Knocking on Heaven's Door - Lisa Randall [114]
[ FIGURE 35 ] Cinzia da Via and an engineer, Domenico Dattola, standing on scaffolding in front of one of the bulkheads of the CMS silicon tracker, to which the cables are connected.
Most charged particles, however, live long enough to make it to the next tracker component, so detectors record a much greater length path. Therefore, outside the inner pixel detectors with fine resolution in two directions are silicon strips with asymmetric size in the two directions, much coarser in one of the two. The longer strips are consistent with the cylindrical shape of the experiment and make covering a larger area (remember the area gets far bigger with bigger radius) feasible.
The CMS silicon tracker consists of a total of 13 layers in the central region and 14 layers in the forward and backward regions. After the first three finely pixilated layers we just described, the next four layers, consisting of silicon strips, extend to 55 centimeters radius. The detector elements here are 10-centimeter-long, 180-micrometer-wide strips. The remaining six layers are even less precise in the coarser orientation, consisting of strips up to 20 centimeters long and varying in width between 80 and 205 micrometers, with the strips extending out to a radius of 1.1 meters. The total number of strips in the CMS inner detector is 9.6 million. These strips are essential to reconstructing the tracks of most charged particles that pass through. In total, CMS has silicon covering essentially the area of a tennis court—a significant advance over the previous largest silicon detector of only two square meters.
The ATLAS inner detector extends to a slightly smaller radius of one meter and is seven meters long longitudinally. As with CMS, outside the three inner silicon pixel layers, the Semiconductor Tracker (SCT) consists of four layers of silicon strips. In ATLAS’s case, they are 12.6 centimeters by 80 micrometers in size. The total area of the SCT is also enormous, covering 61 square meters. Whereas the pixel detectors are useful for reconstructing fine measurements near the interaction points, the SCT is most critical to overall track reconstruction because of the large region it covers with high precision (albeit in one direction).
Unlike CMS, the outer detector of the ATLAS apparatus is not made of silicon. The transition radiation tracker (TRT), the outermost component of the inner detector, consists of tubes filled with gas and acts as both a tracking device and a transition radiation detector. Charged particle tracks are measured when they ionize the gas in the straws, which are 144 centimeters by 4 millimeters in size, with wires down the center to detect the ionization. Here again there is highest resolution in the transverse direction. The straws measure the tracks with a precision of 200 micrometers, which is less precise than with the innermost tracker but covers a far greater region. The detectors also discriminate among particles moving very close to the speed of light that produce so-called transition radiation. This discriminates among particles of different mass, since lighter particles will generally be moving faster. This helps identify electrons.
If you’re finding all these details a bit overwhelming, keep in mind that this is more information than even most physicists need to know. They give a sense of the magnitude and precision, and are of course important to anyone working on a particular detector component. But even those who have extreme familiarity with one