Knocking on Heaven's Door - Lisa Randall [61]
However, the nonzero masses of elementary particles, which are an intrinsic property they possess, are an enormous mystery. Not only quarks and leptons, but also weak gauge bosons—the particles that communicate the weak force—have nonzero mass. Experimenters have measured these masses, but the simplest physics rules simply don’t allow them. Standard Model predictions work if we just assume particles have these masses. But we don’t know where they came from in the first place. Clearly the simplest rules don’t apply and something more subtle is afoot.
Particle physicists believe these nonvanishing masses arise only because something very dramatic occurred in the early universe in a process that is most commonly called the Higgs mechanism in honor of the Scottish physicist Peter Higgs who was among the first to show how masses could arise. At least six authors contributed similar ideas, however, so you might also hear about the Englert-Brout-Higgs-Guralnik-Hagen-Kibble mechanism, though I will stick with the name Higgs.36 The idea—whatever we call it—is that a phase transition (perhaps like the phase transition of liquid water bubbling into gaseous steam) took place that actually changed the nature of the universe. Whereas early on, particles had no mass and zipped around at the speed of light, later on—after this phase transition involving the so-called Higgs field—particles had masses and traveled more slowly. The Higgs mechanism tells how elementary particles go from having zero mass in the absence of the Higgs field to the nonzero masses we have measured in experiments.
If particle physicists are correct and the Higgs mechanism is at work in the universe, the LHC will reveal telltale signs that betray the universe’s history. In its simplest implementation, the evidence is a particle—the eponymous Higgs boson. In more elaborate physical theories in which the Higgs mechanism is nonetheless at work, the Higgs boson might be accompanied by other particles with about the same mass, or the Higgs might be replaced by some other particle altogether.
Independently of how the Higgs mechanism is implemented, we expect the LHC to produce something interesting. It might be a Higgs boson. It might be evidence of a more exotic theory such as technicolor that we will discuss later on. Or it could be something completely unforeseen. If all goes as planned, experiments at the LHC will discern what it was that implemented the Higgs mechanism. No matter what is found, the discovery will tell us something interesting about how particles acquire their masses.
The Standard Model of particle physics, which describes matter’s most basic elements and their interactions, works beautifully. Its predictions have been confirmed many times at a high level of precision. This Higgs particle is the last remaining piece of the Standard Model puzzle.37 We now assume particles have masses. But when we understand the Higgs mechanism, we’ll know how those masses came about. The Higgs mechanism, which is explored further in Chapter 16, is essential to a more satisfactory understanding of mass.
And there is another, even bigger, puzzle in particle physics where the LHC should help. Experiments at the Large Hadron Collider are likely to illuminate the solution to a question known as the hierarchy problem of particle physics. The Higgs mechanism addresses the question of why fundamental particles have mass. The hierarchy problem asks the question why those masses are what they are.
Not only do particle physicists believe that masses arose because of a so-called Higgs field that permeates the universe, we also believe we know the energy at which the transition from massless to massive particles occurred. That’s because the Higgs mechanism gives masses to some particles in a predictable manner that depends only on the strength of the weak nuclear force and the energy at which the transition occurs.
The peculiar thing is that this transition energy doesn’t really make sense from an underlying