secondary particle
Technical term in physics for a particle produced by the interaction of primary radiation or particles with matter.
When cosmic rays collide with the atmosphere, secondary particles are produced.
一次粒子 refers to primary particles (the original radiation or particles), while 二次粒子 are the secondary particles generated from interactions.
Compound of 二次 (secondary) and 粒子 (particle). The term is a direct conceptual translation of the English 'secondary particle'.