Electroweak Unification

The discovery of the W and Z particles, the intermediate vector bosons, in 1983 brought experimental verification of particles whose prediction had already contributed to the Nobel prize awarded to Weinberg, Salam, and Glashow in 1979. The photon , the particle involved in the electromagnetic interaction, along with the W and Z provide the necessary pieces to unify the weak and electromagnetic interactions. With masses around 80 and 90 Gev, respectively, the W and Z were the most massive particles seen at the time of discovery while the photon is massless. The difference in masses is attributed to spontaneous symmetry breaking as the hot universe cooled. The theory suggests that at very high temperatures where the equilibrium kT energies are in excess of 100 GeV, these particles are essentially identical and the weak and electromagnetic interactions were manifestations of a single force. Theories of the early 1960s suggested that all these exchange particles should be massless. In 1964 theoretical efforts by Robert Brout and Francois Englert in Brussels and Peter Higgs at the University of Edinburgh developed a mechanism whereby mass could be given to elementary particles while maintaining the structure of their original interactions. The Brout-Englert-Higgs (BEH) mechanism employed the properties of a field (the Higgs field) to break the symmetry, but it predicted another massive particle, the Higgs boson. A comment in the Atlas bulletin: "The Higgs boson would become the most sought-after particle in all of particle physics."

The next step is the inclusion of the strong interaction in what is called grand unification.

Spontaneous Symmetry Breaking

The weak and electromagnetic fundamental forces seem very different in the present relatively low temperature universe. But when the universe was much hotter so that the equilibrium thermal energy was on the order of 100 GeV, these forces may have appeared to be essentially identical - part of the same unified "electroweak" force. But since the exchange particle for the electromagnetic part is the massless photon and the exchange particles for the weak interaction are the massive W and Z particles, the symmetry was spontaneously broken when the available energy dropped below about 80 GeV and the weak and electromagnetic forces take on a distinctly different look. The model is that at an even higher temperature, there was symmetry or unification with the strong interaction, the grand unification. And higher still, the gravity force may join to show the four fundamental forces to be a single unified force. Trefil invokes some interesting analogies to illustrate the concept of spontaneous symmetry breaking.

Symmetry Breaking: Analogies

The concept of spontaneous symmetry breaking is important to the understanding of electroweak unification and further unifications. Trefil invokes some analogies in the realm of classical physics.

The snowflake: Both the hydrogen and oxygen molecules are quite symmetric when they are isolated. The electric force which governs their actions as atoms is also a symmetrically acting force. But when their temperature is lowered and they form a water molecule, the symmetry of the individual atoms is broken as they form a molecule with 105 degrees between the hydrogen-oxygen bonds. When they freeze to form a snowflake, they form another type of symmetry, but the symmetry of the original atoms has been lost. Since this loss of symmetry occurs without any external intervention, we say that it has undergone spontaneous symmetry breaking.

Magnetic Symmetry Breaking

A magnet can be used as an analogy to illustrate the concept of spontaneous symmetry breaking which is important to the understanding of electroweak unification and further unifications. When the magnet is strongly magnetized in one direction, it would be hard to guess that the underlying interaction is actually symmetric under rotation. The magnetic field from the magnet is certainly very different if it is rotated 90 degrees, or 180 degrees. The underlying symmetry can only be seen if the energy of the system is raised - heating the magnet to its Curie temperature would remove the directional magnetic field and restore the rotational symmetry of the material. This is an apt analogy for the electroweak unification, since the symmetry between the Coulomb force and the weak interaction is certainly not evident at low temperatures. Only at high enough temperatures so that the available energies are in excess of the mass energies of the W and Z exchange particles for the weak interaction, or in the neighborhood of 100GeV do the weak and electromagnetic forces appear to be unified.

Grand Unification

Grand unification refers to unifying the strong interaction with the unified electroweak interaction. The basic problem of "restoring the broken symmetry" between the strong and electroweak forces is that the strong force works only on colored particles and the leptons don't have color. You have to be able to convert quarks to leptons and vice versa. But this violates the conservation of baryon number, which is a strong experimental nuclear physics principle. Baryon number minus lepton number (B-L) would still be conserved as a quark is changed to an anti-lepton. The required mass of the exchange boson is 10¹⁵ eV, which is more like the mass of a visible dust particle than that of a nuclear entity. This particle is called the X-boson.

One prediction of the grand unified theories is that the proton is unstable at some level.

In the 1970's, Sheldon Glashow and Howard Georgi proposed the grand unification of the strong, weak, and electromagnetic forces at energies above 10¹⁴ GeV. If the ordinary concept of thermal energy applied at such times, it would require a temperature of 10²⁷ K for the average particle energy to be 10¹⁴ GeV.

The unification of the strong force is well beyond our reach at the present time, and the unification of gravity with the other three is out of reach for earthbound experiments. This has led to greater cooperation between high-energy particle physicists and astrophysicists as each group realizes that some of their answers can only come from the other.