While ψ weights the yield y taking into account all n-grams (including those crossing the boundaries of phrases), ω weights edges in isolation.
ω(y[e i ]) = λ ψ q LM (y[e i ]), where y[e i ] returns the sequence of target words (a target phrase) associated with the edge, and q LM (•) is an upperbound on the true LM probability p LM (•) (see §4.3).