__Quarks and QCD__
Note: This paper was originally the cooperative effort of several people: Karl Kowallis, Audrey Bohan, Kayaham Jones, and Michle Witz. The paper was eventually completed, printed and turned in. I have been unable to locate a completed electronic copy. This is an intermediate stage edition. The information in this article may not be accurate! If a final version turns up it will replace this copy.
Introduction
There is an ongoing quest in the scientific field to did out what things are made of and why they work together as they do. What are the most fundamental building blocks of everything? Scientists have been getting closer to the answers but whether or not we're there yet can never fully be ascertained. Years ago it was believed that the fundamental building blocks of matter were atoms. In fact the work atom in Greek means indivisible. Since then we have discovered smaller particles inside the atom, namely protons, neutrons and electrons. The search hasn't stopped there--as scientists continue to search for what really are the elementary and fundamental particles, they have some up with theories of things called quarks--smaller building blocks within protons and neutrons, hadrons leptons--combinations of quarks and families of such particles. How these quarks were discovered and how they interact together, specifically the strong force and Quantum Chromodynamics, is the subject of this paper. The purpose of this paper is not to go too deep in theory etc., but to present the information in an understandable and simple way--given some basic knowledge about the parts of the atom and electromagnetic forces.
Introduction:
There is an ongoing quest in the scientific field to find out what things are made of and why they work together as they do. What are the most fundamental building blocks of everything? Scientists have been getting closer to the answers but whether or not we're there yet can never be ascertained. Years ago it was believed that the fundamental building blocks of matter were atoms. In fact the word atom in Greek means indivisible. Since then we have discovered smaller particles inside the atom, namely protons, neutrons and electrons. The search hasn't stopped there--as scientists continue to search for what really are the elementary and fundamental particles, they have come up with theories of particles called quarks--smaller building blocks within protons and neutrons, hadrons leptons--combinations of quarks and families of such particles. How these quarks were discovered and how they interact together--specifically the strong force and Quantum Chromodynamics--is the subject of this paper.
Elementary Particles
There are three basic families of elementary particles--Leptons, Hadrons and mediator particles. Leptons consist of the electron, muon and neutrino--these are the particles that do not participate in the strong force (the strong force will be discussed a little later in this paper). Hadrons--coming from the Greek root "hadrys" meaning strong--are the particles that do participate in the strong force. The last category of particles we will discuss is the mediator particles. These are particles such as photons and gluons that carry forces between the other elementary particles. This will also be elaborated on later.
How they Found The Quark:
One game physicists like to play is the game of grouping and categorizing--thus the three families of elementary particles discussed above. It was through the process of categorizing particles that they came up with the idea that there must be something more fundamental that the proton, neutron etc. Physicists observed that particles have distinct properties that make each particle unique. These same properties were observed in several different particles but in unique combinations so that the properties made the particles unique also drew many similarities or symmetries between particles.
The periodic table of elements is an example of a chart organizing atoms according to the specific properties they have. These elements are associated with certain charges, masses and electron levels. This organization was not explained until the structure of the atom was discovered. Because we know the structure of atoms we understand why atoms group together the way they do and we can predict certain interactions between them. The elementary particles were organized in a similar way--this being the eight fold way. Elementary particles were organized according to some of their specific properties, just like the atoms have an underlying structure that explains why there exists a symmetry between atoms, physicists also believed that there must be a similar structure of elementary particles. Some of the properties the proton and other such particles can have are their baryon number (B), strangeness (S) and isotopic spin (I). Using these properties a certain "order-without-explanation" was found. The eight fold way is the technical term for the process of organizing these particles. If particles are graphed in such a way that you set up the vertical axis with B + S on the vertical axis and Iz (projection of isotopic spin) on the horizontal axis you can graph particles that consist of different combinations of B, S and Iz that all fit the formula
Example of graph (Missing)
These eight particles all satisfy the equation. From the graph we can notice several similarities. The particles in horizontal rows all have the same strangeness. The particles lined up in the diagonals from the upper left to lower left have the same electrical charge.
Murray Gell-Mann and George Zweig both independently came up with an explanation of the eightfold way. They decided that the elementary particle octets could be explained if they were made up of still more fundamental particles. From this they figured out that hadrons must be made up of three constituent particles which we know as quarks. Each of theses quarks have specific characteristic such as spin and electric charge.
This is not the only evidence for quarks. Richard Feynman conducted an experiment much like Rutherford scattering experiment. The idea behind these experiments is that even though you cannot directly "see" the quarks, you can tell a lot about their structure by looking at what happens to projectiles after they hit. The projectiles fired at the protons were scattered at very large angles; the energy of these scattered particles was also so great that new particles were created in the collision. Both the energy of the projectiles and the angles they were scattered are evidence that there is concentrated charges inside the protons--these concentrated charges are the quarks.
