Sorry to be a giant PITA, but Prime is the interest rate banks charge their best customers. Its not directly controlled by the fed. Its usually US10Y + 2-3%, depending on some other factors. Prime has already gone way down with the fall of the 10 year note.Fed believes it is time to lower the Prime.
The fed controls the fed funds rate, aka the fed overnight rate, which is really bad terminology also given its usually a 90 day note. Gotta love banker terminology

IMHO its debate able that fed funds has had a whole lot of affect on the real economy anyway. It might have at some times in the past because the only ones borrowing a that rate are the banks, except very few banks are borrowing from the fed. There flush with cash already.
But I digress.