# Robert Brout

Wolf Prize Laureate in Physics 2004

The Prize Committee for Physics has unanimously decided that the 2004 Wolf Prize be jointly awarded to: Peter W. Higgs, Robert Brout and Francois Englert.

**Robert Brout**

**Universite Libre de Bruxelles**

“for pioneering work that has led to the insight of mass generation, whenever a local gauge symmetry is realized asymmetrically in the world of sub-atomic particles.”

Four fundamental forces are presently known. Two of them have a long range: electromagnetism and gravity. Weak interactions have a short range, the composite hadronic-nuclear forces are also short-ranged although color interactions among their constituents, the quarks, grow with distance.

The carriers of the interactions are themselves particles–gauge particles. They have zero mass when the forces have a long range. Such is the photon. They are massive, when the forces have a short range. Electromagnetic interactions are very successfully described by a theory, which has a very large degree of symmetry, a local symmetry called gauge symmetry. On the one hand, it seemed that gauge symmetry necessarily implied that gauge particles have zero mass; on the other, in the absence of a local gauge symmetry, the weak and hadronic short-ranged interactions were ill defined.

Professor Robert Brout, in collaboration with Professors Francois Englert and Peter W. Higgs, have discovered how mass can be generated for gauge particles in the presence of a local abelian and non-abelian gauge symmetry. This was demonstrated by them, both classically and quantum mechanically, successfully avoiding theorems initiated by J. Goldstone while indication that the theory would remain well defined (renormalizable). Similar ideas have been developed in the domain of condensed matter physics.

The work has deeply shaped the understanding of the fundamental interactions. It has allowed to unify the long-range electromagnetic forces with the short-range weak ones. This was manifested in the works of Glashow, Salam and Weinberg, determining the laws of the electro-weak forces. The discovery of Brout, Englert and Higgs, was essential to the proof of G. ‘t Hooft that the theory with massive gauge particles is well defined; and subsequent calculations in that theory, verified experimentally, culminating in the discovery of the massive W and Z particles.

Mass generation has been the cornerstone of many key theoretical results, including cosmology at high temperature and density, grand unified theories and the possible realization of the Dirac monopole. Mass generation is independent of the detailed short-distance physics. It could be driven, among others, by an elementary scalar, or by an effective composite scalar. Near future experiments are constructed to shed light on this fundamental question. The above indicates the far- reaching fundamental nature of the Brout-Englert-Higgs contribution.