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ScienceWeek
THEORETICAL PHYSICS: ON THE STRONG FORCE
The following points are made by Ian Shipsey (Nature 2004 427:591):
1) The fundamental particles called quarks exist in atom-like bound states, such as those constituting protons and neutrons, the quarks held together by the strong force. The heavier varieties of quark, such as the bottom quark, can disintegrate to produce other, lighter particles, and the pattern of the decay rates is constrained, but not determined, in the theory of fundamental particles, the so-called "standard model". That pattern, especially the part involving the bottom quark, is sensitive to new physical phenomena. But although accurate measurements of the rates have been made, the window on new physics has been obscured. This is because the binding effect of the strong force between quarks modifies the decay rates: unless correction factors can be accurately worked out, the data cannot be fully interpreted for signs of any physics that is as yet unknown. This has been the case for almost 40 years.
2) The standard model describes all observed particles and their interactions. Particles interact by exchanging other particles that convey force. For example, in an atom, electrons bind to protons by swapping photons. This is the electromagnetic force, described by the theory of quantum electrodynamics (QED). In a proton, two types of quark, called "up" and "down", are bound together so tightly, by exchanging particles called gluons, that this is known as the "strong force". Its associated theory is quantum chromodynamics, or QCD. In the standard model there is a third force, the "weak force", which is the mediator of radioactive beta-decay. Another example of the weak force in action is the decay of a heavy bottom quark into an up quark, through the emission of a W particle (which then itself decays to an electron and an anti-neutrino).
3) Despite its success, the standard model leaves many questions unanswered. For example, although the observable Universe is made of matter and there is no evidence for significant quantities of antimatter, equal amounts of both should have been created in the Big Bang. When matter and antimatter meet, they annihilate each other: if a small asymmetry between matter and antimatter did not exist at the time of the Big Bang, there would be no matter in the Universe today. So how did that asymmetry arise?
4) If heavy particles that existed in the early Universe decayed preferentially into matter over antimatter, that could have created the matter excess. In the standard model, two types of quark, bottom and strange, do decay asymmetrically. But this effect alone is far too small to account for the asymmetry. However, there are many theories that predict the existence of other, massive particles that could readily produce the asymmetry. And because of the connection between asymmetry and mass, these theories also address other puzzles, such as why electrons are almost 10,000 times lighter than bottom quarks.
Nature http://www.nature.com/nature
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ON QUANTUM CHROMODYNAMICS
According to the Standard Model of physics, the fundamental forces comprise the gravitational force, the electromagnetic force, the nuclear strong force, and the nuclear weak force. The strong force is approximately 100 times stronger than the electromagnetic interaction and in general is the force responsible for the stability of the atomic nucleus.
Quantum field theory is the mathematical fusion of quantum mechanics with special relativity theory, and there are essentially 2 branches: quantum electrodynamics (applicable to charged particles involved in electromagnetic interactions) and quantum chromodynamics (applicable to nuclear particles involved in strong force interactions).
A quark is a hypothetical fundamental particle, having charges whose magnitudes are one-third or two-thirds of the electron charge, and from which the elementary particles may in theory be constructed. Quantum chromodynamics (QCD) is a theory that describes the strong interaction in terms of quarks and antiquarks and the exchange of massless "gluons" between them. The "chromo-" in chromodynamics derives from the use of designated "color" attributes of quarks, the various "colors" labels for various quark properties.
The equations of quantum chromodynamics are difficult to solve, but improvements in computing power and computing techniques have made it possible to demonstrate that the theory has considerable success in predicting the outcome of experiments at high energy particle accelerators.
The following points are made by Frank Wilczek (Physics Today August 2000):
1) Whereas in quantum electrodynamics (QED) there is just one kind of charge (electric charge), quantum chromodynamics (QCD) has 3 different kinds of charge labeled by "color". The QCD color names are arbitrary: the color charges of QCD have nothing to do with physical colors. Rather, the color charges have properties analogous to electric charge. In particular, the color charges are conserved in all physical processes, and there are photon-like massless particles, called "color gluons", that respond in appropriate ways to the presence or motion of color charge, very similar to the way photons respond to electric charge.
2) The quarks are a class of particles that carry color charge, and there are 6 different kinds ("flavors") of quarks: up, down, strange, charmed, bottom, top. Of these, only up and down quarks play a significant role in the structure of ordinary matter. The other, much heavier quarks, are all unstable. A quark of any one of the 6 flavors can also carry a unit of any of the 3 color charges. Although the different quark flavors all have different masses, the theory is perfectly symmetrical with respect to the three colors.
