By Abstract nouns, Difference between

Widen" is for both literal and abstract meanings whereas "broaden" is reserved for more abstract sense ------------------ hmm, "broaden his pitch" is the phrase which I read from CNN news recently. Is pitch an abstract noun here? I think it is a common noun, isn't it?

There is no rule. Words with these prefixes have come about through accidents of history. The most usual is "un-", but always consult a dictionary. The following does not really answer your question, but you may find it somewhat useful anyway, especially if you're willing to work to dig some...