strong interaction

History

In the 1930s, scientists realized that there must be a special attraction in the nucleus, otherwise it would fall apart. Japanese physicist Hideki Yukawa predicted the existence of carrier particles—mesons—in 1935. Later, in the 1960s–70s, quantum chromodynamics (QCD) was developed, where the carriers are gluons and the constituents are quarks.

How it works

Imagine two fish in an aquarium with very thick water: when they are far apart, the water hardly bothers them, but as they approach, a powerful attraction arises due to water pressure that draws them together. It's the same with quarks: the farther they are from each other, the more a 'string' of the gluon field stretches between them, and the energy grows. If you try to pull quarks apart, the energy becomes so great that new particles are born—quark-antiquark pairs—and they combine into new stable particles.

History

In the 1930s, scientists realized that there must be a special attraction in the nucleus, otherwise it would fall apart. Japanese physicist Hideki Yukawa predicted the existence of carrier particles—mesons—in 1935. Later, in the 1960s–70s, quantum chromodynamics (QCD) was developed, where the carriers are gluons and the constituents are quarks.

How it works

Imagine two fish in an aquarium with very thick water: when they are far apart, the water hardly bothers them, but as they approach, a powerful attraction arises due to water pressure that draws them together. It's the same with quarks: the farther they are from each other, the more a 'string' of the gluon field stretches between them, and the energy grows. If you try to pull quarks apart, the energy becomes so great that new particles are born—quark-antiquark pairs—and they combine into new stable particles.

History

The first indications of the strong interaction appeared in the 1920s with the discovery of the atomic nucleus. In 1935, Hideki Yukawa proposed the meson theory of nuclear forces. The development of accelerators in the 1950s–60s led to the discovery of many hadrons, necessitating the introduction of the quark model (Gell-Mann, Zweig) in 1964. The color degree of freedom and the confinement hypothesis were formulated in the early 1970s, and quantum chromodynamics as a gauge theory with SU(3) group was constructed by Gross, Wilczek, and Politzer in 1973 (Nobel Prize 2004). Experimental confirmation of asymptotic freedom came from deep inelastic scattering.

How it works

The strong interaction binds quarks through the exchange of gluons. Gluons are massless but carry color, so they interact with each other, forming 'gluon chains' and strings. When trying to separate quarks, the potential energy of the string increases, and upon exceeding the threshold for pair production of quark-antiquark, the string breaks with the formation of new hadrons—this phenomenon is hadronization. In high-energy collisions, quarks and gluons form quark-gluon plasma—a state in which they are weakly bound. The residual strong interaction between colorless hadrons manifests as nuclear forces holding nucleons together in the nucleus (an effective interaction analogous to van der Waals forces in molecular physics). Accompanying laws: Pauli principle, gauge invariance, asymptotic freedom.

Open problems & fun facts

The nature of confinement: a rigorous proof of the impossibility of free quarks existing within QCD is lacking.; The equation of state of quark-gluon plasma and QCD phase transitions.; Computing hadron properties (masses, spins) directly from QCD requires large computational resources (lattice QCD).; The structure of the nucleus at the quark level and the role of gluons in the proton spin (the proton spin problem).

💡 Quark-gluon plasma, which existed in the first microseconds after the Big Bang, is recreated in accelerators, such as the LHC, by colliding heavy ions at near-light speeds—at such points, the temperature exceeds 5 trillion degrees.