“Recursive self-improvement” is the idea that artificial intelligence could learn to build and train itself, creating exponential new progress — and risk.

Minutes after four top researchers said they were leaving, Google said that Demis Hassabis, the Nobel-winning scientist who led the company’s A.I. lab, was stepping into a new job.

Jeff Dean, who for years was one of Google’s most important executives, is leading the new artificial intelligence company with the backing of Google.