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B. J. Holzer et al.
6.9 Ion Colliders 1
W. Fischer · J. M. Jowett
Ion colliders are research tools for high-energy nuclear physics. The collisions of
fully stripped high-energy ions, that is, atomic nuclei, create matter of a temperature
and density that existed in the first microseconds after the Big Bang. The matter
created in these high-energy ion collisions is known as the Quark Gluon Plasma
(QGP), and interactions between the quarks and gluons is the subject of the
theory of quantum chromodynamics (QCD). The basic interactions are studied in
simpler collisions such as e + e − or pp but heavy-ion collisions allow the study of
more complex collective phenomena in QCD. The collisions in ion colliders can
create hadronic matter at much higher densities and temperatures than fixed target
experiments although at a much lower luminosity.
The collisions of heavy ions in RHIC and the LHC have yielded a number of
new results and revealed phenomena that were unexpected on the basis of previous
theoretical understanding. The QGP generated in the heavy ion collisions in RHIC
was expected to be weakly interacting, but found to be strongly interacting like an
almost perfect liquid [73, 74]. Hadronic jets created in the collisions have a rather
short mean free path in the QGP leading to a phenomenon termed “jet quenching”
[74], and the largest ever measured vorticity was seen in heavy ion collisions [75].
The collisions also created the heaviest artificially made antimatter nuclei, antihelium-4 [76, 77]. The higher energies in the LHC create many more hard probes
and heavy bound states such as charmonium (J/ψ) or bottomonium (ϒ) and, in
the highest-energy p-Pb collisions, toponium. Z and W bosons, particles that do not
interact with the QGP via the strong interaction, were never before seen in heavy ion
collisions. The ALICE experiment also reported the highest temperatures directly
measured in the laboratory [78].
The colliding nuclei also have high electric charges (Z ~ 80). Together with the
powerful Lorentz-compression at high energies, these generate enormous electromagnetic fields outside the nuclear radius. As first shown by Fermi, Weizsäcker
and Williams, these fields can be represented as a beam of high energy quasireal photons, leading to so-called ultraperipheral photonuclear and photon-photon
collisions. Besides their intrinsic interest, the high cross-sections for these processes
have consequences for the operation of the collider. The ATLAS experiment at the
LHC has published the first evidence for light-on-light elastic scattering, a longpredicted fundamental process of nonlinear quantum electrodynamics, transcending
Maxwell’s equations.
1 This section has been authored by Brookhaven Science Associates, LLC under Contract No. DESC0012704 with the U.S. Department of Energy. The United States Government retains and the
publisher, by accepting the article for publication, acknowledges that the United States Government
retains a non-exclusive, paid-up, irrevocable, world-wide license to publish or reproduce the
published form of this manuscript, or allow others to do so, for United States Government purposes.
B. J. Holzer et al.
6.9 Ion Colliders 1
W. Fischer · J. M. Jowett
Ion colliders are research tools for high-energy nuclear physics. The collisions of
fully stripped high-energy ions, that is, atomic nuclei, create matter of a temperature
and density that existed in the first microseconds after the Big Bang. The matter
created in these high-energy ion collisions is known as the Quark Gluon Plasma
(QGP), and interactions between the quarks and gluons is the subject of the
theory of quantum chromodynamics (QCD). The basic interactions are studied in
simpler collisions such as e + e − or pp but heavy-ion collisions allow the study of
more complex collective phenomena in QCD. The collisions in ion colliders can
create hadronic matter at much higher densities and temperatures than fixed target
experiments although at a much lower luminosity.
The collisions of heavy ions in RHIC and the LHC have yielded a number of
new results and revealed phenomena that were unexpected on the basis of previous
theoretical understanding. The QGP generated in the heavy ion collisions in RHIC
was expected to be weakly interacting, but found to be strongly interacting like an
almost perfect liquid [73, 74]. Hadronic jets created in the collisions have a rather
short mean free path in the QGP leading to a phenomenon termed “jet quenching”
[74], and the largest ever measured vorticity was seen in heavy ion collisions [75].
The collisions also created the heaviest artificially made antimatter nuclei, antihelium-4 [76, 77]. The higher energies in the LHC create many more hard probes
and heavy bound states such as charmonium (J/ψ) or bottomonium (ϒ) and, in
the highest-energy p-Pb collisions, toponium. Z and W bosons, particles that do not
interact with the QGP via the strong interaction, were never before seen in heavy ion
collisions. The ALICE experiment also reported the highest temperatures directly
measured in the laboratory [78].
The colliding nuclei also have high electric charges (Z ~ 80). Together with the
powerful Lorentz-compression at high energies, these generate enormous electromagnetic fields outside the nuclear radius. As first shown by Fermi, Weizsäcker
and Williams, these fields can be represented as a beam of high energy quasireal photons, leading to so-called ultraperipheral photonuclear and photon-photon
collisions. Besides their intrinsic interest, the high cross-sections for these processes
have consequences for the operation of the collider. The ATLAS experiment at the
LHC has published the first evidence for light-on-light elastic scattering, a longpredicted fundamental process of nonlinear quantum electrodynamics, transcending
Maxwell’s equations.
1 This section has been authored by Brookhaven Science Associates, LLC under Contract No. DESC0012704 with the U.S. Department of Energy. The United States Government retains and the
publisher, by accepting the article for publication, acknowledges that the United States Government
retains a non-exclusive, paid-up, irrevocable, world-wide license to publish or reproduce the
published form of this manuscript, or allow others to do so, for United States Government purposes.