The development of quark theory
- Also called:
- elementary particle
- Related Topics:
- quark
- CP violation
- symmetry
- quantum field theory
- Higgs boson
The beauty of the SU(3) symmetry does not, however, explain why it holds true. Gell-Mann and another American physicist, George Zweig, independently decided in 1964 that the answer to that question lies in the fundamental nature of the hadrons. The most basic subgroup of SU(3) contains only three objects, from which the octets and decuplets can be built. The two theorists made the bold suggestion that the hadrons observed at the time were not simple structures but were instead built from three basic particles. Gell-Mann called these particles quarks—the name that remains in use today.
By the time Gell-Mann and Zweig put forward their ideas, the list of known subatomic particles had grown from the three of 1932—electron, proton, and neutron—to include most of the stable hadrons and a growing number of short-lived resonances, as well as the muon and two types of neutrino. That the seemingly ever-increasing number of hadrons could be understood in terms of only three basic building blocks was remarkable indeed. For this to be possible, however, those building blocks—the quarks—had to have some unusual properties.
These properties were so odd that for a number of years it was not clear whether quarks actually existed or were simply a useful mathematical fiction. For example, quarks must have charges of +2/3e or −1/3e, which should be very easy to spot in certain kinds of detectors; but intensive searches, both in cosmic rays and using particle accelerators, have never revealed any convincing evidence for fractional charge of this kind. By the mid-1970s, however, 10 years after quarks were first proposed, scientists had compiled a mass of evidence that showed that quarks do exist but are locked within the individual hadrons in such a way that they can never escape as single entities.
This evidence resulted from experiments in which beams of electrons, muons, or neutrinos were fired at the protons and neutrons in such target materials as hydrogen (protons only), deuterium, carbon, and aluminum. The incident particles used were all leptons, particles that do not feel the strong binding force and that were known, even then, to be much smaller than the nuclei they were probing. The scattering of the beam particles caused by interactions within the target clearly demonstrated that protons and neutrons are complex structures that contain structureless, pointlike objects, which were named partons because they are parts of the larger particles. The experiments also showed that the partons can indeed have fractional charges of +2/3e or −1/3e and thus confirmed one of the more surprising predictions of the quark model.
Gell-Mann and Zweig required only three quarks to build the particles known in 1964. These quarks are the ones known as up (u), down (d), and strange (s). Since then, experiments have revealed a number of heavy hadrons—both mesons and baryons—which show that there are more than three quarks. Indeed, the SU(3) symmetry is part of a larger mathematical symmetry that incorporates quarks of several “flavours”—the term used to distinguish the different quarks. In addition to the up, down, and strange quarks, there are quarks known as charm (c), bottom (or beauty, b), and top (or truth, t). These quark flavours are all conserved during reactions that occur through the strong force; in other words, charm must be created in association with anticharm, bottom with antibottom, and so on. This implies that the quarks can change from one flavour to another only by way of the weak force, which is responsible for the decay of particles.
The up and down quarks are distinguished mainly by their differing electric charges, while the heavier quarks each carry a unique quantum number related to their flavour. The strange quark has strangeness, S = −1, the charm quark has charm, C = +1, and so on. Thus, three strange quarks together give a particle with an electric charge of −e and a strangeness of −3, just as is required for the omega-minus (Ω−) particle; and the neutral strange particle known as the lambda (Λ) particle contains uds, which gives the correct total charge of 0 and a strangeness of −1. Using this system, the lambda can be viewed as a neutron with one down quark changed to a strange quark; charge and spin remain the same, but the strange quark makes the lambda heavier than the neutron. Thus, the quark model reveals that nature is not arbitrary when it produces particles but is in some sense repeating itself on a more-massive scale.
Colour
The realization in the late 1960s that protons, neutrons, and even Yukawa’s pions are all built from quarks changed the direction of thinking about the nuclear binding force. Although at the level of nuclei Yukawa’s picture remained valid, at the more-minute quark level it could not satisfactorily explain what held the quarks together within the protons and pions or what prevented the quarks from escaping one at a time.
