Serpentine belt pulley?

The vehicle listed above is an LS model. I Need to know the name of a piece on my car. We replaced the serpentine belt and discovered that on the bottom round wheel (pulley) that is to have grooves on it no longer does, it's very smooth. So, I'm told that a part has come off of that the keeps the grooves in that wheel (pulley) (is what I'm being told) for the serpentine belt to have the ability to remain on this wheel it has to have grooves so I'm needing that piece for that wheel (pulley) to put the grooves back into it. What is this called so I can go get one? Please and thank you.