What is The ATLAS Experiment?

ATLAS is an enormous multi-purpose detector situated at one of the crossing points of the two oppositely directed proton beams of the Large Hadron Collider's (LHC).  The LHC accelerator, located at CERN on the French-Swiss border near Geneva, is housed in an enormous tunnel roughly 27 km in circumference and 100 m underground.

The LHC and its detectors were designed to study the smallest fundamental building blocks that make up our universe – to find out what these building blocks are and how they interact (and don't interact) with one another.  In order for a particle to be classified as 'fundamental' in this case, it means that as far as we know, the particle is not itself a composite object, composed of even smaller pieces. 

The answers to some of the questions we can ask about fundamental particles are, at least in part, provided by the long-standing 'Standard Model of Particle Physics', our current theoretical model of how the universe works at the smallest scale.  Although we know the Standard Model is not strictly incorrect insofar as we've been able to measure, we know that it must be incomplete or simply a low-energy approximation to a more general theory.

Somewhat paradoxically, in order to probe nature at its smallest scale, we need to build gigantic machines, such as the LHC at its various detectors, which operate at exceedingly high energies.  ATLAS is one of CERN's flagship experiments, collecting vast amounts of data from the deeply inelastic proton-proton interactions that take place very near to its centre.  The ATLAS detector itself is composed of a number of layers, all acting in concert to provide physicists with as much information as possible about the interactions of interest. 

ATLAS is currently in its fourth year of proton-proton collision data-taking at an unprecedented centre-of-mass energy of √s = 13 TeV.  For more information about how particle accelerators such as the LHC work and to have a better sense of what we could potentially learn from analyzing all of that ATLAS data, have a look at 'An Introduction to Collider Physics'.

 

Click the link to the left to access the main website for the ATLAS experiment – see the latest news & results, the current running conditions of the experiment, and further links to some great multimedia pages for students & teachers.

An exterior view of Building 40 at CERN, situated between Meyrin, Switzerland and Saint-Genis-Pouilly, France.

An exterior view of Building 40 at CERN, situated between Meyrin, Switzerland and Saint-Genis-Pouilly, France.

What is The ATLAS Collaboration?

The ATLAS collaboration is made up of roughly 3000 researchers and engineers from nearly 40 countries.  While many of the collaboration members are often based at CERN, whether it be during a data-taking period for shift work, for an experimental meeting or workshop, or otherwise, a large number are based at their home institutions spread around the world.  Some are involved in hardware upgrades and associated studies, some help to develop simulated so-called Monte Carlo datasets which are crucial in allowing us to make measurements and potential discoveries, some analyze the ATLAS data itself to perform physics measurements, and some help monitor the data quality as it comes in or are involved in the actual operation of the detector during data-taking periods; often it is a mix of all of the above – members divide their time and are involved in several different aspects of the experiment at once.

The Max Planck Institute (MPI) for Physics in Munich, Germany where I currently work as a research associate is just one of the various institutes worldwide which hosts members of the ATLAS collaboration.  MPI and its many ATLAS members contribute to the success of the collaboration in a number of ways. 

For more information on the group's activities with the ATLAS experiment, follow the link here.

 

The ATLAS Forward Calorimeters

The far-forward region of the ATLAS detector – close to the LHC proton beamline – is subject to by far the highest levels of radiation stemming from the proton-proton collisions taking place at the detector's centre.  It is here that the so-called Forward Calorimeter (FCal) modules absorb enormous amounts of the energy from such forward-directed particles.  The FCal is an example of a sampling calorimeter where liquid argon (LAr) acts as the active medium.  The absorbing materials are primarily copper and tungsten.  The electrodes from the modules themselves run parallel to the LHC beam direction.  These electrodes are surrounded by very thin LAr gaps in which electrons liberated from the ionization from incoming particles lead to electrical currents in the detector readouts.  Crucially these currents are what allow physicists to measure the energy of incoming particles.  During the recent LHC data-taking periods (2011 - 2015) the FCal demonstrated excellent performance.

In the coming years the ATLAS detector will undergo a series of upgrades in order to be able to handle the particle fluxes which will be present during the so-called high-luminosity LHC (HL-LHC) phase of data-collection.  A number of modifications to the current FCal setup were initially being considered in order to deal with these higher fluxes, and although the ultimate replacement of the FCal units with upgraded higher-granularity units was ruled out, many of the initial studies (or extensions thereof) are relevant in the context of understanding and anticipating the expected performance of this important part of the detector for future LHC running conditions.

I am currently working on improving the background suppression for hadronic jets in the forward region through the analysis of simulated HL-LHC ATLAS data.

My M.Sc. research focused on an analysis of pion test-beam data collected in 2003 before the FCal was integrated into the full ATLAS detector setup in order to compare different hadronic calibration schemes.

Photo blow: taken at CERN during a visit to the ATLAS detector cavern in the summer of 2008.  The photo itself shows a sample piece of the ATLAS LAr barrel calorimeter (separate from the FCal modules, but part of the integrated liquid argon calorimeter system as a whole).  The LAr barrel calorimeter employs a novel accordion structure and measures the energy of particles traversing regions of the detector which are situated at large angles relative to the LHC proton beam axis.

Accordion structure of part of the ATLAS liquid argon calorimeter system.