Reality → Energy → Particles → Major discoveries
The following particles, listed in historical order of their discovery, are central to our understanding of nature:
The neutrino was originally thought to be massless. The Nobel Prize in Physics 2015 was awarded for the discovery of neutrino oscillations that imply the presence of mass, however small. The discovery is a new manifestation of the incompleteness of the standard model (see also CERN brief).
Because of non-interaction, the neutrino penetrates all matter without hindrance. Enormous amounts of neutrinos are generated inside the sun, pass through the sun, radiating in all directions and passing almost unhindered through the planets. It has been estimated that about 65 billion neutrinos from the sun pass every second through each square centimeter of the earth.
Gell-Mann’s quark model brought symmetry and order to the then existing ‘zoo’ of nuclear particles. The model was proven to be correct in high-energy (deep inelastic scattering) experiments conducted by a team led by Richter at the Stanford Linear Accelerator (SLAC).
Quarks, like other fermions, come in 3 ‘flavors’, also called ‘generations’ or ‘families’, which are characterized by different masses. The mass of a proton is about 100 times greater than the sum of the rest masses of its three constituent quarks. The difference is explained by the mass equivalent of the energy of gluons, carriers of the strong force that holds the quarks tightly bound inside the proton.
The proton is the lightest and the only stable particle composed of quarks (the neutron is only stable as long as it is confined to the nucleus). In addition to the proton and neutron there are hundreds of known other hadrons composed of various combinations of the six different quarks and their antiquarks. These are all unstable, i.e., radioactive.
The strong interaction (or force) mediated by the gluon is by far the strongest of the four known fundamental interactions of nature (strong, weak, electromagnetic, and gravitation). It has an extremely short range (radius of a proton) and, unlike gravitation or the electromagnetic force, it increases with distance from the center (experimental results deliver a picture of quarks moving freely near the center of the proton). The quark-gluon interaction is explained by the theory of quantum chromodynamics. The theory postulates 8 different gluons, based on the principle of symmetry. It works with red, green, blue color and anticolor charges of quarks and antiquarks (a color charge resembles in some respects an electric charge). The color charges are additive, similar to light colors (e.g., the combination of three colors (or anticolors) results in colorless).
Together with the strong interaction, the weak interaction (more than 10 orders of magnitude weaker) is responsible for nuclear forces (acting only over very short distances, about the diameter of a proton). The weak interaction is crucial for explaining radioactive decay as observed on earth and the fusion taking place in the sun.
CERN observed in 2012 LHC experiments a new particle that "is consistent with the Higgs boson predicted by the Standard Model". The important discovery was honored in 2013 with the award of a Nobel prize. For more information see CERN's topic The Higgs boson and a 2015 follow-up report.