The answer to questions like these seems to lie in the property called colour. Colour was originally introduced to solve a problem raised by the exclusion principle that was formulated by the Austrian physicist Wolfgang Pauli in 1925. This rule does not allow particles with spin 1/2, such as quarks, to occupy the same quantum state. However, the omega-minus particle, for example, contains three quarks of the same flavour, sss, and has spin 3/2, so the quarks must also all be in the same spin state. The omega-minus particle, according to the Pauli exclusion principle, should not exist.
To resolve this paradox, in 1964–65 Oscar Greenberg in the United States and Yoichiro Nambu and colleagues in Japan proposed the existence of a new property with three possible states. In analogy to the three primary colours of light, the new property became known as colour and the three varieties as red, green, and blue.
The three colour states and the three anticolour states (ascribed to antiquarks) are comparable to the two states of electric charge and anticharge (positive and negative), and hadrons are analogous to atoms. Just as atoms contain constituents whose electric charges balance overall to give a neutral atom, hadrons consist of coloured quarks that balance to give a particle with no net colour. Moreover, nuclei can be built from colourless protons and neutrons, rather as molecules form from electrically neutral atoms. Even Yukawa’s pion exchange can be compared to exchange models of chemical bonding.
This analogy between electric charge and colour led to the idea that colour could be the source of the force between quarks, just as electric charge is the source of the electromagnetic force between charged particles. The colour force was seen to be working not between nucleons, as in Yukawa’s theory, but between quarks. In the late 1960s and early 1970s, theorists turned their attention to developing a quantum field theory based on coloured quarks. In such a theory colour would take the role of electric charge in QED.
It was obvious that the field theory for coloured quarks had to be fundamentally different from QED because there are three kinds of colour as opposed to two states of electric charge. To give neutral objects, electric charges combine with an equal number of anticharges, as in atoms where the number of negative electrons equals the number of positive protons. With colour, however, three different charges must add together to give zero. In addition, because SU(3) symmetry (the same type of mathematical symmetry that Gell-Mann and Neʾeman used for three flavours) applies to the three colours, quarks of one colour must be able to transform into another colour. This implies that a quark can emit something—the quantum of the field due to colour—that itself carries colour. And if the field quanta are coloured, then they can interact between themselves, unlike the photons of QED, which are electrically neutral.
Despite these differences, the basic framework for a field theory based on colour already existed by the late 1960s, owing in large part to the work of theorists, particularly Chen Ning Yang and Robert Mills in the United States, who had studied similar theories in the 1950s. The new theory of the strong force was called quantum chromodynamics, or QCD, in analogy to quantum electrodynamics, or QED. In QCD the source of the field is the property of colour, and the field quanta are called gluons. Eight gluons are necessary in all to make the changes between the coloured quarks according to the rules of SU(3).
Asymptotic freedom
In the early 1970s the American physicists David J. Gross and Frank Wilczek (working together) and H. David Politzer (working independently) discovered that the strong force between quarks becomes weaker at smaller distances and that it becomes stronger as the quarks move apart, thus preventing the separation of an individual quark. This is completely unlike the behaviour of the electromagnetic force. The quarks have been compared to prisoners on a chain gang. When they are close together, they can move freely and do not notice the chains binding them. If one quark/prisoner tries to move away, however, the strength of the chains is felt, and escape is prevented. This behaviour has been attributed to the fact that the virtual gluons that flit between the quarks within a hadron are not neutral but carry mixtures of colour and anticolour. The farther away a quark moves, the more gluons appear, each contributing to the net force. When the quarks are close together, they exchange fewer gluons, and the force is weaker. Only at infinitely close distances are quarks free, an effect known as asymptotic freedom. For their discovery of this effect, Gross, Wilczek, and Politzer were awarded the 2004 Nobel Prize for Physics.
The strong coupling between the quarks and gluons makes QCD a difficult theory to study. Mathematical procedures that work in QED cannot be used in QCD. The theory has nevertheless had a number of successes in describing the observed behaviour of particles in experiments, and theorists are confident that it is the correct theory to use for describing the strong force.
