What is up and down quarks

Quark-Gluon Plasma

left: unbound quarks and gluons (quark-gluon-plasma);
Middle: Components of atomic nuclei are formed;
right: electrons bind to the atomic nucleus and the first atoms are formed.
Graphics: CERN

The everyday world as we know it is made up of atoms, consisting of an atomic nucleus of neutrons and protons and electrons orbiting around it. Neutrons and protons are formed from quarks that are held together by the force particles of the strong interaction called gluons. The cohesion between quarks and gluons is extremely strong, so strong that it is impossible to isolate quarks. Immediately after the Big Bang it was completely different.

A few millionths of a second after the Big Bang, the energy density was extremely high. Model calculations of the strong interaction predict that at these high energy densities, a phase transition from quarks and gluons into a plasma takes place. In this quark-gluon plasma, the quarks and gluons are not bound to particles of matter, but can move freely. However, this state of affairs immediately after the Big Bang did not last. The quark-gluon plasma cooled down, and at a certain temperature the first bound particles formed: protons and neutrons. As a result, normal matter as we know it was created. This formation of particles from the quark-gluon plasma had a great influence on the subsequent development of the universe. Therefore, the properties of this state should be investigated at the LHC.

The big bang in the accelerator

There can be no quark-gluon plasma in today's universe. But it is possible to create this state for fractions of a second at particle accelerators. To do this, protons or heavy atomic nuclei, such as lead ions, are brought to very high energies and shot at each other. During the collisions, a quark-gluon plasma is created for a very short time, but it disintegrates immediately. Particles are released that can be examined with detectors and provide information about the conditions that prevail in the quark-gluon plasma.

At the LHC, it is the ALICE detector that records and analyzes these processes. Since significantly higher energies are achieved at the LHC during particle collisions and the quark-gluon-plasma state exists for a longer period of time than with any other accelerator in the past, the researchers hope to gain new insights into the properties of the quark-gluon plasma.