principal investigator; PI
Formal term used in academic, medical, and research contexts for the lead researcher responsible for a project.
The principal investigator of this research project is Professor Tanaka.
As the PI, I am responsible for data management.
主任研究員 is a senior researcher or chief researcher, often a rank within an organization, while 研究責任者 specifically refers to the person with overall responsibility for a research project, often the PI.
Compound of 研究 (research) + 責任者 (person in charge). The term is a direct translation of the English 'principal investigator' and is used in formal research settings.