3) Despite their similarities, there are crucial differences between quantum chromodynamics and quantum electrodynamics:
a) The response of gluons to color charge is much more vigorous than the response of photons to electric charge.
b) In addition to just responding to color charge, gluons can also change one color charge into another. All possible changes of this kind are allowed, and yet color charge is conserved. So the gluons themselves can carry unbalanced color charges. For example, if absorption of a gluon changes a blue quark into a red quark, then the gluon itself must have carried one unit of red charge and minus one unit of blue charge.
c) The 3rd difference between QCD and QED is the most profound and follows from the 2nd: Because gluons respond to the presence and motion of color charge, and also carry unbalanced color charge, it follows that gluons, quite unlike photons, respond directly to one another. Photons, in contrast, are electrically neutral.
4) At first sight, it is difficult to accept that the equations of quantum chromodynamics can describe the real world of the strongly interacting particles. None of the particles that have actually been seen appear in QCD, and none of the particles that appear in QCD have every been observed. In particular, we have never seen particles that carry fractional electric charge, which we nonetheless ascribe to the quarks. And certainly we have not seen anything like gluons -- massless particles mediating long-range strong forces. So if quantum chromodynamics is to describe the world, it must explain why quarks and gluons cannot exist as isolated particles. That is the so-called "confinement problem".
Physics Today http://www.physicstoday.org
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PARTICLE PHYSICS: ON GLUONS AND GLUEBALLS
The physics of elementary particles is in the midst of an era of conceptual complexity that some people find unsettling and other people find invigorating. A quark is a hypothetical fundamental particle, having charges whose magnitudes are one-third or two-thirds of the electron charge, and from which the elementary particles that have an apparent internal structure may in theory be constructed. Quarks are believed to be held together through the exchange of gluons, massless particles that carry the *strong force. At the present time, 18 different quarks with various properties are thought to exist, with a corresponding number of *antiquarks. Gluons have a "sticking" property -- they can agglomerate -- and agglomerations of gluons alone are called "glueballs".
The following points are made by F.E. Close and P.R. Page (Scientific American November 1998):
1) Along with fractional electric charge, quarks also have "flavor" in 6 varieties (up, down, charm, strange, top, and bottom), and "color" (red, yellow, or blue). [*Note #1]
2) Quarks may also attach to antiquarks, particles that have opposite charge, and an antiquark comes in anticolors (antired, antiyellow, antiblue). An anticolor is mathematically denoted by negative color, and a color and its anticolor attract.
3) The theory of electromagnetism describes the attraction between opposite electric charges. In the 1940s, physicists merged electromagnetism with relativity and quantum theory, creating quantum electrodynamics (QED). This theory -- the most successful theory known to physics -- holds that the electromagnetic force is transmitted by massless objects called photons. These quanta of light banish the classical idea of action at a distance. It can be said that photons bounce between an electron and an antiparticle (the positron) in such a manner as to draw the two together.
4) The equivalent theory of color charges, which communicate via the *strong force, is called quantum chromodynamics (QCD). Gluons, the massless quanta of the strong force, transmit the color interactions.
5) Gluons are fundamentally different from photons. Photons do not have charge, so one photon cannot push or pull on another photon. Gluons, however, are themselves colored. A red quark, for example, can turn into a blue quark by radiating a red/antiblue gluon. Basically, a gluon can attract another gluon. Another difference between photons and gluons is that while photons uniformly surround electrons, forming a shell with spherical symmetry whose density falls off with distance, gluons are not uniformly distributed and instead clump together into a tube linking a quark and an antiquark. The color originating in the quark can be thought to "flow" through the tube to the antiquark, where it becomes absorbed.
6) In 1972 H. Fritzch and M. Gell-Mann predicted that two or more gluons can combine into a strongly bound, neutral-colored particle of pure "glue". This hypothetical object is called a "glueball". A glueball is thought to have a radius of 0.5 x 10^(-15) meters (less than that of a proton), and exist for less time than light takes to cross a hydrogen atom.
7) The authors state that although the idea of glueballs was elegant, quantum chromodynamics is a "rather messy theory", since the peculiar "sticky" character of the strong force makes it impossible to perform exact calculations. Almost everything known about color and glue comes not from direct calculation but from massive computer simulations known as "lattice QCD". 8)
Finally, the authors discuss various current and planned future attempts to detect the existence of glueballs, and they conclude: "One of these experiments will, we fondly hope, upturn unambiguous evidence of unadulterated glue."
Scientific American http://www.sciam.com
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Notes by ScienceWeek:
strong force: The fundamental forces comprise the gravitational force, the electromagnetic force, the nuclear strong force, and the nuclear weak force.
antiquarks: The antimatter quark entity. In general, antiparticles are homologs of elementary particles but with opposite charge. The positron, for example, is the antimatter particle homologous to the electron. Matter composed entirely of antiparticles is called antimatter.
Note #1: In this context, flavors and colors are labels for specific sets of properties associated with specific types of quarks. Some people call these labels "whimsical", but perhaps there is some sense to the whimsy, since it emphasizes that at the present time we are apparently unable to describe the properties and behaviors of the fundamental particles with classical language (i.e., with the language of old models).
ScienceWeek http://scienceweek.com
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