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thereby converting the proton into a neutron. According to the diagram, the W particle doesn’t hang around. It dies and converts into an antielectron and a neutrino.12 W particles emitted when a deuteron forms always die, and in fact nobody has ever seen W particles except via the stuff they turn into as they exit the world. As a rule of thumb almost all of the elementary particles die, because there is usually a Feynman vertex that allows it. The exception occurs whenever it is impossible to conserve energy or momentum, and that tends to mean that only the lightest particles stick around. That is the reason that protons, electrons, and photons dominate the stuff of the everyday world. They simply have nothing to decay into: The up and down quarks are the lightest quarks, the electron is the lightest charged lepton, and the photon has no mass. For example, the muon is pretty much identical to the electron except that it is heavier. Remember that we encountered it earlier when we were talking about the Brookhaven experiment. Since it starts out with more mass energy than an electron, its decay to an electron will not violate the conservation of energy. In addition, as illustrated in Figure 20, Feynman’s rules allow it to happen and because a pair of neutrinos is also emitted there is no trouble conserving momentum. The upshot is that muons do decay and on average live for a fleeting 2.2 microseconds. Incidentally, 2.2 microseconds is a very long time on the timescale of most of the interesting particle physics processes. In contrast, the electron is the lightest Standard Model particle and it simply has nothing to decay into. As far as anyone can tell, an electron sitting on its own will never decay, and the only way to vanquish an electron is to make it annihilate with its antimatter partner.

Returning to the deuteron, Figure 19 explains how a deuteron can form from the collision of two protons, and it says we should expect to find one antielectron (positron) and one neutrino for every fusion event. As we have already mentioned, the neutrinos interact with the other particles in the universe only very weakly. The master equation tells us that is the case, for the neutrinos are the only particles that interact solely through the weak force. As a result, the neutrinos that are manufactured deep in the core of the sun can escape without too much trouble; they stream outward in all directions and some of them head out toward the earth. As with the sun, the earth is pretty much transparent to them and they pass through it without noticing it is even there. That said, each neutrino does have a very small chance of interacting with an atom in the earth, and experiments like Super-Kamiokande have detected them, as we discussed earlier.

How certain can we be that the Standard Model is correct, at least up to the accuracy of our current experimental capabilities? Over many years now the Standard Model has been put through the most rigorous tests at various laboratories around the world. We don’t need to worry that the scientists are biased in favor of the theory; those conducting the tests would dearly love to find that the Standard Model is broken or deficient in some way, and they are trying hard to test it to destruction. Catching a glimpse of new physical processes, which may open up dazzling new vistas with magnificent views of the inner workings of the universe, is their dream. So far the Standard Model has withstood every test.

FIGURE 20

The most recent of the big machines used to test it is the Large Hadron Collider (LHC) at CERN. This worldwide collaboration of scientists aims to either confirm or break the Standard Model; we shall return to the LHC shortly. The predecessor to the LHC was the Large Electron Positron Collider (LEP), and it succeeded in delivering some of the most exquisite tests to date. LEP was housed inside a 27-kilometer circular tunnel running underneath Geneva and some picturesque French villages, and it explored the world of the Standard Model for eleven years, from 1989 until 2000. Large