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ATLAS experiment

2007 Schools Wikipedia Selection. Related subjects: General Physics

   ATLAS (A Toroidal LHC ApparatuS) is one of the five particle detector
   experiments ( ALICE, ATLAS, CMS, TOTEM, and LHCb) being constructed at
   the Large Hadron Collider, a new particle accelerator at CERN in
   Switzerland. It will be 45 metres long and 25 metres in diameter, and
   will weigh about 7,000 tonnes. The project involves roughly 2,000
   scientists and engineers at 151 institutions in 34 countries. The
   construction is scheduled to be completed in 2007. The experiment is
   expected to measure phenomena that involve highly massive particles
   which were not measurable using earlier lower-energy accelerators and
   might shed light on new theories of particle physics beyond the
   Standard Model.

   The ATLAS collaboration, the group of physicists building the detector,
   was formed in 1992 when the proposed EAGLE (Experiment for Accurate
   Gamma, Lepton and Energy Measurements) and ASCOT (Apparatus with Super
   COnducting Toroids) collaborations merged their efforts into building a
   single, general-purpose particle detector for the Large Hadron
   Collider. The design was a combination of those two previous designs,
   as well as the detector research and development that had been done for
   the Superconducting Supercollider. The ATLAS experiment was proposed in
   its current form in 1994, and officially funded by the CERN member
   countries beginning in 1995. Additional countries, universities, and
   laboratories joined in subsequent years, and further institutions and
   physicists continue to join the collaboration even today. The work of
   construction began at individual institutions, with detector components
   shipped to CERN and assembled in the ATLAS experimental pit beginning
   in 2003.

   ATLAS is designed as a general-purpose detector. When the proton beams
   produced by the Large Hadron Collider interact in the centre of the
   detector, a variety of different particles with a broad range of
   energies may be produced. Rather than focusing on a particular physical
   process, ATLAS is designed to measure the broadest possible range of
   signals. This is intended to ensure that, whatever form any new
   physical processes or particles might take, ATLAS will be able to
   detect them and measure their properties. Experiments at earlier
   colliders, such as the Tevatron and Large Electron-Positron Collider,
   were designed based on a similar philosophy. However, the unique
   challenges of the Large Hadron Collider—its unprecedented energy and
   extremely high rate of collisions—require ATLAS to be larger and more
   complex than any detector ever built.

Background

   ATLAS experiment detector under construction in October 2004 in its
   experimental pit; the current status of construction can be seen here.
   Note the people in the background, for comparison.
   Enlarge
   ATLAS experiment detector under construction in October 2004 in its
   experimental pit; the current status of construction can be seen here.
   Note the people in the background, for comparison.

   The first cyclotron, an early type of particle accelerator, was built
   by Ernest O. Lawrence in 1931, with a radius of just a few centimetres
   and a particle energy of 1 MeV. Since then, accelerators have grown
   enormously in the quest to produce new particles of greater and greater
   mass. As accelerators have grown, so too has the list of known
   particles that they might be used to investigate. The most
   comprehensive model of particle interactions available today is known
   as the Standard Model; except for the Higgs boson, all of the particles
   in this model have been discovered, but the Standard Model will break
   down at energies beyond the current energy frontier of about one TeV
   (set at the Tevatron). A theory of beyond-the-Standard-Model physics,
   which is identical to the Standard Model at energies thus far probed,
   is expected to describe particle physics at higher energies. Most of
   these theories feature new higher-mass particles. The Large Hadron
   Collider (LHC), 27 kilometres in circumference, will collide two beams
   of protons with an energy seven million times that of the first
   accelerator. It will be able to frequently produce particles with
   energies up to about ten times more massive than any particles known
   today, if such particles exist.

   Particles that are produced in accelerators must also be observed, and
   this is the task of particle detectors. Beginning in the 1970s,
   detectors have been designed similarly to onions, completely
   surrounding the interaction point—where the particle beams from the
   accelerator collide—with layers of detectors of different types. The
   different features that particles leave in each layer of the detector
   allow for effective particle identification and accurate measurements
   of energy and momentum. (The role of each layer in the detector is
   discussed below.) As the energy of the particles produced by the
   accelerator increases, the detectors attached to it must grow to
   effectively measure and stop higher-energy particles. Thus ATLAS is the
   largest detector ever built at a particle collider, with the capability
   to measure the properties of the highly massive particles produced at
   the LHC.

