Knocking on Heaven's Door - Lisa Randall [112]
Standard Model particles are characterized by their mass, spin, and the forces through which they interact. No matter what is ultimately created, both experiments rely on detecting it through known Standard Model forces and interactions. That’s all that’s possible. Particles with no such charges would leave the interaction region without a trace.
But when experiments measure Standard Model interactions, they can identify what passed through. So that’s what the detectors are designed to do. Both CMS and ATLAS measure the energy and momentum of photons, electrons, muons, taus, and strongly interacting particles, which get subsumed into jets of closely aligned particles traveling in the same direction. Detectors emanating from the proton collision region are designed to measure energy or charge in order to identify particles, and they contain sophisticated computer hardware, software, and electronics to deal with the overwhelming abundance of data. Experimenters identify charged particles since they interact with other charged stuff that we know how to find. They also find anything that interacts via the strong force.
The detector components all ultimately rely on wires and electrons produced through interactions with the material in the detector to record what passed through. Sometimes charged particle showers occur because many electrons and photons are produced and sometimes material is simply ionized with charges recorded. But either way wires record the signal and send it along for it to be processed and analyzed by physicists at their computers.
[ FIGURE 34 ] Simulation of an event in the ATLAS detector showing the transverse spray of particles though the detector layers. (Note that the person gives a sense of scale, but collisions don’t happen when people are in the cavern.) The distinctive toroidal magnets are clearly visible. (Courtesy of CERN and ATLAS)
Magnets are also critical to both detectors. They are essential to mea-suring both the sign of the charges and the momenta of charged particles. Electromagnetically charged particles bend in a magnetic field according to how fast they are moving. Particles with bigger momenta tend to go straighter, and particles with opposite charges bend in opposite directions. Because particles at the LHC have such large energies (and momenta), the experiments need very strong magnets to have a chance of measuring the small curvature of the energetic charged particle tracks.
The Compact Muon Solenoid (CMS) apparatus is the smaller in size of the two large general-purpose detectors, but it is heavier, weighing in at a whopping 12,500 metric tons. Its “compact” size is 21 meters long by 15 meters in diameter—smaller than ATLAS but still big enough to cover the area of a tennis court.
The distinguishing element in CMS is its strong magnetic field of 4 tesla, which the “solenoid” piece of the name refers to. The solenoid in the inner part of the detector consists of a cylindrical coil six meters in diameter made up of superconducting cable. The magnetic return yoke that runs through the outer part of the detector is also impressive and contributes most of the huge weight. It contains more iron than Paris’s Eiffel Tower.
You might also wonder about the word “muon” in the name CMS (I did too when I first heard it). Rapidly identifying energetic electrons and muons, which are heavier counterparts of electrons that penetrate to the outer reaches of the detector, can be important for new particle detection—since these energetic particles are sometimes produced when heavy objects decay. Since they don’t interact via the strong nuclear force, they are more likely to be something new—since protons won’t automatically make them. These readily identifiable particles could therefore indicate