Knocking on Heaven's Door (36 page)

[
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 component don’t necessarily keep track of all the others, as I accidentally learned when trying to track down some detector photos and make sure some diagrams were precise. So don’t feel too badly if you don’t get it all the first time. Though some experts coordinate the overall operation, even many experimenters don’t necessarily have every detail at their fingertips.

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 or hadronic calorimeters but instead barrel straight through the thick outer region of the detector. (See Figure 38.)

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.