What I "Learned” from Wikipedia, August 8, 2007

In particle physics, the X and Y bosons are hypothetical elementary particles analogous to the W and Z bosons, but corresponding to a new type of force, such as the forces predicted by grand unified theory.

The properties of the bosons vary according to the grand unification
theory in which they are predicted. In most such theories, the X and Y
bosons couple quarks to leptons, allowing violation of the conservation of baryon number and thus proton decay.

An X boson would have the following decay modes:

• 
• 

Where q is a quark and l is a lepton. In these reactions both lepton number and baryon number are not conserved, but B−L is. Different branching ratios between the X boson and its antiparticle (as is the case with the K-meson) would explain baryogenesis.