Knocking on Heaven's Door - Lisa Randall [116]
Energetic muons are very useful when looking for new particles because, unlike hadrons, they are sufficiently isolated that they are relatively clean to detect and measure. Experimenters want to record all events with energetic muons in the transverse direction because muons are likely to be associated with the more interesting collisions. Muon detectors could also prove useful for any heavy stable charged particle that makes it to the outer reaches of the detector.
[ FIGURE 38 ] CMS’s magnetic return coil interlaced with its muon detector—all under construction.
Muon chambers record the signals left by the muons that reach these outermost detectors. They are similar in some respects to the inner detector with its trackers and magnetic fields bending the muon tracks so their trajectories and momenta can be measured. However, in the muon chambers, the magnetic field is different, and the thickness of the detector is much bigger, permitting measurements of smaller curvatures and hence higher-momentum particles (high-momentum particles bend less in a magnetic field). In CMS, the muon chambers extend from about three meters to the outer radius of the detector at about 7.5 meters, while in ATLAS they extend from four meters to the outer reaches of that detector at 11 meters. These huge structures permit 50-micrometer particle track measurements.
ENDCAPS
The last detector elements to describe are the endcaps, the detectors at the forward and backward ends of the experiments. (See Figure 39 to get a sense of the overall structure.) We are no longer working our way radially outward from the beam—the muon detectors were the last step in that direction—but rather we now are proceeding along the axis of the cylindrical detectors to the two ends that cap them off. The cylindrical portions of the detectors are “capped” off there with detectors covering the end regions that ensure that as many particles as possible get recorded. Since the endcaps were the last components of the detector to be moved to their final positions, I could readily see the multiple layers that sit inside the detectors when I visited in 2009.
[ FIGURE 39 ] Computer image of ATLAS showing its many layers and the endcaps separated. (Courtesy of CERN and ATLAS)
Detectors are placed in these end regions to ensure that LHC experiments measure all the particles’ momenta. The goal is to make the experimental apparatuses hermetic, meaning there is coverage in all directions with no holes or missing regions. Hermetic measurements ensre that even noninteracting or very weakly interacting particles can be discovered. If “missing” transverse momentum is observed, one or more particles with no directly detectable interactions must have been produced. Such particles carry momentum, and the momentum they take away makes experimenters aware of their existence.
If you know the detector is measuring all the transverse momentum, and the momentum perpendicular to the beam doesn’t appear to be conserved after a collision, then something must have disappeared undetected and carried away momentum. Detectors, as we have seen, measure momentum in the perpendicular directions very carefully. The calorimeters in the forward and backward regions ensure hermeticity by guaranteeing that very little energy or momentum perpendicular to the beam can escape unnoticed.
The CMS apparatus has steel absorbers and quartz fibers in the end regions, which separate the particle tracks better because they are denser. The brass in the endcaps is recycled material—it was originally used in Russian artillery shells. The ATLAS apparatus uses liquid-argon calorimeters in the forward region to detect not only electrons and photons but also hadrons.
MAGNETS
The remaining pieces of both detectors that remain to be described in more detail are the magnets that give both experiments their names. A magnet is not a detector element in that it doesn’t record particle properties. But magnets