The Higgs Boson: Searching for the God Particle Read Online Free Page B
Author: Scientific American Editors
electroweak theory holds that the symmetry uniting them is apparent only at high energies.
At lower energies it is concealed. An analogy can be drawn to the magnetic behavior of iron. When iron is warm, its molecules, which can be regarded as a set of infinitesimal magnets, are in hectic thermal motion and therefore randomly oriented. Viewed in the large the magnetic behavior of the iron is the same from all directions, reflecting the rotational symmetry of the laws of electromagnetism. When the iron cools below a critical temperature, however, its molecules line up in an arbitrary direction, leaving the metal magnetized along one axis. The symmetry of the underlying laws is now concealed.
The principal actor in the breaking of the symmetry that unites electromagnetism and the weak interaction at high energies is a postulated particle called the Higgs boson. It is through interactions with the Higgs boson that the symmetry-hiding masses of the intermediate bosons are generated. The Higgs boson is also held to be responsible for the fact that quarks and leptons within the same family have different masses. At very high energies all quarks and leptons are thought to be massless; at lower energies interactions with the Higgs particle confer on the quarks and leptons their varying masses. Because the Higgs boson is elusive and may be far more massive than the intermediate bosons themselves, experimental energies much higher than those of current accelerators probably will be needed to produce it.
The three intermediate bosons required by the electro weak theory, however, have been observed. Energies high enough to produce such massive particles are best obtained in head-on collisions of protons and antiprotons.
In one out of about five million collisions a quark from the proton and an antiquark from the antiproton fuse, yielding an intermediate boson.
The boson disintegrates less than 10 -24 second after its formation. Its brief existence, however, can be detected from its decay products.
In a triumph of accelerator art, experimental technique and theoretical reasoning, international teams at CERN led by Carlo Rubbia of Harvard and Pierre Darriulat devised experiments that in 1983 detected the W bosons and the Z 0 particle. An elaborate detector identified and recorded in the debris of violent proton-antiproton collisions single electrons whose trajectory matched the one expected in a W- particle's decay; the detector also recorded electrons and positrons traveling in precisely opposite directions, unmistakable evidence of the Z 0 particle.
For their part in the experiments and in the design and construction of the proton-antiproton collider and the detector Rubbia and Simon van der Meer of CERN were awarded the 1984 Nobel prize in physics.
Unification
----
KINSHIP OF ALL MATTER is implied by unified theories of the fundamental forces; one branch of a unified family of elementary particles is shown here. Particles that are equivalent within a theory can metamorphose into one another. Because leptons, such as the electron and the neutrino, respond to the electroweak force alone whereas quarks also respond to the strong force, the two kinds of particle are not equivalent in current theory, and transformations of one into the other have not been observed (left) . If the simplest unified theories are correct and the fundamental forces are ultimately identical, then at some very high energy quarks and leptons are interconvertible (right) . Known transformations are mediated by force particles such as the W bosons and the glouns; transitions between the quark and lepton groups would be mediated by new force particles, here given as X and Y .
Illustration by Andrew Christie
----
With QCD and the electroweak theory in hand, what remains to be understood?
If both theories are correct, can they also be complete? Many observations are explained only in part, if at all, by the separate theories of the strong and the electroweak
At lower energies it is concealed. An analogy can be drawn to the magnetic behavior of iron. When iron is warm, its molecules, which can be regarded as a set of infinitesimal magnets, are in hectic thermal motion and therefore randomly oriented. Viewed in the large the magnetic behavior of the iron is the same from all directions, reflecting the rotational symmetry of the laws of electromagnetism. When the iron cools below a critical temperature, however, its molecules line up in an arbitrary direction, leaving the metal magnetized along one axis. The symmetry of the underlying laws is now concealed.
The principal actor in the breaking of the symmetry that unites electromagnetism and the weak interaction at high energies is a postulated particle called the Higgs boson. It is through interactions with the Higgs boson that the symmetry-hiding masses of the intermediate bosons are generated. The Higgs boson is also held to be responsible for the fact that quarks and leptons within the same family have different masses. At very high energies all quarks and leptons are thought to be massless; at lower energies interactions with the Higgs particle confer on the quarks and leptons their varying masses. Because the Higgs boson is elusive and may be far more massive than the intermediate bosons themselves, experimental energies much higher than those of current accelerators probably will be needed to produce it.
The three intermediate bosons required by the electro weak theory, however, have been observed. Energies high enough to produce such massive particles are best obtained in head-on collisions of protons and antiprotons.
In one out of about five million collisions a quark from the proton and an antiquark from the antiproton fuse, yielding an intermediate boson.
The boson disintegrates less than 10 -24 second after its formation. Its brief existence, however, can be detected from its decay products.
In a triumph of accelerator art, experimental technique and theoretical reasoning, international teams at CERN led by Carlo Rubbia of Harvard and Pierre Darriulat devised experiments that in 1983 detected the W bosons and the Z 0 particle. An elaborate detector identified and recorded in the debris of violent proton-antiproton collisions single electrons whose trajectory matched the one expected in a W- particle's decay; the detector also recorded electrons and positrons traveling in precisely opposite directions, unmistakable evidence of the Z 0 particle.
For their part in the experiments and in the design and construction of the proton-antiproton collider and the detector Rubbia and Simon van der Meer of CERN were awarded the 1984 Nobel prize in physics.
Unification
----
KINSHIP OF ALL MATTER is implied by unified theories of the fundamental forces; one branch of a unified family of elementary particles is shown here. Particles that are equivalent within a theory can metamorphose into one another. Because leptons, such as the electron and the neutrino, respond to the electroweak force alone whereas quarks also respond to the strong force, the two kinds of particle are not equivalent in current theory, and transformations of one into the other have not been observed (left) . If the simplest unified theories are correct and the fundamental forces are ultimately identical, then at some very high energy quarks and leptons are interconvertible (right) . Known transformations are mediated by force particles such as the W bosons and the glouns; transitions between the quark and lepton groups would be mediated by new force particles, here given as X and Y .
Illustration by Andrew Christie
----
With QCD and the electroweak theory in hand, what remains to be understood?
If both theories are correct, can they also be complete? Many observations are explained only in part, if at all, by the separate theories of the strong and the electroweak
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