Police are looking for witnesses to a single vehicle crash in Barrhaven that killed a 24-year old motorcyclist Thursday evening. They say the man appeared to lose control of his motorcycle and was thrown from the bike.   Paramedics say the motorcycle driver had suffered severe head injuries and was pronounced dead on scene.  No one else was hurt.  The crash occured at the corner of Strandherd and Jockvale about 9:45pm Thursday.  Police have cordonned off sections of both roads while they conduct their investigation.