The Higgs Boson: Searching for the God Particle Read Online Free Page A
Author: Scientific American Editors
creating quark-gluon plasma in the laboratory through collisions of heavy nuclei at very high energy.
Electroweak Symmetry
Understanding of the third interaction that elementary-particle physics must reckon with, the weak interaction, also has advanced by analogy with QED. In 1933 Enrico Fermi constructed the first mathematical description of the weak interaction, as manifested in beta radioactivity, by direct analogy with QED. Subsequent work revealed several important differences between the weak and the electromagnetic interactions. The weak force acts only over distances of less than 10 -16 centimeter (in contrast to the long range of electromagnetism), and it is intimately associated with the spin of the interacting particles.
Only particles with a left-handed spin are affected by weak interactions in which electric charge is changed, as in the beta decay of a neutron, whereas right-handed ones are unaffected.
In spite of these distinctions theorists extended the analogy and proposed that the weak interaction, like electromagnetism, is carried by a force particle, which came to be known as the intermediate boson, also called the W (for weak) particle. In order to mediate decays in which charge is changed, the W boson would need to carry electric charge. The range of a force is inversely proportional to the mass of the particle that transmits it;
because the photon is massless, the electromagnetic interaction can act over infinite distances. The very short range of the weak force suggests an extremely massive boson.
A number of apparent connections between electromagnetism and the weak interaction, including the fact that the mediating particle of weak interactions is electrically charged, encouraged some workers to propose a synthesis. One immediate result of the proposal that the two interactions are only different manifestations of a single underlying phenomenon was an estimate for the mass of the W boson.
The proposed unification implied that at very short distances and therefore at very high energies the weak force is equal to the electromagnetic force.
Its apparent weakness in experiments done at lower energies merely reflects its short range. Therefore the whole of the difference in the apparent strengths of the two interactions must be due to the mass of the W boson. Under that assumption the W boson's mass can be estimated at about 100 times the mass of the proton.
To advance from the notion of a synthesis to a viable theory unifying the weak and the electromagnetic interactions has required half a century of experiments and theoretical insight, culminating in the work for which Sheldon Lee Glashow and Steven Weinberg, then at Harvard University, and Abdus Salam of the Imperial College of Science and Technology in London and the International Center for Theoretical Physics in Trieste won the 1979 Nobel prize in physics. Like QED itself, the unified, or electroweak, theory is a gauge theory derived from a symmetry principle, one that is manifested in the family groupings of quarks and leptons.
Not one but three intermediate bosons, along with the photon, serve as force particles in electro weak theory.
They are the positively charged W+ and negatively charged W- bosons, which respectively mediate the exchange of positive and negative charge in weak interactions, and the Z 0 particle, which mediates a class of weak interactions known as neutral current processes. Neutral current processes such as the elastic scattering of a neutrino from a proton, a weak interaction in which no charge is exchanged, were predicted by the electroweak theory and first observed at CERN in 1973 . They represent a further point of convergence between electromagnetism and the weak interaction in that electromagnetic interactions do not change the charge of participating particles either.
To account for the fact that the electromagnetic and weak interactions, although they are intimately related, take different guises, the
Electroweak Symmetry
Understanding of the third interaction that elementary-particle physics must reckon with, the weak interaction, also has advanced by analogy with QED. In 1933 Enrico Fermi constructed the first mathematical description of the weak interaction, as manifested in beta radioactivity, by direct analogy with QED. Subsequent work revealed several important differences between the weak and the electromagnetic interactions. The weak force acts only over distances of less than 10 -16 centimeter (in contrast to the long range of electromagnetism), and it is intimately associated with the spin of the interacting particles.
Only particles with a left-handed spin are affected by weak interactions in which electric charge is changed, as in the beta decay of a neutron, whereas right-handed ones are unaffected.
In spite of these distinctions theorists extended the analogy and proposed that the weak interaction, like electromagnetism, is carried by a force particle, which came to be known as the intermediate boson, also called the W (for weak) particle. In order to mediate decays in which charge is changed, the W boson would need to carry electric charge. The range of a force is inversely proportional to the mass of the particle that transmits it;
because the photon is massless, the electromagnetic interaction can act over infinite distances. The very short range of the weak force suggests an extremely massive boson.
A number of apparent connections between electromagnetism and the weak interaction, including the fact that the mediating particle of weak interactions is electrically charged, encouraged some workers to propose a synthesis. One immediate result of the proposal that the two interactions are only different manifestations of a single underlying phenomenon was an estimate for the mass of the W boson.
The proposed unification implied that at very short distances and therefore at very high energies the weak force is equal to the electromagnetic force.
Its apparent weakness in experiments done at lower energies merely reflects its short range. Therefore the whole of the difference in the apparent strengths of the two interactions must be due to the mass of the W boson. Under that assumption the W boson's mass can be estimated at about 100 times the mass of the proton.
To advance from the notion of a synthesis to a viable theory unifying the weak and the electromagnetic interactions has required half a century of experiments and theoretical insight, culminating in the work for which Sheldon Lee Glashow and Steven Weinberg, then at Harvard University, and Abdus Salam of the Imperial College of Science and Technology in London and the International Center for Theoretical Physics in Trieste won the 1979 Nobel prize in physics. Like QED itself, the unified, or electroweak, theory is a gauge theory derived from a symmetry principle, one that is manifested in the family groupings of quarks and leptons.
Not one but three intermediate bosons, along with the photon, serve as force particles in electro weak theory.
They are the positively charged W+ and negatively charged W- bosons, which respectively mediate the exchange of positive and negative charge in weak interactions, and the Z 0 particle, which mediates a class of weak interactions known as neutral current processes. Neutral current processes such as the elastic scattering of a neutrino from a proton, a weak interaction in which no charge is exchanged, were predicted by the electroweak theory and first observed at CERN in 1973 . They represent a further point of convergence between electromagnetism and the weak interaction in that electromagnetic interactions do not change the charge of participating particles either.
To account for the fact that the electromagnetic and weak interactions, although they are intimately related, take different guises, the
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