The Strong Force/ QCD
Fundamental Forces
Hadrons
Classification
1976, a table of particles, published in the Review of Modern Physics, was 245 pages long and filled the entire journal. (Trefil, p. 115.) The search was then to find some sort of classification that would provide some structure to the chaos. Classifications led to the concept of Hadrons, which feel the strong force, and leptons, which do not. Hadrons themselves are further classified into baryons and mesons.
Before the quark model was introduced, baryons were identified by decay products. Particles which decayed into protons were baryons. (Trefil pp 120.) Once the quark model was introduced it was able to explain baryons as combinations of three quarks. The lightest quarks, the up and down, form the most stable particle group, the protons and neutrons.
Mesons were the first of the unstable particles discovered. The term meson originally was used to identify intermediate particles, between the masses of the proton and the electron. The pion was studied extensively because it was used as a virtual particle to explain the strong force. It was initially postulated in the 1930's but its actual discovery was delayed by the discovery of the muon, a particle of similar mass that was mistaken for a pion, but which is not even a hadron. K- mesons followed and eventually mesons were discovered that were more massive than the proton.
The quark model successfully explains mesons as quark, anti-quark pairs. By the time the quark theory was proposed and accepted the ranks of particles that it proposed were complete. No particle, before 1974, could not be explained by the quark model, nor did the quark model leave any holes for undiscovered particles. (Trefil, pp 151, 171.) In 1974 two independent groups discovered a new particle, the J/ . The new particle was a c, anti-c () meson, where c was the abbreviation for the charmed quark, newly discovered. This led to the possibility of other combinations of particles with charm quarks. Several of these have been discovered, lending support to the quark theory and the baryon, meson classification system.
Groups of particles are easily formed into families. These families have similar masses and are generally characterized by varying charges. For instance, the pions, + -, form a family, as do ++ + -, the delta family.
One of the sources of the names for quarks came from the quantum numbers used to classify the particles that were rapidly being observed. Unfortunately, the names given can often be misleading in expressing an interpretation. The term strangeness was used to identify particles that had an unusually long lifetime. These particles are made of strange quarks. The electric charge, often abbreviated Q, is also a quantum number. Baryon number, and charm are also quantum numbers. Quantum numbers can only be specific values, generally integers. Spin, a quantum number regarding a form of angular momentum, is able to take on half integer values.
The Pauli Exclusion Principal brought up one of the first challenges to face the quark hypothesis. The principal is similar to the idea that no two cars can park in the same parking spot at the same time. In atoms, or any quantum system, no two particles can be in the same state, with all of the same quantum numbers. The double positive delta seems to be an exception to this. The ++ is composed of three u quarks with aligned spins. This could be overcome if the quarks were somehow not identical.
*Insert sentence.* Color was first introduced by O. W. Greenberg at the University of Maryland in 1964, although it was more than a decade before the model was widely accepted. (Trefil p. 184.) The concept was not simply introduced to overcome an unexplainable error; it actually explains quark combinations. This is not the color that we associate with light and the colors of the rainbow. It is a name for a charge that works similar to an electric charge. Electric charge comes in two varieties: positive and negative, whereas color charge associated with quarks comes in three varieties: red, green and blue and three anti colors. Each of the identical quarks that make up the protons and neutrons have a different associated color charge. This makes the quarks in the ++ different. Quantum Chromodynamics, QCD, is a theory that uses color and the quark model to explain subnuclear interaction and the strong force.
Part of the theory includes that only non colored particles can ever by observed. Red, green and blue light can combine to form white, neutral light. A color and its anti color can do the same. If the analogy carried to electromagnetism, we would find no charged particles in nature. This in itself shows how in some respects the forces are vastly different. The attempt to separate quarks will be discussed later on.
Some of the downfall in the use of the terminology is due to the common impressions it gives. The angular momentum, spin, is not directly related to the type of motion as a ball on an axis either. The qualities of strange and charm also mean specific things to physicists which can be lost in an explanation. A. Grutsch and F. Herrmann, German Physicists, suggest that the color explanation has flaws and should be done away with. (P. 274.) They suggest the term strong charge as a replacement. Mathematically, the strong charge and the perceptible color after which it is named, do not behave similarly. {The use of verbal colors is analogous to using the terms male for positive and female for negative in electromagnetism. Enough of each would be neutral but the terms would lead to inaccurate assumptions. (Ibid p. 273.)} {Throw away?} The main concern with the usage is not that the theory is wrong but that the terminology leads to false assumptions.
Gluons
For every type of force there is a force carrier; the carrier of the strong force is the gluon. The gluon is a particle, much like a photon, the force carrier for electromagnetism (light). Both have no mass and travel at the speed of light, but the similarities end there. The photons do not carry a "charge", while gluons carry a factor that is called color. The color of a gluon is responsible for the force in quark-gluon interactions, much like how charge is responsible for the interactions between charged particles.
