BELFAST, Maine — Ten-year-old Marissa Kennedy clearly experienced torture at the hands of her mother and stepfather before her death in Stockton Springs two years ago, Justice Robert Murray said on Friday as he explained what factored into his decision to sentence her mother to 48 years in prison for her role in the murder. But the evidence presented in the 35-year-old’s murder trial in December failed to show that Kennedy’s participation reached the severity of torture necessary under Maine law to impose the maximum life sentence that prosecutors requested.