Proton decay in intersecting brane models.

We aim to show that in intersecting brane models there are no open string diagrams which represent proton decay into leptons and gauge bosons. We focus on the model of Figure 15.7 and restrict ourselves to string diagrams presented as a disk with boundary punctures, where the set of open string states relevant to the process (incoming and outgoing) are inserted. The punctures split the boundary of the disk into components, which are the loci of open string endpoints, and consequently, are labeled by D-branes.

Prove that no consistent assignment of labels to the boundary components is possi­ ble given that p10ton decay invol ves thr ee quar ks (and a number of leptons and gauge bosons), and a quark has one endpoint on a baryonic brane and the other endpoint elsewhere. [Hint. Tr y draw ing str ing diagrams. Your attempts should evol ve into a clear and ver y br ief argument.]