The ATLAS experiment is one of the detectors at the Large Hadron Collider (LHC) at CERN in Geneva. With the particle accelerator LHC, scientists produce collisions between protons at an unprecedented energy, hoping and expecting that this will generate new forms of matter. For this, hundreds of packages of billions of protons each are shot through the accelerator at practically the speed of light in a ring with a 27 km circumference. One half of the packages goes through the ring clockwise and the other half anti-clockwise, so that they cross each other. Each package has approximately the width of a human hair and is several centimeters long. At the points where the packages cross each others’ paths, collisions will occur between a proton from one package with a proton from another package, the rest of the packages go on for the next round. This way, eventually a large number of collisions are produced.
The ATLAS detector is able to detect these collisions with great precision and high speed. The detector, which is about the same size as the Palace on the Dam square in Amsterdam, consists of a number of components that are built around the collisions like the different layers in an onion. The measurement data is electronically collected and this way a precise picture of the collision is generated. In total, the different components have about 150 million electronic channels – and ATLAS produces a considerable number of petabytes of measurement data each year (a petabyte is a thousand terabyte – a terabyte is a thousand gigabyte). The ATLAS detector was developed in a collaboration between more than 3000 scientists all over the world.
The scientists mainly want to find an answer to the following questions:
1. The Standard Model of elementary particles predicts the existence of the Higgs particle. But does this Higgs particle really exsist? If yes, what are its characteristics? If no, what does that mean for the Standard Model?
2. Never before have there been collisions at such high energies, and what happens exactly in these circumstances is still unknown. Many theorists predict the existence of a great number of other new particles. Do these really exist?
3. Observations in the universe show us that there is such a thing as ‘dark matter’. What is this ‘dark matter’? Are these elementary particles and is the LHC able to produce these particles?
Nikhef contributed in several ways to the development of the ATLAS detector. Components of the muon spectrometer, ATLAS’ outer layer, were designed, built and tested at Nikhef, and these components have been installed in ATLAS. The same goes for assembling the Semi Conducting Tracker, a component in the heart of ATLAS. Nikhef has also greatly contributed to processing the large amount of measurement data produced by ATLAS.
In March 2010 the first high-energy collisions were made and registered. The analysis of the collisions is in full swing now – the Nikhef researchers play a central role in this as well.
The research questions at ATLAS are aimed at the Standard Model of elementary particles (the model that outlines our understanding of the elementary particles and their mutual forces), and on the extensions on that. Each of these questions are fundamental and are being driven by curiosity. Of course, finding an answer to these questions will have technological implications in the long run, but this is not the motive behind this scientific endeavour.