Police are asking for the public’s help in their investigation of the assault of a lifeguard Tuesday at the Breakheart Reservation in Saugus. Anyone with information about the assault should call 978-538-6009, State Police said. The 20-year-old woman, who worked at the reservation, was reported missing at 2:13 p.m. Tuesday, police said. About an hour later, a jogger found her on a trail, where she said she had been assaulted by a man. Police are not yet releasing details about the nature of the assault, said David Procopio, a State Police spokesman. The woman was taken to Massachusetts General Hospital with potentially serious injuries. Police planned to interview her yesterday.