In the study concerning the basic structure of matter, physicists have achieved
a theoretical framework, the so-called Standard Model of particle physics,
describing the elementary building blocks and the interactions amongst them.
Within this model nuclear matter consists of pointlike particles called quarks
and the strong interaction amongst these quarks is mediated by the exchange of
massless particles called gluons.
In addition to the quarks there exists a second group of elementary particles,
the so-called leptons amongst which we find the familiar electron.
Under normal conditions the quarks are confined in groups to form particles like
the proton and neutron which in turn are the entities to form various nuclei.
However, statistical calculations within the Standard Model indicate that at
extremely high densities the quarks and gluons will become deconfined, leading to
a new state of matter, the so-called Quark-Gluon Plasma (QGP).
Such a deconfined state might exist in the cores of neutron stars and is expected
to have been present in the very early universe.
The formation, detection and systematic study of such a QGP state would yield new
information on strong interaction dynamics.
The use of ultra-relativistic heavy-ion beams is well suited for studying nuclear
matter under extreme conditions by means of violent nucleus-nucleus collisions.
While various observations lead to the perception that the initial phase of the
collision consists of a hot and dense system with strong rescattering, a direct
signature of the plasma and its properties is still missing.
To investigate plasma formation in a more direct way, one needs penetrating probes
which reflect the hot early stage of the interaction.
The most promising candidate for this is the thermal radiation from the hot system.
These promptly produced thermal photons are thought to provide an excellent means
for studying the state of nuclear matter at the various stages of the interaction,
since photons decouple from the nuclear system immediately after their production
and are essentially uninfluenced by the hadronisation process.
In this course an overview of the main aspects of heavy-ion physics is provided
and the various observations which indicate the possible creation of a deconfined
state will be addressed. Special attention will be given to the investigation of
prompt thermal photons.
In addition, a brief description will be given concerning the connection with the
Standard Model of cosmology (the Big Bang) and observations of astrophysical
phenomena by means of Cosmic Ray studies.
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