ATLAS
overview
ATLAS is one of the largest and most complex particle detectors ever built, one of the world’s leading tools in the search for beyond-Standard Model physics and a key TRIUMF international collaboration.
Based at CERN as part of the Large Hadron Collider (LHC), ATLAS is one of the largest scientific collaborations in history. As part of the 10-institution ATLAS-Canada collaboration (TRIUMF and nine universities), TRIUMF scientists, engineers and technicians have provided critical expertise to all aspects of ATLAS’ success.
This includes detector design, construction and installation, accelerator science, high-performance scientific computing (TRIUMF’s ATLAS Tier-1 Computing Centre), data analysis, theory and large-scale scientific project management.
TRIUMF’s involvement in ATLAS is an outgrowth of the lab’s contribution to the LHC. From 1995 to 2005, TRIUMF managed Canada’s $41-million contribution to the LHC’s construction. Leveraging TRIUMF’s accelerator expertise, Canada made important in-kind contributions, including to the injection kickers, twin-aperture quadrupole magnets, and injector synchrotrons.
ATLAS is renowned for the historic 2012 discovery of the Higgs boson, the final predicted piece in the puzzle of the Standard Model (SM). TRIUMF staff and technology played an essential role in the Higgs boson discovery, including building parts of the detectors that recorded it, the intensive Higgs boson data analysis, and the high-performance computers and software that provided data reduction and modeling.
However, ATLAS is also designed to explore beyond-SM physics, including searches for dark matter and supersymmetric particles, and completely new and unimagined physics. To this end, the global ATLAS science team is in the midst of a decade-long series of staged upgrades to ATLAS to super-charge it for the High Luminosity LHC (HL-LHC) era, when the collider will provide multiple times the design beam intensity opening the way for postulated beyond-SM physics discoveries.
Beyond its historic physics discoveries, ATLAS is driving and inspiring a broad array of related science and technological advance, including in nuclear and medical research. In particular, ATLAS has come to define the era of big data and its scientists and data analysis techniques are fuelling big data applications from e-commerce to fintech and e-health. TRIUMF’s participation is a key conduit for bringing these ancillary benefits to Canada.
ATLAS + TRIUMF
how ATLAS works
The LHC, the world’s most powerful particle accelerator, is a 27 km ring of superconducting magnets with a number of accelerating structures to accelerate two beams of protons in opposite directions to 99.999999% the speed of light. The beams inside the LHC are made to collide at four detector locations around the accelerator ring – CMS, ALICE, LHCb, and ATLAS.
Almost half as long as a football field and weighing 7,000 tonnes (about the mass of the Eiffel Tower), ATLAS is the largest volume detector ever constructed for a particle collider. Located in a cavern 100 m below ground, the cylindrical ATLAS detector, 46 m long and 25 in diameter, consists of six different detecting subsystems wrapped concentrically in layers around the collision point to record the trajectory, momentum, and energy of particles, allowing some of them to be individually identified as distinct particles, from muons to electrons.
The proton-proton collisions at the centre of the ATLAS detector produce collision debris in the form of new particles which fly out in all directions.
Presently, more than a billion particle interactions take place in the ATLAS detector every second, but only one-in-a-million proton-on-proton collisions triggers the data collection system as interesting physics and thus recorded for further analysis.
Inner Tracker upgrade: silicon strip module fabrication
TRIUMF and ATLAS
The TRIUMF ATLAS scientific group is a Canadian keystone for the participation of over one hundred Canadian researchers, graduate and undergraduate students at nine ATLAS-Canada participating universities. The TRIUMF ATLAS group includes TRIUMF researchers, emeriti, and affiliated scientists; co-appointed researchers based at Canadian universities; post-doctoral research associates based at TRIUMF and CERN; and graduate students from various universities who are supervised by TRIUMF research scientists.
From the design and construction of original ATLAS components to planned upgrades, TRIUMF researchers, detector engineers and technicians have played, and continue to play, key roles.
The ATLAS collaboration is divided into six sub-groups, each responsible for one of the six key detector sub-components. TRIUMF significantly contributes to three of these: the Liquid Argon Calorimeters; the Muon Detectors; and the Computing Systems. TRIUMF brings its detector expertise to both calorimetry (measuring particle energy) and tracking (measuring particle trajectories to determine their charge and momentum). TRIUMF’s ATLAS participation is tightly linked to the ATLAS experimental timeline. The ATLAS detector operates on a schedule of experimental runs, and long shutdowns (LS), when maintenance and major upgrades are performed.
TRIUMF is significantly involved in three of the major ATLAS upgrades: Digitizing the trigger for the Liquid Argon Hadronic End-cap Calorimeters; the new Muon Small Wheel detector; and the replacement of the present Inner Detector with a new Inner Tracker.
Liquid Argon Hadronic End-cap Calorimeters
TRIUMF-ATLAS researchers, engineers and technicians helped design, build, test, and assemble two of ATLAS’ original Liquid Argon Hadronic Endcap Calorimeter (HEC) wheels.
