what would happen if a proton beam hits an electron beam both close to the speed of light?

As strange as this may seem, there will likely be few interaction largely because the beams are mostly space. These are really smal particles and electrostatic force prevent you from packing them very densely so ther eis a relatively small probability of collision between two particles in opposing beams. You definitely would see a lot of high energy photons due to scattering of the electrons - less so due to proton scatterinf since the proton is about 1800x more massive than the electron. You might get some meson and pions due to pair production via the energetic photons. But you noly have three quarks in the proton, so the mesons are iffy. If you want heavy particles, you really need to collide protons with protons.

Most complex thing in physics.

Its d thing which happen in world's largest and complex machine LARGE HADRON COLLIDER.

When such type of particle collide with speed of light, results in radioactive decay n formation of other sub atomic particles hadrons, mesons etc etc...

It is out of our understanding..