A Brief History of Particle Physics, Part II Quiz

Quiz Answer Key and Fun Facts

1. We return to the story of particle physics in 1964, when the famous quark model was proposed independently by George Zweig and Murray Gell-Mann. They explained the Eightfold Way by suggesting that the hadrons (composite particles like protons, neutrons and mesons) were composed of even smaller particles, which Gell-Mann called "quarks." From what James Joyce novel did he take that name?

Answer: Finnegans Wake

Gell-Mann, who won the 1969 Nobel Prize in Physics in part for the quark model, and whom we met in Part I of this quiz, took the name "quarks" from the line "Three quarks for Muster Mark!" in Book 2, Episode 4 of the 1939 novel "Finnegans Wake." That he was able to get away with this playful name shows that the quark model was not yet taken seriously! There was plenty of indirect evidence for the quark model -- for example, the proton charge is concentrated in three tiny lumps -- but there was still the problem that nobody had ever isolated a quark.

Gell-Mann and Zweig postulated that there were three quarks (up, down, and strange); the other three would be discovered in later decades. Gell-Mann later said that the "three quarks" seemed to match perfectly his three fundamental particles, which is part of what drew the Joyce line to mind. He is also very insistent that "quarks" should be pronounced to (sort of) rhyme with "quarts," rather than with "Mark."

2. There was a big problem with the quark model: the Pauli exclusion principle. Quarks are fermions and can never occupy the same quantum state, but some hadrons can only be constructed out of three quarks with identical quantum numbers. Later in 1964, Oscar Greenberg solved the problem by introducing what additional quantum number that can never be observed in the laboratory?

Answer: color

Greenberg suggested that every quark comes in three "colors", conventionally called red, green and blue. These aren't real colors in the way that we understand color, of course; red, green and blue are just convenient labels for the different "charges" of the strong force, just as positive and negative electrical charges don't indicate that protons are optimistic and electrons pessimistic.

In a baryon made of three quarks, each quark must have a different color, so that they're all in different quantum states; in a meson made of a quark and an antiquark, the quark will have some color and the antiquark will have the corresponding anticolor. This makes all particles colorless (with the optics analogy red + green + blue = white). This principle (later explained much more rigorously by the theory of quantum chromodynamics) is why you can't isolate a quark; they must always appear in colorless groups.

3. Even as the quark model was coming together, however, a cherished physical symmetry was dying. Physicists had been stunned when parity (P) was shown to be violated by the weak force in 1956, but they held out hope for CP symmetry. The idea was that the "mirror image" of a particle was its antiparticle: if you changed a particle to an antiparticle, and reflected it from left to right, then the physical laws would remain the same. Alas, this is not the case! In what strange mesons was CP violation discovered in 1964?

Answer: neutral K mesons

James Cronin and Val Fitch observed this effect at Brookhaven while trying to verify that CP symmetry was valid; they won the 1980 Nobel Prize in Physics for their astute discovery. They hit a metal target with a proton beam to produce beams of neutral K mesons. The K converts into an antiK, and vice versa, through the weak interaction, and the two ways in which they can combine have different lifetimes. If CP symmetry is valid, then the short-lived K meson should always decay into two pions, and the long-lived K into three.

Cronin and Fitch let the kaon beam travel for 57 feet before passing through their detector, to allow all the short-lived K mesons to decay so that they could observe a pure beam of long-lived K mesons. This was when they made their shocking discovery: about one in 500 long-lived K mesons decayed in two pions, which should have been forbidden under CP symmetry. Because the effect is so small, it is very difficult to explain theoretically, although it has since also been observed in the B meson system. The current theory is that maybe the "mirror symmetry" of nature is a CPT symmetry: make a particle an antiparticle, have it travel backwards in time, reflect it left to right, and THEN the laws of physics will be the same! Maybe.

4. Theorists working with the strong and weak forces using the quark model soon found that they could make use of a clever type of diagram first introduced for quantum electrodynamics. They draw straight lines for fundamental particles (quarks and leptons), using arrows to indicate the forward time direction (antiparticle arrows are drawn backwards). Force-carrying bosons can be drawn using dashed or squiggly lines, and it is easy to calculate the cross section of a reaction by multiplying the contributions of the various lines and corners of the diagram. These useful diagrams are named after what famous American physicist, notorious for his practical jokes?

