pub const RRF_K: usize = 60;Expand description
The k in 1/(k + rank), from the paper and from §5.9.
It damps the contribution of top ranks so that agreement between the two arms outweighs a single arm’s confidence: at k = 60 the gap between rank 1 and rank 2 is small, so a document both arms rank tenth beats one that is first in one list and absent from the other. Lower it and the fusion approaches “best of either arm”; raise it and it approaches “appears in both”.