STANDARD MODEL OF ELEMENTARY PARTICLES

The Standard Model is the theory that describes the elementary particles that constitute matter and the fundamental forces, i.e. the way in which these interact with each other.This very successful theory was proposed more than fifty years ago by the Nobel Prize winners Steven Weinberg and Abdus Salam, completing the pioneering work of Sheldon Glashow. The model is a quantum theory of fields that is based on “gauge” symmetry.
Each elementary particle is characterised by three quantities: the mass, the electrical charge, and the spin (the rotation around its axis). Each particle has its antiparticle. Particles and antiparticles have the same mass, the same spin, but opposite electrical charges.

There are twelve particles that make up matter and they have a spin equal to 1/2 and belong to the category of fermions (in honour of Enrico Fermi). The force particles have a spin equal to 1 and belong to the category of bosons (in honour of the Indian physicist Satyendranath Bose). The particles that constitute known matter are divided into two groups: quarks (there are six of these: up, down, strange, charm, bottom, top) and leptons (six, again: electron, muon, tau, and the three corresponding neutrinos).There are three forces described by the Standard Model: the electromagnetic force, the strong force, and the weak force. These are generated by the exchange of mediator particles: photons, W bosons (which are of two types W+ and W-), Z bosons, and gluons (of which there are eight different types). The three bosons W+, W-, and Z were discovered in 1983 by the UA1 collaboration, led by Carlo Rubbia, at CERN’s SPS accelerator. The fourth force of nature, gravity, is not included in the Standard Model; it is very small as an interaction between particles and can be ignored at the energies accessible to accelerators.

Infographic of the Standard Model (© INFN)

Giving mass to all the particles is a spin-0 boson, the Higgs boson, which was discoveredin 2012 by the collaborations of the ATLAS and CMS experiments at CERN’s Large Hadron Collider.This particle is associated with a field, the Higgs field, which is indispensable for the standard model to be able to correctly describe nature. The “gauge” symmetry of the standard model implies, in fact, that the mass of all the particles describedby the theory is equal to zero, in contrast with the experimental observations. The mechanism of spontaneous symmetry breaking, proposed by the Nobel Prize winners Peter Higgs, Robert Brout, François Englert, and others, resolves this problem by introducing the Higgs field. This field modifies the theory’s quantum vacuum so as to generate the masses of particles through their coupling with the field itself. The discovery of the Higgs boson definitively confirmed the standard model’s theoretical system. However, questions and open problems remain – not just in relation to cosmological observations, such as, the lack of a candidate for dark matter, but also in the area of the model itself.
Throughout the world, the high-energy physics community is engaged in research in this field, especially at CERN. First, such research involves the high-luminosity phase of LHC and, in the future, it will involve the next-generation FCC accelerator, a 90 km ring that is being designed.

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