Monday, May 13, 2013

History of Science -- Part Twenty-Four: Quarks

Murray Gell-Mann
Physicists Murray Gell-Mann and George Zweig, working separately, each prosed the possibility of the quark in 1964. Its unusual name was chosen by Gell-Mann, who saw the word "quark"  used as a nonsensical term in James Joyce's Finnegans Wake (1939) and liked the sound of it. The line "Three quarks for Muster Mark…" appears in the fanciful book.

Zweig, a 1959 graduate of the University of Michigan, also proposed the existence of quarks while a graduate student in physics at the California Institute of Technology in 1964 (independently of Murray Gell-Mann). Zweig dubbed them "aces" after the four playing cards, because he speculated there were four of them.

Murray Gell-Mann received the 1969 Nobel Prize in physics for his work on the theory of elementary particles. He is a Professor of Theoretical Physics Emeritus at the California Institute of Technology, a Distinguished Fellow and co-founder of the Santa Fe Institute, Professor in the Physics and Astronomy Department of the University of New Mexico, and the Presidential Professor of Physics and Medicine at the University of Southern California.

He introduced the quark constituents of all hadrons, having first identified the SU(3) flavor symmetry of hadrons, now understood to underlie the light quarks, extending isospin to include strangeness, a quantum number which he also discovered.

He developed the V−A theory of the weak interaction in collaboration with Richard Feynman, and he introduced "current algebra" as a method of systematically exploiting symmetries to extract predictions from quark models. This provided starting points underpinning the development of the standard theory of elementary particles.

Gell-Mann was born in Manhattan into a family of Jewish immigrants from the Austro-Hungarian Empire. His parents were Arthur Gell-Mann and Pauline Reichstein. Teaching himself calculus at the age of seven years old, Gell-Mann quickly revealed himself as a child prodigy.

Propelled by an intense curiosity and love for nature and mathematics, he graduated valedictorian from the high school and subsequently entered Yale at the age of 15 where he earned a bachelor's degree in physics in 1948, and a Ph.D. in physics from MIT in 1951.

In 1958, Gell-Mann and Richard Feynman, both professors at CalTech discovered the chiral structures of the weak interaction (in parallel with the independent team of George Sudarshan and Robert Marshak).

Gell-Mann's work in the 1950s involved recently discovered cosmic ray particles that came to be called kaons and hyperons. Classifying these particles led him to propose that a quantum number called strangeness would be conserved by the strong and the electromagnetic interactions, but not by the weak interactions. Another of Gell-Mann's ideas is the Gell-Mann-Okubo formula, which was, initially, a formula based on empirical results, but was later explained by his quark model.

In 1961, this led him (and Kazuhiko Nishijima) to introduce a classification scheme for hadrons, elementary particles that participate in the strong interaction. Gell-Mann referred to the scheme as the Eightfold Way, because of the octets of particles in the classification. (The term is a reference to the eightfold way of Buddhism.) Although now retired from teaching, he is still active in the scientific community and spoke at the “World Economic Forum” in 2012.

As discussed previously, in ordinary matter, the strong force acts only in the nucleus and it is due to the presence of the quarks, the ultimate basic particles from which protons and neutrons are formed. As the electric and magnetic forces are effects arising from electric charges, so is the strong force ultimately due to a new variety of charge, which is carried by quarks but not by leptons. Hence leptons, such as the electron, are not affected by the strong force; conversely, particles such as protons and neutrons that are made of quarks do feel the strong force.

The laws governing this are fundamentally similar to the those for the electromagnetic force. Quarks carry the new charge in what we can define as the positive form, and so antiquarks will carry the same amount but with a negative charge. The attraction of opposites then brings a quark and an antiquark together. The quark-antiquark particles are called mesons. (Note that the antiquark is not the same flavor as the quark. Otherwise the antimatter antiquark and the quark would be destroyed. For example, it might be an u quark with a d antiquark.)

Baryons, which are made of three quarks, don’t bond with this “+” / “−” bond, but another characteristic is the cause of the strong force.

It turns out that there are three distinct varieties of the strong attraction and to distinguish among them we call them red (R), blue (B), and green (G). As such they have become known as color charges, though this has nothing to do with color in the familiar sense — it is just a name.

As unlike colors attract, and like repel, so would two quarks each carrying a red color charge, for example, mutually repel. However, a red and a green would attract, as would three different colors, RBG. Bring a fourth quark near such a trio and it will be attracted to two and repelled by the third which carries the same color charge. The repulsion turns out to balance the attraction such that the fourth quark is in some sort of limbo; however, should it find two other quarks, carrying each of the two other color charges, then this trio can also tightly bind together. Thus we begin to see the attraction of trios, as when forming protons and neutrons, is due to the threefold nature of color charges. As the presence of the electric charges within atoms leads to them clustering together to make molecules, so do the color charges within protons and neutrons lead to the clusters that we know as nuclei.

The underlying similarity in the rules of attraction and repulsion give similar behavior to the electromagnetic and strong forces at distances much less than the size of an individual proton or neutron. However, the threefold richness that positive or negative color charges have in comparison with their singleton electric counterparts leads to a different behavior in these forces at larger distances. The color-generated forces saturate at distances of around 10-15 meters, the typical size of a proton or neutron, and are very powerful, but only so long as the two particles encroach within this distance — figuratively “touch” one another — hence the color-induced forces act only over nuclear dimensions. The electromagnetic force, in contrast, acts over atomic dimensions of some 10-10 meters when building stable atoms, and can even be felt over macroscopic distances, for example the magnetic fields surrounding the earth.

