The “God Particle” (Higgs boson)

The existence of the Higgs boson was predicted (on the basis of the quantum theory that underlies the Standard Model) by Peter Higgs and several other physicists as early as 1964, though the particle was not actually detected in any experiments until 2012.See here for a handy timeline of discoveries at the LHC.

In popular literature, the Higgs boson is sometimes called the “God particle,” but there is nothing divine about it. This nickname originated with Leon Lederman, former director of the Fermi National Accelerator Laboratory (Fermilab), who led many unsuccessful attempts to detect the elusive Higgs boson. In his 1993 book The God Particle: If the Universe Is the Answer, What Is the Question? Lederman explains the origin of the nickname:

This boson is so central to the state of physics today, so crucial to our final understanding of the structure of matter, yet so elusive, that I have given it a nickname: the God Particle. Why God Particle? Two reasons. One, the publisher wouldn’t let us call it the Goddamn Particle, though that might be a more appropriate title, given its villainous nature and the expense it is causing. And two, there is a connection, of sorts, to another book, a much older one...Leon Lederman, The God Particle: If the Universe Is the Answer, What Is the Question? (1993; repr., New York: Houghton Mifflin, 2006), 23.

Lederman goes on to liken his futile search for the Higgs boson to the building of the Tower of Babel described in Genesis 11:1-9.

The search for the Higgs boson was one of the primary reasons (if not the primary reason) for constructing the largest and most expensive experimental apparatus in the world: the Large Hadron Collider, which finally succeeded in detecting the particle. Why did physicists care so much about finding this exotic particle, you ask? The discovery of the Higgs boson confirmed the existence of a theoretical entity called the Higgs field, which plays a unique and important role in particle physics. As we’ll see in chapter 7, each type of elementary particle is associated with a field. Electrons are associated with an electron field, quarks are associated with quark fields, and so on. But the Higgs field is special. According to the currently accepted theory, the Higgs field gives elementary particles their mass! In other words, the mass of each elementary particle is determined (at least in part) by the way it interacts with the Higgs field.

In classical mechanics, as you may recall from chapter 2, mass was a fundamental property—a property that was not explained in terms of other properties. In the Standard Model of particle physics, however, that is no longer true. Mass isn’t regarded as a fundamental property anymore; it is explained in terms of more basic properties. The details of this new theory of mass are not easy to grasp, but here’s the general idea. Recall from chapter 2 that the fundamental property called “mass” plays two distinct roles in Newton’s laws. In Newton’s law of universal gravitation, mass is the property that determines how strongly two objects attract each other by the gravitational force: the strength of the gravitational force is directly proportional to the product of the masses of the two objects. In Newton’s second law of motion, on the other hand, mass has nothing to do with gravity. Instead, it is the quantity that determines how much force is required to accelerate an object at any given rate, or the quantity that determines how quickly an object will accelerate when any given force is applied to it. In other words, mass is the thing that determines an object’s resistance to acceleration. The more an object resists being accelerated, the greater its mass.

The fact that mass can play both of these seemingly unrelated roles is known as the “equivalence principle.” The equivalence principle puzzled Albert Einstein, who believed it couldn’t be sheer coincidence that an object’s susceptibility to gravity always happens to match its resistance to acceleration. This conviction led Einstein to formulate his general theory of relativity, as we’ll see in chapter 6.

This second way of understanding mass—the idea that mass is simply resistance to acceleration—is crucial to the explanation of mass in contemporary particle physics. The degree to which a particle resists acceleration depends on its interaction with the Higgs field. A simple analogy is often used to illustrate this idea. Think of the Higgs field as a sticky syrup, pervading the whole universe. Particles that get “stuck” in the syrup can’t be accelerated as quickly as particles that are slippery and can easily slide through it. This syrup analogy isn’t technically accurate. The Higgs field isn’t really sticky, and it doesn’t cause friction or resistance to motion. (Only the acceleration of a particle is affected.) But the analogy provides at least a hint of the way in which a particle’s mass depends on its interaction with the Higgs field. If we think of mass simply as resistance to acceleration, it’s not too hard to imagine that a particle’s mass could be explained in terms of its interaction with a field.

The more strongly a particle interacts with the Higgs field, the harder it is to accelerate that particle. Elementary particles that interact with the Higgs field very strongly have a lot of mass, while particles that interact weakly have little mass. For example, electrons are affected only a little by the Higgs field, so electrons can be accelerated easily. (In other words, they have little mass.) W and Z bosons are affected more strongly by the Higgs field, and can’t be accelerated as easily as electrons. (Thus, W and Z bosons have more mass.) And particles that don’t interact with the Higgs field at all—namely, photons and gluons—don’t have to be accelerated; they’re already moving at the maximum possible speed, the speed of light! (According to Albert Einstein, the speed of light is the maximum possible speed for any particle. We’ll see why in chapter 6.)

A couple of technical clarifications are in order here. First, the Higgs field doesn’t give particles mass by slowing them down; rather, the mass comes from potential energy that is transfered from the Higgs field to the particle during the interaction. Secondly, not all massive particles get their mass from the Higgs field. Protons, for example, get only 1% of their mass from the Higgs field; the other 99% comes from other forms of potential and kinetic energy. In fact, the mass of any particle comes from the energy it contains, in accordance with Einstein’s most famous equation: E = mc2. We’ll examine how this works in chapter 6.