A tachyon or tachyonic particle is a hypothetical particle that always travels faster than light. Most physicists believe that faster-than-light particles cannot exist because they are not consistent with the known laws of physics.
Do Tachyons travel back in time? The answer lies in tachyons. These are particles with imaginary mass that can be thought of as travelling backwards in time. They move faster than light, and slowing down to the speed of light would be as impossible for them as it is for us to accelerate to light speed. Wikipedia