High energy Heavy- Ion Collisions
An introduction
Obiettivi:
Discutere, dal punto di vista di un fisico sperimentale, le
problematiche e i risultati relativi alle collisioni tra ioni pesanti ad
alta energia, dalle energie intermedie alle energie relativistiche e
ultrarelativistiche, con attenzione anche ai possibili segnali di
formazione di quark-gluon-plasma.
Presentare una introduzione alle tecniche di rivelazione e alle
metodologie di analisi in uso nella fisica degli ioni pesanti ad alta
energia, con particolare riferimento alle tecnologie innovative di
rivelazione di eventi ad alta molteplicità e ai problemi software di
ricostruzione di tali eventi.
Organizzazione didattica
Il corso comprende una serie di lezioni, corredate da alcune
esercitazioni su aspetti discussi nel corso, in modo da
familiarizzarsi anche con l’utilizzo di procedure di simulazione e
analisi.
Il contenuto del corso potrà essere adattato al curriculum
effettivo degli studenti interessati, anche in base agli altri
corsi seguiti.
Una bibliografia di massima è riportata alla fine del programma,
e potrà essere ulteriormente precisata in base all’attività
effettivamente svolta.
Collisions between nucleons or light nuclei at low energy vs
heavy ion collisions at higher energies
At low energy and for light systems, few body final
states dominate the phenomenology. Elementary
processes take place between the involved nuclei.
In contrast, heavy ion physics at high energies is
characterized by a large number of particles in the final
state. This is determined by the overlap region of
projectile and target, depending on energy density and
temperature.
4He
p
7Li
4He
A typical low energy, two body reaction
Au+Au collision at 1.5 GeV/nucleon
Low energy heavy ion physics was (and is being) carried out
by Tandem accelerators, which provide heavy ion beams
with energies around a few MeV/nucleon
From 1970 on, some accelerator used by particle physicists
was converted to accelerate heavy ions.
Examples: Bevatron (Berkeley)
Syncrophasatron (Dubna)
In the same period, heavy ion accelerators were built to
produce beams from 10 MeV/A to a few GeV/A
Examples: NSCL (MSU, East Lansing)
100 MeV/A
GANIL (Caen)
100 MeV/A
GSI (Darmstadt)
1-2 GeV/A
Saturne (Saclay)
few 100 MeV/A
CELSIUS (Uppsala)
500 MeV/A
LNS (Catania)
50-100 MeV/A
Around mid ’80s, heavy ions were injected into the
highest energy proton accelerators, producing heavy ion
beams in the energy range 10-200 GeV/A
Examples: Cosmotron (Dubna)
few AGeV
AGS (Brookhaven)
10 AGeV
SPS (CERN)
160 AGeV
The new era: The Colliders
Year 2000:
RHIC (Brokhaven)
200 AGeV c.m.
2009:
LHC (CERN)
pp @ 7 TeV
2010:
LHC (CERN)
PbPb
One of the historical reasons for heavy ion physics at low
energy was the hope to sintetize new (superheavy) elements,
and to exploit a possible stability valley beyond transuranic
elements.
In recent years, high energy nuclear physics attracted much
attention, since heavy ion reactions are the only way to
exploit the two additional degree of freedoms:
density
temperature
The main reason is the search for a deconfined phase of
quarks and gluons (QGP, Quark-Gluon-Plasma)
However, even if QGP is not formed in a given reaction, what
is the behaviour of the nuclear matter under extreme energy
densities and temperature?
The intermediate step of a heavy ion collision may
involve as many as 500 particles in a small volume even at
100 MeV/A
At higher energies (100 GeV/A) thousands of particles
and antiparticles are produced
For such systems, statistical approaches are possible
Since the collision is a dynamical process, both
equilibrium and non-equilibrium effects are present
Thermodynamical properties of nuclear matter in statistical
equilibrium may be described by an equation-of-state (EOS).
If we want to change the normal nuclear density (compression), we
have to pump compressional energy into the system. The EOS
describes which compressional energy corresponds to which density.
This density dependence of the compression is in principle unknown.
Interesting aspects of nuclear EOS:
The phase transition between nuclear liquid into vapor of fragments
and nucleons (liquid-gas phase transition)
Compressibility of nuclear matter up to densities much higher than
the standard density
The phase transition to the quark-gluon-plasma
What are the
phenomena from the
initial stage of the
collision to the final
one?
The collision
history
Simulation of a Ca+Ca collision at 500
MeV/nucleon.
Time step between frames: 10 fm/c
Density
evolution
Momentum
space
The collision
history/2
Simulation of a Ca+Ca collision at 40
MeV/nucleon.