Physics programme

   A schematic, called a Feynman diagram, of two virtual gluons from
   colliding LHC protons interacting to produce a hypothetical Higgs
   boson, a top quark, and an antitop quark. These in turn decay into a
   specific combination of quarks and leptons that is very difficult to
   fake in other processes. Collecting sufficient evidence of signals like
   this one may eventually allow ATLAS collaboration members to discover
   the Higgs boson.
   Enlarge
   A schematic, called a Feynman diagram, of two virtual gluons from
   colliding LHC protons interacting to produce a hypothetical Higgs
   boson, a top quark, and an antitop quark. These in turn decay into a
   specific combination of quarks and leptons that is very difficult to
   fake in other processes. Collecting sufficient evidence of signals like
   this one may eventually allow ATLAS collaboration members to discover
   the Higgs boson.

   ATLAS is intended to investigate many different types of physics that
   might become detectable in the energetic collisions of the LHC. Some of
   these are confirmations or improved measurements of the Standard Model,
   while many others are searches for new physical theories.

   One of the most important goals of ATLAS is to investigate the final
   missing piece of the Standard Model, the Higgs boson. The Higgs
   mechanism, which includes the Higgs boson, is invoked to give masses to
   elementary particles, giving rise to the differences between the weak
   force and electromagnetism by giving the W and Z bosons masses while
   leaving the photon massless. If the Higgs boson is not discovered by
   ATLAS, it is expected that another mechanism of electroweak symmetry
   breaking that explains the same phenomena, such as Technicolour, will
   be discovered. The Standard Model is simply not mathematically
   consistent at the energies of the LHC without such a mechanism. The
   Higgs boson would be detected by the particles it decays into; the
   easiest to observe are two photons, two bottom quarks, or four leptons.
   Sometimes these decays can only be definitively identified as
   originating with the Higgs boson when they are associated with
   additional particles; for an example of this, see the diagram at right.

   The asymmetry between the behaviour of matter and antimatter, known as
   CP violation, will also be investigated. Current CP-violation
   experiments, such as BaBar and Belle, have not yet detected sufficient
   CP violation in the Standard Model to explain the lack of detectable
   antimatter in the universe. It is possible that new models of physics
   will introduce additional CP violation, shedding light on this problem;
   these models might either be detected directly by the production of new
   particles, or indirectly by measurements made of the properties of B-
   mesons. ( LHCb, an LHC experiment dedicated to B-mesons, is likely to
   be better suited to the latter.)

   The top quark, discovered at Fermilab in 1995, has thus far had its
   properties measured only approximately. With much greater energy and
   greater collision rates, LHC will produce a tremendous number of top
   quarks, allowing ATLAS to make much more precise measurements of its
   mass and interactions with other particles. These measurements will
   provide indirect information on the details of the Standard Model,
   perhaps revealing inconsistencies that point to new physics. Similar
   precision measurements will be made of other known particles; for
   example, ATLAS may eventually measure the mass of the W boson twice as
   accurately as has previously been achieved.

   Perhaps the most exciting lines of investigation are those searching
   directly for new models of physics. One theory that is the subject of
   much current research is broken supersymmetry. The theory is popular
   because it could potentially solve a number of problems in theoretical
   physics and is present in almost all models of string theory. Models of
   supersymmetry involve new, highly massive particles; in many cases
   these decay into high-energy quarks and stable heavy particles that are
   very unlikely to interact with ordinary matter. The stable particles
   would escape the detector, leaving as a signal one or more high-energy
   quark jets and a large amount of "missing" momentum. Other hypothetical
   massive particles, like those in Kaluza-Klein theory, might leave a
   similar signature, but its discovery would certainly indicate that
   there was some kind of physics beyond the Standard Model.

   One remote possibility (if the universe contains large extra
   dimensions) is that microscopic black holes might be produced by the
   LHC. These would decay immediately by means of Hawking radiation,
   producing all particles in the Standard Model in equal numbers and
   leaving an unequivocal signature in the ATLAS detector. In fact, if
   this occurs, the primary studies of Higgs bosons and top quarks would
   be conducted on those produced by the black holes.

Components

   The ATLAS detector consists of a series of ever-larger concentric
   cylinders around the interaction point where the proton beams from the
   LHC collide. It can be divided into four major parts: the Inner
   Detector, the calorimeters, the muon spectrometer and the magnet
   systems. Each of these is in turn made of multiple layers. The
   detectors are complementary: the Inner Detector tracks particles
   precisely, the calorimeters measure the energy of easily stopped
   particles, and the muon system makes additional measurements of highly
   penetrating muons. The two magnet systems bend charged particles in the
   Inner Detector and the muon spectrometer, allowing their momenta to be
   measured.

   The only stable particles that cannot be detected directly are
   neutrinos; their presence is inferred by noticing a momentum imbalance
   among detected particles. For this to work, the detector must be "
   hermetic", and detect all non-neutrinos produced, with no blind spots.
   Maintaining detector performance in the high radiation areas
   immediately surrounding the proton beams is a significant engineering
   challenge.

