A Large Ion Collider Experiment
Encyclopedia
ALICE is one of the six detector
experiments at the Large Hadron Collider
at CERN
. The other five are: ATLAS
, CMS
, TOTEM
, LHCb
, and LHCf
. ALICE is optimized to study heavy ion
collisions. Pb
-Pb nuclei
collisions will be studied at a centre of mass energy of 2.76 TeV
per nucleon
. The resulting temperature and energy density are expected to be large enough to generate a quark-gluon plasma
, a state of matter wherein quark
s and gluon
s are deconfined.
ITS layers (counting from the interaction point):
(TPC) is the main particle tracking device in ALICE. Charged particles crossing the gas of the TPC ionize the gas atoms along their path, liberating electrons that drift towards the end plates of the detector. An avalanche effect in the vicinity of the anode wires strung in the readout, will give the necessary signal amplification. The positive ions created in the avalanche will induce a positive current signal on the pad plane. The readout is done by the 557 568 pads that form the cathode plane of the multi-wire proportional chambers (MWPC) located at the end plates. This gives the r and
coordinates. The last coordinate, z, is given by the drift time.
s and positron
s can be discriminated from other charged particles using the emission of transition radiation
, X-rays emitted when the particles cross many layers of thin materials. To develop such a Transition Radiation Detector
(TRD) for ALICE many detector prototypes were tested in mixed beams of pion
s and electrons.
Spectrometer (PHOS) is designed to measure the temperature of collisions by detecting photons emerging from them. It will be made of lead tungstate crystals. When high energy photons strike lead tungstate, they make it glow, or scintillate, and this glow can be measured. Lead tungstate is extremely dense (denser than iron), stopping most photons that reach it.
to determine the speed of particles beyond the momentum range available through energy loss (in ITS and TPC, p = 600 MeV) and through time-of-flight measurements (in TOF, p = 1.2–1.4 GeV). Its momentum range is up to 3 GeV for pion/kaon
discrimination and up to 5 GeV for kaon/proton
discrimination. It is the world's largest caesium iodide
RICH detector, with an active area of 11 m². A prototype was successfully tested at CERN in 1997 and currently takes data at the Relativistic Heavy Ion Collider
at the Brookhaven National Laboratory
in the US.
spectrometer measures pairs of muons, in particular those coming from the decays of J/ψ and Upsilon
particles. Tracking chambers to detect the muons and reconstruct their trajectories will be made from a special composite material, which is highly rigid but very thin. A set of resistive plate chambers (RPC) will act as a triggering device.
detector which measures the multiplicity and spatial distribution of photons produced in the collisions.
Particle detector
In experimental and applied particle physics, nuclear physics, and nuclear engineering, a particle detector, also known as a radiation detector, is a device used to detect, track, and/or identify high-energy particles, such as those produced by nuclear decay, cosmic radiation, or reactions in a...
experiments at the Large Hadron Collider
Large Hadron Collider
The Large Hadron Collider is the world's largest and highest-energy particle accelerator. It is expected to address some of the most fundamental questions of physics, advancing the understanding of the deepest laws of nature....
at CERN
CERN
The European Organization for Nuclear Research , known as CERN , is an international organization whose purpose is to operate the world's largest particle physics laboratory, which is situated in the northwest suburbs of Geneva on the Franco–Swiss border...
. The other five are: ATLAS
ATLAS experiment
ATLAS is one of the six particle detector experiments constructed at the Large Hadron Collider , a new particle accelerator at the European Organization for Nuclear Research in Switzerland...
, CMS
Compact Muon Solenoid
The Compact Muon Solenoid experiment is one of two large general-purpose particle physics detectors built on the proton-proton Large Hadron Collider at CERN in Switzerland and France. Approximately 3,600 people from 183 scientific institutes, representing 38 countries form the CMS collaboration...
, TOTEM
TOTEM
TOTal Elastic and diffractive cross section Measurement is one of the six detector experiments at the Large Hadron Collider at CERN. The other five are: ATLAS, ALICE, CMS, LHCb, and LHCf. It shares intersection point IP5 with the Compact Muon Solenoid...
, LHCb
LHCb
LHCb is one of six particle physics detector experiments collecting data at the Large Hadron Collider accelerator at CERN. LHCb is a specialized b-physics experiment, that is measuring the parameters of CP violation in the interactions of b-hadrons...
, and LHCf
LHCf
The LHCf is a special-purpose Large Hadron Collider experiment for astroparticle physics, and one of seven detectors in the LHC accelerator at CERN. The other six are: ATLAS, ALICE, CMS, MoEDAL, TOTEM, and LHCb...
