How do subatomic particles convey forces?

Particle Physics: Quarks - the stuff that particles are made of

The particle zoo is systematized

The so-called particle zoo grew and grew. Today it houses several hundred subatomic particles. But subatomic is not elementary. The real building blocks of the matter still had to be found. That is why the new particles initially required an explanation and classification based on the laws of physics.

When a particle breaks down into smaller components or transforms into others, for example, energy, momentum, angular momentum (spin) and the electrical elementary charge must all be retained. This fact is described with the corresponding conservation laws. If a neutron breaks down into its components proton, electron and anti-electron neutrino, the entire electrical charge of all the particles involved must be retained before and after the reaction, as well as the total mass.

The behavior of the atomic building blocks proton, neutron and electron can be adequately described with the well-known conservation laws. But the new zoo members evaded such a classification. Conservation laws of a new type became necessary, in which new quantum numbers flowed, which to a certain extent describe the internal properties of the particles.

A complete set of quantum numbers uniquely characterizes a particle. As a practical matter, physicists introduce new quantum numbers when experimentally different particles cannot be differentiated by the available quantum numbers.

Werner Heisenberg (1932 Nobel Prize in Physics) introduced isospin symmetry into nuclear physics in this way. This enabled him to group some particles together. With this differentiation, the brilliant physicist described the independence of the forces acting between the nucleons - that is, protons and neutrons - from the electrical charge.

However, it was left to the American Murray Gell-Mann (Nobel Prize 1969) to provide an explanation to counter the riddle of the stability of nucleons. Gell-Mann, the child prodigy who classified birds, coins, words and much more and studied physics at the age of 14, could imagine the really elementary particles, the source of matter.

At the same time and independently of the Israeli researcher Yuval Ne'eman, he first summarized the elementary particles into octets and called this the "eightfold way" according to a Buddhist aphorism. His considerations are based on a complicated mathematical structure, the SU (3) algebra. This allows the particles to be classified in triangular multiplets (Fig. 1).

up, down, strange ...

Since it was suddenly possible to bring the elementary particles into an ordered structure similar to the periodic table, it was obvious that there had to be a substructure for this. Gell-Mann called these hypothetical building blocks of matter "Quarks" after the young guys ("Three quarks for muster Mark") in James Joyce's "Finnegan's Wake". The name has endured, although his colleague Georg Zweig wanted to call it "aces".

According to Gell-Mann, matter should consist of the three quarks "up", "down" and "strange" (Fig. 2). Proton, neutron and the well-known mesons could be explained with it. But it soon became clear that the concept of the three quarks had weaknesses. Gell-Mann expanded his ideas on three-dimensional SU (4) algebra and postulated a fourth quark, which he called "charm". This thought scheme made it possible to postulate new particles such as the omega (Fig. 1).

When, at the same time as this hypothesis was presented in 1974, the meson J / Psi was discovered, which has twenty times the mass of a pion and consists of a charm-anticharm quark doublet, it quickly became apparent that there had to be other mesons whose properties were predicted quite accurately could become. In 1978 a meson was surprisingly discovered which has ten times the mass of a proton and whose properties could not be explained other than the existence of a fifth quark called "bottom" or "beauty".

It was believed by now that quarks only occur in pairs. That is why the name "top" has already been reserved for the still undiscovered sixth quark. Its discovery was a long time coming in 1995; "top" has the ludicrously large mass of 175 protons.

This completes the quark family. It is very likely that there are no other quark pairs besides the six known quarks and their antiparticles (Fig. 2).

Quarks - building blocks of mesons and baryons

Quark research has so far been of no practical importance. But that can change. Because the processes in the atomic nucleus are not yet fully understood.

All particles that are made up of quarks are called hadrons, which are divided into mesons (medium particles) and baryons (heavy particles). The baryons each consist of three quarks; these include protons and neutrons. The mesons each consist of a quark and an antiquark (Fig. 3, Tab. 1).

The mesons convey the strong force that holds the core particles together, just as the photon is responsible for the electromagnetic interaction. Pion mesons fly back and forth between the nucleons about 1020 times per second. They decay at the speed that light needs to travel the distance one atomic nucleus diameter - that's 10-8 until 10-23 Seconds - and arise again and again.

These processes can be clearly explained with the Quark model. It fits seamlessly into the standard model, which describes how the elementary particles are organized and how they interact with one another.

The quark model is now being called into question by the particle Ds (2317), a meson that was discovered in California in early 2003. It is one of the theoretically predicted eight charm-and-strange mesons. Compared to the four particles in this group that have already been detected, however, it is ten percent too small. If this deviation from the theoretically estimated mass is confirmed, the previous theory of quarks would have to be reconsidered.


For more than 30 years it has been assumed that there must be particles that consist of five quarks. Five years ago, German and Russian researchers calculated the properties of such a particle. The experimental proof of a pentaquark was recently achieved by Japanese researchers.

At Osaka University, a carbon block was bombarded with high-energy gamma rays. After a long effort, it was possible to detect the merging of a meson and a neutron to form a pentaquark. Proof of the fact that after 10-20 Seconds of decaying pentaquarks were only possible through the fragments: a neutron and a positive kaon.

Takashi Nakano, the leader of the group, hopes the discovery will fertilize the theory about the young universe shortly after the Big Bang. It is very unlikely to find pentaquarks anywhere in the universe, at most in a black hole.

Color charge

Each quark can be called red, yellow, or blue. This color charge - a fairly new quantum number - plays a role similar to that of the electrical charge. She holds the quarks z. B. together in a proton. The associated eight exchange particles are called gluons. According to the theory, the strong interaction, which is after all the strongest force in the universe, is the remainder of these very strong color forces.

The pursuit of the great synthesis of supernuclear interaction, that is, of a unified theory of electromagnetic, weak and strong interactions, raises many questions. Among other things, it is about whether a quark can transform into a lepton, and above all about why the proton is so stable.


Károly Simonyi: Cultural History of Physics. Harry Deutsch, Frankfurt am Main, 1990. Volkard Linke: Quarks, basic building blocks of matter. Lecture at the Free University of Berlin, 1995.