The Naturalness of the Fourth SM Family

TL;DR

The paper discusses the theoretical motivation, expected properties, and potential experimental signatures of a hypothetical fourth Standard Model family, emphasizing its implications for Higgs production and detection at colliders.

Contribution

It proposes flavor democracy as a basis for the fourth family and analyzes its possible masses, production mechanisms, and detection prospects at current and future colliders.

Findings

Fourth family fermions likely have masses around 450 GeV.

Higgs production via gluon fusion could be significantly enhanced.

Early Higgs detection at the LHC or Tevatron is possible due to increased cross-sections.

Abstract

The necessity of the fourth family follows from the SM basics. According to flavor democracy the Dirac masses of the fourth SM family fermions are almost equal with preferable value 450 GeV, which corresponds to common (for all fundamental fermions) Yukawa coupling equal to SU(2) gauge coupling gW. In principle, one expect u4 a little bit lighter than d4, while nu4 could be essentially lighter than l4 due to Majorana mass terms for right-handed components of neutrinos. Obviously, the fourth family quarks will be copiously produced at the LHC. However, the first indication of the fourth SM family may be provided by early Higgs boson observation due to almost an order enhancement of the gluon fusion to Higgs cross-section. For the same reason the Tevatron still has a chance to observe the Higgs boson before the LHC. Concerning the fourth family leptons, in general, best place will be NLC/CLIC. However, for some mass regions and MNS matrix elements double discovery of both the nu4 and H could be possible at the LHC.