losing oneself (in); being absorbed (in); forgetting oneself
彼女は無我夢中で走り続けた。
She kept running, utterly lost in the moment.
締め切り前で無我夢中になって資料を作った。
Facing the deadline, I threw myself into preparing the documents with total absorption.
火事の現場で消防士は無我夢中で子供を救い出した。
At the scene of the fire, the firefighter rescued the child in utter self-forgetful focus.