A Haverhill woman was arraigned Tuesday on murder charges in the stabbing death of her boyfriend at their home Monday night, prosecutors said Tuesday. A not-guilty plea was entered on behalf of Susan Lovejoy, 64, at her arraignment in Haverhill District Court. She was ordered held without bail, said Carrie Kimball-Monahan, a spokeswoman for Essex District Attorney Jonathan W. Blodgett. Lovejoy was arrested after police responded to her apartment on Water Street just after 11:30 p.m. Monday, Kimball-Monahan said. Lovejoy’s arrest was made in the stabbing death of her boyfriend, Kevin Paul, 51, who was killed in the apartment they shared, Kimball-Monahan said.