Can quarks have potential energy?


The use of particle accelerators enabled the advance into the interior of the protons: It was discovered that these particles consist of even smaller particles, the quarks - which we now consider elementary particles. With this the hadron physics was founded. It deals with the study of all particles that contain quarks and are known as hadrons.

The elementary building blocks of protons and neutrons - the quarks - are held together by the strong force. There are six different types of quarks and corresponding antiquarks, which can be linked in different ways and thus form a large number of so-called hadrons. Except for the protons and the neutrons bound in atomic nuclei, however, all hadrons are unstable and can only be generated for a short time in accelerator experiments. Protons and neutrons are made up of the two lightest quarks, called up and down quarks.

Elementary particles and basic forces

Unlike the molecules, which we can largely understand from the properties of the atoms, the hadrons still pose great puzzles to us. To clear this up, a much deeper understanding of the Strong Force is required. The theory of strong forces - quantum chromodynamics - has not yet been able to describe how the quarks are trapped in hadrons. Approaches in which the properties of hadrons are calculated using high-performance computers are promising for solving this question.

Hadrons cannot be split into individual quarks. Despite an intensive search, no isolated quarks have been observed in nature so far. Phenomenologically, this property of quarks can be explained by the fact that the strong force mediated by gluons acts like a rubber band. Pulling the quarks apart requires work that is stored as potential energy in the rubber band - that is, in the gluon field between the quarks. The rubber band only breaks when the energy is sufficient to form a quark-antiquark pair, which leads to the formation of new hadrons, but does not release any quarks. In doing so, according to Einstein's formula E = mc², energy is converted into matter. This behavior of the quarks in the hadrons is called Confinement designated. It is one of the great challenges of modern physics that Confinement To be understood not only qualitatively but also quantitatively within the framework of the theory of the strong force.

The mass of the proton

Actually, the mass of a proton should result from the sum of the masses of its constituent parts, except for small corrections due to binding effects that slightly reduce the mass of the composite system. All the more surprising was the observation that the quarks contribute less than two percent to the proton or neutron mass. The mass of the quarks is generated by their interaction with the so-called Higgs particle, which is predicted by the standard model of particle physics. In experiments at the Large Hadron Collider at CERN, a new particle has now been discovered that has the properties of the postulated Higgs particle. However, the mass of the nucleons - and thus the mass of the visible universe - cannot be explained by the Higgs mechanism.

Composite quarks

According to today's understanding, the mass of the nucleons results mainly from the kinetic energy of the quarks and the energy of the gluon field between them. Here, too, the equivalence of energy and mass applies. Physicists believe that another process played an important role in the creation of the hadron mass: the violation of chiral symmetry. The so-called chirality is one of the most important symmetries in nature. It describes the fact that there are objects that relate to one another like a picture and a mirror image, i.e. despite their similarity, they can never be brought into line, like the two hands of a person. Hence the term chirality, which is derived from the Greek "χειρ" for "hand" and can be translated as "handedness".

The fundamental theory of the strong force, quantum chromodynamics, is based on chiral symmetry. This means that there are right-handed and left-handed quarks, with the handedness being defined by the combination of the direction of flight and a quantum mechanical property called the spin of the quarks. With right-handed quarks the direction of flight and spin point in the same direction, with left-handed quarks they are opposite. Right and left-handed quarks cannot be made to coincide because they are massless and therefore move like photons at the speed of light. The interaction with the Higgs particles gives the quarks a mass, which physicists refer to as an explicit breaking of the chiral symmetry. In this state, the quarks existed shortly after the Big Bang when they moved freely in the hot quark-gluon plasma. This plasma of light quarks and massless gluons condensed into hadrons that are many times heavier than the sum of their components. Physicists speak of a spontaneous breaking of the chiral symmetry of the strong force. The creation of the mass of hadrons and matter is thus closely linked to the breaking of the chiral symmetry in the early universe.

Further evidence was found that the chiral symmetry in our hadronic world has been violated. There are certain pairs of hadrons that behave like an image and a mirror image. These so-called chiral partners would have to have the same masses if the chiral symmetry were fulfilled. The observed hadron masses of chiral partners are clearly different. This can be explained by the spontaneous breaking of the chiral symmetry, which causes a shift and splitting of the masses of the chiral partners and thus contributes to their hadronic mass.

Strong interaction in focus

Protons also have a spin. For a long time it was assumed that the spin of the proton is simply composed of the spins of the quarks. However, scattering experiments with high-energy electrons then showed that the spins of the quarks contribute less than thirty percent to the spin of the proton. Physicists therefore suspected that the orbital angular momentum of the quarks and the spin of the gluons also contribute to the total angular momentum of the proton. However, new experimental data from COMPASS at CERN and HERMES at DESY show that the contribution of the gluons is small.

On the other hand, there are first experimental indications that the orbital motion of the quarks makes a finite contribution to the total angular momentum of the proton. New theoretical descriptions allow a three-dimensional picture of the internal structure of the hadrons. In order to check this picture experimentally, one needs extremely intense beams of polarized electrons, i.e. electrons with a defined spin direction relative to the direction of flight. However, these are currently not available. Planning for corresponding new accelerator facilities and detection systems has begun.

The inside of the proton

The strong force has a special property that distinguishes it from all other forces: the gluons, which mediate the attraction between the quarks, also attract each other. This attraction of the gluons to one another is the cause of the extreme strength of the strong force and leads to a complex picture for the hadrons observed in nature, which are composed of quarks, antiquarks and gluons.

In qualitative terms, many of the hadrons observed so far can be described in simplified form as two- or three-particle systems of quarks, in which case one speaks of constituent quarks. The question of which structures can actually be produced by the strong interaction is open and is currently being intensively researched. Theoretically, it should also be possible that, for example, hadrons exist that are hybrid states composed of two constituent quarks and one gluon. It cannot be ruled out that there are particles that only consist of gluons.

The - still distant - goal of physicists is to find an effective description of the strong interaction with which they could understand the structure of the hadrons. This description should be able to explain the experimentally obtained data as well as satisfy the physical principles that are summarized in quantum chromodynamics. To this end, the physicists are looking for suitable abstract properties of the hadrons, which they call degrees of freedom. An everyday example of such a degree of freedom is the center of gravity of a tennis ball, which is also referred to as the center of mass. With its help, the trajectory of the ball can be calculated well.

If the physicists found suitable degrees of freedom for the strong interaction, they could predict which hadron shapes or particle combinations arise under which conditions. Collisions between protons and antiprotons at high energies, as will be available at the future FAIR accelerator facility in Darmstadt, offer unique opportunities for the generation and study of new hadronic states.