Quarks and Leptons Trivia Quiz

Answer: It has no known substructure.

The early history of particle physics is a story of looking deeper for the structure underneath. In the early 1800s, atoms were supposed to be the most fundamental unit of matter; by the early 1900s, physicists knew that atoms consisted of a positively charged nucleus orbited by negatively charged electrons. In the coming decades, the nucleus was found to consist of protons and neutrons, and the protons and neutrons were found to consist of quarks -- but quarks, like electrons, show no evidence of having constituents of their own.

Discovered by J. J. Thomson in 1897, the electron is now classified as the first (lightest) generation of lepton -- a type of spin-1/2 particle that doesn't feel the strong nuclear force.

Answer: J/psi meson

At the valence level, a proton has two up quarks and a down (uud); a neutron has two downs and an up (udd); and a pion has either an up and an antidown, a down and an antiup, or a quantum superposition of up/antiup and down/antidown states. (Pions are where things start to get fun!) Its presence in protons and neutrons, of course, makes up quarks one of the top two practical quark flavors for human beings.

The J/psi meson is made up of two quarks, a charm and an anticharm. It got its name because it was discovered simultaneously by two research groups (one at Brookhaven, one at SLAC) and neither group leader was willing to accept the other's proposed name.

Answer: -1/3 e

Despite what students are often taught in their courses on electricity and magnetism, there ARE fractional charges -- charges with magnitude less than an electron or proton charge! These fractional charges are carried only by quarks, elementary spin-1/2 particles that experience all four fundamental forces of nature (the strong nuclear force, the weak nuclear force, electromagnetism, and gravity). Down quarks have charge -1/3 e, while up quarks have charge +2/3 e.

But you can't find quarks by themselves, due to the mysteries of the strong nuclear force. Instead, you always find them in composite particles, which have integer charge. A proton, for example, has two ups and a down, and the total electric charge is (+2/3 e + 2/3 e - 1/3 e) = e. A neutron, with two downs and an up, has a total electric charge of 0.

Answer: A radioactive gamma source

Electron neutrinos and antineutrinos are created copiously in various types of nuclear reaction, including nuclear fusion (as in the Sun), nuclear fission (as in a power plant; in fact, that's where electron antineutrinos were first discovered), and radioactive, beta-decaying nuclei. Other types of radioactive decay, such as gamma decay or alpha decay, proceed via other physical pathways and do NOT produce neutrinos. (Gamma decays are so named because they produce photons, or gammas; the eponymous product of an alpha decay, meanwhile, is a helium-4 nucleus, also known as an alpha particle.)

Neutrinos are hard to detect because they're electrically neutral and have a very small mass, so they are "ghostly" particles that tend to pass through detectors unseen. If you have enough of them, though, and a large enough, sensitive enough detector, you'll spot a few just fine.

Answer: The Eightfold Way

Murray Gell-Mann, one of the discoverers of the Eightfold Way, named the pattern while thinking about the Noble Eightfold Path by which Buddhists attempt to achieve liberation. (Gell-Mann had a knack for memorable names; he also came up with the word "quark.") Much as the Periodic Table of the Elements revealed underlying structure by organizing elements into rows and columns, the Eightfold Way revealed underlying structure by organizing spin-1/2 composite particles into an octagon shape, with charge on one axis and strangeness on the other. This shape, along with a triangle shape that could be built from the ten spin-1/2 baryons, eventually led to the development of quark theory.

In quark theory, "strangeness" is due to strange quarks -- a quark flavor that complements up quarks and down quarks. Hadrons containing strange quarks had longer lifetimes because they couldn't decay to lighter, strangeness-free hadrons via the fast-acting strong nuclear force; instead, they had to decay via the slower method of the flavor-changing weak nuclear force.

The official physics name for the Eightfold Way is now SU(3) flavor symmetry, which connects the physics content to mathematics (specifically, group theory). The Eightfold Way will always have a special place in physicists' hearts, however.

Answer: Charm

Sheldon Glashow, one of the theorists who predicted the charm quark, explained its name this way: "We called our construct the 'charmed quark', for we were fascinated and pleased by the symmetry it brought to the subnuclear world." The name was well-chosen. The prediction and subsequent discovery of the charm quark (as a component of the J/psi meson) finally led the physics community to accept the quark model, which until then had largely been regarded as a cute but silly idea. That thrilling sea change in physics history, following a simultaneous discovery by two groups in November 1974, picked up a memorable moniker of its own: the "November Revolution."

