Intersecting Brane Worlds

It is known that chiral fermions naturally appear at certain intersections of branes at angles. Motivated by this fact, we propose a string scenario in which different standard model gauge interactions propagate on different (intersecting) brane worlds, partially wrapped in the extra dimensions. Quarks and leptons live at brane intersections, and are thus located at different positions in the extra dimensions. Replication of families follows naturally from the fact that the branes generically intersect at several points. Gauge and Yukawa couplings can be computed in terms of the compactification radii. Hierarchical Yukawa couplings appear naturally, since amplitudes involving three different intersections are proportional to exp(-A_{ijk}), where A_{ijk} is the area of a string world-sheet extending among the intersections. The models are non-supersymmetric but the string scale may be lowered down to 1-10 TeV. The proton is however stable due to a set of discrete symmetries arising from world-sheet selection rules, exact to all orders in perturbation theory. The scenario has some distinctive features like the presence of KK, winding and other new excited states (`gonions'), with masses below the string scale and accessible to accelerators. The models contain scalar tachyons with the quantum numbers of standard $SU(2)\times U(1)$ Higgs doublets, and we propose that they induce electroweak symmetry breaking in a somewhat novel way. Specific string models with D4-branes wrapping on T^2 x (T^2)^2/Z_N, leading to three-family realistic spectra, are presented in which the above properties are exemplified.