The First 48, a new A&E series, trails real-life cops during the initial 48 hours of a murder investigation. In Miami, we meet a jaded girl who barely blinks when told her ex-boyfriend is dead; we see a criminal’s childhood photos and realize he was once a kid in a spotless karate uniform. In Kansas City, Luminol lights up a wall like a bloody galaxy and an officer returns home to a dark house, saying wistfully, ”My last wife — I loved her very dearly ? just needed more love and affection and attention than I could give her.” Good cops and bad cops play their games; some cases are solved — others remain baffling. No matter the outcome, the show never wavers from its aching immediacy: A beautiful young music teacher — ID’d from charred remains found in a Philly park — joltingly becomes human when her car trunk reveals a girlish scatter of MetroCards, makeup, and gum. ”48” flies by, but moments along the way linger.