Charm physics, focused on the production and study of composite particles that contain charm quarks, is still an active subfield of particle physics.

Answer: Airline pilots

Muons are produced in our atmosphere, when cosmic rays -- high-energy protons and nuclei from outer space -- interact with atoms in the air. The average muon lifetime is 2.2 millionths of a second, so muons decay to electrons as they pass through the air. The total number of muons passing through a given area is thus higher at high altitudes (in airplanes and on mountaintops) than it is on the ground. Nickel miners have the lowest exposure to muons of all, since they work underground where they are shielded by massive amounts of rock. That's why sensitive physics experiments are often located underground, in mine shafts or tunnels -- to limit the number of muons passing through.

The decay of cosmic-ray muons in flight is a nice problem in special relativity, actually. Muons created in the atmosphere typically have a lot of energy, and move at a significant fraction of the speed of light. The resulting time dilation means that the apparent muon lifetime, as seen by detectors at rest on Earth, is longer than the actual muon lifetime.

Answer: Neutrinos have mass greater than zero.

The leaders of two experiments, SNO in Canada and SuperKamiokande in Japan, received the 2015 Nobel Prize in Physics for establishing neutrino oscillation, but many other experiments have filled in critical data as well. The explanation for neutrino flavor oscillation (electron neutrinos turning into muon neutrinos or into tau neutrinos, and vice versa in all the combinations) is wonderfully weird and magnificently quantum mechanical.

There are three distinct flavors of neutrino, which we already knew about: electron neutrino, muon neutrino, and tau neutrino. The new information is that there are also three distinct mass states of neutrino (creatively called m1, m2, m3), which are different from the mass states. Each electron neutrino is a quantum superposition of m1, m2, and m3; each m1 neutrino is a quantum superposition of electron, muon and tau flavors.

Neutrinos are created with a definite flavor state, but they travel through space -- they propagate -- as mass states. And, as they travel, the different mass states get a little out of phase with each other, depending on the neutrino energy, and that means that the probabilities of measuring a given flavor state change too. If you put your neutrino detector at the right spot, you can see a clear signal for the oscillation: a "disappearance" of one flavor of neutrino where there should be more, or an "appearance" of a neutrino flavor where there should be none. Neutrino oscillation explains a longstanding mystery -- why were there only 2/3 as many electron neutrinos coming from the Sun as nuclear fusion theory predicted that there should be? -- and opens exciting mysteries of its own.

Answer: Bottom

"Baryon" is the generic name for composite particles with three constituent quarks, such as protons and neutrons. Belle and Babar are particle-physics experiments specially tuned to produce huge amounts of b quarks. The common name for the b quark is "bottom"; there were early attempts to call it "beauty," but this was considered too precious even for a community that had named quarks "charm" and "strange."

Unlike their heavier third-generation counterparts, the top quarks, bottom quarks live long enough to form composite particles bound by the strong force. Quite a few interesting physics problems can be investigated with bottom-containing particles, which is why when Belle and Babar shut down, they were replaced with Belle II and LHCb.

Answer: Hadrons

Bosons are particles with integer spin, and include some elementary particles (like photons, or the hypothetical gravitons). Fermions are particles with half-integer spin; every particle in this quiz has been an elementary fermion, but composite fermions also exist (like protons and neutrons). "Hadron," derived from a Greek word meaning "thick," is the particle-physics term for any composite particle that's built out of quarks and bound by the strong nuclear force. This could be a two-quark system (a meson), a three-quark system (a baryon), or perhaps even more (a pentaquark state was discovered at CERN in 2015).

The electron is a stable particle. The muon decays into electrons, but it doesn't have enough mass energy to create hadrons, which are very massive; one muon makes one electron, one electron antineutrino, and one muon neutrino. Only the tau, with a mass of 1776.82 MeV/c^2 (17 times a muon mass and 3475 times an electron mass) has enough mass energy for its decay to create hadrons. (A tau does have about a 35% chance of decaying in a way that doesn't create any hadrons, but its huge variety of decay modes sets it apart from the lower-generation leptons.)