The Higgs Boson: Searching for the God Particle Read Online Free
Author: Scientific American Editors
mediates a different transformation.
The mediating gluon is itself colored, bearing both a color and an anticolor.
The fact that the gluons are colorcharged, in contrast to the electrically neutral photons of QED, accounts for the differing behaviors over distance of the electromagnetic and strong interactions.
In QCD two competing effects govern the effective charge:
screening, analogous to the screening of QED, and a new effect known as camouflage. The screening, or vacuum polarization, resembles that in electromagnetic interactions. The vacuum of QCD is populated by pairs of virtual quarks and antiquarks, winking into and out of existence. If a quark is introduced into the vacuum, virtual particles bearing contrasting color charges will be attracted to the quark; those bearing a like charge will be repelled.
Hence the quark's color charge will be hidden within a cloud of unlike colors, which serves to reduce the effective charge of the quark at greater distances.
Camouflage
Within this polarized vacuum, however, the quark itself continuously emits and reabsorbs gluons, thereby changing its color. The color-charged gluons propagate to appreciable distances.
In effect they spread the color charge throughout space, thus camouflaging the quark that is the source of the charge. The smaller an arbitrary region of space centered on the quark is, the smaller will be the proportion of the quark's color charge contained in it. Thus the color charge felt by a quark of another color will diminish as it approaches the first quark. Only at a large distance will the full magnitude of the color charge be apparent.
In QCD the behavior of the strong force represents the net effect of screening and camouflage. The equations of QCD yield a behavior that is consistent with the observed paradox of quarks: they are both permanently confined and asymptotically free. The strong interaction is calculated to become extraordinarily strong at appreciable distances, resulting in quark confinement, but to weaken and free quarks at very close range.
In the regime of short distances that is probed in high-energy collisions, strong interactions are so enfeebled that they can be described using the methods developed in the context of QED for the much weaker electromagnetic interaction. Hence some of the same precision that characterizes QED can be imparted to QCD. The evolution of jets of hadrons from a quark and an antiquark generated in electron-positron anhthilation, for example, is a strong interaction. QCD predicts that if the energy of the collision is high enough, the quark and the antiquark moving off in opposite d irections may generate not two but three jets of hadrons. One of the particles will radiate a gluon, moving in a third direction. It will also evolve into hadrons, giving rise to a third distinct jet–a feature that indeed is common'
ly seen in high-energy collisions.
The three jets continue along paths set by quarks and gluons moving within an extremely confined space, less than 10 -13 centimeter. The quark-antiquark pair cannot proceed as isolated particles beyond that distance, the limit of asymptotic freedom. Yet the confinement of quarks and of their interactions is not absolute. Although a hadron as a whole is color-neutral, its quarks do respond to the individual color charges of quarks in neighboring hadrons. The interaction, feeble compared with the color forces within hadrons, generates the binding force that holds the protons and neutrons together in nuclei.
Moreover, it seems likely that when hadronic matter is compressed and heated to extreme temperatures, the hadrons lose their individual identities.
The hadronic bubbles of the image used above overlap and merge, possibly freeing their constituent quarks and gluons to migrate over great distances.
The resulting state of matter, called q uark-gluon plasma, may exist in the cores of collapsing supernovas and in neutron stars. Workers are now studying the possibility of
The mediating gluon is itself colored, bearing both a color and an anticolor.
The fact that the gluons are colorcharged, in contrast to the electrically neutral photons of QED, accounts for the differing behaviors over distance of the electromagnetic and strong interactions.
In QCD two competing effects govern the effective charge:
screening, analogous to the screening of QED, and a new effect known as camouflage. The screening, or vacuum polarization, resembles that in electromagnetic interactions. The vacuum of QCD is populated by pairs of virtual quarks and antiquarks, winking into and out of existence. If a quark is introduced into the vacuum, virtual particles bearing contrasting color charges will be attracted to the quark; those bearing a like charge will be repelled.
Hence the quark's color charge will be hidden within a cloud of unlike colors, which serves to reduce the effective charge of the quark at greater distances.
Camouflage
Within this polarized vacuum, however, the quark itself continuously emits and reabsorbs gluons, thereby changing its color. The color-charged gluons propagate to appreciable distances.
In effect they spread the color charge throughout space, thus camouflaging the quark that is the source of the charge. The smaller an arbitrary region of space centered on the quark is, the smaller will be the proportion of the quark's color charge contained in it. Thus the color charge felt by a quark of another color will diminish as it approaches the first quark. Only at a large distance will the full magnitude of the color charge be apparent.
In QCD the behavior of the strong force represents the net effect of screening and camouflage. The equations of QCD yield a behavior that is consistent with the observed paradox of quarks: they are both permanently confined and asymptotically free. The strong interaction is calculated to become extraordinarily strong at appreciable distances, resulting in quark confinement, but to weaken and free quarks at very close range.
In the regime of short distances that is probed in high-energy collisions, strong interactions are so enfeebled that they can be described using the methods developed in the context of QED for the much weaker electromagnetic interaction. Hence some of the same precision that characterizes QED can be imparted to QCD. The evolution of jets of hadrons from a quark and an antiquark generated in electron-positron anhthilation, for example, is a strong interaction. QCD predicts that if the energy of the collision is high enough, the quark and the antiquark moving off in opposite d irections may generate not two but three jets of hadrons. One of the particles will radiate a gluon, moving in a third direction. It will also evolve into hadrons, giving rise to a third distinct jet–a feature that indeed is common'
ly seen in high-energy collisions.
The three jets continue along paths set by quarks and gluons moving within an extremely confined space, less than 10 -13 centimeter. The quark-antiquark pair cannot proceed as isolated particles beyond that distance, the limit of asymptotic freedom. Yet the confinement of quarks and of their interactions is not absolute. Although a hadron as a whole is color-neutral, its quarks do respond to the individual color charges of quarks in neighboring hadrons. The interaction, feeble compared with the color forces within hadrons, generates the binding force that holds the protons and neutrons together in nuclei.
Moreover, it seems likely that when hadronic matter is compressed and heated to extreme temperatures, the hadrons lose their individual identities.
The hadronic bubbles of the image used above overlap and merge, possibly freeing their constituent quarks and gluons to migrate over great distances.
The resulting state of matter, called q uark-gluon plasma, may exist in the cores of collapsing supernovas and in neutron stars. Workers are now studying the possibility of
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