CT: Forty years ago, protons and neutrons were thought to be fundamental particles like electrons. In other words, it was thought that they could not be divided into smaller particles. In the 1960s, physicists working at the Stanford Linear Accelerator Center found that electrons traveling near the speed of light sometimes changed their direction abruptly, losing much of their energy, when they collided with matter. It was as though they were encountering small, but very hard pieces of matter, on their voyage through nucleons. These results (see Figure 1) were similar to Rutherford’s experiment using alpha particles showing that atoms were mostly empty space. In this case, observations indicated that protons and neutrons are composed of smaller particles.
Just as you modeled in the activity, protons and neutrons are each made of three quarks. A proton consists of two “up” quarks and one “down” quark, whereas a neutron contains two” down” and one “up.” “Up” and “down” are two “flavors,” or varieties, of quarks. There are four others “flavors,” but they have not been found to form stable matter.
Why are there three quarks in each nucleon?
You have learned that there are two kinds of electric charge—positive and negative—and that like-charged objects repel each other while unlike-charged objects attract. So, a proton with a +1 charge and an electron with a -1 charge can form a stable hydrogen atom.
Quarks possess a characteristic not found in any other type of particle. Each “flavor” of quark is able to carry three different types of charges—red, green, and blue “color” charges. Quarks of the same “color” repel each other because they have similar forces. And, like
trying to balance a three-legged stool on two legs, no two quarks can form a particle because their “color” forces would not be balanced. So, it takes one red, one blue, and one green quark to form an object that is stable. These stable particles are said to be “color-neutral” or “white.”
S: NASA – https://go.nasa.gov/2CCpvRx (last access: 9 January 2019)
N: 1.1964, applied by U.S. physicist Murray Gell-Mann (b.1929), who said in correspondence with the editors of the OED in 1978 that he took it from a word in James Joyce’s “Finnegans Wake” (1939), but also that the sound of the word was in his head before he encountered the printed form in Joyce.
German Quark “curds, rubbish” has been proposed as the ultimate inspiration (Barnhart; Gell-Mann’s parents were immigrants from Austria-Hungary); it is from Old Church Slavonic tvarogu “curds, cottage cheese,” from a suffixed form of PIE root *teue- “to swell” (source also of Greek tyros“cheese”). George Zweig, Gell-Mann’s co-proposer of the theory, is said to have preferred the name acefor them.
2. Any of a group of sub-atomic particles (originally three in number) conceived of as having a fractional electric charge and making up in different combinations the hadrons, but not detected in the free state.
3. Quarks are central to what is now known in physics as the standard model. The standard model holds that all matter is made of two kinds of particle: quarks (which constitute protons, neutrons and many other particles) and leptons (among which are electrons).
4. Protons and neutrons are made up of simpler and more basic entities known as quarks and gluons.
5. The theory predicts the existence of six quarks, known as up, down, strange, charm, bottom and top, that differ from one another in their mass and charge characteristics. Quarks appear to be true elementary particles; that is, they have no apparent structure and cannot be resolved into something smaller.
6. However, quarks always seem to occur in combination with other quarks or with antiquarks, their antiparticles, to form all hadrons—the so-called strongly interacting particles that encompass both baryons and mesons.
S: 1. OED – https://bit.ly/2TVErB3 (last access: 9 January 2019). 2 to 5. TERMIUM PLUS – https://bit.ly/2TZ3pzF; https://bit.ly/2TZ3pzF; EncBrit – https://bit.ly/2TTMbU7 (last access: 9 January 2019). 6. EncBrit – https://bit.ly/2TTMbU7 (last access: 9 January 2019).