ATLAS
(A Toroidal LHC Apparatus)
at the Large Hadron Collider at CERN
Muon Detectors
The LHC
Calorimeters
Muons are particles very much like electrons, but which are 200 times heavier. They
are the only detectable particles that can traverse all the calorimeter absorbers without
being stopped. The muon spectrometer surrounds the calorimetry and measures the
muon trajectories to determine their
momenta with high precision. It
consists of large chambers placed in
the magnetic field produced by giant
superconducting toroidal coils. The muon system comprises several
types of detection chamber. These
chambers are, typically, enclosed
volumes of gas in which traversing
muons produce ionization. A strong
electric field accelerates the electrons
freed through the ionization, causing
a cascade of electrons which are then
detected on sense wires.
The muon system chambers extend
over an area of more than 12000 m2.
They are constructed of almost 4000
chambers and more than 1 million
readout-channels.
Within a 27 km tunnel, about
150 m below the Franco-Swiss
border, two beams of protons are
accelerated to up to 7 TeV, and
collide, unveiling new particles.
Electromagnetic (Liquid Argon) Calorimeter
The Electromagnetic calorimeter consists of thin lead plates (about 1.5 mm thick) immersed in a bath of
liquid argon. When high-energy photons or electrons traverse the lead, their energies are transformed into a
shower of low-energy electrons and positrons. These particles ionize the liquid argon between the plates.
The charge collected is a measure of the deposited energy.
Hadronic (Tile) Calorimeter
The hadronic calorimeter surrounds the electromagnetic calorimeter. It absorbs and measures the energies
of high-energy hadrons, including protons, neutrons, pions and kaons. (Electrons and photons stop before
reaching it.) The main ATLAS hadronic calorimeter consists of steel absorbers separated by tiles of
scintillating plastic. The hadrons interact in the plates transforming their incident energy into “showers" of
many low-energy hadrons. These showers, in traversing the scintillating tiles, causes them to emit light in an
amount proportional to the incident energy.
ATLAS Overview
ATLAS is about 45 m long, more than 25 m in diameter. It weighs about 7000
tons. For comparison, the nave of Winchester Cathedral in England measures
80 m in length and 24 m in height. The iron structure of the Eiffel tower in
Paris weighs 7300 tons.
Trigger System
In the LHC, proton-proton collisions occur at a frequency of more than
1 GHz (109 per second). Only events where interesting physics processes
occur need to be kept. To select these events, a three level “trigger” is used.
Level-1 Trigger
The first-level comprises purpose-build electronics which analyses
information from the calorimeter and muon systems. This level accepts up
to 100000 events per second and requires 2 µs to complete. Level-2 Trigger
The second-level trigger considers regions of interest defined by level-1. At
this level the full-granularity information from all detectors is available to
help refine the decision. Events are analyzed concurrently in processor
farms. Typical processing times are 10 ms per event, which results in an
output rate of about ~5 kHz.
Event Filter
The Event Filter completes the selection of events. Complicated selection
criteria are applied, using processor farms acting on the full-event data. The
processing time per event is several seconds, reducing the final rate to about
500 Hz.
Inner Detector
Magnets
The ATLAS superconducting magnet systems provide magnetic fields strong
enough to bend the tracks of even very energetic charged particles as they
emerge from collisions, thereby allowing the measurement of their momenta.
Solenoid
The central ATLAS solenoid has a length of 5.3 m with a bore of 2.4 m. It
creates a field of 2 Tesla which bends particles around the direction of the
incoming LHC beams.
Toroid
Each of three ATLAS Toroid systems consists of eight coils, assembled
radially and symmetrically around the beam axis. There are two end-cap
toroids and a 25 m long barrel toroid. The Barrel system (below) comprises
eight separate cryostats. The Toroids each carry a current of 20 kA,
generating a magnetic field of 4 Tesla.
The ATLAS Inner Detector combines high-resolution detectors at the inner radii with continuous tracking
elements at the outer radii, all contained within the Solenoid. The relative precision of the measurements
are well matched, so that no single measurement dominates the momentum resolution. In the barrel region
the high-precision detectors are arranged in concentric cylinders around the beam axis, while the end-cap
detectors are mounted on disks perpendicular to the beam axis. The outer radius of the Inner Detector is
1.15 m, and the total length 7 m.
Pixel Detector
The ATLAS Pixel Detector provides a very high granularity, high precision, set of measurements very close
to the interaction point. The system provides three precision measurements over the full acceptance, and
largely determines the ability of the Inner Detector to find short-lived particles such as B-Hadrons. The
system consists of three barrels and three disks, on each side, with a total of 80 million pixels. Semiconductor Tracker (SCT)
The SCT system provides eight precision measurements per track in the intermediate radial range,
contributing to the measurement of momentum and vertex position. In the barrel SCT, this is achieved by
pairs of silicon micro-strip detectors mounted on four carbon-fiber cylinders. Transition Radiation Tracker (TRT)
The Transition Radiation Tracker (TRT) incorporates high-rate straw detectors. Electron identification is
made by employing Xenon gas to detect transition radiation photons created in an inter-straw radiator.
Each straw is 4 mm in diameter. The barrel contains about 50000 straws, and the end-caps 320000 straws.
The detector can discriminate between tracking hits and transition radiation hits. Text, photographs and figures courtesy CERN and the ATLAS collaboration.
ATLAS UK supported by the Science and Technologies Facilities Council.
Poster created by: Dr. B.M. Barnett, PPD, Rutherford Appleton Laboratory