He became Director-General of the international high energy physics CERN facility (January 2004 to December 2008). His term of office was marked by the completion of construction and the first commissioning of the Large Hadron Collider (LHC). His experience of complex industrial projects proved to be crucial, as the CERN teams had to overcome numerous challenges linked to the LHC's innovative technologies and their industrial production.

As Head of the Material Sciences division at the Commissariat à l'énergie atomique et aux énergies alternatives (French Alternative Energies and Atomic Energy Commission, CEA) from 1990 to 1994, he set out to bring together the physics of the infinitely large and the infinitely small, as well as the associated instrumentation, in an institute that has now become the Institut de recherche sur les lois fondamentales de l'Univers (Institute for Research into the Fundamental Laws of the Universe, CEA IRFU). He served on many national and international committees, and in 1993 he chaired the LHC External Review Committee, whose recommendation proved decisive in the project's approval. In 2001, the CERN Council called on his expertise once again by entrusting him with the chairship of the LHC programme's External Review Committee.

When he took over as Director-General of CERN in 2004, the construction of the LHC was well under way, but there were many industrial and financial challenges still to overcome. During his tenure, the machine was completed and the first beams circulated. That first start-up in 2008 was followed by a major technical problem that led to a shutdown lasting several months. But the LHC had demonstrated that it could run, and in 2009 the machine was successfully restarted.

Robert Aymar's term of office also saw a simplification of CERN's structure and procedures, aimed at making the Laboratory more efficient. He also set about reducing costs and secured additional funding to complete the construction and optimise the operation of the LHC.

The current ITER director, Pietro Barabaschi, said of Aymar "He was the director of the ITER project between 1994 and 2004. A person of great integrity, knowledge, intellect. A strong leader that shaped the early phases of ITER - a project he loved and fostered with strong passion. I always felt priviledged for having worked with and for him. We shall miss you dearly.