Elementary Particle, Particle
noun
A particle (粒子) that is the element version of it is an elementary particle.
そりゅうし
This is a jukugo word that uses the on'yomi readings of the kanji. You should be able to read this on your own.
「素粒子」って言葉は物理の教科書で見た事がある気がするよ。
I feel like I’ve seen the word "elementary particle" in my physics textbook.