What particle can travel at the speed of light?
When it comes to speed limits, the ultimate one set by the laws of physics themselves is the speed of light.
When scientists talk about the speed of light — 299,792,458 m/s — we implicitly mean "the speed of light in a vacuum." Only in the absence of particles, fields, or a medium to travel through can we achieve this ultimate cosmic speed. Even at that, it's only the truly massless particles and waves that can achieve this speed. This includes photons, gluons, and gravitational waves, but not anything else we know of.