GUT Breaking on the Brane ∗

We present a five-dimensional supersymmetric SU(5) theory in which the gauge symmetry is broken maximally (i.e. at the 5D Planck scale M∗) on the same 4D brane where chiral matter is localized. Masses of the lightest Kaluza-Klein modes for the colored Higgs and X and Y gauge fields are determined by the compactification scale of the fifth dimension, MC ∼ 1015 GeV, rather than by M∗. These fields’ wave functions are repelled from the GUT-breaking brane, so that proton decay rates are suppressed below experimental limits. Above the compactification scale, the differences between the standard model gauge couplings evolve logarithmically, so that ordinary logarithmic gauge coupling unification is preserved. The maximal breaking of the grand unified group can also lead to other effects, such as O(1) deviations from SU(5) predictions of Yukawa couplings, even in models utilizing the Froggatt-Nielsen mechanism. ∗ This work was supported in part by the U.S. Department of Energy under Contracts DE-AC0376SF00098, in part by the National Science Foundation under grant PHY-95-14797. Research fellow, Miller Institute for Basic Research in Science.