High energy Heavy- Ion CollisionsAn 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.

7Li

p

4He

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?

Simulation of a Ca+Ca collision at 500 MeV/nucleon.

Time step between frames: 10 fm/c

Density evolution

Momentum space

The collision history

Simulation of a Ca+Ca collision at 40 MeV/nucleon.

Time step between frames: 20 fm/c

Density evolution

Momentum space

The collision history/2

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 non-equilibrium 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-gluon-plasma

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.

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

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