A calorimeter is a detector that measures a particle’s energy. At either end of ATLAS’ 360-degree Liquid Argon Hadronic Calorimeter system is a HEC, used to measure the energy of hadrons (particles composed of quarks, such as pions) emerging at low angle (less than 25º) from proton-proton collisions.
Each HEC is a massive, integrated detector weighing 250 tons. At slightly more than four metres in diameter, it’s made up of two wheels, (0.8m and 1.0m thick) each composed of 32 wedge-shaped modules arranged to form the wheel shape. Each module is a layered design of dozens of 25 or 50 mm thick layers of copper, interspersed with 8.5 mm gaps filled with liquid argon and read-out electronics, all cryogenically cooled to -186ºC (87 K).
When high energy hadrons from a proton collision traverse the copper, they produce a hadronic shower in which the collision debris’ initial energy is transformed into lower energy hadrons, including protons and neutrons ejected from copper nuclei. These charged particles subsequently strip electrons from argon atoms, leaving a trail of electron-argon ion pairs. A strong electric field causes the free electrons to rapidly drift to the positive side, producing a distinctive electric signal that’s collected by HEC’s front-end electronics. Based on the shape and combinations of these electric signals, ATLAS scientists can help identify the particles involved.
TRIUMF worked closely with Canadian university-based ATLAS team members, including a number of cross-appointments, on the design, construction and assembly of HEC components. With University of Alberta researchers, TRIUMF designed radiation-hard front-end electronics for the HEC and supplied and contributed to the machining of the copper plates. TRIUMF participated in the design and engineering of the liquid argon cryostat feedthrough project at the University of Victoria. TRIUMF-ATLAS members also worked with ATLAS-team members at Carleton University and the University of Toronto to produce the tungsten components for two of the ATLAS forward calorimeters, located inside the central bore of the HECs.
The TRIUMF-built components operated at 99.6% efficiency during Run 1.
Muon Small Wheel detector
In collaboration with five Canadian universities in the ATLAS-Canada team, TRIUMF made major contributions to the New Small Wheels (NSWs), building one-quarter of the upgraded small-strip Thin Gap Chambers (sTGCs), gas wire detectors that are one of the NSWs’ two key detector technologies.
The NSWs are capable of simultaneous precision tracking and high-speed triggering in order to both weed out fake muons and capture the trajectories of interesting ones. In coordination with the other muon detectors, the NSWs’ triggers provide location resolution of about 10 microns, about the diameter of a human hair. This provides the overall muon trigger system with reconstructed track segments of good angular resolution, clearly indicating whether or not the triggered muons originated from the collision point.
Inner Tracker
The Inner Detector (ID) is a cylindrical detectore with endcap discs immediately surrounding the LHC proton-proton collision site. The ID experiences intense radiation and, over time, radiation damage. To prepare for even higher radiation levels of ATLAS’ High Luminosity era, the ID will be completely replaced by a new, all-silicon Inner Tracker (ITk). The ITk will provide both significantly higher detector resolution and more radiation tolerant sensors.
This upgrade is a major international collaboration, involving 20 countries, with strong participation from the ATLAS-Canada group (TRIUMF and seven Canadian universities). Collaboratively, ATLAS-Canada members are providing 1500 of the approximately 7,000 sophisticated silicon strip modules that will form the ITk endcaps.
Each module consists of a silicon strip sensor (about 10×10 cm2) with radiation-resistant, customized read-out electronics (ASICs and power boards) attached.
Each sensor consists of two or four rows of strips, about 25 mm long and only 80 micrometers wide. Particles crossing the sensor leave signals in the strips they cross which can be detected by the electronics. Knowing which strip was crossed allows the trajectory of the particles to be very precisely determined.
New Small Wheel upgrade: Small-strip Thin Gap Chamber fabrication
Partners and support
ATLAS was built, and is being upgraded, through a globally distributed collaboration that involves thousands of individual components. These components are designed, constructed, tested and assembled at various national particle physics laboratories, universities and industries in 38 countries, and then shipped to CERN for assembling into final ATLAS structures.
Today, ATLAS involves more than 3000 scientists from about 182 institutions in 38 countries, with more than 1200 doctoral students involved in detector development, data collection and analysis.
The TRIUMF-ATLAS scientific group is a keystone for the participation of over one hundred Canadian researchers, graduate and undergraduate students at nine ATLAS-Canada participating universities. The TRIUMF ATLAS group includes TRIUMF researchers, emeriti, and affiliated scientists; co-appointed researchers based at Canadian universities; post-doctoral research associates based at TRIUMF and CERN; and graduate students from various universities who are supervised by TRIUMF research scientists.
We gratefully acknowledge support from the Natural Sciences and Engineering Research Council of Canada and the Canada Foundation for Innovation.