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ALICE - A Large Ion Collider Experiment
The ALICE Collaboration is building a dedicated heavy-ion detector to exploit the unique physics potential of nucleus-nucleus interactions at LHC
energies. Our aim is to study the physics of strongly interacting matter at extreme energy densities, where the formation of a new phase of matter, the
quark-gluon plasma, is expected. The existence of such a phase and its properties are key issues in QCD for the understanding of confinement and of
chiral-symmetry restoration. For this purpose, we intend to carry out a comprehensive study of the hadrons, electrons, muons and photons produced in
the collision between heavy nuclei.
Time Of Flight
Time Projection Chamber
Particle tracking will continue outside the ITS in a large
detector called the Time Projection Chamber (TPC). The
TPC will be full of gas with an electric field applied
across it. When charged particles pass through, they will
knock electrons out of atoms in the gas and these will
drift in the electric field. By measuring the arrival time
of the electrons at the end of the chamber, the TPC will
reconstruct the path of the original charged particles in
all three spatial dimensions.
High-Momentum Particle
Identification Detector
A special task of the ALICE experiment is to identify
the mass of the particles emitted. If the low energy
particles may be identified by the loss of energy, the
higher ones are detected by measuring the time it takes
for a particle to reach from the collision point to the
detector barrel which is 3.5 meters away. The sensor for
the arrival of the particles will be Parallel Plate
Counters - 160,000 of them distributed over 150 square
meters. Using the tracking information from other
detectors, every track firing a sensor is identified. An
early prototype is shown.
At high energies, where the yield of particles is low and
it makes little sense to measure particles on an event-byevent basis, there is a smaller detector (14 square
meters) for identifying the mass of the particles. This
detector is based on the detection of Cherenkov photons
emitted by the particles in a dielectric medium. Hence
the detector is called a RICH (Ring Imaging CHernkov)
because the pattern of the photons detected by a CsI
photocatode is ring like. The two-third prototype
successfully tested at CERN in November 1997 is
shown. The large area CsI photocathodes are the result
of a CERN R&D program.
Absorber
Dipole
Magnet
L3 Magnet
Inner Tracking System
The Inner Tracking System (ITS) will be made from six
cylindrical layers of silicon detectors. They will surround
the collision point and measure the properties of particles
emerging from the collision, pin-pointing their positions to a
fraction of a millimeter. The ITS will look for particles
containing strange and charm quarks by identifying the
points at which they decay. Shown is a prototype of one of
the silicon detectors used, Silicon Drift Detectors.
Photon Spectrometer
Muon Chamber
The photon spectrometer (PHOS) is designed to
measure the temperature of the fireball produced by the
collisions of the ions by detecting the photons emerging
from it. It is made of lead- tungstate crystals like those
shown here. When high energy photons strike lead
tungstate, they cause it to glow, or scintillate, and this
glow can be measured. Lead tungstate is extremely
dense, which means that it stops most photons that reach
it.
The tracking chambers of the ALICE muon arm will be
made of 'sandwich composite' technology like this.
Sandwich composites are highly rigid but use very little
material. One job of the muon arm will be to measure
J/ particles via their decay into muons. Since these
particles can decay into a pair of muons, two muons
coming from the same place signal a possible J/
decay.
ALICE Detectors Not Shown
Centauro and STrange Object Research (CASTOR) - A specialized detector system dedicated to the search for
Centauros and Strangelets in the baryon dense, very forward phase space region in central Pb + Pb collisions at the CERN
LHC.
Forward Multiplicity Detector (FMD) - The FMD system provides information on the number of charged particles in the
forward rapidity range, primary vertex z location, and allowing for the rejection of beam gas events.
Photon Multiplicity Detector (PMD) - Measures the number of photons in the forward region adding information about
the event 'shape' and its spin fluctuations.
START - The START detector is designed to give a high quality fast indication of the starting time of an event. This starting
time is used by other detectors like the TOF, TPC, ITS, and the TRD.
Transition Radiation Detector (TRD) - Greatly improves the identification of electrons and consequently improves the
identification of heavy meson decays. It will also provide an additional input signal to the trigger.
Zero Degree Calorimeter (ZDC) - Detects spectator nucleons, and thereby measures the relative impact parameter,
centrality, between the two nuclei.
Ohio State Participants in the ALICE Project
Helen Caines
Howard Dyke
Jason Hubbell
Tom Humanic
Mike Lisa
Bjorn Nilsen
Guy Paić
Evan Sugarbaker