CERN Accelerating science

CERN Alice Slider
A Large Ion Collider Experiment

 

1927 members / 174 institutes / 39 countries

ALICE is the acronym for A Large Ion Collider Experiment, one of the largest experiments in the world devoted to research in the physics of matter at an infinitely small scale. Hosted at CERN, the European Laboratory for Particle Physics, ALICE relies on an international collaboration of more than 1900 physicists, engineers and technicians from 174 physics institutes in 39 countries across the world. The main goal of the ALICE collaboration is to characterize the physical properties of the Quark-Gluon Plasma (QGP), a state of matter created under the extreme conditions of temperature and energy density created in nuclear collisions, at the Large Hadron Collider (LHC), the world's largest accelerator.

 

 

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Physics

ALICE is optimized to study collisions of nuclei at the ultra-relativistic energies provided by the LHC. These collisions offer the best experimental conditions to produce the quark–gluon plasma. Such conditions are believed to have existed up to a few millionths of a second after the Big Bang before quarks and gluons were bound together to form protons and neutrons. Recreating this primordial state of matter in the laboratory and understanding how it evolves will allow us to shed light on questions about how matter is organized and the mechanisms that confine quarks and gluons.

Quarks and Gluons

Ordinary matter is made up of atoms, each of which consists of a nucleus surrounded by a cloud of electrons. Nuclei consist of hadrons (such as protons and neutrons), which in turn are made of quarks. Quarks are bound together in hadrons by the strong interaction mediated by gluons.  The strong interaction is one of the four fundamental forces.

Heavy-ion collisions

To recreate conditions similar to those of the early universe, powerful accelerators deliver head-on collisions between massive ions, such as gold or lead nuclei. In these heavy-ion collisions the hundreds of protons and neutrons from these nuclei smash into one another at energies of upwards of a few trillion electronvolts each.  The extreme energy density that is then reached causes the formation of the quark-gluon plasma. The QGP quickly cools until the individual quarks and gluons recombine into a blizzard of ordinary matter that speeds away in all directions. The ALICE detectors are designed to efficiently detect the particles produced in such lead-lead collisions at the LHC.

Confinement

No isolated quark has ever been observed: quarks, as well as gluons, seem to be bound permanently together and confined inside hadrons. This phenomenon is known as confinement. It relates to the fact that, in addition to an electric charge (which is a fraction of the electron's one), each quark has a colour charge, which can only take 3 values (red, blue or green).  For normal conditions of temperature and density, quarks only exist within hadrons - aggregates of two or three quarks that are colour neutral (i.e. white).  Confinement is also the fundamental property that gives each hadron a mass much larger than the sum of the masses of its constituents: in fact, confinement generates about 99% of the nuclear mass.

Quark – Gluon Plasma

The QGP is a state of matter wherein quarks and gluons are no longer confined in hadrons.  Such a state is predicted by the current theory of the strong interaction (called quantum chromodynamics, QCD) for very high temperatures and very high densities.  The transition between confined matter and the QGP would occur when the temperature exceeds a critical value estimated to be around 2000 billion degrees (about 100 000 times the temperature of the core of the Sun). Such extreme temperatures have not existed in Nature since the birth of the Universe: it is believed that for a few millionths of a second after the Big Bang the temperature was above the critical value, and the entire Universe was in a quark-gluon plasma state.

Experiment

ALICE is a 10,000-tonnes detector – 26 m long, 16 m high, and 16 m wide. It sits in a vast cavern 56 m below ground in the territory of Sergy on the border with St Genis-Pouilly in France. The detector is designed to measure, in the most complete way possible, the particles produced in the collisions which take place at its centre, so that the evolution of the system produced during these collisions can be reconstructed and studied. To do so, many different subdetectors have to be used, each providing a different piece of information. To understand such a complex system, one needs to observe it from different points of view, using different instruments at the same time, in the same way that a satellite looks at the earth combining detectors sensitive to different wavelengths, allowing us to see forests or clouds or archeological sites…

Detectors

The detectors sit within a magnetic field that bends the tracks of charged particles. The momentum of a particle –…