. ALICE is optimized to study heavy ion
Heavy ion
Heavy ion refers to an ionized atom which is usually heavier than helium. Heavy-ion physics is devoted to the study of extremely hot nuclear matter and the collective effects appearing in such systems, differing from particle physics, which studies the interactions between elementary particles...
collisions. Pb
Lead
Lead is a main-group element in the carbon group with the symbol Pb and atomic number 82. Lead is a soft, malleable poor metal. It is also counted as one of the heavy metals. Metallic lead has a bluish-white color after being freshly cut, but it soon tarnishes to a dull grayish color when exposed...
-Pb nuclei
Atomic nucleus
The nucleus is the very dense region consisting of protons and neutrons at the center of an atom. It was discovered in 1911, as a result of Ernest Rutherford's interpretation of the famous 1909 Rutherford experiment performed by Hans Geiger and Ernest Marsden, under the direction of Rutherford. The...
collisions will be studied at a centre of mass energy of 2.76 TeV
TEV
TEV may refer to:* TeV, or teraelectronvolt, a measure of energy* Total Enterprise Value, a financial measure* Total Economic Value, an economic measure* Tobacco etch virus, a plant pathogenic virus of the family Potyviridae....
per nucleon
Nucleon
In physics, a nucleon is a collective name for two particles: the neutron and the proton. These are the two constituents of the atomic nucleus. Until the 1960s, the nucleons were thought to be elementary particles...
. The resulting temperature and energy density are expected to be large enough to generate a quark-gluon plasma
Quark-gluon plasma
A quark–gluon plasma or quark soup is a phase of quantum chromodynamics which exists at extremely high temperature and/or density. This phase consists of asymptotically free quarks and gluons, which are several of the basic building blocks of matter...
, a state of matter wherein quark
Quark
A quark is an elementary particle and a fundamental constituent of matter. Quarks combine to form composite particles called hadrons, the most stable of which are protons and neutrons, the components of atomic nuclei. Due to a phenomenon known as color confinement, quarks are never directly...
s and gluon
Gluon
Gluons are elementary particles which act as the exchange particles for the color force between quarks, analogous to the exchange of photons in the electromagnetic force between two charged particles....
s are deconfined.
Inner Tracking System
The Inner Tracking System (ITS) consists of six cylindrical layers of silicon detectors. The layers surround the collision point and measure the properties of the emerging particles, pin-pointing their positions to a fraction of a millimetre. The ITS will recognize particles containing heavy quarks by identifying the points at which they decay.ITS layers (counting from the interaction point):
- 2 layers of SPD (Silicon Pixel Detector),
- 2 layers of SDD (Silicon Drift DetectorSilicon drift detectorSilicon drift detectors are X-ray radiation detectors used in x-ray spectrometry and electron microscopy . Their chief characteristics compared with other X-ray detectors are:*high count rates*comparatively high energy resolution Silicon drift detectors (SDDs) are X-ray radiation detectors used...
), - 2 layers of SSD (Silicon Strip Detector).
Time Projection Chamber
The ALICE Time Projection ChamberTime projection chamber
In physics, a time projection chamber is a particle detector invented by David R. Nygren, an American physicist, at Lawrence Berkeley Laboratory in the late 1970s...
(TPC) is the main particle tracking device in ALICE. Charged particles crossing the gas of the TPC ionize the gas atoms along their path, liberating electrons that drift towards the end plates of the detector. An avalanche effect in the vicinity of the anode wires strung in the readout, will give the necessary signal amplification. The positive ions created in the avalanche will induce a positive current signal on the pad plane. The readout is done by the 557 568 pads that form the cathode plane of the multi-wire proportional chambers (MWPC) located at the end plates. This gives the r and
coordinates. The last coordinate, z, is given by the drift time.Transition Radiation Detector
ElectronElectron
The electron is a subatomic particle with a negative elementary electric charge. It has no known components or substructure; in other words, it is generally thought to be an elementary particle. An electron has a mass that is approximately 1/1836 that of the proton...
s and positron
Positron
The positron or antielectron is the antiparticle or the antimatter counterpart of the electron. The positron has an electric charge of +1e, a spin of ½, and has the same mass as an electron...
s can be discriminated from other charged particles using the emission of transition radiation
Transition radiation
Optical Transition radiation is produced by relativistic charged particles when they cross the interface of two media of different dielectric constants. The emitted radiation is the homogeneous difference between the two inhomogeneous solutions of Maxwell's equations of the electric and magnetic...
, X-rays emitted when the particles cross many layers of thin materials. To develop such a Transition Radiation Detector
Transition radiation detector
A transition radiation detector is a particle detector using the \gamma-dependent threshold of transition radiation in a stratified material. It contains many layers of materials with different indices of refraction. At each interface between materials, the probability of transition radiation...
(TRD) for ALICE many detector prototypes were tested in mixed beams of pion
Pion
In particle physics, a pion is any of three subatomic particles: , , and . Pions are the lightest mesons and they play an important role in explaining the low-energy properties of the strong nuclear force....
s and electrons.
