Nicholson, Hannele B. M. (2007) DISFLUENCY IN DIALOGUE: ATTENTION, STRUCTURE AND FUNCTION PhD. thesis, University of Edinburgh, Edinburgh, Scotland, UK, available online: https://era.ed.ac.uk/bitstream/handle/1842/1763/Nicholson%20...
But "um" may not be quite the same as "aha" for English native speakers (and Japanese native speakers may use both "um" and "ahm" as disagrement).
Nicholson, Hannele B. M. (2007) DISFLUENCY IN DIALOGUE: ATTENTION, STRUCTURE AND FUNCTION PhD. thesis, University of Edinburgh, Edinburgh, Scotland, UK, available online: https://era.ed.ac.uk/bitstream/handle/1842/1763/Nicholson%20...
But "um" may not be quite the same as "aha" for English native speakers (and Japanese native speakers may use both "um" and "ahm" as disagrement).