# Ultrarelativistic heavy-ion collision experiments

Why to make particles collide?

Okay, if you’re on this website you probably do not need to know this and you already have an idea of the physics studied at the $\textsf{LHC}$ and by other particles accelerator experiments.

Strongly interacting particles at very high temperatures and energy densities compose a new state of matter called quark-gluon plasma (QGP). The main purpose of ultrarelativistic heavy-ion collisions (URHICs) is the investigation of the QGP.

It is believed that such a state could be found in nature in the core of neutron stars and that the very very early Universe was in a QGP state for some instants at around $10^{-6}\ s$  after the Big Bang.

To recreate such conditions, head-on collisions between massive ions are made up by particle accelerators.
In these heavy-ion collisions protons and neutrons composing the two nuclei smash into one another, forming a deconfined state of quarks and gluons.

The initial shape of the collision zone is typically an ellipse.
Pressure in the liquid seeks to make the matter round, so it makes the liquid flow faster in the shorter directions.
As the fireball instantly expands and cools down, the quarks hadronize and the system freezes out. Furthermore, the individual partons recombine into ordinary matter that typically speeds away in all directions.
The debris contains nuclei of atoms or antiatoms which are made up by quarks and antiquarks yielded by the collision.
What takes place during the expansion and hadronization phases can be inferred from the study of the dynamical properties of the resulting hadrons.

The first ideas for experiments to search for the QGP emerged in the 1970s, when the QCD state equations (*) began to prospect the likelihood of a quark state of matter at temperatures and densities accessible to high-energy heavy-ion collisions experiments.

The experimental attempts to create the QGP in the laboratory and to measure its properties started around 1975 at the $\textsf{Bevalac}$ (Billions of eV Linear Accelerator), Lawrence Berkeley Laboratory (Long Island, USA), with a centre-of-mass energy per pair of colliding nucleons of $\sqrt{s_\mathrm{NN}}\simeq 1\ GeV$.
Energies rised to $\sqrt{s_\mathrm{NN}}= 5\ GeV$ at the Alternating Gradient Synchrotron ($\textsf{AGS)}$ at $\textsf{BNL}$ (Brookhaven National Laboratory, USA), and to $\sqrt{s_\mathrm{NN}}= 17\ GeV$ at the Super Proton Synchrotron ($\textsf{SPS}$) at the $\textsf{CERN}$, in the 1980s and 1990s.
The results obtained at $\textsf{SPS}$ led the $\textsf{CERN}$ to announce indirect evidence for a “new state of matter”, in 2000.

Four experiments ($\textsf{Star}$, $\textsf{BRAMHS}$, $\textsf{PHENIX}$, $\textsf{PHOBOS}$) have been put into action at the Relativistic Heavy Ion Collider ($\textsf{RHIC}$) at$\textsf{BNL}$ in the 2000s with $\sqrt{s_\mathrm{NN}}\simeq 200\ GeV$.
Their results have provided clearer indications of the formation of the QGP at high temperature and low baryon density.

Further confirmations came from the experiments of the $\textsf{LHC}$, at the $\textsf{CERN}$, where the $\textsf{ALICE}$ experiment is specifically optimized to study Pb-Pb collisions.