Contact interactions in D-brane models

TL;DR

This paper calculates four-point scattering amplitudes in D-brane string models, identifying dimension-six operators that set new lower bounds of 2-3 TeV on the string scale, especially relevant for intersecting brane scenarios.

Contribution

It provides the first detailed computation of dimension-six four-fermion operators from D-brane intersections, refining bounds on the string scale in brane-world models.

Findings

Dimension-six operators dominate in intersecting brane models.

Lower bounds on the string scale are increased to 2-3 TeV.

Results are independent of the number of large extra dimensions.

Abstract

We compute the tree-level four-point scattering amplitudes in string models where matter fields live on D-brane intersections. Extracting the contribution of massless modes, we are left with dimension-six four-fermion operators which in general receive contributions from three different sources: exchange of massive Kaluza--Klein excitations, winding modes and string oscillator states. We compute their coefficients and extract new bounds on the string scale in the brane-world scenario. This is contrasted with the situation where matter fields arise from open strings with both ends confined on the same collection of D-branes, in which case the exchange of massive string modes leads to dimension-eight operators that have been studied in the past. When matter fields live on brane intersections, the presence of dimension-six operators increases the lower bound on the string scale to 2--3 TeV, independently of the number of large extra dimensions.