Quarks, Discovery of

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QUARKS, DISCOVERY OF

The twentieth century began with the confirmation that matter was not continuous but made of tiny atoms and molecules. It ended with the confirmation that matter is made, in part, of even tinier objects called quarks.

Atoms consist of nuclei and electrons, and nuclei consist of neutrons and protons. However, in 1950 the proton and neutron were considered to be the final elementary constituents of matter. The pion was the carrier of the strong force that attracted protons and neutrons to form nuclei, just as the photon was the carrier of the electromagnetic force that bound electrons and nuclei into atoms. But by 1962 many new unexpected particles had been discovered. They were first grouped into families called multiplets and described by the Eightfold Way. By 1966 it became clear that none of the new particles could be really elementary. The neutron, proton, and pion were not qualitatively different like the electron and the photon; they and all the new strongly interacting particles called baryons and mesons were built of the same even smaller building blocks now called quarks.

The Eightfold Way itself had been puzzling because it gave no reason why any particular multiplets should be found. Like the Mendeleev table of the chemical elements, it provided a way to classify the so-called "elementary constituents of matter," but their very number suggested that they could not all be elementary.

In 1963 Hayim Goldberg and Yuval Ne'eman pointed out that all the known particles could be constructed mathematically from the same three building blocks, now called the up (u ), down (d ), and strange (s ) quarks, together with their antiparticles, now called antiquarks.

In 1964 Murray Gell-Mann and George Zweig dared to propose that these were indeed the basic building blocks of matter. But a serious difficulty arose. The electron, neutron, proton, and pion were all discovered experimentally as isolated particles that could be detected and created individually and whose paths through space could be determined. However, with current technology, scientists are still not able to create or study individual quarks. But scientists already believed that matter consisted of atoms and molecules long before anyone had created or detected them individually. Perhaps future discoveries will make the creation and detection of quarks possible.

There are no simple answers to the questions who discovered the atom, who discovered the quark, and how the reality of atoms and quarks was established. One possible answer appears in the book by E. D. Hirsch Jr. The Schools We Need and Why We Don't Have Them: "The scientific community reaches conclusions by a pattern of independent convergence (a kind of intellectual triangulation), which is along with accurate prediction, one of the most powerful confidence-building patterns in scientific research. There are few or no examples in the history of science when the same result, reached by three or more truly independent means, has been overturned" (p.159). Hirsch quotes Abraham Pais's biography of Einstein for an example of this convergence:

The debate on molecular reality was settled once and for all because of the extraordinary agreement in the values of N [Avogadro's number] obtained by many different methods. Matters were clinched not by a determination of N but by an overdetermination of N. From subjects as diverse as radioactivity, Brownian motion, and the blue in the sky, it was possible to state, by 1909, that a dozen independent ways of measuring N yielded results in remarkable agreement with one another.

In 1966 this kind of circumstantial evidence already convinced Richard Dalitz that matter was made of quarks, when he gave his invited review at the annual International Conference on High Energy Physics in Berkeley, California. This evidence included the existence of experimentally observed regularities in the properties of particles created at high-energy accelerators, the fact that collisions between different kinds of particles were simply related, the fact that the electromagnetic properties of different mesons and baryons were simply related, the observed experimental ratio of the magnetic moments of the neutron and proton, and the fact that the annihilation of a proton and an antiproton at rest nearly always produced three mesons. These were otherwise unexplained and converged on the same conclusion: mesons and baryons were built from the same elementary building blocks. This independent convergence eventually convinced everyone that all of the many particles described by the Eightfold Way were not the basic building blocks of matter, as had been formerly believed, but were themselves built of even smaller building blocks.

Many particle physicists could not understand why quarks were not generally accepted until well into the 1970s. One problem was that the values of the electric charges of the quarks were smaller than the electric charge of the electron. The u quark has a positive electric charge two-thirds of the value of the electron's charge, and the d and s quarks have negative charges one-third of the electron's charge. So far all known particles have values of electric charge that are integral multiples of the charge of the electron and its antiparticle the positron. Neither fractionally charged particles nor isolated quarks have ever been observed.

Yet more and more circumstantial evidence for the existence of quarks as the building blocks from which all matter is constructed has accumulated since1966. All the particles that are continually being discovered and that fit into the multiplets defined by the Eightfold Way behave as if they are either built from three quarks or from a single quark and a single anti-particle of the quark called an antiquark.

The Search for Evidence for Individual Quarks

Ever since the first quark proposal in 1964, experimenters have searched for particles with electric charges less than the charge of the electron. But none have been found. All the overwhelming evidence for the existence of quarks came from properties of the mesons and baryons that indicated that they were built from quarks.

In the 1970s experiments shooting high-energy electrons at a proton target produced evidence that the electrons were striking and being scattered by single quarks. Here again the evidence was still circumstantial. The quark itself was never observed. But an electron scattered by a pointlike object with an electric charge changes its direction of motion and changes its energy in a well-defined and well-known way. Studying the changes of direction and energy in the electron scattering experiments indicated that the electrons were scattered from pointlike constituents in the proton with the fractional electric charges predicted by the quark model.

These experiments helped to confirm that the peculiar quarks really existed. But they raised two new questions. Although the quarks were hit very hard by the electron, and they absorbed a very high energy and momentum, they were never knocked out of the proton. Isolated free quarks were never observed. This indicated that the quarks were bound by very strong forces inside the proton that kept them confined. But the electron scattering data indicated that the objects scattering the electrons transferred energy and momentum like a free particle, with no evidence of being constrained by any strong forces. These two puzzles have been clarified in the new Standard Model and given the names of confinement and asymptotic freedom.

