The US and ATLAS

ATLAS detector
View of one end of the under-construction ATLAS detector showing the End Cap Calorimeter. Image © CERN
Approximately 700 physicists, engineers and graduate students from 44 US institutions participate in the ATLAS collaboration. This number includes 169 US graduate students and ATLAS participants represent 40 universities and four Department of Energy national laboratories. The whole ATLAS collaboration includes 2800 physicists from 37 countries and 169 institutions.

The US ATLAS collaboration completed the contribution of $163.75 million to the construction of the ATLAS detector in 2008. US groups have contributed components to all of the ATLAS detector subsystems, each dedicated to measuring different properties of different types of particles. US scientists have also contributed to the development and testing of the data acquisition system, which takes the raw data from the ATLAS detector, filters it, and stores it in a form that physicists will use to search for and measure fundamental particles and forces.

Tracking systems

Closest to the point where the protons collide are the tracking detectors. These detectors, which are immersed in a magnetic field, combine to precisely measure the curved path—and thus the momentum—of the many charged particles emitted from the proton-proton collisions. US scientists have contributed to the design, construction and testing of the detectors and electronics for the three tracking systems: the pixel, semiconductor and transition radiation tracker systems. The pixel detector and semiconductor tracker use silicon detectors to measure the momentum and position of charged particles. The transition radiation tracker uses straw detectors filled with Xenon gas to measure photons created when a particle passes between the straws.

Calorimeters

Calorimeters measure the energies of particles such as photons, electrons, protons and neutrons. ATLAS has two types of calorimeters: the Liquid Argon Calorimeter, which uses liquid argon to transform a particle’s incident energy into a shower of particles detected by sensors; and the Tile Calorimeter, which uses steel plates to produce particle showers, and scintillating plastic to transform particles’ energies into light that is later digitized.

US groups are responsible for approximately 20 percent of the total cost of the Liquid Argon Calorimeter and are involved at various levels in all aspects of the system’s construction, including the electronics to convert detector signals to digital data, and the cryogenics system that keeps the calorimeter at -321 degrees Fahrenheit. For the Tile Calorimeter, US institutions provided parts of the steel fabric of the calorimeter, electronics to digitize the detector signals, and other specialized pieces such as light-reflecting wrapping for the scintillator tiles.

Muon Spectrometer

The muon spectrometer surrounds the inner detectors—calorimeters and tracking systems—and its function is to identify and measure muons. The spectrometer measures the paths in a magnetic field of these heavy electron-like particles so that their momenta can be determined with high precision. US scientists designed, fabricated and installed more than 250 of the chambers that make up the spectrometer, which included fabrication and testing of more than 88,000 individual tubes that allow the muons to be measured. US groups also participated in development of the electronics that convert detector signals to digital signals, and the design and layout of more than 3,600 cables to carry the signals.

Data Acquisition and Computing

Approximately one billion proton-proton collisions every second send particles flying through the ATLAS detector. Due to computer storage limitations, only about 200 of these "events " can be permanently saved. The job of the data acquisition and trigger systems is to filter through billions of events to find the several hundred that may yield evidence for Higgs boson, supersymmetry, or other phenomena scientists wish to study. US groups have contributed to the development of hardware and software for these systems, and are participating in the worldwide computing grid that will distribute data from the ATLAS experiment to scientists in the US and around the globe.

List of US institutions participating in the ATLAS experiment