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mass should lie within a particular range now that we know the masses of the W particle and the top quark. LEP could have seen the Higgs if it had been at the lighter end of the predicted range, but since none were seen, we might presume it is too heavy to have been produced at LEP (remember that heavier particles need more energy to produce them, by virtue of E = mc2). At the time of writing, the Tevatron collider at the Fermi National Accelerator Laboratory (Fermilab) near Chicago is hunting for the Higgs, but again it has not to date seen a hint. It is again very possible that the Tevatron has insufficient energy to deliver a clear Higgs signal, although it is very much in the race. The LHC is the highest-energy machine ever built, and it really should settle the question of the Higgs’s existence once and for all because it has enough energy to reach well beyond the upper limits set by the Standard Model. In other words, the LHC will either confirm or break the Standard Model. We’ll return shortly to explain why we are so sure that the LHC will do the job the earlier machines have failed to do, but first we would like to explain just how the LHC expects to make Higgs particles.

The LHC was built within the same 27-kilometer-circumference tunnel that LEP used but, apart from the tunnel, everything else has changed. An entirely new accelerator now occupies the space LEP once occupied. It is capable of accelerating protons in opposite directions around the tunnel to an energy equal to more than 7,000 times their mass energy. Smashing the protons into each other at these energies advances particle physics into a new era, and if the Standard Model is right, it will produce Higgs particles in large numbers. Protons are made up of quarks, so if we want to figure out what should happen at the LHC, then all we need to do is identify the relevant Feynman diagrams.

FIGURE 22

The most important vertices corresponding to interactions between the regular Standard Model particles and the Higgs particle are illustrated in Figure 22, which shows the Higgs as a dotted line interacting with the heaviest quark, the top quark (labeled t), and with the also pretty heavy W or Z particles. Perhaps it will come as no surprise that the particle responsible for the origin of mass prefers to interact with the most massive particles around. Knowing that the protons furnish us with a source of quarks, our task is to figure out how to embed the Higgs vertex into a bigger Feynman diagram. Then we’ll have figured out how Higgs particles can be manufactured at the LHC. Since quarks interact with W (or Z) bosons, it is easy to work out how the Higgs could be produced via W (or Z) particles. The result is shown in Figure 23: A quark from each of the colliding protons (labeled “p”) emits a W (or Z) particle, and these fuse together to make the Higgs. The process is called weak boson fusion, and it is expected to be a key process at the LHC.

FIGURE 23

The case of the top quark production mechanism is a little trickier. Top quarks do not exist inside protons, so we need a way to go from the light (up or down) quarks to top quarks. Well, top quarks interact with the lighter quarks through the strong force—i.e., mediated by emitting and absorbing a gluon. The result is shown in Figure 24. It is rather similar to the weak boson fusion process except that the gluons replace the W or Z. In fact, because this process proceeds through the strong force, it is the most likely way to produce Higgs particles at the LHC. It goes by the name of gluon fusion.

FIGURE 24

This then is the Higgs mechanism, the currently most widely accepted theory for the origin of mass in the universe. If all goes according to plan, the LHC will either confirm the Standard Model description of the origin of mass or show that it is wrong. This is what makes the next few years such an exciting time for physics. We are in the classic scientific position of having a theory that predicts precisely what should happen in an experiment, and will therefore stand or fall depending