Time step between frames: 20 fm/c
Density
evolution
Momentum
space
In both cases the system passes through a
compression phase and then, later on, it expands
Some difference is observed in momentum space: in
the high energy case the two momentum spheres are
well separated in the initial stage, and the system
tends to equilibrium in longer times
Such simulations are carried out by microscopic model
calculations (transport theories), with codes which are
usually called BUU (Boltzmann-Uhling-Uhlenbeck), QMD
(Quantum Molecular Dynamics), and so on.
These approaches allow to follow the dynamical
evolution of the system from the initial stage to the
final break-up stage.
Transport equations describe the evolution from nonequilibrium phase to a thermalized phase
What happens during
such a collision?
Temperature,
pressure and
energy density
vary with time
Density increases a
factor 2-3 w.r.t.
normal density
Energy density increases
up to 350 MeV/fm3
(standard = 150 MeV/fm3)
New collective phenomena
One of the most impressive results of high energy
heavy ion physics is the importance of new collective
phenomena discovered in these processes
The hot and compressed nuclear matter behaves like
a compressible fluid, so that dynamical fluid effects
are observed (sideward flow and squeeze-out)
Particle production
Another relevant aspect is the production of new
particles. For some energy regime, cooperative effects
may lead to the production of particles below the
threshold (subthreshold production)
At the highest energies, production of exotic particles
is also predicted
Heavy-Ion dynamics
Low energy ( ≈ 20 MeV/A):
Nuclear mean field effects
Intermediate energy (20-200 MeV/A):
Both nuclear mean field and two-body collisions
Relativistic energy (several hundred MeV/A- few GeV/A):
Two-body collisions dominate
Ultra-Relativistic energy (10 GeV/A-10 TeV/A):
Typical phenomena in the
low energy regime:
Fusion reactions
Fission reactions
Few nucleon transfer
Break-up
…
Intermediate energy regime
Non-equilibrium processes and dynamical
instabilities start to become increasingly
important
First stages of nuclear collisions are important to
understand pre-equilibrium phenomena, high
density fireball formation and evolution towards
equilibrated systems
Which probes to use?
Hard photons
Very energetic nucleons
Pions and other mesons
Typical phenomena of the low-energy ->
intermediate energy transition regime (few
tens of MeV/A)
Fragment emission (IMF)
Multifragmentation
Liquid-gas phase transition
Pre-equilibrium emission
Flow
Nuclear stopping
Subthreshold production of particles
The study of such phenomena may require
Centrality evaluation
Event characterization and selection
Determination of reaction plane
HBT analysis of interaction zone
Exclusive vs inclusive measurements
Relativistic energies (200 MeV/A - 10 GeV/A)
Particle production
Density and temperature of the participant
zone
The compressibility and other basic properties
of EOS may be tested
Other items: In-medium cross sections,
momentum dependence of nucleon-nucleon
interaction
Study of flow
UltraRelativistic energies (10 GeV/A - 10 TeV/A)
Most important question: the search for quark-gluonplasma
Did we already observe such state at SPS?
In the low energy part of this regime (stopping region)
baryons are fully stopped, forming a baryon-rich matter
In the high-energy part of this regime (transparent
region) baryons are not slowed down completely. The
large energy density may lead to the formation of a
baryon-free QGP.
This requires energies in excess of SPS (RHIC or LHC)
Connections to other
fields of physics
Nuclear physics
Of course high energy nuclear physics IS nuclear physics!
The most important connection is probably related to the
study of the nuclear Equation-Of-State (EOS).
While in low energy nuclear physics, the EOS is probed at
low temperatures and at densities close to the ground state,
in high energy heavy ion collisions, a larger domain of T and
density may be explored.
Connections to other
fields of physics
Particle physics
Elementary processes determine the evolution of heavy ion
reactions
The phenomenology of hadronic interactions is the basis for
many heavy ion models
Collective processes may be extracted only understanding
the superposition of independent hadronic collisions
Heavy ion reactions test the features of non perturbative
QCD, not completely known
Subthreshold particle production plays a role also in heavy
ion collisions
Connections to other
fields of physics
Statistical physics
Heavy ion reactions involve dynamical systems of a few
hundred nucleons.
This is a large number, but still far from the continuum
Possibility to study deviations from infinite matter limit but
also signs of collective matter
Phase transitions in a dynamical system is an open field of
research
Heavy ion physics contributes also to transport theory at
high energies
Connections to other
fields of physics
Astrophysics
Informations which can be extracted from heavy ion physics
help to understand models of early Universe, neutron stars,
supernova explosions, quark stars,…
A common ingredient is the Equation-Of-State (EOS). As an
example, compressibility extracted from heavy ion data may
be used for neutron stars
Models of early Universe strongly depend on the phase
transition from QGP to hadronic matter and on its dynamics
Reference:
L.P.Csernai, Introduction to Relativistic
Heavy Ion Collisions, Chapter I