subatomic particle
Physics term for particles smaller than an atom, such as protons, neutrons, and electrons.
Protons and neutrons are types of subatomic particles.
素粒子 refers specifically to elementary particles (fundamental, indivisible), while 亜原子粒子 is a broader term for any particle smaller than an atom, including composite ones like protons.
Compound of 亜 (sub-), 原子 (atom), and 粒子 (particle). A modern calque from English 'subatomic particle'.