Knocking on Heaven's Door - Lisa Randall [113]
ATLAS, like CMS, features its magnet in its name since a big magnetic field is also critical to its operation. As noted earlier, ATLAS is the acronym for A Toroidal LHC ApparatuS. The word “toroid” refers to the magnets, whose field is less strong than that of CMS but extends over an enormous region. The huge magnetic toroids help make ATLAS the larger of the two general-purpose detectors and in fact the largest experimental apparatus ever constructed. It is 46 meters long and 25 meters in diameter and fits rather snugly into its 55-meter-long, 40-meter-high cavern. At 7,000 metric tons, ATLAS is a little more than half the weight of CMS.
To measure all the particle properties, increasingly large cylindrical detector components emanate from the region where collisions occur. The CMS and ATLAS detectors both contain several embedded pieces designed to measure the trajectory and charges of the particles as they pass through. Particles emerging from the collision first encounter the inner trackers that precisely measure the paths of charged particles close to the interaction point, next the calorimeters that measure energy deposited by readily stopped particles, and finally the muon detectors that are at the outer edges and measure the energy of highly penetrating muons. Each of these detector elements has multiple layers to increase the precision for each measurement. We’ll now tour the experiments from the innermost detectors to the outermost as measured radially from the beams and explain how the spray of particles leaving a collision turns into recorded identifiable information.
THE TRACKERS
The innermost portions of the apparatuses are the trackers that record the positions of charged particles as they leave the interaction region so that their paths can be reconstructed and their momenta measured. In both ATLAS and CMS, the tracker consists of several concentric components.
The layers closest to the beams and interaction points are the most finely segmented and generate the most data. Silicon pixels, with extremely tiny detector elements, sit in this innermost region, starting at a few centimeters from the beam pipe. They are designed for extremely precise tracking very close to the interaction point where the particle density is highest. Silicon is used in modern electronics because of the fine detail that can be etched into each tiny piece, and particle detectors use it for the same reason. Pixel elements at ATLAS and CMS are designed to detect charged particles with extremely high resolution. By connecting the dots to one another and to the interaction points from which they emerged, experimenters find the paths the particles followed in the innermost region very near to the beam.
The first three layers of the CMS detector—out to 11 centimeter radius—consist of 100 by 150 micrometer pixels, 66 million in total. ATLAS’s inner pixel detector is similarly precise. The smallest unit that can be read out in the ATLAS innermost detector is a pixel of size 50 by 400 micrometers. The total number of ATLAS pixels is about 82 million, a little more than the number in CMS.
The pixel detectors, with their tens of millions of elements, require elaborate electronic readouts. The extent and speed required for the readout systems, as well as the huge radiation the inner detectors will be subjected to, were two of the major challenges for both of the detectors. (See Figure 35.)
Because there are three layers in these inner trackers, they record three hits for any long-lasting enough charged particle that passes through. These tracks will generally continue to an outer tracker beyond the pixel layers to create a robust signal that can be definitively associated with a particle.
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