What exists between nucleons

Structure of matter

In their search for an explanation of “what holds the world together at its core”, physicists ventured into ever smaller dimensions. In the process, not only new structures of matter, but also previously unknown forces were discovered.

All galaxies, stars and planets, the things of our everyday life and also we humans are made up of matter that only consists of three types of particles: the positively charged protons, the electrically neutral neutrons and the negatively charged electrons. Protons and neutrons are the building blocks of the atomic nucleus, which is surrounded by a shell of electrons. The various chemical elements differ in the number of protons in the nucleus, and the electrons connect the atoms to form crystals or biomolecules, creating the complex structures of inanimate and animate nature.

Four basic forces in physics

The cohesion of matter is caused by four natural forces. The galaxies of the universe and our solar system were created and exist through gravity, which we experience as gravity on earth. At the atomic level, the electromagnetic force dominates between the positively charged atomic nuclei and the negatively charged electrons. We have known for about thirty years that protons and neutrons, collectively called nucleons, themselves have an internal structure and are composed of elementary particles, the quarks.

The atomic nuclei are stabilized by the strong force that holds the nucleons together as well as the quarks in the nucleons. At the nuclear level, another force acts, the so-called weak interaction. It is responsible for the radioactive decay of atomic nuclei and for the fusion of light atomic nuclei which, as an energy source for stars like our sun, enable life on earth.

The strong force in particular still poses many puzzles to physicists. The strong force is generated between the quarks through the exchange of particles, the so-called gluons. glue = Glue). The gluons connect quarks like rubber bands, and this has amazing consequences: If the distance between two quarks is very small, the attractive force between them is very weak. However, the greater the distance between the quarks, the stronger the force between them becomes.

Quarks can therefore never occur in isolation, physicists speak of Confinement. This puzzling property distinguishes quarks from all other particles. Quarks are trapped in groups of three in the nucleons. Linked to this is another puzzling phenomenon: a nucleon is heavier than the sum of its components. The mass of the quarks contributes less than two percent to the nucleon mass, and the gluons are massless. The mass of the point-like quarks is generated by their interaction with the so-called Higgs particle, which is predicted by the standard model of elementary particle physics.

Where does the crowd come from?

In experiments at the Large Hadron Collider at CERN, a new particle was recently discovered that has properties of the long-sought Higgs particle. However, the mass of the nucleons - and thus the mass of the visible universe - cannot be explained by the Higgs particle. According to the theory of strong forces, a large part of the mass of nucleons is found in the kinetic energy of the quarks and in the energy of the gluon field that holds the quarks together. The atomic nuclei and matter as a whole would thus mainly consist of kinetic and field energy! Albert Einstein described this equivalence of energy and mass in his famous formula E = mc².

In cosmic rays and in laboratory experiments with accelerators, further - albeit very short-lived - particles were discovered. They consist either of three quarks or of pairs of quarks and their antiparticles, the so-called antiquarks. There are even particles that consist of three antiquarks, these form the basic building blocks of antimatter.

Today six different types of quark are known, which differ greatly in their mass. Nucleons consist of light quarks that have a mass of only about one percent of a nucleon. The heaviest quark, on the other hand, has almost 200 times the mass of a nucleon. All particles composed of quarks or antiquarks are called hadrons.