05/11/2017 - 16:02
Christine Nattrass, Assistant Professor at the University of Tennessee,...
05/11/2017 - 16:02
Christine Nattrass, Assistant Professor at the University of Tennessee,...
05/11/2017 - 11:44
In a paper published on 24 April 2017 in Nature Physics, ALICE reports on...
07/22/2014 - 11:29
Scientists leading the work of the ALICE collaboration have been awarded the...
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 1800 physicists, engineers and technicians from 174 physics institutes in 42 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.
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.
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.
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.
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.
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.
ALICE recorded the very first proton collisions provided by the LHC in November 2009. In March 2010, the first high-energy proton run started at a centre-of-mass energy of 7 TeV. In November 2010, LHC provided for the first time Pb-Pb collisions at a centre-of-mass energy of 2.76 TeV per nucleon pair. This was followed by a second lead-lead run at the end of 2011 when much higher statistics data were collected. In February 2013, ALICE collected data from the asymmetric p-Pb collisions provided by the LHC. In 2015 the second LHC run started and ALICE is therefore taking data with increased energy beams.
From the data collected so far a wealth of interesting results has been produced. A selection of these results is presented below.
In heavy-ion collisions at the LHC, the ALICE collaboration found that the hot matter created in the collision behaves like a fluid with little friction, with almost zero viscosity.Read more
A technique called Bose-Einstein or HBT Interferometry allows us to measure the size and lifetime of the fireball created in heavy-ion collisions. The fireball formed in heavy ion collisions at the LHC is hotter, lives longer and expands to a larger size than at lower energies.Read more
ALICE data on photons produced in lead-lead collisions (Phys. Lett. B 754 (2016) 235, arXiv:1509.07324) are in line with models which assume the formation of a quark–gluon plasma with a temperature of about 5 trillion degrees. This suggests that for a fleeting moment the hottest matter on earth is created in these collisions.Read more
The strange quark is heavier than u and d, yet close enough in mass to undergo production and modification processes in similar manner. That, and the relative abundance of the strange quark in high-energy interactions, make the s-quark a very useful study tool for proton-proton and heavy nucleus collisions.Read more
The J/ψ is composed of a heavy quark–antiquark pair with the two objects orbiting at a relative distance of about 0.5 fm, held together by the strong colour interaction. However, if such a state were to be placed inside a QGP, it turns out that its binding could be screened by the huge number of colour charges (quarks and gluons) that make up the QGP freely roaming around it. This causes the binding of the quark and antiquark in the J/ψ to become weaker so that ultimately the pair disintegrates and the J/ψ disappears – i.e. it is "suppressed".Read more
A basic process in QCD is the energy loss of a fast parton in a medium composed of colour charges. This phenomenon, "jet quenching", is especially useful in the study of the QGP, using the naturally occurring products (jets) of the hard scattering of quarks and gluons from the incoming nuclei. ALICE has recently published the measurement of charged particles in central heavy-ion collisions at the LHC.Read more
Quarkonia are bound states of heavy flavour quarks (charm or bottom) and their antiquarks. The first ALICE results for charm hadrons indicate strong in-medium energy loss for charm and strange quarks that is an indication of the formation of the hot medium of QGP.Read more
The analysis of the data from the p-Pb collisions at the LHC revealed a completely unexpected double-ridge structure with so far unknown origin. The proton–lead (pPb) collisions in 2013, two years after its heavy-ion collisions opened a new chapter in exploration of the properties of the deconfined, chirally symmetrical state of the QGP.Read more
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…
ITS consists of six cylindrical layers of silicon detectors and is used to track the particles produced in the collisions before they quickly decay.
The FMD extends the coverage for the multiplicity of charged particles into the forward regions with high resolution - giving ALICE the widest coverage of the four LHC experiments for these measurements.
V0 is the dedicated detector that estimates how central the collisions of heavy ions in ALICE were.
T0 serves as a start, trigger, and luminosity detector allowing fast particle identification at ALICE.
The main device in ALICE's central barrel for the tracking and identification of charged particles, coping with unprecedented densities.
The main electron detector in the GeV/c momentum range in ALICE. Electrons and positrons coming out of the collision points of heavy ions can be distinguished from other charged particles by the emission of transition radiation.
The TOF measures the flight times of charged particles over a given distance to determine their velocity and identify them.
The HMPID identifies charged particles with large momentum, which is a physical quantity related to the particle's mass and speed.
EMCal is a large electromagnetic calorimeter that extends ALICE's reach to study jets and other hard processes.
Photons tell us about the temperature of the system. To measure them, special detectors are necessary: the crystals of the PHOS calorimeter.
The ALICE underground cavern provides an ideal place for the study of cosmic rays. The ALICE Cosmic Ray Detector (ACORDE) detects cosmic rays, making ALICE a high-tech telescope.
It is optimized for the detection of heavy quark resonances that are an important tool for the study of the early and hot stage of heavy-ion collisions.
Diffractive processes represent more than 25% of the cross-section for inelastic proton–proton collisions at the LHC. The AD detector will help study such processes.
The DCal will extend significantly the jet quenching measurements enabled by the EMCal in ALICE, by providing large acceptance for back-to-back correlation measurements of jets and hadrons.
The Photon Multiplicity Detector (PMD) is a particle shower detector, which measures the multiplicity and spatial distribution of photons produced in collisions.
The ZDCs are calorimeters that detect the energy of the spectator nucleons in order to determine the overlap region of the two colliding nuclei.
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 1550 members coming from 151 institutes in 37 countries. For a full list click here.