Section 5: The Discovery of Quarks
The accelerators at Berkeley and Brookhaven were designed to accelerate protons. Physicists at Stanford University had a different idea: an electron accelerator. After all, they reasoned, the proton was not a fundamental particle. And because the electron appeared to have no substructure, it should make a cleaner probe. So Stanford designed and built several generations of linear electron accelerators, culminating in the Mark III accelerator, which grew to over 300 feet in length.
Then in 1951 a diminutive firebrand named Wolfgang Panofsky arrived from Berkeley, after refusing to sign the McCarthy-era loyalty oath required by the state of California. Panofsky led the Stanford faculty in developing a proposal to construct a new two-mile-long linear accelerator, dubbed Project M—for Monster. In 1962, the Atomic Energy Commission provided $114 million to build the Monster under the more benign name of the Stanford Linear Accelerator Center (SLAC). Four years later, the linac (for linear accelerator) began accelerating intense beams of electrons up to energies of 20 billion electron volts.
Figure 11: Overview of the Stanford Linear Accelerator Center.
Source: © SLAC National Accelerator Laboratory historical photo index. More info
A beam switchyard at the end of the linac directed the beam to different experimental areas, or end stations, much like a railroad switchyard. In End Station A, an enormous version of Rutherford's scattering experiment used liquid hydrogen and deuterium (or heavy hydrogen) as targets. And just as Rutherford had discovered a small hard nucleus that occasionally caused an alpha particle to scatter at a large angle or even backwards, researchers at SLAC observed electrons scattering at wide angles much more frequently than expected. By the early 1970s, detailed analyses of the distribution of the scattered electrons measured in the giant magnetic spectrometers in End Station A revealed three scattering centers within the nucleon—the first experimental evidence that quarks were in fact real. Physicists Jerome Friedman, Henry Kendall, and Richard Taylor received the Nobel Prize for this discovery in 1990.
Unfortunately, physicists can't take the next step of observing isolated individual quarks. The reason: a property known as color confinement. If you try to pluck a single quark out of a proton, a new quark-anti-quark pair will suddenly pop out of the vacuum; it turns the single quark into a hadron and shields its nakedness from view. Particles called gluons bind the quarks together and play the same role in strong interactions that the photon plays in electromagnetic interactions. We shall discuss this in more detail in the next unit.
Rapid development of quark theory
Despite their aggregation into composite particles, the confirmation of fractionally charged particles inside the neutron and proton set the stage for rapid development in the next two decades. The three flavors of quark—up, down, and strange—were soon augmented by the discovery of a fourth. In 1974, two scientific teams almost simultaneously discovered the so-called "charm quark," in the form of a meson made up of a charm and an anti-charm quark. The fact that the teams used entirely different approaches to the discovery gave the find added credibility.
Figure 12: Computer reconstruction of a psi-prime decay in the SLAC Mark I detector.
Source: © SLAC National Accelerator Laboratory. More info
A team at SLAC headed by Burton Richter caused collisions between beams of electrons and their antiparticles, positrons, creating showers of particle-antiparticle pairs. The SLAC team tuned the beam energy, watching for any change in the amount of particles produced in the collision. The new meson revealed itself as a huge spike called a resonance in the probability of interactions between particles. The resonance appeared when the energy produced in the collision was near the new meson's mass. The other group, led by Samuel Ting of MIT, took a different tack. They fired protons onto a fixed target at Brookhaven National Laboratory and identified the meson's signature against the background of other particles.
Intriguingly, the two teams first gave the new meson different names. The SLAC physicists called it the "psi particle" because one of its characteristic decay modes produced four particles that curved in their detector's magnetic field to look like the Greek letter psi. Ting took an equally symbolic approach. He chose the name "J," owing to the similarity in shape between that letter and the ideogram for his Chinese name. Once they realized that they had discovered the same particle, the two teams agreed to name it "J/psi."
At this point in the story, the fundamental constituents of matter were once again manageable in number. We have two generations of particles, each of which consists of a lepton with charge -1, and two quarks with charges +2/3 and -1/3. The first generation has the three fundamental building blocks of entirely stable matter: the electron and the up and down quarks. The second generation consists of the muon, charm, and strange quarks. All are unstable and eventually decay into particles of the first generation. Why does a second generation exist? This remains a mystery that has only deepened with the discoveries that followed.
More surprising particles
Surprises continued beyond the 1970s. The next was the third lepton, the tau (after the Greek letter for or third). SLAC made the find within a couple of years after the discovery of the charm quark. Initially, the tau lepton confused the situation by making it much more difficult for experimenters to understand the detailed properties of mesons containing a charm quark. Eventually, however, the story fell into place. It became clear that the electron and muon had a third, much heavier cousin. While the muon is about 200 times heavier than the electron, the tau is about 3,500 times more massive. This immediately begged the question of the existence of a third generation of quarks, setting off another of those rushes to be the first to discover the missing puzzle pieces that were so clearly waiting to be found.
Experimenters at the Fermi National Accelerator Laboratory (Fermilab) near Chicago sought evidence of the bottom quark using the Tevatron, a new and bigger accelerator with higher energy protons. Fermilab physicists looked for evidence of the bottom quark in the particles produced in proton collisions with a stationary target, a process known as "bump hunting." The resonances appear as small bumps in the probability of particles being produced in a collision. Identifying the bumps requires careful statistical analysis.
Figure 13: Aerial view of the Tevatron at Fermilab.
Source: © Fermilab. More info
The Tevatron team searched for a resonance bump that would reveal the existence of the meson known as the "upsilon," consisting of bottom and anti-bottom quarks. After a false alarm due to statistical fluctuations that became known as the "Oops-leon," the team led by Leon Lederman was finally successful in discovering the upsilon.
Tracking down the top quark
The existence of the sixth quark, known as the "top quark," was now all but a certainty. Several groups around the world built accelerators that theorists regarded as energetic enough to produce and detect it, but not until 1995 did the top quark finally reveal itself. The Tevatron revealed it by producing top-anti-top quark pairs. Measurements showed that the top quark is about as heavy as a nucleus of gold. That's 40 times more massive than the bottom quark.
Figure 14: The Collider Detector at Fermilab (CDF).
Source: © Fermilab. More info
If creating enough energy to produce the top quark presented a huge challenge, so did identifying it. The top quark decays immediately to a bottom quark, which then usually decays to a charm quark. That, in turn, usually decays to a strange quark. These quarks are "clothed" as mesons, and the decay chain produces a variety of particles that finally live long enough to be seen inside the enormous detectors built around the collision point. Physicists must reconstruct the decay chain in order to determine if it reveals a top quark rather than a random combination of unrelated particles. Digging this rare signal out of the much noisier background caused by random combinations was a major success of the Fermilab program. It put the capstone on the Standard Model of fundamental particles.
The discovery of the sixth quark also completed the three families of quarks. It still leaves some unanswered questions, however. Why three families, when only the first generation of up and down quarks is necessary for ordinary matter? What does the pattern of masses mean, especially the very heavy top quark? And is there a fourth generation of quarks and leptons? Numerous searches have failed to find one, indicating that it must be very heavy if it exists. And evidence coming from the neutrino sector indicates that there are probably only three generations of quarks and leptons, as we shall now explain.