---
title: "ATLAS experiment"
chunk: 5/5
source: "https://en.wikipedia.org/wiki/ATLAS_experiment"
category: "reference"
tags: "science, encyclopedia"
date_saved: "2026-05-05T13:02:57.471723+00:00"
instance: "kb-cron"
---

  
    
      
        r
        =
        
          
            p
            
              q
              
              B
            
          
        
        .
      
    
    {\displaystyle r={\frac {p}{q\,B}}.}
  

where 
  
    
      
        p
        =
        γ
        
        m
        
        v
      
    
    {\displaystyle p=\gamma \,m\,v}
  
 is the relativistic momentum of the particle. As a result, high-momentum particles curve very little (large 
  
    
      
        r
      
    
    {\displaystyle r}
  
), while low-momentum particles curve significantly (small 
  
    
      
        r
      
    
    {\displaystyle r}
  
). The amount of curvature can be quantified and the particle momentum can be determined from this value.

==== Solenoid Magnet ====
The inner solenoid produces a two tesla magnetic field surrounding the Inner Detector.  This high magnetic 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.

==== Toroid Magnets ====
The outer toroidal magnetic field is produced by eight very large air-core superconducting barrel loops and two smaller end-caps air toroidal magnets, for a total of 24 barrel loops all situated outside the calorimeters and within the muon system.  This magnetic field extends in an area 26 metres long and 20 metres in diameter, and it stores 1.6 gigajoules of energy.  Its magnetic field is not uniform, because a solenoid magnet of sufficient size would be prohibitively expensive to build. It varies between 2 and 8 Teslameters.

=== Forward detectors ===

The ATLAS detector is complemented by a set of four sub-detectors in the forward region to measure particles at very small angles. 

LUCID (LUminosity Cherenkov Integrating Detector)  is the first of these detectors designed to measure luminosity, and located in the ATLAS cavern at 17 m from the interaction point between the two muon endcaps;
ZDC (Zero Degree Calorimeter)  is designed to measure neutral particles on-axis to the beam, and located at 140 m from the IP in the LHC tunnel where the two beams are split back into separate beam pipes;
AFP (Atlas Forward Proton)  is designed to tag diffractive events, and located at 204 m and 217 m;
ALFA (Absolute Luminosity For ATLAS)  is designed to measure elastic proton scattering located at 240 m just before the bending magnets of the LHC arc.

=== Data systems ===

==== Data generation ====
Earlier particle detector read-out and event detection systems were based on parallel shared buses such as VMEbus or FASTBUS. Since such a bus architecture cannot keep up with the data requirements of the LHC detectors, all the ATLAS data acquisition systems rely on high-speed point-to-point links and switching networks. Even with advanced electronics for data reading and storage, the ATLAS detector generates too much raw data to read out or store everything: about 25 MB per raw event, multiplied by 40 million beam crossings per second (40 MHz) in the center of the detector. This produces a total of 1 petabyte of raw data per second. By avoiding to write empty segments of each event (zero suppression), which do not contain physical information, the average size of an event is reduced to 1.6 MB, for a total of 64 terabyte of data per second.

==== Trigger system ====
The trigger system uses fast event reconstruction to identify, in real time, the most interesting events to retain for detailed analysis.  In the second data-taking period of the LHC, Run-2, there were two distinct trigger levels:

The Level 1 trigger (L1), implemented in custom hardware at the detector site. The decision to save or reject an event data is made in less than 2.5 μs. It uses reduced granularity information from the calorimeters and the muon spectrometer, and reduces the rate of events in the read-out from 40 MHz to 100 kHz. The L1 rejection factor in therefore equal to 400.
The High Level Trigger trigger (HLT), implemented in software, uses a computer battery consisting of approximately 40,000 CPUs. In order to decide which of the 100,000 events per second coming from L1 to save, specific analyses of each collision are carried out in 200 μs. The HLT uses limited regions of the detector, so-called Regions of Interest (RoI), to be reconstructed with the full detector granularity, including tracking, and allows matching of energy deposits to tracks.  The HLT rejection factor is 100: after this step, the rate of events is reduced from 100 to 1 kHz. The remaining data, corresponding to about 1,000 events per second, are stored for further analyses.

==== Analysis process ====
ATLAS permanently records more than 10 petabytes of data per year.
Offline event reconstruction is 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 is being used extensively 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 refinements are ongoing, even after data collection has begun.
Individuals and groups within the collaboration are continuously writing their own code to perform further analyses of these objects, searching the patterns of detected particles for particular physical models or hypothetical particles. This activity requires processing 25 petabytes of data every week.

== References ==

== Further reading ==
ATLAS Technical Proposal. Archived 2012-08-02 at the Wayback Machine CERN: The Atlas Experiment.  Retrieved on 2007-04-10
"ATLAS Detector and Physics Performance Technical Design Report Archived 2003-05-16 at the Wayback Machine". CERN: The Atlas Experiment.  Retrieved on 2007-04-10
N. V. Krasnikov; V. A. Matveev (September 1997). "Physics at LHC". Physics of Particles and Nuclei. 28 (5): 441–470. arXiv:hep-ph/9703204. Bibcode:1997PPN....28..441K. doi:10.1134/1.953049. S2CID 118907038.
The Atlas Experiment Monica Lynn Dunford and Peter Jenni,  Scholarpedia 9(10):32147. doi:10.4249/scholarpedia.32147

== External links ==

Official ATLAS Public Webpage at CERN (The "award winning ATLAS movie" is a very good general introduction!)
Official ATLAS Collaboration Webpage at CERN (Lots of technical and logistical information)
ATLAS Cavern Webcams Archived 2017-07-20 at the Wayback Machine
Time lapse video of the assembly
ATLAS section from US/LHC Website
New York Times article on LHC and experiments
United States Department of Energy article on ATLAS Archived 2021-03-01 at the Wayback Machine
Large Hadron Collider Project Director Dr Lyn Evans CBE on the engineering behind the ATLAS experiment, Ingenia magazine, June 2008
Aad, G.; et al. (The ATLAS Collaboration) (2008-08-14). "The ATLAS Experiment at the CERN Large Hadron Collider". Journal of Instrumentation. 3 (S08003) S08003. Bibcode:2008JInst...3S8003A. doi:10.1088/1748-0221/3/08/S08003. hdl:2027.42/64167. S2CID 250683252. (Full design documentation)
LEGO model of ATLAS, by an ATLAS-scientist at the Niels Bohr Institute
Padilla, Antonio (Tony). "ATLAS at the Large Hadron Collider". Sixty Symbols. Brady Haran for the University of Nottingham.
"ATLAS celebrates results of 1000 collision papers". ATLAS. Retrieved 2021-08-03.
Record for ATLAS experiment on INSPIRE-HEP