Warped Passages - Lisa Randall [87]
From Figure 52 you can see that although there were only three known flavors of quark when Gell-Mann first proposed them, we now know of six: three “up types” and three “down types”—one in each generation. In addition to the up quark itself, there are two identically charged up-type quarks—the charm and the top. Similarly, the down, strange, and bottom quarks are different flavors of down-type quark. And the muon and tau leptons are heavier versions of the electron.
Physicists are still trying to understand the reason for three generations and why particles have their particular masses. These are major questions about the Standard Model that fuel the research being conducted today. Along with many others, I’ve worked on these problems throughout my career, but we’re still searching for the answers.
The heavier flavors are significantly heavier than the lighter ones. Although the next heaviest quark, the bottom, was discovered in 1977, the very heavy top quark eluded discovery until 1995. Two particle experiments, including the remarkable one that discovered the top quark, are the subject of the following chapter.
What to Remember
The Standard Model consists of the nongravitational forces and the particles that experience those forces. In addition to the well-known force of electromagnetism, there are two forces that act within a nucleus: the strong force and the weak force.
The weak force poses the most important remaining mystery about the Standard Model. Whereas the other two forces are communicated by massless particles, the gauge bosons that communicate the weak force have mass.
In addition to the particles that communicate forces, the Standard Model contains particles that experience those forces. These particles are divided into two categories: quarks, which experience the strong force, and leptons, which do not.
The light quarks and leptons found in matter (the up quark, the down quark, and the electron) are not the only known particles. Heavier quarks and leptons also exist: the up quark, the down quark, and the electron each have two heavier versions.
These heavy particles are unstable, which means that they decay to lighter quarks and leptons. But experiments at particle accelerators have produced them and shown that these heavier particles experience the same forces as the familiar light, stable particles.
Each of the groups of particles that include a charged lepton, an up-type quark, and a down-type quark is known as a generation. There are three generations, each of which contains successively heavier versions of each particle type. These particle varieties are known as flavors. There are three up-type quark flavors, three down-type quark flavors, three charged lepton flavors, and three neutrino flavors.
I won’t use the details or names of any particular quark or lepton later on. However, you will need to know about flavors and generations because of the strong constraints on the particles’ properties, which give us vital clues and constraints on the physics that lies beyond the Standard Model.
Chief among these constraints is that different flavors of quarks and leptons with the same charges rarely, if ever, turn into one another. Theories in which particles readily change flavor are ruled out. We will see later that this poses a big challenge for models of broken supersymmetry and other proposed extensions of the Standard Model.
8
Experimental Interlude: Verifying the Standard Model
One way, or another
I’m gonna find you…
Blondie
Ike once again dreamed he met the quantum detective. This