Answer: Richard Feynman

Feynman, who shared the 1965 Nobel Prize in Physics with Julian Schwinger and Sin-Itiro Tomonaga for their development of quantum electrodynamics in the 1940s, is a bit of a folk hero to physicists. He was called the Great Explainer for his clear introductory physics lectures (which are still published in three volumes as a hot commodity for physics students today); he used to claim that if a physical concept couldn't be explained in a freshman-level lecture, then it wasn't fully understood yet by anyone. (I wonder what he would have thought about FunTrivia quizzes!) His two autobiographical books, "Surely You're Joking, Mr. Feynman" and "What Do You Care What Other People Think?" are well worth reading.

Quantum electrodynamics is, to date, the most accurate theory as compared to experimental results that humanity has ever devised, and Feynman diagrams make complicated particle physics problems easy to understand.

5. The quark model was not fully accepted until the autumn of 1974, when a pair of papers gave rise to the November Revolution. Groups at Brookhaven (in New York) and the Stanford Linear Accelerator (SLAC, in California) had independently discovered a new particle, now called the J/Psi, that provided evidence for what new flavor of quark?

Answer: charm

Samuel Ting's group at Brookhaven was the first to discover what they called the J, in the summer of 1974, but they kept it a secret for several months while they checked and double-checked their experimental apparatus. Then, while Ting was visiting Stanford in November, he learned that Burton Richter's SLAC group was probing the same energy range and was sure to find it. Ting and Richter rushed to publish (Richter called the particle a Psi), their papers came out simultaneously, and they shared the 1976 Nobel Prize in Physics. The particle is now called the J/Psi in recognition of both groups.

Ten years earlier, Sheldon Glashow (1979 Nobel Prize in Physics) and John Bjorken had suggested that there ought to be a fourth quark, for symmetry with the four leptons (electron, muon, and electron and muon neutrinos). The J/Psi, which is a bound state of a charm and an anticharm, ushered in an era of discovery of other charmed particles (the D mesons) and led at last to the acceptance of the quark model.

6. Everything seemed to fall into place after the November Revolution. There were four quarks, there were four leptons, and all the hadrons could be arranged in three-dimensional "periodic tables." Then, in 1975, Martin Perl and his group at SLAC had to go and discover an unexpected fundamental particle. What was it?

Answer: a fifth lepton (the tau)

Perl, who won the 1995 Nobel Prize in Physics for this discovery, had been looking for heavy leptons in order to try to shed some light on why electrons and muons have different masses. He shared the same data as scientists looking for charmed hadrons, but he was looking for a very different set of events, and he found them. An electron and positron annihilate and a tau-antitau pair are produced. Each then decays into a lighter lepton plus two neutrinos. "We have no conventional explanation for these events," Perl's group wrote in their December paper. By 1979 other groups had confirmed the existence of the tau.

Incidentally, Perl used the very same collider -- the Stanford Positron-Electron Accelerating Ring (SPEAR) at SLAC -- that Richter had used to discover the J/Psi in the previous question. SPEAR later was the pioneer synchrotron accelerator (synchrotron radiation is used to probe small structures, from transistors to biological cells). There's a good reason Stanford calls it "the most cost-effective machine ever built in the field of high energy physics!"

7. Physicists soon began to make progress understanding the weak force, which changes the flavors of quarks and is responsible for beta decay (a type of radioactivity). Sheldon Glashow, Abdus Salam, and Steven Weinberg predicted that the weak force was mediated by three "intermediate vector bosons," the W+, W-, and Z. They reached this conclusion by unifying the weak force with what other force?

Answer: Electromagnetic force

Glashow and Weinberg (both Americans) and Salam (a Pakistani) developed electroweak theory from 1961-1967 and shared the 1979 Nobel Prize in Physics. In this theory, which sounds bizarre but is supported by ample experimental evidence, electromagnetism and the weak force are both manifestations of the same force, which has two charged carriers (W+ and W-) and two neutral carriers (Z and the photon). All of these particles should be massless at high energies, but the symmetry of the theory is broken at the lower energies of our universe.

