Warped Passages - Lisa Randall [119]
However, even the most idyllic-seeming families can reveal undercurrents of tension when investigated more closely. Despite well-coordinated behavior and a happy, harmonious appearance, a devastating hidden family secret can be lurking underneath. The Standard Model has just such a skeleton in the closet. Everything agrees with predictions if you uncritically assume that the strength of the electromagnetic force, the strength of the weak force, and the gauge boson masses take the values that have been measured in experiments. But we’ll soon see that the mass parameter (the weak scale mass that determines the elementary particle masses), though very well measured, is ten million billion times, or sixteen orders of magnitude, lower than the mass that physicists would expect from general theoretical considerations. Any physicist who would have guessed the value of the weak scale mass based on a high-energy theory would have gotten it (and therefore all particle masses) completely wrong. The mass seems to come out of thin air. This puzzle—the hierarchy problem—is a gaping hole in our understanding of particle physics.
In the Introduction I explained the hierarchy problem as the question of why gravity is so weak, but we will now see that this problem can be restated as the question of why the Higgs particle’s mass, and hence the weak gauge boson masses, are so small. For those masses to take their measured values, the Standard Model has to incorporate a fudge that is as unlikely as someone winning the guessing game against Ike and randomly choosing a sixteen-digit number correctly. Despite its many successes, the Standard Model relies on this unconscionable fudge to accommodate the known elementary particle masses.
This chapter explains this problem, and why I, and most other particle theorists, think it’s so important. The hierarchy problem tells us that whatever is responsible for electroweak symmetry breaking is bound to be more interesting than the two-field Higgs example presented in Chapter 10. Possible solutions all involve new physical principles, and the solution will very likely guide physicists to more fundamental particles and laws. Identifying what plays the role of the Higgs field and breaks electroweak symmetry will reveal some of the richest new physics we are likely to nail down in my lifetime. New physical phenomena will almost certainly appear at an energy of about a TeV. Experimental tests of competing hypotheses are near at hand, and within a decade there should be a dramatic revision in our understanding of fundamental physical laws that will incorporate whatever is discovered there.
The hierarchy problem tells us that before extrapolating physics to extremely high energies, we have at least one urgent low-energy problem to attend to. For the last thirty years or so, particle theorists have been searching for the structure that predicts and protects the weak scale energy, the relatively low energy at which electroweak symmetry breaks. I and others think that there must be a solution of the hierarchy problem, and that it will provide one of the best clues about what lies beyond the Standard Model. To understand the motivation for some of the theories I’ll soon present, it is useful to know a little about this somewhat technical but very important problem. The search for its solution has already led us to investigate