Just another bubble

 

Section 4: From Cloud Chambers to Bubble Chambers

Physicists became impatient waiting for cosmic rays to produce the rare events that led to new discoveries. So after World War II, research shifted to national laboratories where accelerators were built to produce intense beams of energetic protons. To record the particles and their decay tracks, physicists built large bubble chambers. These liquid versions of cloud chambers recorded thousands of photographs of particle tracks.

Figure 8: An abandoned bubble chamber at Fermilab.
Source: © Fermilab. More info

The new accelerators represented greatly improved versions of the crude accelerators that J.J. Thomson and Ernest Rutherford had used in their pioneering studies of atomic structure. Those original instruments had a significant disadvantage: The naturally produced alpha and beta particles that provided the projectiles for the accelerators had relatively little energy. In 1927, Rutherford upped the ante by calling for ways of creating "a copious supply" of higher-energy particles. Ernest Lawrence, a young physics professor at the University of California, Berkeley, found a unique way to take up the challenge. It involved a circular device in which a magnetic field confined particles to orbiting in a horizontal plane while an alternating electric potential applied to each half of the circular plane would give the particles an energy boost twice per orbit. This ingenious technique avoided the use of very high voltages—an achievement both difficult and dangerous. Instead, it applied a modest voltage many times.
The first cyclotron built by Lawrence and his student M. Stanley Livingston measured 4.5 inches in diameter. As soon as they proved that it worked, they built a larger version. With a diameter of 11 inches, this accelerated protons to energies of more than one million electron volts. Eventually, Lawrence founded the Radiation Laboratory at Berkeley (now the Lawrence Berkeley National Laboratory) and oversaw the construction of ever-larger cyclotrons. That group of devices, which included an accelerator called the Bevatron, led to the discovery of new mesons, enabled the first detection of the antiproton, created transuranic elements, and even provided beams of particles for cancer treatment.

New species for the particle zoo

The Bevatron at Berkeley and the Cosmotron at Brookhaven National Laboratory on Long Island led the way to the new surge of discovering subatomic particles. Reaching full power in 1953, the Cosmotron became the first particle accelerator to give single particles kinetic energies of more than 1 giga-electron volt (GeV, or 10⁹ electron volts). Once it started operation in 1954, meanwhile, the Bevatron accelerated protons at energies up to 6.2 GeV into a fixed metal target.

Figure 9: The first cyclotron, the Bevatron, and particle tracks.
Source: Cyclotron: © Lawrence Berkeley National Laboratory, courtesy AIP Emilio Segre Visual Archives, Bevatron and Particle Tracks: © Lawrence Berkeley National Laboratory. More info

The studies added several new species to the particle zoo, with names like sigma (), cascade (), and delta (). Since these particles were heavier than the proton, physicists dubbed them baryons (meaning heavy ones in Greek). The research also revealed particles of different electrical charge—positive, negative, and neutral—with the same mass and decay properties, suggesting that they were members of a family. Physicists even identified a ⁺⁺ particle that had a charge of +2 (i.e., twice the proton charge)!
The situation now resembled that faced by chemists before the advent of the Rutherford-Bohr model of the atom. To impose some order, physicists followed Dmitri Mendeleev's example and constructed tables that organized the eight known mesons and nine known baryons according to their electric charges and amounts of strangeness (as determined by the number of kaons in the decay chain). They plainly needed a new theory to find the underlying symmetry in this particle zoo.

Three fundamental building blocks

In 1964, theorists Murray Gell-Mann and George Zweig independently suggested that all of the observed mesons and baryons could be constructed from just three fundamental building blocks. The pair regarded these quarks as mathematical constructs that were useful for explaining the observed data, but not necessarily as fundamental particles corresponding to physical reality.

The Naming of the Quark

"What's in a name?" asked Juliet Capulet in William Shakespeare's Romeo and Juliet. "That which we call a rose by any other name would smell as sweet." As with roses, so it is with subatomic particles. Murray Gell-Mann and George Zweig, who independently developed the concept of three truly fundamental particles in 1964, came up with different names for the entities. Zweig called them "aces." Gell-Mann trumped him with a more literary approach. He appropriated the word "quark" from the line, "Three quarks for Muster Mark!" in the James Joyce novel Finnegans Wake. The term had natural appeal because the concept itself envisioned just three of the entities as basic building blocks. While American physicists pronounce quark to rhyme with quart, most European physicists rhyme it with arc. However they pronounce the term, they know that the concept has come up aces.

The model postulated that the three types, or flavors, of quark—that physicists named up, down, and strange—had fractional electric charges. It assigned the up quark a charge of +2/3 (two-thirds of the charge on the proton), and the down and strange quarks charges of -1/3 (one-third of the electron's charge). All baryons, the model suggested, consisted of three quarks, combined in such a way that they have integral or zero electric charge. Protons, for example, contained two up quarks and a down quark, providing a net electric charge of +1. Neutrons stemmed from one up and two down quarks, netting out at zero charge.
Mesons, meanwhile, were created from just two constituent quarks. They gained their integral electric charges by combining quarks and anti-quarks. Anti-quarks are quarks' antimatter partners; they have the opposite electric charge and bear the same relation to quarks as positrons to electrons. For example, the pi+ consisted of an up quark and an anti-down quark with a charge of +1; the pi-zero stemmed from an up and an anti-up (or down and anti-down) quark; and the pi- from a down quark and an anti-up quark. And if you wanted kaons, you simply changed the down quarks to strange quarks.

Table 1: How quarks create baryons.

Quark 1	Quark 2	Quark 3	Baryon
up	up	down	proton
up	down	down	neutron
up	down	strange	lambda

Elegant in its simplicity, the theory echoed the atomic model that had posited the proton, neutron, and electron as the basic building blocks for more than 100 different elements. The quark model saw the proton and neutron as no longer fundamental but composite particles created from quarks. The model accounted for the entire particle zoo by combining three types of quarks and anti-quarks in all possible allowed combinations.
However, one combination had so far defied observation: the tenth baryon, constructed from three strange quarks, that Gell-Mann dubbed the "Omega minus (-)." Just as a gap in the periodic table suggested an element waiting to be discovered, the prediction of the quark model set off a search to find the missing baryon. Within the year, it culminated in the discovery of the Omega minus in the 80-inch bubble chamber at Brookhaven National Laboratory's 80-inch bubble chamber. Just like the periodic table, the quark model had predictive power.

Figure 10: The periodic table for heavier mesons and baryons.
Source: © Wikimedia Commons, GNU license version 1.2. Authors: Laurascudder, 2007 (Meson octet and Baryon decuplet) and Dr_Eric_Simon, 2006 (Baryon octet). More info

Despite this triumph, most physicists still did not believe that quarks really existed. Rather, they merely provided a useful artifice to explain the pattern of particles observed in nature. That opinion gained strength when experimentalists failed to find fractionally charged particles. But a new and powerful electron accelerator in California overturned that view.

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