Photons are massless particles according to the Standard Model of particle physics. They do carry energy and momentum, which can be transferred to other particles during interactions, but they themselves do not possess rest mass. This is a fundamental property of photons and is a consequence of their special status as particles of light and electromagnetic radiation.
The energy and momentum of a particle are related to its mass through the famous equation E = mc², where E is energy, m is mass, and c is the speed of light. However, for massless particles like photons, the equation simplifies to E = pc, where p represents momentum. In this case, the energy of the photon is directly proportional to its momentum, but there is no contribution from mass since the mass is zero. Photons always travel at the speed of light in vacuum, and their energy and momentum are determined solely by their frequency (or wavelength) according to the equation E = hf, where h is Planck's constant and f is the frequency.
It is worth noting that the concept of massless particles and their behavior is a fundamental aspect of our current understanding of physics, but new discoveries or theories may emerge that could revise our understanding in the future.