Trans World Airlines (TWA) flight 150 had just landed and was taxing to the gate. A sweeper truck was stopped on taxiway 'A' across from taxiway 'T', with the operator out of the truck picking up debris. The flight crew saw the truck and operator, but the first officer 'determined that the sweeper was located well clear of our aircrafts path.' The aircraft struck the sweeper truck. The flight crew thought they hit a pothole, and did not report it to Air Traffic Control (ATC). Another TWA flight saw the sweeper operator on the ground and notified ATC.