children, Albert, Friedrich, Elisabeth, and Ulrich. After retirement, he moved together with his second wife Barbara Klie to Schliersee, a pastoral village in the Bavarian mountains. He died on 5 January 2016, in Fischhausen-Neuhaus, in southern Bavaria. Scientific career At the beginning of his career, Haag contributed significantly to the concepts of quantum field theory, including Haag's theorem, from which follows that the interaction picture of quantum mechanics does not exist in quantum field theory. A new approach to the description of scattering processes of particles became necessary. In the following years Haag developed what is known as Haag–Ruelle scattering theory. During this work, he realized that the rigid relationship between fields and particles that had been postulated up to that point, did not exist, and that the particle interpretation should be based on Albert Einstein's principle of locality, which assigns operators to regions of spacetime. These insights found their final formulation in the Haag–Kastler axioms for local observables of quantum field theories. This framework uses elements of the theory of operator algebras and is therefore referred to as algebraic quantum field theory or, from the physical point of view, as local quantum physics. This concept proved fruitful for understanding the fundamental properties of any theory in four-dimensional Minkowski space. Without making assumptions about non-observable charge-changing fields, Haag, in collaboration with Sergio Doplicher and John E. Roberts, elucidated the possible structure of the superselection sectors of the observables in theories with short-range forces. Sectors can always be composes with one another, each sector satisfies either para-Bose or para-Fermi statistics and for each sector there is a conjugate sector. These insights correspond to the additivity of charges in the particle interpretation, to the Bose–Fermi alternative for particle statistics, and to the existence of antiparticles. In the special case of simple sectors, a global gauge group and charge-carrying fields, which can generate all sectors from the vacuum state, were reconstructed from the observables. These results were later generalized for arbitrary sectors in the Doplicher–Roberts duality theorem. The application of these methods to theories in low-dimensional spaces also led to an understanding of the occurrence of braid group statistics and quantum groups. In quantum statistical mechanics, Haag, together with Nicolaas M. Hugenholtz and Marius Winnink, succeeded in generalizing the Gibbs–von Neumann characterization of thermal equilibrium states using the KMS condition (named after Ryogo Kubo, Paul C. Martin, and Julian Schwinger) in such a