New Standard-like Models from Intersecting D4-Branes

Abstract

We construct a new set of intersecting D4-brane models that yield the (non-supersymmetric) standard model up to vector-like matter and, in some cases, extra U(1) factors in the gauge group. The models are constrained by the requirement that twisted tadpoles cancel, and that the gauge boson coupled to the weak hypercharge U(1)_Y does not get a string-scale mass via a generalised Green-Schwarz mechanism. We find six-stack models that contain all of the Yukawa couplings to the tachyonic Higgs doublets that are needed to generate mass terms for the fermions at renormalisable level, but which have charged-singlet scalar tachyons and an unwanted extra U(1) gauge symmetry after spontaneous symmetry breaking. There is also a six-stack model without any unwanted gauged U(1) symmetries, but which only has the Yukawa couplings to generate masses for the u quarks and charged leptons. A particular eight-stack model is free of charged-singlet tachyons and has gauge coupling strengths whose ratios at the string scale are close to those measured at the electroweak scale, consistent with the string scale being at most a few TeV.