A New Bedford man charged with beating and killing his girlfriend’s 9-month-old daughter has pleaded not guilty. Ethen Harrison was ordered held without bail at his arraignment on a murder charge Tuesday in Fall River Superior Court. Prosecutors said that Harrison, 21, killed 9-month-old Ariel Eluziario in January when he was alone with the child. Court documents said Harrison told police he was angry when Ariel became ‘‘fussy.’’ She died at a hospital of head trauma. A defense lawyer said he has questions about Harrison’s psychological background and his experience in caring for children.