9. THE HIGGS BOSON
So what is the Higgs particle? To understand that one needs to know about the Higgs field.We have talked about the fundamental forces and the force carrying particles that carry gravity, electromagnetism, strong, and the weak nuclear forces. We have implicitly talked about a force field which is the region of space surrounding a body within which it can exert a force on another similar body not in contact with it. Examples are electromagnetic field for a charged particle or a magnet, and gravitational field that has an attractive effect, extending throughout space, of matter on other matter. Then there is the “Higgs field” which is a field pervading space. Everything in the universe is submerged in the Higgs field.
The Higgs field has the very interesting property that it interacts with the particles moving through it and giving some of them mass. The Higgs boson is like a vibration in the Higgs field—the Higgs field becomes locally distorted whenever a particle moves through it. The distortion, or the clustering of the field around the particle, generates the particle’s mass.
The Higgs boson, the associated particle of the Higgs field, is a “scaler” boson meaning it has zero spin and can be exchanged back and forth by other particles, giving rise to a force of nature. The Higgs is unlike any other particle. It is the lynch pin of the Standard Model and it is responsible for holding matter together. It is connected to a field that fills all of space and which gives particles, like the electron, mass and allowed them to be caught in atoms and thus is responsible for creating atoms, molecules, planets, and people. Without the Higgs, life as we know it would not exist.
An excellent non-technical explanation of the Higgs Boson was provided by physicist David Miller of the University College London.
Imagine a cocktail party of political party workers who are uniformly distributed across the floor, all talking to their nearest neighbors. The ex-Prime- Minister enters and crosses the room. All of the workers in her neighborhood are strongly attracted to her and cluster round her. As she moves she attracts the people she comes close to, while the ones she has left return to their even spacing. Because of the knot of people always clustered around her she acquires a greater mass than normal, that is, she has more momentum for the same speed of movement across the room. Once moving she is harder to stop, and once stopped she is harder to get moving again because the clustering process has to be restarted.
With the introduction of the Higgs particle, the Standard Model table which nicely separated particles and force carriers, became “skewed” (Figure 9-1). Sean Carrol has the following lines in his book The Particle at the End of the Universe: “The Higgs boson is different. Named after Peter Higgs, who was one of the several people who proposed the idea back in the 1960s, the Higgs boson is somewhat of an ugly duckling. Technically speaking it’s a force-carrying particle, but it is a different kind of force carrier from the ones we’re most familiar with. From the view point of a theoretical physicist the Higgs seems like an arbitrary and whimsical addition to an otherwise beautiful structure. If it weren’t for the Higgs boson, the Standard Model would be the epitome of elegance and virtue; as it is it’s a bit of a mess. And finding the mess-maker has proven to be quite a challenge.”
So far we talked about the Higgs field and how vibrations in the Higgs field manifest as the Higgs particle. Without the Higgs field, many elementary particles would be identical to each other, but instead they have different masses and lifetimes. But how did the physicists predict an odd ball like the Higgs boson?
It was Nambu’s work that inspired Peter Higgs, Robert Brout, François Englert, and somewhat later (but independently), Gerry Guralnik, C. R. Hagen, Tom Kibble to develop the theoretical mechanism (now called the Higgs mechanism) for the Higgs boson in 1960. They found a loophole in a version of Goldstone theorem when it is was applied to relativistic gauge theories. In this mechanism the massless spin-zero particle (the Nambu-Goldstone boson) does not show up. It gets “eaten” by the massless gauge bosons giving them a mass. In the case of the weak interactions, these massive gauge bosons are the two charged W^± particles and the neutral Z particle, discovered at CERN in the early 1980’s (1984 Nobel Prize to Carlo Rubbia and Simon van der Meer!). A second massive spin-zero particle, the Higgs boson, is a consequence of this Higgs mechanism and represents excitation of the Higgs field about its new vacuum state. Higgs mechanism allows calculation of the interactions of the Higgs boson with all other particles, but unfortunately does not predict the mass of the Higgs particle itself.
The story of the discovery of Higgs mechanism will not be complete without mentioning the very significant contribution by Philip Anderson, a condensed matter physicist and one of the leading experts in superconductivity. In the summer of 1962, Anderson realized that in analogy with superconductivity, when you have both gauge symmetry and spontaneous symmetry breaking, the Nambu-Goldstone massless mode can combine with the massless gauge field modes to produce a physical massive vector field. His paper appeared in the particle physics section of the April 1963 issue of Physical Review. In Anderson’s words:
It is likely, then, considering the superconducting analog, that the way is now open for a degenerate-vacuum theory of the Nambu type without any difficulties involving either zero-mass Yang-Mills gauge bosons or zero-mass Goldstone bosons. These two types of bosons seem capable of “canceling each other out” and leaving finite mass bosons only.
Anderson did not provide an explicit relativistic example to supplement the non-relativistic superconductivity case. This was provided by several authors in 1964, with Higgs giving the first explicit relativistic model. Higgs was also the first to explicitly discuss the possible existence of a massive particle, that we now call a “Higgs particle.
Higgs mechanism is a part of the unification of electromagnetic and weak forces into a single theory, called the electroweak theory, which is a central tenet of the Standard Model of particle physics. It refers specifically to the generation of masses for the W^±, and Z weak gauge bosons through electroweak symmetry breaking. The electroweak theory was first suggested by Sheldon Glashow who incorporated these masses into the theory by hand, but did not explain their origin. Steven Weinberg (and independently, Abdus Salam), incorporated the Higgs mechanism into Glashow’s electroweak theory producing a consistent, unified electroweak theory. Gerard t’Hooft proved that the electroweak theory was renormalizable . The electroweak theory included a new particle, dubbed the Higgs boson, which, when included in the scattering calculations, completed a new theory—the Standard Model—which made sensible predictions even for very high-energy scattering. Below some extremely high temperature, the Higgs field causes spontaneous symmetry breaking during interactions. Particle masses are thought to come from interactions with the Higgs particle, which arises out of the spontaneous symmetry breaking.