2003/05/21 by Andrzej J. Buras, P. Q. Hung
Physics and Astronomy · #Dark Matter and Cosmic Phenomena #Neutrino Physics Research #Particle physics theoretical and experimental studies #hep-ph
paper · pdf · doi:10.1103/physrevd.68.035015
published as Phys.Rev. D68 (2003) 035015 · Main latex-file, 2 figures, 18 pages
arxiv created 2003/05/21 · openalex publication_date 2003/08/28 · arxiv updated 2009/11/30 · openalex created_date 2025/10/10 · openalex updated_date 2026/07/28
A recent surge of interest in the novel ideas of large extra dimensions and their implications, such as the early unification of quarks and leptons, has prompted us to revive a paper written 22 years ago. In that paper, we provided a general discussion of quark-lepton unification characterized by the gauge group GS\ensuremath\bigotimesGW with two couplings gS and gW and by the unification mass scales M=10TeV--1000TeV. The constraint from sin2\ensuremathθW restricts the choices for GW and our favorite model for petite unification was chosen to be SU(4)PS\ensuremath\bigotimesSU(2)4. In the present paper, we review the main results of our earlier paper and propose two new models based on the groups SU(4)PS\ensuremath\bigotimesSU(2)3 and SU(4)PS\ensuremath\bigotimesSU(3)2, for which the consistency with the measured value of sin2\ensuremathθW(MZ2) determines the unification scale to be roughly 1 TeV and 3--10 TeV, respectively. The implication of this very early unification is the existence of new quarks and leptons with charges up to 4/3 (for quarks) and 2 (for leptons) and masses O(250GeV). Interestingly, in these models the rare decay KL\ensuremath→\ensuremathμe is automatically absent at the tree level and the one-loop contributions are consistent with the experimental upper bound for this decay. On the other hand, the original SU(4)PS\ensuremath\bigotimesSU(2)4 model can be made consistent with the measured value of sin2\ensuremathθW(MZ2) and the unification scale M=O(1TeV) only provided there exist at least nine ordinary quark and lepton generations, with four generations in the case of the supersymmetric version. Moreover, the solution to the KL\ensuremath→\ensuremathμe problem is not as natural as in the two other scenarios. We comment on the recent papers on early unification in the context of large extra dimensions.