Experimental Nuclear Physics
In our everyday life we are surrounded by matter, which at the most fundamental level is made up of atoms that consist of a nucleus surrounded by a cloud of electrons. The building blocks of atomic nuclei are protons and neutrons, which in turn are built up of quarks and gluons. In nuclear physics we study the nucleus, its properties, and the strong nuclear force which holds it together.
This force is also the force responsible for confining the quarks into protons and neutrons. In fact, quarks and gluons have never been observed as free particles only as confined constituents of composite particles. This is one of the fascinating properties of the strong force called “confinement”. The strong force is described by a quantum field theory called Quantum Chromodynamics (QCD). There are still many aspects of QCD which are not yet fully understood, among them quark confinement and how to treat dense systems with collective interactions between many quarks and gluons.
Here in Bergen, the experimental nuclear physics group is a part of one of the large CERN experiments at the Large Hadron Collider (LHC) called ALICE (A Large Ion Collider Experiment). Below we will describe some of the topics the nuclear physics group is focusing on.
The ALICE experiment is specifically designed to study heavy ion collisions at the highest energies, where hadronic matter undergoes a phase transition into a “soup of free quarks and gluons” or more formally a Quark Gluon Plasma (QGP). The QGP is ideal for further studying (and hopefully understanding) the many complicated properties of the strong force..
In a heavy ion collision, two ions collide and produce a system of strongly interacting matter with extremely high temperature and density. At densities and temperatures high enough, the matter transitions into a QGP phase. After production, the system expands and cools down causing the “free” quarks and gluons to recombine into confined particles (known as “hadronization”, a hadron being a bound state of either a quark and an anti-quark, 3 quarks, or 3 anti-quarks). These hadrons (or their decay particles) are detected by the experiment.
This whole process happens during a small fraction of a second. In order to get some understanding of what happened before and during the QGP phase one must reconstruct the detected particle tracks and identify which particles produced them. From this one hopes to deduce the key properties of the QGP.
A schematic illustration of the time development of a collision between two heavy ions. The figure to the right shows the tracks the emitted particles produce in the detector.
One of the signatures for quark gluon plasma production is the J/psi (from here on J/ψ) meson. This particle is composed of a charm quark and its antiquark, held together by the strong force. The quark pair orbits at a relative distance of about 0.5 fm (1fm = 10-15m). Thus, the quark pair of a J/ψ placed inside the QGP could lose sight of each other due to the high number of deconfined quarks and gluons that are splashing around in the medium. This causes the binding to become weaker so that ultimately the pair disintegrates and the J/ψ vanishes. The probability of dissociation depends on the temperature of the QGP, therefore the observation of the J/ψ suppression is equivalent to placing a «thermometer» inside the medium. When the density of quarks in the QGP becomes very high, J/ψ mesons can, however, also be formed through recombination – a merger between two arbitrary charm-anticharm pairs in the plasma. This latter process thus enhances rather than reduces the number of J/ψs. Results from the ALICE collaboration has shown that one has to take both these processes into account to understand J/ψ production at different collision energies.
A J/ψ meson consisting of a charm and anti-charm quark in vacuum (left). A dissociated charm and anti-charm quark inside a quark-gluon plasma (middle). The contribution from melting and recombination (regeneration) to J/ψ prodcution as a function of the energy density (right).
Collisions between heavy ions do, however, not always involve the strong nuclear force. The ions can also interact electromagnetically. The electromagnetic fields associated with a charged particle are enhanced and compressed at high energies, and the fields effectively correspond to a flux of high energy photons (this is known as the Weizsäcker-Williams method of equivalent photons). The electromagnetic field has an infinite range, so electromagnetic interaction can occur also when the ions pass by each other separated by several hundreds of fm. In these co called ultra-peripheral collisions no strong interactions are possible because of the short range of the nuclear force. The heavy ion beams at the LHC provide collisions between two photons or between a photon and a nucleus at energies higher than at any other accelerator. Various types of interactions can occur in an ultra-peripheral collision, production of J/ψ mesons is an example also in this case.
One of the most commonly used models for ultra-peripheral collisions, STARLIGHT, has partly been developed in Bergen.
Master's or PhD student in the Nuclear Physics group?
The group can provide master's and PhD projects within the ALICE experiment. The projects can involve work on hardware and read-out electronics as well as physics and data analysis. As part of the group at UiB, you get to work with basic research contributing to a better understanding of the fundamental mechanisms of the strong interaction and the matter subject to this force. You will become part of an international team and parts of the work will in most cases be done at CERN or at other international laboratories.