7 Relativistic Nucleus-Nucleus Collisions and the QCD Matter Phase Diagram
429
qualitative consideration to oftentimes quantitative comprehension. We shall briefly
revisit some of these topics below, sketching the present state of the art, and
mentioning some completely new developments (such as the study of light system
collisions and, more spectacularly, the possible advent of Equation of State(EOS)
information from neutron star mergers). This will, however, not be a real review but
merely a narrative of recent progress.
The development of the field took a decisive turn by the startup of the CERN
LHC collider which did not only move the energy frontier up to truly asymptotic
values, 2.76 and 5.02 TeV per projectile nucleon pair in P b + P b collisions,
but also introduced a completely new generation of nuclear collision experiments,
offering an extended reach of physics observables and, moreover, a vastly increased
event statistics capability ranging upward well into the 10 9 domain. On the other
hand, the “blockbuster” innovations from the late RHIC period, minimally viscous
hydrodynamics with specific viscosity η/s tantalizing close to the fundamental field
theoretical limit (establishing the QGP as a near-ideal liquid), transport coefficients
of the QGP from in-medium-jet quenching, statistical production of charmonia and
the impression that charmed quarks are also thermalized in the deconfined QCD
phase, the saturation of the QCD hadronization temperature near 160 MeV, just
to mention the highlights, have remained the cornerstones of LHC physics to a
large degree. This holds, also, for the notoriously evasive gluon saturation and
Colour Glass Condensate physics, and to the inconclusive critical point searches.
In retrospect, one may state that one very central and important element of the
RHIC progress (handed on to the LHC physics) consisted of a clear separation of
the collisional initialization time period (the first fm/c interval) from the ensuing
hydrodynamical evolution and its in-medium effects. This provided for a much
clearer relationship between data and theory: it is the simple feature that projectiletarget interpenetration which took several fm/c at SPS energies is now shrunk to
subfractions of a fm/c thus providing for a sharp, global synchronization of the
successive eras of collisional dynamics: initialization, flow expansion, hadronization, final hadron/resonance “afterburning”. This is essential as one wants to tie the
formation of the physics observables to a specific stage of the dynamical evolution,
thus making them clearcut diagnostic tools.
One last thing. Neither RHIC nor the LHC experiments were built to explore
phenomena typical of a large baryochemical potential, such as the critical point of
QCD or the existence of a first order phase transition toward yet lower energies
and, quite generally, the QCD phase diagram at high μ B . However, at second
thought a vigorous development at RHIC, the Beam Energy Scan(BES) program,
as well as a re-vitalization of SPS experiments(the NA61/Shine experiment), and
a concurrent brilliant extension of lattice QCD technique concerning (“critical”)
fluctuation observables, were undertaken. Also, the low energy, high μ B domain
will come under renewed focus with new facilities, FAIR at GSI Darmstadt and
NICA at JINR Dubna, both under construction.
In the following we will present brief sketches of the recent and concurrent
work, theoretical and experimental, concerning a couple of (subjectively selected)
observables: reaction dynamics decomposed into 4 characteristic, separate stages,
429
qualitative consideration to oftentimes quantitative comprehension. We shall briefly
revisit some of these topics below, sketching the present state of the art, and
mentioning some completely new developments (such as the study of light system
collisions and, more spectacularly, the possible advent of Equation of State(EOS)
information from neutron star mergers). This will, however, not be a real review but
merely a narrative of recent progress.
The development of the field took a decisive turn by the startup of the CERN
LHC collider which did not only move the energy frontier up to truly asymptotic
values, 2.76 and 5.02 TeV per projectile nucleon pair in P b + P b collisions,
but also introduced a completely new generation of nuclear collision experiments,
offering an extended reach of physics observables and, moreover, a vastly increased
event statistics capability ranging upward well into the 10 9 domain. On the other
hand, the “blockbuster” innovations from the late RHIC period, minimally viscous
hydrodynamics with specific viscosity η/s tantalizing close to the fundamental field
theoretical limit (establishing the QGP as a near-ideal liquid), transport coefficients
of the QGP from in-medium-jet quenching, statistical production of charmonia and
the impression that charmed quarks are also thermalized in the deconfined QCD
phase, the saturation of the QCD hadronization temperature near 160 MeV, just
to mention the highlights, have remained the cornerstones of LHC physics to a
large degree. This holds, also, for the notoriously evasive gluon saturation and
Colour Glass Condensate physics, and to the inconclusive critical point searches.
In retrospect, one may state that one very central and important element of the
RHIC progress (handed on to the LHC physics) consisted of a clear separation of
the collisional initialization time period (the first fm/c interval) from the ensuing
hydrodynamical evolution and its in-medium effects. This provided for a much
clearer relationship between data and theory: it is the simple feature that projectiletarget interpenetration which took several fm/c at SPS energies is now shrunk to
subfractions of a fm/c thus providing for a sharp, global synchronization of the
successive eras of collisional dynamics: initialization, flow expansion, hadronization, final hadron/resonance “afterburning”. This is essential as one wants to tie the
formation of the physics observables to a specific stage of the dynamical evolution,
thus making them clearcut diagnostic tools.
One last thing. Neither RHIC nor the LHC experiments were built to explore
phenomena typical of a large baryochemical potential, such as the critical point of
QCD or the existence of a first order phase transition toward yet lower energies
and, quite generally, the QCD phase diagram at high μ B . However, at second
thought a vigorous development at RHIC, the Beam Energy Scan(BES) program,
as well as a re-vitalization of SPS experiments(the NA61/Shine experiment), and
a concurrent brilliant extension of lattice QCD technique concerning (“critical”)
fluctuation observables, were undertaken. Also, the low energy, high μ B domain
will come under renewed focus with new facilities, FAIR at GSI Darmstadt and
NICA at JINR Dubna, both under construction.
In the following we will present brief sketches of the recent and concurrent
work, theoretical and experimental, concerning a couple of (subjectively selected)
observables: reaction dynamics decomposed into 4 characteristic, separate stages,