In this sentence, what does probably modify? “I am probably not going to the party.” Also, do adverbs modify verb phrases or just the main verb? For example, in, “I am walking quickly.” Does quickly modify “am walking” or “walking?”
There is a consensus among linguists that “probably” is a sentence adverb. In your example, it modifies the sentence “I am not going to the party.” The best reference I know is the chapter on adverbs in McCawley’s book The Syntactic Phenomena of English.
Here is an interesting paper from 1970 by George Lakoff on the logic of adverbs: Adverbs and modal operators.