Knocking on Heaven's Door - Lisa Randall [120]
At this point, experimenters anywhere can go through their data to sleuth out what the high-energy proton collisions might reveal. This can be something new and exciting. But in order to establish whether or not this is the case, the first task for the experiments—which we’ll explore further in the following chapter—is deducing what was there.
CHAPTER FOURTEEN
IDENTIFYING PARTICLES
The Standard Model of particle physics, compactly categorizes our current understanding of elementary particles and their interactions (summarized in Figure 40).53 It includes particles like the up and down quarks and the electrons that sit at the core of familiar matter, but it also accommodates a number of other heavier particles that interact through the same forces, but which are not commonly found in nature—particles that we can study carefully only at high-energy collider experiments. Most of the Standard Model’s ingredients, such as the particles the LHC is currently studying, were rather thoroughly buried until the clever experimental and theoretical insights that revealed them in the latter half of the twentieth century.
At the LHC, the ATLAS and CMS experiments are designed to detect and identify Standard Model particles. The real goal, of course, is to go beyond what we already know—to find new ingredients or forces that address outstanding mysteries. But to do so, physicists need to be able to distinguish Standard Model background events and identify the Standard Model particles into which any exotic new particles might decay. Experimenters at the LHC are like detectives who analyze data to piece together clues and ascertain what was there. They will be able to deduce the existence of something new only after they have ruled out everything that is familiar.
Having toured the general-purpose experiments, we will now revisit them in this chapter to better understand how LHC physicists identify individual particles. A bit more familiarity with the particle physics status quo and how Standard Model particles are found will help when we discuss the discovery potential of the LHC in Part IV.
HANDEDNESS
Particles are left-handed or right-handed according to which way they appear to spin about the axis of their direction of motion.
[ FIGURE 40 ] The elements of the Standard Model of particle physics, with masses shown. Also shown are separate left- and right-handed particles. The weak force that changes particle type acts only on the left-handed ones.
FINDING LEPTONS
Particle physicists divide the elementary matter particles of the Standard Model into two categories. One type is called leptons, which includes particles such as the electron that don’t experience the strong nuclear force. The Standard Model also includes two heavier versions of the electron, which have the same charge but much bigger masses, and which are called the muon and the tau. It turns out that every Standard Model matter particle has three versions, all with the same charge but with each successive generation heavier than the next. We don’t know why there should be three versions of these particles, all with the same charges. The Nobel Prize—winning physicist Isidor Isaac Rabi, on hearing of the muon’s existence, notably expressed his bafflement with the exclamation, “Who ordered that?”
The lighter leptons are the easiest to find. Although both electrons and photons deposit energy in the electromagnetic calorimeter, because the electron is charged and the photon is not, the electron is readily distinguished from a photon. Only an electron leaves a a track in the inner detector before depositing energy in the ECAL.
Muons too are relatively straightforward to identify. Like all the other heavier Standard Model particles, muons decay so quickly that they aren’t found in ordinary matter, so we rarely find them on Earth. However, muons live long enough to travel to the outer reaches of the detectors before they decay. They therefore leave long clearly visible tracks throughout that experimenters