What is the term for unchanged elementary particles with a mass nearly equal to that of a proton?

The term that describes unchanged elementary particles with a mass nearly equal to that of a proton is neutrons. Neutrons are neutral particles found in the nucleus of an atom alongside protons. They play a crucial role in the stability of atomic nuclei and have a mass that is almost identical to that of protons, which makes them significant in nuclear physics and chemistry.

In contrast to neutrons, electrons have a much smaller mass and carry a negative charge, which distinguishes them in atomic structure. Photons, being massless quantum particles of light, are entirely different in terms of mass and charge. Quarks, while fundamental constituents of protons and neutrons, do not exist freely in nature; they combine in groups to form the particles we encounter in the atomic nucleus, such as protons and neutrons. Thus, neutrons are the correct response due to their mass and neutral charge characteristics that closely match those of protons.