Experimental particle physics is increasingly requiring bigger and bigger experiments to carry out new research, and U of T’s physics department is involved in the most ambitious one to date. The Large Hadron Collider (LHC) is currently being constructed at CERN (Conseil Européen pour la Recherche Nucléaire), located on the outskirts of Geneva. When switched on in three or four years, it will collide protons head-on to produce energies larger than any previous particle accelerators have been capable of.
Most tantalising is the prospect of seeing the Higgs particle for the first time. Its existence has been postulated in order to explain why the particles that make up the ordinary matter around us have mass. If there is such a particle, it should be produced in these higher energy collisions. Many researchers also hope to find evidence of super-symmetry, the theory that all known particles have super-symmetric partners. Some think that this could lead us to an understanding of dark matter-invisible matter that betrays its presence only by the gravitational pull it exerts on ordinary matter, but which makes up the majority of the universe’s mass.
U of T, Carleton, the University of Arizona and the Institute of Theoretical and Experimental Physics in Moscow are collaborating on the building and testing of parts of the ATLAS detector, which will measure the energies of particles coming out of collisions. This summer, the university sent me and another undergraduate, Mohammad Hamidian, to CERN to assist them in their work there.
CERN’s two main sites cover an area roughly the size of the St. George campus. CERN itself employs about 3,000 people and 6,500 scientists from 500 universities-half of the world’s particle physicists-carry out research there. Some 2,000 are involved in ATLAS, one of the four detectors that will be placed on different sites around the LHC. The accelerator tunnel, buried beneath Switzerland and France, is 27 kilometres in circumference. It will be able to guide bunches of protons along the circular track with liquid-nitrogen cooled superconducting magnets, as they travel close to the speed of light, making more than 10,000 circuits per second.
Our group spent the summer calibrating detectors, using a test beam of particles. The goal was to compare the energies of incident particles to the strength of the electric signals they generated inside the detector. Knowing this relationship, we will be able to measure the unknown energies of particles when it is put together with the rest of ATLAS on the completed LHC.
The calibration was carried out in a hall resembling the technological lair of a James Bond villain. Data was taken around the clock in three eight-hour shifts for about a month. Most of this meant letting the computer programs run their courses, but sometimes interesting (and frustrating) problems arose.
The analysis of the data was unfinished when I left, and is still ongoing. Meanwhile, the detectors are currently in another test beam. Later in the year, all the detectors from our group will be assembled and joined with a bigger array of instruments, to be tested next summer.
In the following years, all of ATLAS’s components will be lowered into an underground pit along the path of the accelerator, which, at the size of a five-storey building, will barely be able to hold the whole detector when assembled. And a small part of it, located somewhere near its middle, will have begun its life in the basement of the McLennan building here at U of T.
