"The experiment that confirmed the four-vector representation of the energy-momentum relation of a particle with rest mass 0 is the Compton_effect { Compton effect 
Compton effect 
Compton effect }.
Compton effect Compton effect }.The Compton effect demonstrates the particle nature of light and provides evidence for the relativistic energy-momentum relation for photons, which have zero rest mass. In the Compton effect, X-rays scatter off electrons, resulting in a change in wavelength that can be explained using the relativistic energy-momentum four-vector formalism. This effect was crucial in establishing the concept of photons as particles with energy and momentum but zero rest mass, in accordance with the principles of special relativity." (ChatGPT)