ATLAS
Higgs, super symmetry and dark matter
Nikhef is member of the ATLAS experiment, one of the detectors at the Large Hadron Collider (LHC) at CERN in Geneva. Here, over 3000 scientists perform research into elementary particles such as the Higgs particle, and look for answers to questions regarding dark matter.
The LHC is a particle accelerator: it generates collisions among protons with unprecedented energy, which as a result break up into fragments. By analyzing these fragments, researchers hope to discover yet unknown types of matter.
To this end, hundreds of bunches consisting of billions of protons each travel around in a ring of 27 km circumference at almost the speed of light. One half of the packages travel clockwise, the other half counter clockwise, so that they cross each other. Each bunch has the width of a human hair and a length of a few centimeters. Where the bunches cross each other, collisions occur between a proton of one bunch and a proton of the other, while the rest of the bunch move on for the next round. This way, a very large number of collisions are produced.
The LHC includes several detection systems, called experiments, that each use different techniques to look for clues in the fragments resulting from the collisions, and that each focus on different fundamental questions. Two of them, ATLAS and CMS, are general purpose particle detectors. The ATLAS detector is designed to register these collisions with great precision and at high speed. The detector, which roughly has the size of the Amsterdam Royal Palace on the Dam Square, consists of a number of parts constructed around the collision point, just as layers of an onion. The measurement data are collected electronically and in this way a precise photo is made of the collision. ATLAS produces a substantial number of petabytes worth of data every year (a petabyte is one thousand terabytes; a terabyte is one thousand gigabyte). These data are collected using about 150 million electronic channels.
At this moment, ATLAS is looking for answers to the following questions:
- For years, the search has been on for a missing particle to complete the Standard Model of particle physics (the model that summarizes our understanding of the elementary particles and their mutual forces). In July 2012, the existence of the Higgs boson was proven by two detector experiments at the LHC (read the official press release). Now that the existence of the Higgs boson has been proven, we are left with numerous questions, for instance: What are the properties of this particle? What does this discovery imply for the Standard Model?
- Many theoreticians predict the existence of a large number of new particles. Do these really exist?
- Observations in the Universe reveal that something called ‘dark matter‘ should exist. One of the theories is that a large part of the Universe consists of this mysterious matter, which can’t be detected directly. What is ‘dark matter’? Does it consist of elementary particles and is the LHC capable of producing them?
ATLAS and the LHC have been built by a large international community of scientists. Here, Dutch scientists play an important role:
- Muons are particles produced in large quantities when protons collide in the LHC. The central particle detection system of ATLAS however fails to detect them. For that reason, a special muon spectrometer was built. Components of this spectrometer, the outer layer of ATLAS, were designed, built and tested by Nikhef and installed in ATLAS.
- The Semi Conducting Tracker (SCT) was also designed, built and tested by Nikhef. This component at the heart of ATLAS is essential to the detection of the fragments resulting from the collisions. It consists of 1000 sensors with 1500 channels each.
- Nikhef has designed several electronic modules of the trigger and data-acquisition system, which read out the detectors and decide which of the 40 million collisions per second are stored on disk.
- Nikhef has contributed extensively to processing the enormous stream of measured data.
- On the software side, Nikhef has played an important role in the muon reconstruction and in the identification of jets from b-quarks.
- Finally, researchers from Nikhef are the main developers of the analysis framework used to combine the individual Higgs measurements.
This research programme is a prime example of fundamental scientific research, aimed at gathering basic knowledge about everything around us. At the heart of this type of research is curiosity about what our Universe is made of and how it came to be. There’s much that we know already, for example that all visible matter is built up from atoms, yet many questions remain unanswered.
Fundamental research is not aimed at realizing applications in the short term. Still, one thing is for sure: no one can predict which ground-breaking applications will eventually emerge from this research. History shows that today’s fundamental knowledge forms the breeding ground for tomorrow’s discoveries.
