A Brief History of Particle Physics, Part I Quiz

Answer: photoelectric effect

Einstein won the 1921 Nobel Prize in Physics primarily for this explanation of the photoelectric effect, in which light strikes a metal surface and current-carrying electrons come out. Only light above a certain frequency (depending on the metal) can induce this effect; Einstein deduced that each photon carried a certain amount, or quantum, of energy depending on its frequency or color, which had to be sufficient to overcome the electron binding energy. He was inspired by Planck's explanation of blackbody radiation, which held that electromagnetic emission was quantized; Einstein expanded this idea to apply to electromagnetic fields in general.

The Compton effect (a shift in wavelength of light scattered off a particle at rest) is what finally proved Einstein's theory to the satisfaction of the scientific community. We now understand the photon as something with properties both of a wave and of a pointlike particle: it propagates as a wave, and interacts with matter like a particle. And it only gets wilder from here!

Answer: electron

Thomson initially believed that these were the only constituents of the atom. He envisioned an atom in which tiny, negatively charged corpuscles floated in a positively charged, massless cloud. (This is the famous "plum pudding" model, so named for the excellent British dessert.) His vision had very little in common with the atom as we now understand it, but his clever cathode ray experiments set the stage for an exciting new era of physics. Thomson won the 1906 Nobel Prize in Physics for this discovery.

Answer: nucleus

Rutherford and his student (Ernest Marsden) had discovered the nucleus, a tiny ball of protons and neutrons (although they wouldn't find out about the neutrons until later. Rutherford, who won the 1908 Nobel Prize in Chemistry, later described how shocking this discovery was: "It was almost as incredible as if you fired a 15-inch shell at a piece of tissue paper and it came back and hit you." J. J. Thomson's plum pudding model was dead forever.

Answer: neutron

Since neutrons don't interact electrically with matter, they are much more difficult to detect than charged particles -- they tend to pass right through detectors! Chadwick won the 1935 Nobel Prize in Physics for his clever multi-step experiment and analysis. First he bombarded beryllium with alpha radiation (an alpha particle is a helium nucleus, two neutrons and two protons).

The beryllium emitted "penetrating neutral radiation" (neutrons), which he used to bombard paraffin, nitrogen, and other targets. Protons, which he could easily detect, were emitted from the targets, and by examining their momenta and scattering angles he could measure the mass of the neutrons that had collided with them. Particle physicists still use these "recoil protons" to detect neutrons.

Answer: All the negative-energy states are filled with an infinite "sea" of electrons.

All matter tends to decay to lower energy states, and the energy has to go somewhere, so the lack of a ground state was a serious problem! Dirac's explanation relied on the fact that electrons - both positive- and negative-energy - are fermions, so the Pauli exclusion principle says that no two electrons can occupy the same state. So if there's an infinite sea of electrons already filling the negative-energy states, none of the other electrons can decay! And if the sea is uniform, then it would exert no net force and we'd never detect it.

It was a brilliant but desperate idea, but one thing was missing. There should be a particle with the same mass as an electron, but with positive energy and positive charge, interpreted as a "hole in the sea" ... but none had been detected. Cue Anderson's experiment!

Answer: positron

Cloud chambers are venerable particle detectors: charged particles passing through a gas leave a trail of ions. Water vapor condenses on the ions, and so the trail is visible in a photograph. Knowing that charged particles curve in a magnetic field, Anderson placed magnets around his cloud chamber. By measuring the direction (clockwise or counterclockwise) of the curved positron track, Anderson determined that the particle was positively charged; by measuring the track's radius of curvature, he could tell that it had the same mass as the electron. Anderson won the 1936 Nobel Prize in Physics for his discovery, but Dirac's interpretation of the positron would be rejected in the 1940s. Feynman (1965 Nobel Prize in Physics) and Stuckelberg argued persuasively that the negative-energy solutions of the Dirac equation could be expressed as positive-energy states of the positron. Dirac's electron sea, and its holes, are no longer necessary -- but Dirac's equation now has the powerful implication that every particle has an antiparticle.

Answer: muon

The muon, which was discovered independently by J. C. Street and E. C. Stevenson in 1937, had not been predicted in theory and its role in particle physics was at first elusive. (Isidor Rabi, a brilliant atomic physicist who won the 1944 Nobel Prize in Physics, famously asked "Who ordered that?" when first told of the discovery.) Decades later, however, the muon turned out to be a fundamental particle, now understood as the charged lepton in the second generation of elementary particles. As such, it's critical in our understanding of the Standard Model; in fact, it played a key role in the development of the particle physics conservation laws as we'll see in Question 14.

The positron is an antielectron, discussed in the previous question. The tau is the heavyweight lepton, and muonium is a neat, short-lived substance of great interest to atomic physicists: it's essentially a hydrogen atom in which the electron has been replaced by a muon.

Answer: It violated conservation of energy.

When something decays, the energies of the decay products had better add up to the energy of the parent state! This is why it was so problematic that the final state energy of beta decay seemed to be variable. Niels Bohr (1922 Nobel Physics Prize) immediately argued that the law of conservation of energy should be abandoned (it should be noted that, while a brilliant man, he is on the record as having ridiculed no fewer than five theories of particle physics now regarded as fundamental). Wolfgang Pauli (1945 Nobel Physics Prize) argued that perhaps a "silent", electrically neutral particle was also emitted, carrying off the energy, and in 1933 Enrico Fermi (1938 Nobel Physics Prize) incorporated this suggestion into his new theory of beta decay, predicting a massless particle which he called the neutrino (it was actually, technically, an antineutrino). I mention the Nobel Prizes to demonstrate that even provably brilliant people don't have all the answers all the time! You should hear some of the stories told about arguments between Bohr and Einstein ...

