Remarks on Limits on String Scale from Proton Decay and Low-Energy amplitudes in Braneworld scenario

We discuss IR limit of four-fermion scattering amplitudes in braneworldmodels including intersecting-branes and SUSY SU(5) GUT version of it. Withcertain compactification where instanton effect is negligible, grandunification condition in D6-D6 intersecting-branes scenario subject toexperimental constraint on proton decay provides possibility for upper limit onthe string scale, $M_S$, through relationship between the string coupling,$g_s$, and the string scale. We discuss how IR divergence is related to numberof twisted fields we have to introduce into intersection region and how it canchange IR behaviour of tree-level amplitudes in various intersecting-branesmodels. Using number of twisted fields, we identify some intersecting-branesmodels whose tree-level amplitudes are purely stringy in nature andautomatically proportional to $g_s/M^2_{S}$ at low energy. They areconsequently suppressed by the string scale. For comparison, we also derivelimit on the lower bound of the string scale from experimental constraint onproton decay induced from purely stringy contribution in the coincident-branesmodel, the limit is about $10^5$ TeV.Comment: 14 page