Warped Passages - Lisa Randall [86]
Collectively, the groups of particles composed of quarks and gluons that move in unison in a particular direction are known as jets. Once an energetic jet is formed, it is like a rope in that it will never disappear. When you cut a rope, all you do is create two new pieces of rope. Similarly, when interactions divide jets, the pieces can only form new jets: they will never separate into individual, isolated quarks and gluons. Stephen Sondheim was presumably not thinking about high-energy particle colliders when he wrote the lyrics to the Jet song from West Side Story, but his words apply admirably to jets of strongly interacting particles. Energetic, strongly interacting particles remain together. “They’re never alone…they’re well protected.”
The Known Fundamental Particles
This chapter has described three of the four known forces: electromagnetism, the weak force, and the strong force. Gravity, the remaining force, is so weak that it would not change particle physics predictions in an experimentally observable way.
But we have not yet finished introducing the particles of the Standard Model. They are identified by their charges, and also by their handedness. As I described earlier, the left-and right-handed particles can (and do) have different weak charges.
Particle physicists categorize these particles as either quarks or leptons. Quarks are fundamental fermionic particles that experience the strong force. Leptons are fermionic particles that do not. Electrons and neutrinos are examples of leptons. The word “lepton” derives from the Greek leptos, which means “small” or “fine,” referring to the tiny mass of the electron.
The bizarre thing is that in addition to the particles that are essential to the structure of the atom, such as the electron and the up and down quarks, there are additional particles that, though heavier, have the same charges as the particles we have already introduced. All of the lightest stable quarks and leptons have heavier replicas. No one knows why they are there, or what they are good for.
When physicists first realized that the muon, a particle first seen in cosmic rays, was nothing other than a heavier version of the electron (200 times heavier), the physicist I.I. Rabi asked, “Who ordered that?” Although the muon is negatively charged, like the electron, it is heavier than the electron, into which it can decay. That is, a muon is unstable (see Figure 53) and quickly converts into an electron (and two neutrinos). So far as we know, it serves no purpose to matter here on Earth. Why does it exist? This is one of the many mysteries of the Standard Model that we hope scientific progress will solve.
Figure 53. In muon decay, the muon turns into a muon neutrino and a virtual W- gauge boson, which then converts to an electron and an electron antineutrino.
In fact, there are three copies of the full set of particles with the same Standard Model charges (see Figure 52). Each of these copies is called a generation, or sometimes a family. The first generation of particles contains a left-and a right-handed electron, a left-and a right-handed up quark, a left-and a right-handed down quark, and a left-handed neutrino. This first generation contains all the stable stuff of which atoms, and therefore all stable matter, is composed.
The second and third generations contain particles that decay and are not present in “normal” known matter. They are not exact copies of the first generation; they have charges identical to those of their first generation counterparts but are heavier. They were discovered only when they were produced at high-energy particle colliders, and their purpose remains obscure. The second generation consists of a