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Modern Subatomic Physics, 7.5p ECTS
 
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  FYS246: Modern Subatomic Physics, 7.5p ECTS

Subatomic research in Lund

On this page, we introduce some of the many subatomic physics research projects being pursued at the Divisions of Nuclear Physics and High Energy Physics at Lund University. Many of the topics presented here will be covered in more detail during the course.

In the following, the projects are coarsely ordered according to the energy needed to study them. If you want more information about a specific topic, get in touch with the listed contact persons!

Absorption of photons in nuclei
Lennart Isaksson

The research programme of the photonuclear group at the Division of Nuclear Physics is performed mainly at the Maxlab accelerator in Lund. The accelerator supplies a nearly continuous electron beam which is converted to a photon beam through bremsstrahlung. The energy of each photon is determined by measuring the remaining energy of the corresponding electron (photon tagging). By combining an excellent photon energy-resolution with state-of-the-art detection systems it has been possible to achieve the highest resolution ever in studies of photonuclear reactions.

Schematic illustration of a photon-induced reaction The research programme includes studies of correlations between nucleons, for which photons are excellent probes. As a result of the photon being massless, a second body must participate in the reaction in order to conserve momentum. At higher photon energies, absorption on proton-neutron pairs is a common reaction mechanism (even in reactions where a single nucleon is emitted) as illustrated by the similarities (probability, population of states, angular distributions) of proton- and neutron-knockout reactions. The properties (quantum numbers) of the proton-neutron pairs absorbing the photon have been determined in reactions where a proton and a neutron are emitted in coincidence.

Photon scattering on nuclei indicates that nucleons bound in a nucleus react in the same way as free nucleons to the external electromagnetic field of the photon. This is not yet a final result, however, and the investigations continue. Recently, a series of measurements of the electric and magnetic polarizabilities of the neutron have been commenced. These are fundamental properties with direct connections to low-energy QCD models.

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Rotating and deformed nuclei
Dirk Rudolph

One way to collect information about the inner structure of atomic nuclei is to make them rotate at very high speeds (over 1020 rotations per second!) and then measure the short pulses of gamma-radiation that are emitted as the nucleus slows down. Such fast rotating nuclei are produced in heavy ion collisions where part of the projectile's kinetic energy is transferred to the reaction products as angular momentum. The illustration below shows how a rapidly rotating nucleus (far right) deexcites stepwise by emitting gamma-rays.

A rotating nucleus deexcites by gamma emission

Most atomic nuclei are not even spherical even in their ground states. The deformation is caused by the interaction between the nucleons inside and can change dramatically with excitation energy, angular momentum, for example, from flattened disks to elongated cigars. More complex shapes ("pears" and "bananas") are also possible. Periodic oscillations ("vibrations") can also be overlaid on the static deformation, leading to a complex behaviour.

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Nuclear structure far from stability
Dirk Rudolph

The "chart of nuclides" provides a map over the nuclear landscape ordered by neutron and proton number. The nuclides in black are stable, while the red, blue, yellow, orange and green squares represent radioactive nuclides that have at some point been observed experimentally. The grey area indicates the large number of predicted, but up to now unknown, nuclei.

The chart of the nuclides

From the figure it is obvious that the vast majority of nuclides that have been observed are built up from combinations of neutrons and protons that are quite different from the "stable" isotopes we find in nature. By studying the evolution of nuclear structure as a function of the distance from the stability line, we seek the answer to the question of whether these "exotic" nuclei can be described with the same physical relationships as the ones closer to stability.

Most theoretical models of basic properties such as mass and half-life are based on data obtained from isotopes at or close to the stability line. Comparisons between different model predictions and with measured values often show large deviations. The fact that the extrapolations do not work very well is a clear indication that our basic understanding of the mechanisms underlying nuclear structure is far from perfect.

"Exotic" nuclei are also interesting since they are thought to play a large role in the astrophysical processes that built all elements heavier than helium. These processes, referred collectively as nucleosynthesis, occur in supernovae and other very extreme stellar environments. By producing and studying nuclei far from stability, we can learn much about our own origins.

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Excited nuclear matter
Bo Jakobsson

Heavy ion collisions help us investigate the properties of excited nuclear matter by studying, for instance, the formation of resonances and the production of new particles. The figure below illustrates what a direct hit (a so-called central collision) between two gold nuclei could look like, as modeled by a "Quantum Molecular Dynamics" simulation.

