Rolf-Dieter Heuer, a crinkly-eyed German with a snowy goatee, showed up at the Four Seasons, the perennial Manhattan power lunch spot, dressed like the physicist he is — in a sweater and baggy jeans for the red-eye to Geneva — rather than the diplomat he had just been playing.

The United Nations was in session, and Dr. Heuer, the director-general of the European Organization of Nuclear Research, or CERN, had a featured role in events celebrating the laboratory’s 60th anniversary.

In 2009, when he took over, dark clouds were hanging over CERN, the world’s largest physics lab.

A few months before, the lab’s new Large Hadron Collider, the most expensive particle accelerator ever built, had blown up, indefinitely delaying the hunt for new particles, new forces and even perhaps new dimensions of nature. Some scientists were taking their research to a competing collider at the Fermi National Accelerator Laboratory in the United States. The worldwide economy was collapsing, as if into the black hole some alarmists had predicted the collider would make.

Now he looks fondly on those days. “If you start with such a low point, you can show your team was able to bring everything out of a low point up to a high point,” he said.