Electroweak Force
Electroweak force is the name given to the unified electromagnetic and weak interactions. The two interactions are viewed as different combinations of one underlying force, hence the term unified. The electroweak force acts differently between particles depending on the direction of their spins. Along with the familiar photon, three more particles, called the Z, W+ , and W- , mediate the electroweak force.
Electromagnetic interactions involve electric charge and electric currents, as well as electric and magnetic fields. Weak interactions are mainly involved in the decay of unstable particles, such as nuclear beta decay or muon decay. At first glance, weak interactions may seem different and unrelated. However, in much the same way that electricity and magnetism were found to be differenct aspects of the same interaction by James Clerk Maxwell, a correct description of the weak force cannot be done without also including the electromagnetic force in the theory.
Progress on finding the correct theory of the weak force was slow. In the 1950s, quantum electrodynamics (QED) was spectacularly successful and was verified by many experiments. A generalization of QED, called non-Abelian gauge theory, was discovered in the 1950s, and many scientists thought it would easily find use as the theory of the weak force. There were complications involved, however, that were yet to be discovered.
Probably the largest leap in knowledge of the weak force came when it was realized that it violated parity. Parity is a discrete space-time symmetry; under a parity transformation all position vectors are reversed. QED was invariant under parity, and it was believed at the time that all theories must also be invariant under parity. But decays of two strange particles (theta and tau) with identical masses and lifetimes into different final states convinced physicists that the weak force must violate parity.
The consequence of parity violation is that particles of different spin interact differently. A common definition of spin in particle physics is to say that a particle is left-handed or right-handed. A left-handed particle has spin pointing opposite to its momentum, and a right-handed particle has spin pointing along its momentum. Only massless particles can be completely left- or right-handed. Left-handed particles have an internal SU(2) isospin symmetry, which is a mathematical way to say that electrons can be traded for neutrinos, or down quarks for up quarks without changing the appearance of the theory.
As a result, in electroweak interactions right-handed particles can interact only via photons and Z bosons, but left-handed particles can interact through all the electroweak particles. Another result is that there are no right-handed neutrinos, because their existence would cause disagreement between the theory and the experimental evidence.
The final, useful form of the electroweak theory was constructed by physicists Sheldon Glashow, Abdus Salam, and Steven Weinberg in the early 1970s, for which they eventually recieved the Nobel Prize. This theory contained one extra ingredient that overcame an important difficulty. As stated above, only massless particles can be completely left- or right-handed, but the electroweak force makes a distinction between left and right, so particles cannot be mixtures of left- and right-handed states. Also, the W and Z bosons were not discovered experimentally until long after the theory was finished, meaning that they also had to be very massive. Explicit mass terms cannot be included into the theory without spoiling its good points. Another solution, therefore, had to be found.
The elegant solution was termed the Higgs mechanism. A new field was postulated, the Higgs field, which interacts with all particles. The quanta of the field are known as Higgs bosons. The Higgs field has a nonzero vacuum expectation value, meaning that even if there are no Higgs bosons around, other particles still interact with the Higgs field. This constant interaction yields an effective mass term for every particle that does not spoil the successes of the electroweak theory.
As of 2000, the Higgs boson is the last undiscovered ingredient of the electroweak theory (the discovery of the tau neutrino was announced in July 2000). There is, however, much indirect evidence that it exists. The success of the theory in explaining the results of scattering experiments, and the agreement of the masses of the W and Z bosons with the theoretical predictions, would be difficult (perhaps impossible) to reproduce without the existence of Higgs bosons.
Together with quantum chromodynamics, the theory of the strong force, the electroweak theory makes up the standard model of particle physics. The success of unifying the electromagnetic and weak interactions has led physicists to attempt to further unify the strong and electroweak forces. So far, their attempts have not been verified, but have led to further features that may be discovered in future experiments.
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