Introduction to the ATLAS Detector at the LHC

The ATLAS detector site provides a comprehensive presentation of the detector and its physics.  This is just a brief presentation for my OLLI classes.

I don’t mean to slight the other general detector at CERN which is CMS, but I only thought that we would have time for one detector description in class.  CMS is the Compact Muon Solenoid, and its results are a partner of those from ATLAS, and are equally quoted in the presentations.  Here is a neat demonstration of how the CMS subsystems work.

This is a schematic of the ATLAS detector at the LHC proton-proton collider at CERN.  Notice the two people talking at the lower left, where one is saying to the other “are you sure you connected wire 1 to wire 2?”  The ATLAS collaboration includes 3,000 physicists, including 1,000 students.  It includes 38 countries and 174 universities and labs – a truly international collaboration and a model for large scale collaboration.  The detector weighs 7,000 tons, is 1/2 the size of Notre Dame, and the size of a 5 story building.  It collects only the most outstanding events at the rate of 27 CD fulls a minute, which must be around 15 Gigabytes a minute, or about a Terabyte an hour, where a Terabyte is a thousand Gigabytes, and a common storage size for a home computer today.  The CERN site says that the total data load for all experiments is 100 Petabytes a year, where a Petabyte is a thousand Terabytes.  As the event rate increases, so does the storage rate.  Very fast electronics and algorithms give a preliminary analysis to every event and decide which ones to keep.  The solenoids and toroids are magnets to bend the charged particles and measure their momentum.

The above generic detector sandwich in the massive detectors shows charged particles of electrons, muons, protons, and pions leaving tracks in the inner silicon tracking chambers.  Next, light electrons and photons make showers leading to their total energy measurement in the electromagnetic calorimeter.  These are the parts of the inner detector which are covered by a uniform magnetic field created by the surrounding solenoid magnet.

In the outer detector is a donut shaped magnetic field created by a large toroid magnet.  In the hadronic calorimeter, the strongly interacting protons, neutrons, and pions are stopped and their energy measured by their particle showers.  Finally, the muon tracks in the muon chamber with a magnetic field that bends their paths measures their momentum and energy.

The search for massive elementary particles between 100 GeV and several TeV is done by observing their decay particles, which can have momentum of order half of their masses.  In order to stop these decay particles and measure their energy, large calorimeters are needed.  In order to bend the particles in magnetic fields to measure their momentum, strong, large and heavy magnets are needed.  These account for the large size and weight of the LHC detectors.

This is a view of the end of the ATLAS detector with some of the insides removed.

UC Irvine has an ATLAS group, and Prof. Andrew Lankford at UCI is the Deputy Spokesperson for ATLAS.  The UCI LHC webpage is at

There is a set of entertaining ATLAS detector presentations on youtube, co-produced by Michael Barnett, who once was a postdoc at UC Irvine, and who is now a leading spokesperson for particle physics, for science and science teaching, and a leader of lobbying efforts for particle physics and science.  He is at Lawrence Berkeley National Lab.  Here are the Youtube videos:

CERN-ATLAS:  Episode 1, A New Hope, overall view of the detector;

CERN-ATLAS:  Episode 2, The Particles Strike Back (Part 1), a detailed explanation of the inner detectors; and

CERN-ATLAS:  Episode 2, The Particles Strike Back (Part 2), a detailed explanation of the outer detectors.

For extra credit, here is a picture of a solenoidal magnet that surrounds the inner detector, and the constant magnetic field B along its axis.  The current I flows in circles around the axis in the windings of the coil.



Here is the toroidal magnet with a green magnetic B field that winds around the axis like it would inside a donut.  It provides the field in the outer muon detector.


The current I wraps around the outer surface of the donut.  In the ATLAS detector, the donut is stretched sideways to cover the length of the detector.


About Dennis SILVERMAN

I am a retired Professor of Physics and Astronomy at U C Irvine. For a decade I have been active in learning about energy and the environment, and in lecturing and attending classes at the Osher Lifelong Learning Institute (OLLI) at UC Irvine.
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