2000/10/31 by Michio Hashimoto, Masaharu Tanabashi, Koichi Yamawaki · 3 citations
Physics and Astronomy · #Black Holes and Theoretical Physics #Particle physics theoretical and experimental studies #Quantum Chromodynamics and Particle Interactions #hep-ph #hep-th
paper · pdf · doi:10.1103/physrevd.64.056003
published as Phys.Rev. D64 (2001) 056003 · 36 pages, 7 figures. Version to appear in PRD. One reference added, some typos corrected
arxiv created 2001/06/15 · openalex publication_date 2001/07/16 · arxiv updated 2009/11/30 · openalex created_date 2025/10/10 · openalex updated_date 2026/07/28
We critically examine a version of the top mode standard model recently cast in extra dimensions by Arkani-Hamed, Cheng, Dobrescu, and Hall, based on the (improved) ladder Schwinger-Dyson (SD) equation for the D- (=6,8\ensuremath-) dimensional gauge theories. We find that the bulk QCD cannot have larger coupling beyond the nontrivial ultraviolet (UV) fixed point, the existence of which is supported by a recent lattice analysis. The coupling strength at the fixed point is evaluated by using the one-loop renormalization group equation. It is then found that, in a version with only the third family (as well as the gauge bosons) living in the D-dimensional bulk, the critical (dimensionless) coupling for dynamical chiral symmetry breaking to occur is larger than the UV fixed point of the bulk QCD coupling for D=6, while smaller for D=8. We further find that the improved ladder SD equation in D dimensions has an approximate scale invariance due to the running of the coupling and hence has an essential-singularity scaling of the ``conformal phase transition,'' similar to Miransky scaling in the four-dimensional ladder SD equation with a nonrunning coupling. This essential-singularity scaling can resolve the fine-tuning even when the cutoff (``string scale'') is large. Such a theory has a large anomalous dimension \ensuremathγm=D/2\ensuremath-1 and is expected to be free from the flavor-changing-neutral-current problem as in walking technicolor for D=4. Furthermore, the induced bulk Yukawa coupling becomes finite even at infinite cutoff limit (in the formal sense), similar to the renormalizability of the gauged Nambu--Jona-Lasinio model. Comments are made on the use of the ``effective'' coupling, which includes finite renormalization effects, instead of the MS running coupling in the improved ladder SD equation.