International Physics Masterclasses

The Standard Model

High Energy Particle Physics is the study of the basic constituents of matter and the interactions among them. The current theory which elegantly summarizes the understanding of the field, the Standard Model (SM), describes interactions between fundamental constituents of matter, grouped in 3 families, or generations, of quarks and leptons, as shown in the figure below, and their antiparticles.

The electromagnetic, weak and strong forces are mediated by exchange bosons and are generated by fundamental symmetries related to conservation laws. The infinite range electromagnetic force is mediated by the mass-less photon and electric charge is conserved. The short-range strong force between quarks is carried by the mass-less colored gluons and the color charge is conserved.

At high energies electromagnetic and weak forces are described as a unified electroweak force. Massive W and Z exchange bosons are compatible with the short-range nature of the weak force. The weak charge, which is hidden from us, contrary to the electric charge, is not conserved.

The fact that particle masses are very small makes gravity negligible compared to the other three forces.

Leptons are free particles. They can be charged (e-, μ-, τ-), in which case they feel both electromagnetic and weak forces, or neutral (neutrinos: νe, νμ, ντ), then they interact only weakly. Quarks are particles that are sensitive to all three interactions. There are no free quarks in nature: We observe only composite states of quarks called hadrons, of which the proton and the neutron are the most widely known examples.

Any introduction of particle masses would break the electroweak symmetry and render the theory unpredictable. To avoid this, the Higgs mechanism spontaneously breaks the symmetry by postulating that the vacuum is filled with a new field that only carries weak charge. Particles that carry weak charge (W and Z bosons, quarks and leptons, and the Higgs particle, H, associated to the Higgs field) are slowed down by interacting with the Higgs field and acquire mass. The Higgs mass, mH, is not predicted by the Higgs mechanism.

The discovery of the long awaited Higgs boson was announced by the ATLAS and CMS collaborations in July 2012. The Higgs boson mass is approximately 125 GeV.

While scientists at CERN continue to study the Higgs boson to see whether its properties are really as predicted by the Higgs mechanism, you have the opportunity through the “Z-Path” to search for and find the Higgs boson just as scientists did just a couple of years ago!