Inner Detector

   The ATLAS TRT central section, the outermost part of the Inner
   Detector, as of September 2005, assembled on the surface and taking
   data from cosmic rays.
   Enlarge
   The ATLAS TRT central section, the outermost part of the Inner
   Detector, as of September 2005, assembled on the surface and taking
   data from cosmic rays.

   The Inner Detector begins a few centimetres from the proton beam axis,
   extends to a radius of 1.2 metres, and is seven metres in length along
   the beam pipe. Its basic function is to track charged particles by
   detecting their interaction with material at discrete points, revealing
   detailed information about the type of particle and its momentum. The
   magnetic field surrounding the entire inner detector causes charged
   particles to curve; the direction of the curve reveals a particle's
   charge and the degree of curvature reveals its momentum. The starting
   points of the tracks yield useful information for identifying
   particles; for example, if a group of tracks seem to originate from a
   point other than the original proton–proton collision, this may be a
   sign that the particles came from the decay of a bottom quark (see
   B-tagging). The Inner Detector has three parts, which are explained
   below.

   The Pixel Detector, the innermost part of the detector, contains three
   layers and three disks on each end-cap, with a total of 1744 modules,
   each measuring two centimetres by six centimetres. The detecting
   material is 250 µm thick silicon. Each module contains 16 readout chips
   and other electronic components. The smallest unit that can be read out
   is a pixel (each 50 by 400 micrometres); there are roughly 47,000
   pixels per module. The minute pixel size is designed for extremely
   precise tracking very close to the interaction point. In total, the
   Pixel Detector will have over 80 million readout channels, which is
   about 50% of the total readout channels; such a large count created a
   design and engineering challenge. Another challenge was the radiation
   the Pixel Detector will be exposed to because of its proximity to the
   interaction point, requiring that all components be radiation hardened
   in order to continue operating after significant exposures.

   The Semi-Conductor Tracker (SCT) is the middle component of the inner
   detector. It is similar in concept and function to the Pixel Detector
   but with long, narrow strips rather than small pixels, making coverage
   of a larger area practical. Each strip measures 80 micrometres by 12.6
   centimetres. The SCT is the most critical part of the inner detector
   for basic tracking in the plane perpendicular to the beam, since it
   measures particles over a much larger area than the Pixel Detector,
   with more sampled points and roughly equal (albeit one dimensional)
   accuracy. It is composed of four double layers of silicon strips, and
   has 6.2 million readout channels and a total area of 61 square meters.

   The Transition radiation tracker (TRT), the outermost component of the
   inner detector, is a combination of a straw tracker and a transition
   radiation detector. It contains many small straws, each four
   millimetres in diameter and up to 144 centimetres long. This gives it a
   much coarser resolution than the other two detectors, a necessary
   sacrifice for covering a larger volume and having a different,
   complementary design. Each straw is filled with gas that becomes
   ionized when a charged particle passes through. The ions produce a
   current in a high-voltage wire running through the straw, creating a
   pattern of signals in many straws that allow the path of the particle
   to be determined. It also contains alternating materials with very
   different indices of refraction, causing charged particles to produce
   transition radiation and leave much stronger signals in each straw.
   Since the amount of transition radiation produced is greatest in highly
   relativistic particles (those with a speed near the speed of light),
   and particles of a particular energy have a higher speed the lighter
   they are, particle paths with many very strong signals can be
   identified as the lightest charged particles, electrons. The TRT has
   about 351,000 straws in total.

Calorimeters

   September 2005: the main barrel section of the ATLAS hadronic
   calorimeter, waiting to be moved inside the toroid magnets.
   Enlarge
   September 2005: the main barrel section of the ATLAS hadronic
   calorimeter, waiting to be moved inside the toroid magnets.
   One of the sections of the extensions of the hadronic calorimeter,
   waiting to be inserted in late February 2006
   Enlarge
   One of the sections of the extensions of the hadronic calorimeter,
   waiting to be inserted in late February 2006

   The calorimeters are situated outside the solenoidal magnet that
   surrounds the inner detector. Their purpose is to measure the energy
   from particles by absorbing it. There are two basic calorimeter
   systems: an inner electromagnetic calorimeter and an outer hadronic
   calorimeter. Both are sampling calorimeters; that is, they absorb
   energy in high-density steel and periodically sample the shape of the
   resulting particle shower, inferring the energy of the original
   particle from this measurement.

   The electromagnetic (EM) calorimeter absorbs energy from particles that
   interact electromagnetically, which include charged particles and
   photons. It has high precision, both in the amount of energy absorbed
   and in the precise location of the energy deposited. The angle between
   the particle's trajectory and the detector's beam axis (or more
   precisely the pseudorapidity) and its angle within the perpendicular
   plane are both measured to within roughly 0.025  radians. The
   energy-absorbing materials are lead and stainless steel, with liquid
   argon as the sampling material, and a cryostat is required around the
   EM calorimeter to keep it sufficiently cool.

