How does protons collide and produce heavier particles if light particles can not decay into heavier particles ? considering particles can not decay into heavier particles irrespective of their energy. How can we see heavier particles at CERN if only the protons are colliding ?
You can understand that ONE light particle cannot decay into heavier particles by remembering that you can observe a process in the rest frame of this particle, where it has no kinetic energy. If there is no kinetic energy, the decay to heavier particles is not possible, because the only accessible energy is the mass of the particle.
For TWO particles that move relatively to one another, you still can move into a frame where ONE of the particles is at rest, but still, the other one will have kinetic energy. This energy can be converted into mass of heavier particles.