It is with great sadness that we learned of the death of Jean‑Claude Le Guillou on 6 January 2026, at the age of 86.
Jean‑Claude Le Guillou became a professor at the Pierre‑and‑Marie Curie University (Paris VI) and finished his career as a professor of exceptional class at the University of Savoie, conducting his research at the LAPTH in Annecy. He was a senior member of the Institut Universitaire de France and received, in 1996, the “Prix des Trois Physiciens” awarded by the École Normale Supérieure of Paris.
A theoretical physicist specializing in quantum field theory, phase transitions, and strongly correlated fermions, he regularly visited the Service de physique théorique du CEA Saclay from the early 1970s to the early 1990s, actively collaborating with the SPhT researchers.
In particular we cite his numerous works with E. Brézin and J. Zinn‑Justin on calculating critical exponents using field‑theory methods, as well as his condensed‑matter research together with Th. Jolicoeur.
Jean‑Claude Le Guillou and J. Zinn‑Justin are the authors of the paper “Critical Exponents from Field Theory” (Physical Review B 21, 3976 (1980)), which has been cited ≈ 1 600 times and was selected as a “Milestone Paper” by Physical Review for the journal’s 50th anniversary.
The Institute of Theoretical Physics as a whole joins in the sorrow of his family and loved ones.