Detectors

First, one needs to know the initial conditions, namely how powerful the collision was: this is done by measuring the…


Detectors

The main tasks of the ITS are to reconstruct the primary and secondary vertices, to track and identify charged…


Detectors

The ALICE Time Projection Chamber (TPC) is a large cylindrical volume filled with gas. Charged particles crossing the…


Detectors

The identification of electrons and positrons is achieved by a transition radiation detector (TRD). In a similar manner…


Detectors

Charged particles in the intermediate momentum range are identified in ALICE by the Time Of Flight (TOF) detector. The…


Detectors

Particle identification plays a key role in the complete understanding of heavy-ion collisions at the LHC. The HMPID…


Detectors

The Photon Multiplicity Detector (PMD) measures the multiplicity and spatial distribution of photons produced in…


Detectors

PHOS is a high-resolution electromagnetic calorimeter which measures the photons coming out of the extremely hot plasma…


Detectors

The EMCal is a lead-scintillator sampling calorimeter with large acceptance (110 degrees azimuthal, from -0.7 to 0.7 on…


Detectors

The new calorimeter, called the “DCal”, is a large lead-scintillator detector with photo-diode readout placed in the…


Detectors

T0 is the fast timing and trigger detector of ALICE. It gives the key trigger and timing signals, measures on-line…


Detectors

Heavy ion collisions can be central or peripheral and it is crucial to distinguish between them. In peripheral…


Detectors

The V0 detector provides the basic, so called minimum-bias, trigger in ALICE. The measured time of arrival of the…


Detectors

The Forward Multiplicity Detector (FMD) has a special role in ALICE. It is designed to measure the charged particles…


Detectors

Diffractive and photon induced physics is a research area with a remarkable discovery potential at the LHC. ALICE has…


Detectors

The Muon Spectrometer is optimized for the detection of heavy quark resonances, such as J/Ψ, Ψ', Υ΄, Υ΄΄ via their…


Detectors

The ALICE underground cavern provides an ideal place for the detection of high energy atmospheric muons coming from…


Detectors

ALICE is performing a major upgrade during the Long Shutdown 2 that started end of 2018. The new ALICE will have…


Detectors

The smooth function of the ALICE detector depends on a number of systems and tools that have been developed to ensure…


Detectors

Data analysis is performed on all permanently stored events, turning the pattern of signals from the detector into…


Detectors

The Muon Forward Tracker is a tracking detector, aimed to enhance the vertexing capability of the ALICE muon…


Detectors

As a result of the LHC injectors upgrade after the second Long Shutdown (LS2), the expected Pb-Pb luminosity and…


Collaboration

The idea of building a dedicated heavy-ion detector for the LHC was first aired at the historic Evian meeting "Towards the LHC experimental programme" in March 1992. From the ideas presented there, the ALICE collaboration was formed in 1993. The wealth of published scientific results and the upgrade programme of ALICE have attracted numerous institutes and scientists from all over the world. The ALICE Collaboration has more than 1900 members coming from 174 institutes in 39 countries.

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Results

ALICE recorded the very first proton collisions provided by the LHC in November 2009.  During the LHC Run 1 (2009-2013), ALICE participated in all data-taking campaigns, recording proton-proton collisions at centre-of-mass energies of 0.9, 2.76, 7 and 8 TeV, proton-lead collisions at 5 TeV and lead-lead collisions at 2.76 TeV. In the case of proton-lead and lead-lead collisions, this is the energy for each collision between two nucleons. In Run 2 (2015-2018) the centre of mass energy was increased to 13 TeV for proton-proton, 8 for proton-lead and 5 for lead-lead. ALICE also recorded comparison data in proton-proton and proton-lead at the same energy as Pb-Pb, 5 TeV. In addition, a short data-taking campaign with xenon-xenon collisions was carried out. From the data collected so far a wealth of interesting results has been produced, spanning from the characterization of the high-density system formed in Pb-Pb collisions and the study of Quark-Gluon Plasma manifestations, to the surprising finding that also in proton-proton and proton-lead collisions some features of particle production resembles those seen in lead-lead.

A selection of these results is presented below: