Hello, just a quick question.   In a Feynman diagram for beta- decay, there is a neutron (or down quark) coming in,...

Hello, just a quick question.

In a Feynman diagram for beta- decay, there is a neutron (or down quark) coming in, and a proton (or up quark coming out) on the right side. All is good so far. On the right side of the diagram, an electron and antineutrino come out. Seems about right.

The problem I'm having is with the directions of the lines. The neutron comes in at a 45 degree angle, but the proton comes out parallel to the y-axis. The electron comes out at 45 degrees, but the antineutrino comes out parallel to the x-axis. Why is this?

I suspect it's something to do with deflection due to charges, but I can't find anything on it on the internet and my textbook isn't helpful. Please help!

Thanks :)