The breakdown of detritus into smaller particles by earthworm is a process called

The important steps of decomposition are fragmentation, leaching, catabolism, humification and mineralisation.
Detritivores such as earthworms break down detritus into smaller particles; this process is called fragmentation.
Catabolism is the degradation of detritus into simpler inorganic substances by bacterial and fungal enzymes.
Humification is the accumulation of dark, amorphous humus, and mineralisation is the release of inorganic nutrients from humus by microbes.
Hence the breakdown of detritus into smaller particles by earthworms is fragmentation.