QMD simulation of a Au+Au collision at 1 GeV/nucleon

The two nuclei are approaching each other with energy high enough to pass over the "Coulomb barrier" (the repulsive energy wall formed by the Coulomb interaction of the positively charged nucleons). After overcoming the barrier, the nuclei start to interact with each other via the nucleon-nucleon short range strong interaction (100 times stronger than the Coulomb force at the same distance). A very short-lived, hot, high-density region is formed, which afterwards expands and cools.

The collision is represented in the center-of-mass reference system, for a bombarding energy of 1 GeV per nucleon. Nucleons (protons and neutrons) are represented by blue bullets, Deltas (the first excitation level of nucleons) by green ones and pions (mesons produced by the decay of a Delta) by red ones.

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The nuclear equation of state
Bo Jakobsson

Heavy ion collisions give information on how much energy is needed to compress nuclear matter. This information is very important in astrophysics to understand supernova explosions and the stability of a neutron star.

plot showing different nuclear equations of state Nuclear matter has an energetic minimum at about 0.15 to 0.17 particles per cubic fermi (normal nuclear matter density, rho-0). If we try to change the density (e.g. by compression, like in heavy ion collisions) we have to overrule the repelling forces, i.e. we have to pump compressional energy into the system. The figure shows the nuclear equation of state. It describes how much energy it is needed to compress nuclear matter, i.e. to raise its density.

The density dependence of the compression is mostly unknown. The knowledge is mainly limited to the behaviour of the density close to and around the ground state density (the so-called compressibility). Theoretical models utilize different parameter sets corresponding to different compressibility values ranging from about 200 MeV (less repulsive, a so-called soft eos) up to about 400 MeV (more repulsive, a so-called hard eos).

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Search for quark-gluon plasma
Evert Stenlund

All particles that have been experimentally observed, except for the leptons, contain either three quarks or one quark and one antiquark. The search for free quarks has been going on for quite some time but so far this search has been fruitless. Quarks have either one or two thirds of the electric charge of an electron and free quarks should thus be possible to observe.

Artistic view of a quark-gluon plasma Nowadays one believes that the quarks cannot be liberated under normal circumstances and it requires enormous temperatures and densities to achieve this. Such conditions can be obtained when two heavy ions collide at very high energies and the result is called a Quark Gluon Plasma. The gluons are those particles that are responsible for the strong force between the quarks, in about the same way as photons (i.e. "particles of light") are responsible for the electromagnetic forces. It is believed that the universe, during its first few quivering moments, consisted of such a plasma of quarks and gluons.

The picture is an artist's rendition of a violent heavy ion collision resulting in the formation of a quark-gluon plasma. The quarks inside the participating nucleons can be seen as colored balls, whereas the gluons are represented by black wiggles.

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Dynamics of heavy ion interactions at high energies
Evert Stenlund

When two atomic nuclei collide, the outcome of the collision is to a large extent determined by the collision geometry. If the impact parameter is large, the overlap between the two nuclei will be small. When the impact parameter is small, large parts of the nuclei will hit each other and the participant part will become larger. The collisions with small impact parameters are the most interesting since the possibility to create a Quark Gluon Plasma is most favourable here. The impact parameter cannot be directly measured in the experiment, but can be indirectly estimated by the total number of emitted particles.

Most of the particles produced in collisions between two heavy ions are hadrons, i.e. particles that interact via the strong force. The particles hitting the detectors are mainly protons, consisting of three quarks, and pions and kaons, consisting of a quark and an antiquark. Electrically neutral particles are more difficult to observe. A particle which was created in the collision and then decayed can sometimes be reconstructed from its decay products.

NA49 ultra-relativistic collision This picture shows the reconstruction of an actual high energy heavy ion collision as seen by the central part of the NA49 detector at CERN. Two nuclei have collided and the created particles have left their imprints in the detector. The particle tracks are visualized as orange curve segments pointing back to the target (top right).

An important tool for evaluating the results of an experiment are so-called event generators, computer programs that simulate events (individual collisions) according to the expectations of various theoretical models. The simulated detector signals can then be analyzed in the same way as the real events. The event generators are based on different models for how the collisions develop and can thus be used to test different hypotheses.

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© 2000 Margareta Hellström | Department of Physics | Lund University.