Knocking on Heaven's Door - Lisa Randall [124]
The other type of quark that is distinctive from an experimental vantage point is the top quark, which is special because it is so heavy. The top quark is the heaviest of the three quarks that have the same charge as the up quark (the other one is called charm). Its mass is about 40 times heavier than the differently charged bottom quark and more than 30,000 times the mass of the up quark, which has the same charge as the top.
Top quarks are sufficiently heavy that their decay products leave distinct tracks. When lighter quarks decay, the decay products, like the initial particle, travel so close to the speed of light that they are rushed along together into what appears to be a single jet—even if the jet had its origin in two or more distinct decay products. Unless they are extremely energetic, top quarks, on the other hand, visibly decay into bottom quarks and W bosons (the charged weak gauge bosons) and can be identified by finding both of them. Because the top quark’s heavy mass implies that it interacts most closely with the Higgs particle and other particles involved in weak scale physics that we are hoping to soon understand, the properties of top quarks and their interactions might provide valuable clues to physical theories underlying the Standard Model.
FINDING THE WEAK FORCE CARRIERS
Before we finish discussing how to identify Standard Model particles, the final particles to consider are the weak gauge bosons, the two Ws and the Z, that communicate the weak nuclear force. The weak gauge bosons have the peculiar property that, unlike the photon or gluons, they have nonvanishing mass. The masses associated with the weak gauge bosons that communicate the weak force pose some major fundamental mysteries. The origin of this mass—as with the masses of the other elementary particles this chapter has discussed—is rooted in the Higgs mechanism that we will get to shortly.
Because the Ws and Z are heavy, these gauge bosons decay. This means that the W and Z bosons, as with the top quark and other un-stable heavy particles, can be identified only by finding the particles into which they decay. Because heavy new particles are also likely to be unstable, we’ll use the weak gauge boson decays to exemplify one other interesting property of decaying particles.
A W boson interacts with all particles that are sensitive to the weak force (namely, all the particles we have discussed). That gives the W plenty of decay options. It can decay into any charged lepton (the electron, the muon, or the tau) and their associated neutrino. It can also decay into an up and down quark or into a charm and strange quark pair, as illustrated in Figure 44.
[ FIGURE 44 ] The W boson can decay into a charged lepton and its associated neutrino, or into an up and down quark, or a charm and strange quark. In reality, the physical particles are superpositions of different types of quarks or neutrinos. This allows the W to some-times decay into particles from different generations simultaneously.
Particle masses are also critical in determining allowed decays. A particle can decay only into other particles whose masses add up to a smaller mass than the initial particle. Although the W also interacts with the top and bottom quarks, the top quark is heavier than the W, so this decay isn’t allowed.56
Let’s consider the W decaying into two quarks, since in that case the experimenters measure both decay products (not true for lepton and neutrino since the neutrino is “missing”). Because energy and momentum are conserved, measuring the total energy and momentum of both final state quarks tells us the energy and momentum of the particle that decayed into them, namely, the W.
At this point both Einstein’s special theory of relativity combined with quantum mechanics make the story