   The hadron calorimeter absorbs energy from particles that pass through
   the EM calorimeter, but do interact via the strong force; these
   particles are primarily hadrons. It is less precise, both in energy
   magnitude and in the localization (within about 0.1 radians only). The
   energy-absorbing material is steel, with scintillating tiles that
   sample the energy deposited. Many of the features of the calorimeter
   are chosen for their cost-effectiveness; the instrument is large and
   comprises a huge amount of construction material: the main part of the
   calorimeter—the tile calorimeter—is eight metres in diameter and covers
   12 metres along the beam axis. The far-forward sections of the hadronic
   calorimeter are contained within the EM calorimeter's cryostat, and use
   liquid argon as it does.

Muon spectrometer

   The muon spectrometer is an extremely large straw tracker, extending
   from the calorimeters out to the full diameter of the detector. Its
   tremendous size is required to accurately measure the momentum of
   muons, which penetrate other elements of the detector; the effort is
   vital because one or more muons are a key element of a number of
   interesting physical processes, and because the total energy of
   particles in an event could not be measured accurately if they were
   ignored. It functions similarly to the inner detector, with muons
   curving so that their momentum can be measured, albeit with a different
   magnetic field configuration, lower spatial precision, and a much
   larger volume. It also serves the function of simply identifying
   muons—very few particles of other types are expected to pass through
   the calorimeters and subsequently leave signals in the muon
   spectrometer. It has roughly one million readout channels, and its
   layers of detectors have a total area of 12,000 square meters.

Magnet system

   The ends of four of eight ATLAS toroid magnets, seen from the surface,
   about 90 metres above, in September 2005.
   Enlarge
   The ends of four of eight ATLAS toroid magnets, seen from the surface,
   about 90 metres above, in September 2005.

   The ATLAS detector uses two large magnet systems to bend charged
   particles so that their momenta can be measured. This bending is due to
   the Lorentz force, which is proportional to velocity. Since all
   particles produced in the LHC's proton collisions will be traveling at
   very close to the speed of light, the force on particles of different
   momenta is equal. (In the theory of relativity, momentum is not
   proportional to velocity at such speeds.) Thus high-momentum particles
   will curve very little, while low-momentum particles will curve
   significantly; the amount of curvature can be quantified and the
   particle momentum can be determined from this value.

   The inner solenoid produces a two tesla magnetic field surrounding the
   Inner Detector. This strong field allows even very energetic particles
   to curve enough for their momentum to be determined, and its nearly
   uniform direction and strength allow measurements to be made very
   precisely. Particles with momenta below roughly 400 MeV will be curved
   so strongly that they will loop repeatedly in the field and most likely
   not be measured; however, this energy is very small compared to the
   several TeV of energy released in each proton collision.

   The outer toroidal magnetic field is produced by eight very large
   air-core superconducting barrel loops and two end-caps, all situated
   outside the calorimeters and within the muon system. This magnetic
   field is 26 metres long and 20 metres in diameter, and it stores 1.2
   gigajoules of energy. Its magnetic field is not uniform, because a
   solenoid magnet of sufficient size would be prohibitively expensive to
   build. Fortunately, measurements need to be much less precise to
   measure momentum accurately in the large volume of the muon system.

Data systems and analysis

   The trigger system uses simple information to identify, in real time,
   the most interesting events out of the 40 million beam crossings that
   occur every second in the centre of the detector. There are three
   trigger levels, the first based in electronics on the detector and the
   other two primarily run on a large computer cluster near the detector.
   After the first-level trigger, about 100,000 events per second have
   been selected. After the third-level trigger, a few hundred events
   remain to be stored for further analysis. This amount of data will
   require over 100 megabytes of disk space per second—at least a petabyte
   each year.

   Offline event reconstruction will be performed on all permanently
   stored events, turning the pattern of signals from the detector into
   physics objects, such as jets, photons, and leptons. Grid computing
   will be extensively used for event reconstruction, allowing the
   parallel use of university and laboratory computer networks throughout
   the world for the CPU-intensive task of reducing large quantities of
   raw data into a form suitable for physics analysis. The software for
   these tasks has been under development for many years, and will
   continue to be refined once the experiment is running.

   Individuals and groups within the collaboration will write their own
   code to perform further analysis of these objects, searching in the
   pattern of detected particles for particular physical models or
   hypothetical particles. These studies are already being developed and
   tested on detailed simulations of particles and their interactions with
   the detector. Such simulations give physicists a good sense of which
   new particles can be detected and how long it will take to confirm them
   with sufficient statistical certainty.
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