![]() The current results of the ATLAS and CMS experiments indicate that the Higgs boson does indeed seem to have all the characteristics of the elementary particle predicted by the spontaneous symmetry breaking mechanism at the origin of the mass of particles in the Universe. LLR researchers have also demonstrated that the Higgs boson couples to other particles of matter with an intensity proportional to their mass. It has also been confirmed that it is a ‘scalar’ boson, in a kind of its own, because it has no “spin’”: it is neither a matter particle, such as the electron (spin = ½), nor the vehicle of an interaction such as the photon (spin = 1). It has since contributed to the precise determination of the intrinsic properties of the Higgs boson and its couplings to other elementary particles. The Laboratoire Leprince-Ringuet (LLR), with the support of CNRS and École Polytechnique, was among the main players of this “discovery of the century” – as part of the international collaboration, CMS. On July 4, 2012, the large CMS and ATLAS experiments at CERN, both announced the discovery of a new particle in the mass region around 125 GeV. For many years, however, there was one major problem: no experiment had ever observed the Higgs boson to confirm this theory. The Higgs boson is the visible manifestation of the Higgs field, much like a wave on the surface of the sea. Like all fundamental fields, the Higgs field is associated with a particle – in this case, the Higgs boson. Transverse view of one end of the current CMS detector with its « tip » at the center left © CERN With the appearance of the mass of elementary particles, such as the electron or quarks, and that of the electromagnetic interaction, which makes it possible to define the electric charge as we know it, the ingredients necessary for the formation of the atoms of ordinary matter finally appear in the Universe. As a result, the weak interaction, responsible for radioactivity, acts only at very short distances, while the electromagnetic interaction has an infinite range. At the same time the photon of zero mass appeared, as a vehicle of the electromagnetic interaction. This sudden increase of the Higgs field led to the spontaneous breaking of electroweak symmetry, with the striking consequence that the weak interaction was suddenly carried by the W and the Z. ![]() Particles like the photon that do not interact with it remain massless while the W and the Z have mass. The more a particle interacts with this field, the heavier it becomes. Just after the Big Bang, the Higgs field was zero, but when the Universe cooled down and its temperature fell below a certain critical temperature, the Higgs field spontaneously increased so that any particle interacting with it acquired a mass. Spontaneous breaking of the electroweak symmetry To solve this problem, theoretical physicists Robert Brout, François Englert and Peter Higgs proposed a mechanism that gives mass to the W and Z particles when they interact with an invisible field, called the ‘Higgs field’, which permeates the entire universe. If the photon is indeed massless, the W and Z have a mass almost 100 times that of a proton. If the basic equations of the unified theory correctly describe the electroweak force and the particles that carry it, namely the photon and‘vector bosons’, W and Z, there is a major hitch: all these particles appear to have no mass in calculations. ![]() ![]() This ‘unification’ implies that electricity, magnetism, light and radioactivity are all manifestations of a single underlying force, known as the electroweak force. These two forces can be described within the framework of a single theory, which forms the basis of the Standard Model of particle physics. Physicists have known since the 1970s that two of the four fundamental forces of nature – weak force and electromagnetic force – are closely related. ![]()
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