This theory became widely accepted in 1971 after some further work by the Dutch theorists Gerardus 't Hooft and Martinus Veltman, who shared the 1999 Nobel Prize in Physics for their contribution. It became an integral part of the Standard Model, and so it was a huge triumph for particle physicists in general when a group at CERN (Centre europ�en de recherches nucl�aires, in Switzerland), led by Italian physicist Carlo Rubbia and Dutch physicist Simon van der Meer, discovered the W+, W- and Z bosons in 1983. Rubbia and van der Meer shared the 1984 Nobel Prize in Physics for their experimental masterpiece.

8. After Perl's discovery, particle physicists strongly suspected that there should be another quark. In 1977, a team at Fermilab found it. What is the name of this negatively-charged quark, the lightest of the third generation?

Answer: bottom

Leon Lederman, who won the 1988 Nobel Prize in Physics for his role in the discovery of the mu neutrino (see Part I of this quiz), led the team at Fermilab, an accelerator located just outside Chicago.

The bottom and top quarks (the top was discovered at Fermilab in 1994) make up the third and presumably last generation of quarks, to match the three generations of leptons. They are extremely heavy (the top quark is about 500 times the mass of the proton!) but their electrical charges, -e/3 and +2e/3 respectively, are the same as for any other generation of quarks.

These discoveries were enormous efforts involving hundreds of people; the paper announcing the discovery of the top quark had more than 300 authors! It is fairly common for the first two or three pages of particle physics papers to be taken up by the author list.

9. As discoveries of heavy mesons intensified, it became clear that -- despite the cleverness of Yukawa's 1930s ideas about the pi meson -- the strong force had to be mediated by something else. In 1979, at the DESY accelerator in Hamburg, Germany, that something else was discovered. What is the massless particle that mediates the strong or color force?

Answer: gluon

One of the most bizarre aspects of particle physics is the hadron "jet". Because of quark confinement, quarks can never appear alone -- so if, by some huge input of energy, you manage to separate two quarks of a meson, each quark ends up setting off a cascade producing several dozen hadrons as additional quarks are created out of the vacuum. These hadrons are all emitted in more or less the same direction, leading to the name "jet". The scientists at DESY (Deutsches Elektronen Synchrotron) observed three jets: one from the quark, one from the antiquark, and one from what could only be an energetic gluon emitted at the same time as the quarks.

Gluons carry color charge (both a color and an anticolor), so gluons interact with each other (unlike the other massless force mediator, the photon, which carries no electric charge). This explains the short range of the strong force: gluons tend to cluster together instead of propagating out to infinity. There are even systems (glueballs) consisting of several interacting gluons with no quarks in sight.

10. In 1998, the SuperKamiokande collaboration, a group of Japanese and American experimentalists, made a startling announcement. It has long been known that neutrinos come in three generations, or "flavors": electron, muon, and tau. But the SuperK results showed that neutrinos oscillate between flavors: over a long distance, a muon neutrino may become an electron neutrino, then back to a muon. What does this result imply?

Answer: Neutrinos have mass.

The SuperKamiokande finding, arising from data from a multiyear experiment involving a 12.5-million-gallon tank of water more than a kilometer underground, showed that the different neutrino flavors must have different masses -- so not all of them have nonzero mass. It gets even stranger than that: each neutrino flavor exists in a superposition of mass states. The "neutrino mass" is not well-defined in the same way that we talk about Schroedinger's cat being both alive and dead at the same time. The discovery that neutrinos have mass (about one ten millionth of the mass of an electron) affects a wide number of physics problems, from the Standard Model (which incorporates massless neutrinos) to dark matter. More neutrino mass experiments are in the planning and construction phases.

I hope you've enjoyed this foray through the history of my chosen field of physics. If you missed it, please check out Part I of this quiz for information on the early days of particle physics! Thank you for playing!

Source: Author CellarDoor

This quiz was reviewed by FunTrivia editor crisw before going online.
Any errors found in FunTrivia content are routinely corrected through our feedback system.

As the 1960s began, much more remained for particle physicists to investigate. Here we examine a thrilling four decades of discovery, from the quark model of particle physics to a shocking upset in 1998 ...

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