Based on this new theory of “color” charges, the study of quarks and the understanding of the nucleus and the strong force is called Quantum Chromodynamics or QCD in analogy to Quantum Electrodynamics or QED.

Since quarks can’t be isolated singularly due to a phenomenon known as color confinement, quarks are never directly observed or found in isolation; they can be found only within hadrons, such as baryons (of which protons and neutrons are examples), and mesons. For this reason, much of what is known about quarks has been drawn from observations of the hadrons themselves.

In seeking a deeper explanation for the regularities of the SU(3) classification scheme, Gell-Mann invented quarks. In this approach there are three fundamental quarks dubbed “up,” or u, “down,” or d, and “strange,” or s — and their antiparticles, the antiquarks. Mesons are built from a quark plus an antiquark, while baryons are composed of three quarks. The proton is a combination of two up quarks plus a down quark (written uud), for example, while the neutron is made of an up quark plus two down quarks (udd).

By assigning a charge to the up quark of + 2/3 e (where − e is the charge on the electron) and − 1/3 e to the other two, the charges on all the known mesons and baryons came out correctly.

But the idea of fractional charges was not accepted by physicists of the day; in his original paper, Gell-Mann even wrote that “a search for stable quarks of charge − 1/3 or + 2/3 at the highest energy accelerators would help to reassure us of the nonexistence of real quarks.”

After several years of fruitless searches, most particle physicists agreed that although quarks might be useful mathematical constructs, they had no innate physical reality as objects of experience.

However, experiments run at the Stanford Linear Accelerator (SLAC) from 1967 to 1973 were eventually interpreted to indicate the actual existence of fractional charged objects within the proton and neutron. In 1973, experimental and theoretical developments had produced a coherent picture of the nucleon as composed of fractionally charged quarks plus neutral gluons. Quarks do exist!

Quarks and Leptons are the building blocks which build up matter, i.e., they are seen as the "elementary particles". In the present standard model, there are six "flavors" of quarks. They can successfully account for all known mesons and baryons (there are over 200).

The most familiar baryons are the proton and neutron, which are each constructed from up and down quarks. Quarks are observed to occur only in combinations of two quarks (mesons), three quarks (baryons). (There is a theoretical combination of five quarks, but it has not been found in experimental data … at least not yet.)

So a proton, consisting of two up and one down, has a total charge of

2/3 + 2/3 − 1/3 = + 1.

A neutron, on the other hand, is two down and one up for

−1/3 −1/3 + 2/3 = 0.

These were the simple equations written on napkins by Gell-Mann when he supposed that the nucleons were made up of three charged particles. It was a simple math trick … yet it turned out to be what Nature had hidden in the nucleus all along.

Each of the six "flavors" of quarks can have three different "colors". The quark forces are attractive only in "colorless" combinations of three quarks (baryons), quark-antiquark pairs (mesons) and possibly larger combinations such as the pentaquark that could also meet the colorless condition. Quarks undergo transformations by the exchange of W bosons, and those transformations determine the rate and nature of the decay of hadrons by the weak interaction.

The property of quarks labeled color is an essential part of the quark model. The force between quarks is called the color force. Since quarks make up the baryons, and the strong interaction takes place between baryons, you could say that the color force is the source of the strong interaction, or that the strong interaction is like a residual color force which extends beyond the proton or neutron to bind them together in a nucleus.

Inside a baryon, however, the color force has some extraordinary properties not seen in the strong interaction between nucleons. The color force does not drop off with distance and is responsible for the confinement of quarks. The color force involves the exchange of gluons and is so strong that the quark-antiquark pair production energy is reached before quarks can be separated. Therefore, you won’t see any quarks in isolation.

Another property of the color force is that it appears to exert little force at short distances so that the quarks are like free particles within the confining boundary of the color force and only experience the strong confining force when they begin to get too far apart. The term "asymptotic freedom" is sometimes invoked to describe this behavior of the gluon interaction between quarks.

We now recognize six flavors of quarks (plus six more antiquarks).

Quark Symbol Charge
Up u + 2/3
Down d − 1/3
Charm c + 2/3
Strange s − 1/3
Top t + 2/3
Bottom b − 1/3

So if protons and neutrons are not fundamental particles, but are actually made up of even smaller particles called quarks, why not suppose that quarks are made of even smaller particles … possibly blue and called “smirfs”? What indeed?

There is an ancient cosmological myth that describes a flat earth riding on the back of a turtle. When asked, “what holds up the turtle,” the jocular answer given was “turtles all the way down.” That is, the turtle rode on the back of another turtle, sort of an infinite regression. Stephen Hawking made the expression famous in a 1988 book. It is sort of the chicken and egg conundrum, but in the case of sub-atomic particles, we don’t believe there are any more turtles.

The latest theories propose that quarks are actually made up of strings … vibrating strings … vibrating in more than three or four dimensions; maybe as many as 29 dimensions. String theory naturally incorporates gravity, and is therefore a candidate for a theory of everything, a self-contained mathematical model that describes all fundamental forces and forms of matter.

Isn’t it odd that I started this journey with the Greeks and their vibrating musical instrument strings suggesting a simple order to the universe … and we now end with vibrating “super strings.”

However, that theory will require a lot more powerful atom smashers to explore. The latest and largest smasher is the Large Haldron Collider in Switzerland at CERN. Our story continues there … next time.

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