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Unlocking the Mystery of Matter
How does nature work? What is the universe made of and what is its fate? These are questions being studied by researchers involved in ATLAS. ATLAS is a particle physics experiment at the LHC at the European Organization for Nuclear Research (CERN) in Geneva, Switzerland with contributors from around the world, including Canada.
“Water, for example, when divided over and over again is eventually described by H2O, which is a water molecule. If H20 is divided, it becomes hydrogen and oxygen atoms. When the atoms are studied more closely they reveal a number of electrons circling a nucleus, and if you split the nucleus, protons and neutrons are revealed,” says Michel Vetterli, Professor in the Department of Physics at Simon Fraser University and TRIUMF, Computing Coordinator for ATLAS-Canada, and Project Leader of the ATLAS Tier-1 Data Analysis Centre at TRIUMF. “If you are a nuclear physicist, the building blocks you would need to construct the universe are protons, neutrons and electrons.”
The particle physicists at ATLAS are taking an even closer look at matter, which could reveal more characteristics about quarks, including whether they are composed of even smaller particles. Vetterli and the Simon Fraser University particle physics group will study the heaviest of the quarks, the top quark, using the ATLAS detector.
“The top quark is a very unusual object,” says Vetterli, who is also a founding Co-Principal Investigator of WestGrid. “It weighs as much as a gold nucleus, which is very heavy, even though it is a fundamental constituent of nature. It’s also heavier than most whole nuclei or composites, so this gives it special properties.”
This leads to another item ATLAS scientists are looking for, which is the mechanism by which matter attains mass and why mass exists. The particle that is the beacon to understanding this is the Higgs boson.
“The Higgs boson is like the holy grail of ATLAS,” says Vetterli. “If we find it, and it has the properties we expect, then we’ll understand the mechanism by which matter gets mass.”
To look for the Higgs boson, a collision is initiated between two very high-energy beams of protons. Beams made of nuclei of hydrogen atoms are accelerated to enormous energy then bashed together. When protons collide at extremely high energy, they are converted into new particles with extremely large mass.
A Higgs boson lives for a tiny fraction of a second before it decays into several other subatomic particles. The patterns of these other particles coming out of the collision point are examined for a specific combination that suggests the Higgs boson was produced.
“This looks like a starburst in a fireworks show,” says Vetterli. “We use the ATLAS detector to take electronic pictures of these proton collisions. Then, we look at the patterns to determine whether we made a Higgs boson or something else, like a top quark.”
These pictures are where the computing comes in. The proton collisions occur 40 million times per second, but only the pictures that look the most interesting are saved. The size of each picture is only 1.6 megabytes, but 200 pictures are taken per second for about 200 days a year, leading to billions of pictures being stored.
“When two proton beams collide, most of what happens we already know about, so we have to sift through the results to find the few unique collisions, maybe one of them being a Higgs boson or another interesting process,” says Vetterli. “We’re looking for one event in 10 billion.”
The amount of storage required in a year when the accelerator is working at full intensity is somewhere between three-and-a-half and four petabytes and the experiment will run for 10-15 years. This enormous amount of data will be stored around the world, including at WestGrid’s facilities.
The experiment is done at CERN, where the preliminary analysis is also performed in a large computing centre, referred to as the Tier-0 centre. The interpretation of the electronic signals, or development of the pictures, and detailed analysis are part of an international effort at distributed Tier-1 centres.
“There are 10 very large Tier-1 centres around the world that take the electronic signals created by the collisions and make the pictures that look like fireworks starbursts,” says Vetterli. “One of these Tier-1 centres is at TRIUMF in Vancouver, BC.”