Intersecting brane worlds

It is known that chiral fermions naturally appear at certain intersections ofbranes at angles. Motivated by this fact, we propose a string scenario in whichdifferent standard model gauge interactions propagate on different(intersecting) brane worlds, partially wrapped in the extra dimensions. Quarksand leptons live at brane intersections, and are thus located at differentpositions in the extra dimensions. Replication of families follows naturallyfrom the fact that the branes generically intersect at several points. Gaugeand Yukawa couplings can be computed in terms of the compactification radii.Hierarchical Yukawa couplings appear naturally, since amplitudes involvingthree different intersections are proportional to exp{-A_{ijk}}, where A_{ijk}is the area of a string world-sheet extending among the intersections. Themodels are non-supersymmetric but the string scale may be lowered down to 1-10TeV. The proton is however stable due to a set of discrete symmetries arisingfrom world-sheet selection rules, exact to all orders in perturbation theory.The scenario has some distinctive features like the presence of KK, winding andother new excited states (`gonions'), with masses below the string scale andaccessible to accelerators. The models contain scalar tachyons with the quantumnumbers of standard SU(2) x U(1) Higgs doublets, and we propose that theyinduce electroweak symmetry breaking in a somewhat novel way. Specific stringmodels with D4-branes wrapping on T^2 x (T^2)^2/Z_N, leading to three-familysemirealistic spectra, are presented, in which the above properties areexemplified.Comment: 36 pages, 6 figures, (v2) reference added, explicit examples changed; (v3) one figure changed, typos correcte