Yukawa originally explained the attraction between two nucleons as the exchange of virtual pions. QCD goes further in explaining the attraction by describing exactly what happens. "The three quarks in each nucleon are continually emitting and absorbing virtual gluons. When two nucleons approach within 1fm or so, the virtual emissions and absorptions within either nucleon can interact with those in the other. One of the u quarks in the proton emits a virtual gluon, which subsequently emits a d anti-d pair. The u and anti-d both move across to the neutron, where the anti-d is annihilated by one of the neutron's d quarks and the u is captured. The effect is that the proton changes into a neutron and vice-versa." The two quarks exchanged (u and anti-d) are the constituents of a pion, so the QCD explanation of the interaction between the nucleons is basically the same as Yukawa's, but more detailed.
While gluons have not been directly observed in the laboratory, experimental evidence for their existence does exist. Electron-nucleon scattering experiments, which showed the presence of three quarks inside the nucleons, indicate that the total momentum of the three quarks is less than that of the nucleon. It is believed that this missing momentum is carried by the virtual gluons bouncing back and forth among the quarks. Further evidence comes from electron-positron collisions. If gluons did not exist, one would expect the collision to produce two jets of hadrons, yet, the collisions sometimes produce three jets of hadrons. The third jet is generally accepted as proof for the existence of gluons. (However, the third jet is not made of gluons; rather it is a stream of q anti-q pairs created in the wake of the gluon.)
Quark Separation
The strength of the strong charge accounts for quarks only being found in bound states. Many people have observed that a magnet can be suspended in the air above another magnet, or stuck to a fridge, seeming to defy the law of gravity. In either case gravity is still acting on the magnets, but the electromagnetic force is much more powerful. Just as electromagnetic force is more powerful than gravity, the strong force is more powerful than the electric force. When two large magnets of opposite polarity are stuck together the most difficult thing is the first little bit of separation. The further they are separated the easier it becomes. For quarks, separation does not lessen the attraction. This is why quarks are always observed in bound states; because their potential energy is so much less than it would be if they were apart.
The difficulty in observing individual quarks arises from the force of attraction as quarks are separated. Quarks are said to be confined. Separating quarks may be thought of as pushing, requiring energy to be added to the system. As the separation and therefore the energy in the system are increased, Einstein's relation, E=mc2, becomes important. It is easier for the energy added to the system to go towards the creation of new particles rather than the increasing separation. As the quarks are separated new complementary quarks are formed such that baryons or mesons can be formed. The system is like a spring. Separation increases the attraction but so does compression. When in the size constraints of the nuclear dimensions quarks behave much like free particles, like marbles in a coffee can. They bump away from each other but also from the edge of the confinement dimensions.
It may be wondered why we would care about particles that cannot be seen, even in principal. Science has a dislike of theories that are unverifiable. The quark picture has explained observations. Although the quarks are tightly bound they are not so tight that particles cannot be fired between them. A 1969 joint experiment between the Massachusetts Institute of Technology and the Stanford Linear Accelerator Center fired electrons at a proton target. The measured scattering indicated individual parts for the proton, an experiment mentioned that lead to the quark hypothesis. (Trefil pp 157-9).
Knowing that QCD describes the strong attraction of quarks, what about the interaction of baryons, in a nucleus for instance. Because of the overall neutrality of the color charge in a baryon, when they are largely separated they experience almost no influence due to the strong force. Only when baryons are within close proximity to each other is there a significant attraction. Nucleons in a nucleus are therefore bound only to their neighbors and not to the entire nucleus. As the nucleus becomes larger it becomes more prone to split because of the isolated nature of the strong attraction. QCD therefore accounts for most of the observed interactions and behaviors in the world of high energy particle physics.
What's Left To Find?
While it might seem as if all of the research on quarks and the strong force has already been completed, this area is one of the most active segments of modern physics. The recent discovery of the top quark was the result of years of work by many physicists. A standard model has been proposed based upon the six leptons and six quarks known to exist. While this theory does appear to be well established, many problems remain, including the inability of the standard model to account for the "lightness" of the electron in comparison to the other massive particles. Many physicists are proposing grand unified theories (GUTs), theories that incorporate both the electroweak (electromagnetic and weak interactions) and strong interactions. However, essential to these GUT's is that we establish whether the proton is unstable and whether or not magnetic monopoles exist. Other physicists speculate that the leptons and quarks may be constructed of other, smaller, truly elementary particles. At the other end of the scale, astro-physics and cosmology are also currently involved with particle physics. They are currently trying to solve two dilemmas: first, to explain why the universe apparently consists primarily of matter, rather than having equal amounts of matter and anti-matter, and second, to explain why the mass of the universe appears to be considerably greater than the mass of the visible stars and interstellar gases. Finally, particle physicists are trying to learn more about the creation of the universe by using particle accelerators with higher and higher energies to try to approximate the conditions present at the beginning of the universe.
Copyright 1996
Last updated 9 Nov 1996
