Knocking on Heaven's Door - Lisa Randall [115]
THE ELECTROMAGNETIC CALORIMETER (ECAL)
Once through the three types of trackers, the next section of detector a particle encounters on its outward radial journey is the electromagnetic calorimeter (ECAL), which records the energy deposited by charged and neutral particles that stop there—electrons and photons in particular—and the position where they left it. The detection mechanism looks for the spray of particles that incident electrons or photons produce when they interact with the detector material. This piece of the detector yields both precise energy and position tracking information for these particles.
The material used for the ECAL in the CMS experiment is a wonder to behold. It is made of lead tungstate crystals, chosen because they are dense but optically clear—exactly what you want for stopping and detecting electrons and photons as they arrive. You can perhaps get a sense of this from my photograph in Figure 36. The reason they are fascinating is their incredible clarity. You’ve never seen anything this dense and this transparent. The reason they are useful is that they measure electromagnetic energy incredibly precisely, which could turn out to be critical to finding the elusive Higgs particle as Chapter 16 will describe.
The ATLAS detector uses lead to stop electrons and photons. Interactions in this absorbing material transform the energy from the initial charged track into a shower of particles whose energy will then be detected. Liquid argon, which is a noble gas that doesn’t chemically interact with other elements and is very resistant to radiation, is then used to sample the energy of the shower to deduce the incident particle energy.
[ FIGURE 36 ] Photograph of the lead tungstate crystal that is used in CMS’s electromagnetic calorimeter.
Despite my theoretical inclinations, I was fascinated to see this detector element at ATLAS on my tour. Fabiola participated in the pioneering development and construction of this calorimeter’s novel geometry with radial layers of accordion-shaped lead plates separated by thin layers of liquid argon and electrodes. She described how this geometry makes readout of the electronics much faster, since the electronics is much closer to the detector elements. (See Figure 37.)
[ FIGURE 37 ] The accordion-like structure of ATLAS’s electromagnetic calorimeter.
THE HADRONIC CALORIMETER (HCAL)
Next in line along our radial outward journey from the beam pipe is the hadronic calorimeter (HCAL). The HCAL measures the energy and positions of hadronic particles—those particles that interact through the strong force—though it does so less precisely than the electron and photon energy measurements made by the ECAL. That’s by necessity. The HCAL is huge. In ATLAS, for example, the HCAL is eight meters in diameter and 12 meters long. It would be prohibitively expensive to segment the HCAL with the precision of the ECAL, so the precision of the track measurement is necessarily degraded. On top of that, energy measurements are simply harder for strongly interacting particles, independent of segmentation, since the energy in hadronic showers fluctuates more.
The HCAL in CMS contains layers of dense material—brass or steel—alternating with plastic scintillator tiles that record the energy and position of the hadrons that pass through, based on the intensity of the scintillating light. The absorber material in the central region of ATLAS is iron, but the HCAL there works pretty much the same way.
MUON DETECTOR
The outermost elements in any general-purpose detector are the muon chambers. Muons, you will remember, are charged particles like electrons, but they are 200 times heavier. They don’t stop in the electromagnetic