Answer: meson

The muon (see Question 7) was initially thought to be Yukawa's meson, but it interacted very weakly with protons and neutrons. Yukawa's meson is actually the pi meson, which comes in three charges (neutral, positive, and negative) and is produced in large amounts by cosmic ray collisions with particles in Earth's upper atmosphere.

The massive pions arise from a symmetry breaking of the strong force; there is also a massless mediator, the gluon, which we'll discuss when we get to the quark model in Part II of this quiz. Yukawa won the 1949 Nobel Prize in Physics for his prediction.

Answer: strangeness S, conserved by the strong force but not by the weak force

Murray Gell-Man (1969 Nobel Prize in Physics; we'll be hearing much more from him later!) and Kazuhiko Nishijima arrived at the idea independently; it was Gell-Man who coined the term strangeness. This was the moment when particle physicists realized that they had a zoo of "elementary" particles on their hands, and no understanding of how they all fit together.

In his 1955 Nobel Physics Prize acceptance speech, atomic physicist Willis Lamb recounted a popular joke that "the finder of a new elementary particle used to be rewarded by a Nobel prize, but such a discovery now ought to be punished by a $10,000 fine."

Answer: The Savannah River nuclear reactor in South Carolina

Nuclear reactors are intense sources of antineutrinos. Cowan and Reines placed a water tank at the reactor to look for recoil neutrons (much like Chadwick's experiment described in Question 4). An antineutrino collides with a proton in the water, resulting in a neutron and a positron -- inverse beta decay! Since antineutrinos are massless and electrically neutral, they don't often interact with matter and the odds of this reaction were extremely small. The Savannah River reactor produced a calculated flux of 50 million million antineutrinos per square centimeter per second at their location, but even with a 200-liter tank of water they only detected about three antineutrino events every hour!

Reines and Cowan observed enough events to prove the existence of the neutrino and antineutrino by 1956, inaugurating a subfield of particle physics that's extremely active to this day. Reines won the Nobel Prize in Physics for this discovery in 1995, but unfortunately Cowan had died 21 years before.

Answer: All neutrinos are left-handed.

If the parity symmetry was valid for the weak interaction (for which neutrinos and antineutrinos are the signature), we would expect that half of all neutrinos would be left-handed (spin axis against the direction of motion) and half would be right-handed (spin axis along the direction of motion). But Wu, investigating neutrino helicity by analyzing the direction of the electrons emitted in the beta decay of Cobalt 60 (neutrino physics is necessarily a bit indirect), discovered that all neutrinos are left-handed and all antineutrinos are right-handed. The weak force violates parity maximally. Her result was confirmed within days by a group at Columbia led by Leon Lederman.

Wu did not win the Nobel Prize for her momentous experiment, a bizarre decision still talked about by particle physicists. The two Chinese-American theorists who had suggested the experiment to Wu (a friend of theirs), Chen Ning Yang and Tsung-Dao Lee, shared the Nobel Physics Prize in 1957. At 31, Lee was the second-youngest scientist ever to win a Nobel Prize.

Answer: No

Neutrinos and antineutrinos are distinct. This was first theoretically predicted in 1953 by Konopinski and Mahmoud, who came up with a conservation law of lepton number to explain why some reactions didn't seem to occur. Electrons, muons and neutrinos had L=1, positrons, antimuons, and antineutrinos would have L=-1, and all other particles had L=0. L would have to be the same on both sides of the reaction equation (this is why Fermi's neutrino in Question 8 actually had to be an antineutrino). Thus the neutrino and antineutrino quantum numbers are distinct: they're different particles.

In 1959, Raymond Davis (who won the 2002 Nobel Prize in Physics for subsequent work on solar neutrinos) and Daniel Harmer proved this experimentally. It was already known that the reaction neutrino + neutron -> proton + electron was possible; Davis and Harmer looked for the same reaction, but with an antineutrino. When they didn't see the reaction, they knew that the particles weren't interchangeable.

Answer: When muons decay into electrons, a neutrino and an antineutrino are produced.

A muon is heavier than an electron, so muon decay is energetically favored. But muons never decayed into an electron plus a photon, as one might expect; this implied that "mu-ness" was a conserved quantity. How then to explain the observed decay of a muon into an electron plus a neutrino plus an antineutrino? Pontecorvo had the idea that the neutrino was a mu neutrino (mu number 1 to balance the muon) and the antineutrino an electron neutrino (electron number -1 to balance the electron). (Tsung-Dao Lee, more famous for the parity violation in Question 12, had the same idea in 1960, before Pontecorvo's work had been translated from the Russian and republished.)

In 1962, at the Brookhaven Laboratory in New York, Leon Lederman (who also confirmed parity violation), Melvin Schwartz, and Jack Steinberger were able to confirm the existence of a distinct mu neutrino. They shared the 1988 Nobel Prize in Physics for this experimental tour de force, which included a 44-foot-thick radiation shield made of steel from a decommissioned warship.

Answer: two hexagons and a triangle

This is what Gell-Man won his 1969 Nobel Prize in Physics for, and he deserved it! He arranged the baryons and mesons according to their charge and strangeness. The eight lightest baryons formed the baryon octet: six baryons on the points of a hexagon, and two in the center. The eight lightest mesons were similarly arranged in the pseudo-scalar meson octet, and the ten heavier baryons were arranged in a triangular array, the baryon decuplet (one baryon on each corner, two on each side, and one in the middle). The Eightfold Way became widely accepted when Gell-Man accurately predicted the mass and lifetime of the omega-minus particle, which had not yet been discovered but which belonged on the downward point of the baryon decuplet.

I hope you've enjoyed this quiz on the history of my chosen field of physics. Watch for Part II of this quiz, which will cover the last 40 years of the twentieth century.