。
She is utterly immersed in her research.
Literal
She [topic-は] research [in-に] is-immersing.
没頭 (literally 'sinking the head') is a sturdier, more bookish synonym for 夢中 — both translate as 'absorbed,' but 没頭する suggests purposeful, total commitment of one's faculties (academic work, training, deep practice), while 夢中 has more of a 'lost-in-the-fun-of-it' flavor. 没頭する takes its target with に, the same as 夢中 and many other inward-facing verbs. The ~ている here is a resulting-state reading: 'she has thrown herself in and remains there.'