimental data, even for heavier Higgs boson masses.
How does a fourth generation modify Higgs boson production and decay at the LHC? A fourth generation significantly enhances the production of Higgs bosons through gluon fusion, increasing the rate by roughly a factor of 9. This enhancement affects various search channels, making some more sensitive and others less so. For example, the “golden mode” H → ZZ → 4µ becomes accessible for a wider range of Higgs masses, while weak-boson-fusion channels are relatively suppressed.
What is the impact of a fourth generation on the Higgs decay to two photons (H → γγ)? The presence of a fourth generation suppresses the branching ratio of the Higgs boson decay to two photons (H → γγ) by roughly a factor of 1/9. This suppression arises from the additional contributions of the fourth-generation particles to the loop diagrams that mediate the decay.
Can a fourth generation lead to exotic Higgs decay signals? Yes, if the Higgs boson is sufficiently heavy, it can decay to fourth-generation particles. Depending on the mixing between the fourth-generation leptons and the SM leptons, these decays can lead to unusual signals, such as decays to same-sign dileptons (H → ν4ν4 → ℓ±ℓ±W∓W∓) when the fourth-generation neutrino has a Majorana mass.
Does a fourth generation affect the stability of the electroweak vacuum? Yes, the large Yukawa couplings of the fourth generation quarks can drive the Higgs quartic coupling to negative values at high energy scales. This negative quartic coupling can lead to an unstable electroweak vacuum, implying the need for new physics at a scale not far above the TeV scale to stabilize the vacuum.
How does the triviality bound come into play with a fourth generation? The large Yukawa couplings can also drive the Higgs quartic coupling and/or the Yukawa couplings themselves to a Landau pole, indicating the breakdown of the theory at a certain energy scale. This triviality bound sets an upper limit on the cutoff scale of the theory, which becomes increasingly restrictive for heavier Higgs boson masses.

Significance

Understanding these findings helps advance our knowledge and inform better decisions. This research represents an important contribution to the field. For the full details, watch the video above and explore the linked resources.

Resources & Further Watching

Read the research paper written by Graham D. Kribs, Tilman Plehn, Michael Spannowsky and Tim M.P. Tait: https://inspirehep.net/literature/753949

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