Time of Flight
Charged particles are identified in ALICE by Time-Of-Flight (TOF); heavier particles are slower and so take longer to reach the outer layers of the detector. For its TOF system ALICE uses detectors called Multigap Resistive Plate Chambers (MRPC). There are approximately 160 000 MRPC pads with time resolution of about 100 ps distributed over the large surface of 150 square meters. Using the tracking information from other detectors every track firing a sensor is identified.Photon Spectrometer
The PhotonPhoton
In physics, a photon is an elementary particle, the quantum of the electromagnetic interaction and the basic unit of light and all other forms of electromagnetic radiation. It is also the force carrier for the electromagnetic force...
Spectrometer (PHOS) is designed to measure the temperature of collisions by detecting photons emerging from them. It will be made of lead tungstate crystals. When high energy photons strike lead tungstate, they make it glow, or scintillate, and this glow can be measured. Lead tungstate is extremely dense (denser than iron), stopping most photons that reach it.
High Momentum Particle Identification Detector
The High Momentum Particle Identification Detector (HMPID) is a RICH detectorRing imaging Cherenkov detector
A Ring Imaging Cherenkov detector is a particle detector that can determine the velocity, v , of a charged particle. This is done by an indirect measurement of the Cherenkov angle, \theta_c , i.e. the angle between the emitted Čerenkov radiation and the particle path...
to determine the speed of particles beyond the momentum range available through energy loss (in ITS and TPC, p = 600 MeV) and through time-of-flight measurements (in TOF, p = 1.2–1.4 GeV). Its momentum range is up to 3 GeV for pion/kaon
Kaon
In particle physics, a kaon is any one of a group of four mesons distinguished by the fact that they carry a quantum number called strangeness...
discrimination and up to 5 GeV for kaon/proton
Proton
The proton is a subatomic particle with the symbol or and a positive electric charge of 1 elementary charge. One or more protons are present in the nucleus of each atom, along with neutrons. The number of protons in each atom is its atomic number....
discrimination. It is the world's largest caesium iodide
Caesium iodide
Caesium iodide is an ionic compound often used as the input phosphor of an x-ray image intensifier tube found in fluoroscopy equipment....
RICH detector, with an active area of 11 m². A prototype was successfully tested at CERN in 1997 and currently takes data at the Relativistic Heavy Ion Collider
Relativistic Heavy Ion Collider
The Relativistic Heavy Ion Collider is one of two existing heavy-ion colliders, and the only spin-polarized proton collider in the world. It is located at Brookhaven National Laboratory in Upton, New York and operated by an international team of researchers...
at the Brookhaven National Laboratory
Brookhaven National Laboratory
Brookhaven National Laboratory , is a United States national laboratory located in Upton, New York on Long Island, and was formally established in 1947 at the site of Camp Upton, a former U.S. Army base...
in the US.
Muon spectrometer
The muonMuon
The muon |mu]] used to represent it) is an elementary particle similar to the electron, with a unitary negative electric charge and a spin of ½. Together with the electron, the tau, and the three neutrinos, it is classified as a lepton...
spectrometer measures pairs of muons, in particular those coming from the decays of J/ψ and Upsilon
Upsilon particle
The Upsilon meson is a flavorless meson formed from a bottom quark and its antiparticle. It was discovered by the E288 collaboration, headed by Leon Lederman, at Fermilab in 1977, and was the first particle containing a bottom quark to be discovered because it is the lightest that can be produced...
particles. Tracking chambers to detect the muons and reconstruct their trajectories will be made from a special composite material, which is highly rigid but very thin. A set of resistive plate chambers (RPC) will act as a triggering device.
Forward Multiplicity Detectors
The Forward Multiplicity Detectors (FMD) consist of 5 large silicon discs with each 10 240 individual detector channels to measure the charged particles emitted at small angles relative to the beam. The forward detectors also comprise the main trigger detectors for timing (T0) and for collision centrality (V0). Another important forward detector in ALICE is the Photon Multiplicity Detector (PMD). This is a pre-showerParticle shower
In particle physics, a shower is a cascade of secondary particles produced as the result of a high-energy particle interacting with dense matter. The incoming particle interacts, producing multiple new particles with lesser energy; each of these then interacts in the same way, a process that...
detector which measures the multiplicity and spatial distribution of photons produced in the collisions.
Electro-Magnetic Calorimeter
The Electro-Magnetic Calorimeter (EM-Cal) will add greatly to the high momentum particle measurement capabilities of ALICE.External links
- Official ALICE Public Webpage at CERN
- ALICE section on US/LHC Website
- ALICE photography panorama
- Photography panorama of ALICE detector center (Full design documentation)