The forces that bind quarks together into mesons and baryons are so strong at large distances that separating a quark from its neighbors costs a tremendous amount of energy. When a quark in a proton is struck with an energy sufficient to create new particles, a new quark-antiquark pair is created. The created antiquark then combines with the struck quark to create a pion or other meson, and the created quark returns to the other constituents of the original proton. The energy produced by striking a quark in a proton does not drive the quark by itself out of the proton; the quark picks up an antiquark which has been created by the large energy transfer and then goes off as a meson. Thus isolated quarks are never observed as products of high-energy collisions; rather they always find partners created in the collisions and combine with them to form mesons and baryons. They are thus always confined by being bound into mesons or baryons and are never observed as isolated free quarks.

More recent experiments with high-energy collisions show how a struck quark creates quark-anti-quark pairs that recombine in different ways to create a chain of mesons and baryons. The struck quark combines with a created antiquark to form a meson, leaving the quark partner of the antiquark to seek a new created antiquark, etc. This appears in the detector of the experiment as a "jet" of particles going out from the initial proton to the struck or leading quark.

An analog to this jet phenomenon from our everyday experience is lightning. When the electric charge on a cloud becomes sufficiently large, the strong force on the air atoms becomes so great that they break up into positively and negatively charged ions. If the cloud is negatively charged, it attracts the positive ions, leaving the negative ions to search for new partners and create a chain or "jet" through the air that one sees as lightning.

The Standard Model now explains how these strong forces do not disturb the electron scattering experiments that give information about the electric charges of the quarks. The field theory called quantum chromodynamics (QCD) states that although the forces between quarks become very strong at long distances, they become so weak at short distances that they are completely negligible in high-energy electron scattering. This difference between short and long distance behavior is called asymptotic freedom.

The Circumstantial Evidence Supporting the Quark Picture

There is much circumstantial evidence supporting the existence of the quark: the agreement with the experimental values of the electric charge, spin, and magnetic moments of particles with quark model predictions have provided striking evidence.

The electric charges of baryons made from three quarks with electric charge values +⅔ and -⅓ can only be +2, +1, 0, and -1. The electric charges of mesons made from a quark and its charge-conjugate antiquark can only be 1, 0, and -1. Many hundreds of particles are now known, and so far all have only these values for electric charge.

The spinning motion of the particles and their displaying of behavior similar to tiny magnets provided important clues to their structure. A spinning electrically charged top behaves like a magnet. The strength of the tiny magnet of the electron, called its magnetic moment, was successfully described by Paul Dirac's famous theory and equation.

The magnetic moments of the proton and neutron gave the first indication that they were not elementary but had a more complicated structure. The neutron has no electric charge but behaves like a magnet made of spinning negative charge. This suggests that the neutron is not an elementary object with no electric charge but consists of smaller building blocks having both positive and negative charges spinning in opposite directions. The proton magnetic moment is much larger than that described by Dirac's theory.

One of the first successes of the quark model was showing how the right experimental values of particle spins and magnetic moments were obtained by adding up the contributions of the quark spins and magnetic moments in each. A baryon made of three quarks will have a spin three times the spin of the electron or proton if the spins are parallel and will have a spin equal to the electron spin if the spin of one is opposite to the spin of the other two. A meson made of a quark and an antiquark will have a spin equal to twice the electron spin if the spins are parallel and zero spin if they are opposite and cancel. The spins of all measured particles fit this picture.

To obtain the values of the magnetic moments in the proton and neutron, one must first note that the proton consists of two u quarks with parallel spins and one d quark with opposite spin. The u and d quarks have opposite signs of electric charge, their magnets point in the same direction when they are spinning in opposite directions. Each quark magnetic moment is proportional to its electric charge. Thus the two u quarks in the proton with charge +⅔ each contribute +⅔ Dirac units of magnetic moment, while the d quark with charge -⅓ is spinning in the opposite direction and contributes -⅓ Dirac unit. In a crude approximation one adds these to get the proton magnetic moment as +5/3 Dirac units. The neutron has two d quarks with charge -⅓ units and parallel spins each contributing -⅓ units, and one u quark with charge -⅔ and opposite spin contributing -⅔ units to give a neutron magnetic moment of -4/3 Dirac units. This gives -5/4 for the ratio of the proton and neutron magnetic moments. A more accurate calculation using the quantum mechanical adding of spins gives -3/2, which agrees remarkably well with the experimental value of -1.46. The sum of the neutron and proton moments is ⅓ Dirac unit. A reasonable assumption for the value of the quark Dirac unit gives an experimental value of 0.33.

This is typical of the accumulation of circumstantial evidence supporting the belief that quarks are the correct building blocks of matter. First, the electric charges of the neutron and proton and all other particles come out right. Second, the spins and very precise correct values for the magnetic moments of the neutron and proton are explained. All these confirm the picture that particles behave "as if they were made out of quarks." Their electricity, magnetism, and spin would be very hard to understand if they were not built from these building blocks. It would not be clear, for example, why the neutron, which has no electric charge, has a magnetic moment similar to the proton, which has electric charge, or why the neutron also has the opposite sign and the correct ratio to the proton moment predicted by the quark model.

This is only one example of the circumstantial evidence supporting the conclusion that quarks are the basic building blocks of all matter. The Standard Model that guides all theoretical and experimental investigations in particle physics begins with this knowledge, even though isolated individual quarks have never been observed.