Case of the Missing Particles

How can you look inside the sun to see how it shines? In the mid-1960s, Ray Davis and John Bahcall thought they had a way. Drawing on advances made by other physicists earlier in the century, they intended to use notoriously elusive particles called neutrinos to verify ideas about the sun's inner workings. Theorist Bahcall calculated the number of neutrinos they expected to find, and experimentalist Davis tried to catch them. But for more than three decades, their results didn't jibe. In the chronology below, follow the case of the missing neutrinos, which ultimately led not only to a triumph for Davis and Bahcall but also to a surprising breakthrough in particle physics.—Susan K. Lewis

First Steps

1920: Theory of sunshine
British astrophysicist Sir Arthur Eddington proposes that the sun generates heat and light by "burning" hydrogen into helium. According to Eddington, every time four hydrogen atoms fuse to become a single atom of helium at the sun's core, a tiny bit of mass is converted into energy, just as Einstein indicated was possible in his famous equation E = mc².

1930: Neutrino "invented"
Austrian physicist Wolfgang Pauli conjures up the notion of a novel subatomic particle to solve a puzzle about the apparent non-conservation of energy in radioactive beta decays. A few years later, Italian physicist Enrico Fermi dubs the particle, which has no electrical charge, the neutrino, or "little neutral one." But there is no conclusive evidence that the particle exists, and most scientists think it may be impossible to ever detect.

1939: Theory of sunshine refined
In his landmark paper "Energy Production in Stars," Hans Bethe lays out details of how hydrogen is fused into helium in stars like the sun. His work leads to an understanding that the fusion process releases not only energy but also the particles Pauli "invented." Each time four hydrogen nuclei change into a helium nucleus, two neutrinos are emitted.

1956: Neutrino detected
In an endeavor dubbed "Project Poltergeist" conducted at the Savannah River nuclear reactor, Frederick Reines and Clyde Cowan prove that the neutrino actually exists.

Grand Experiment

1964: Davis and Bahcall launch test
Ray Davis and John Bahcall propose that a study of neutrinos emitted from the sun can show that nuclear fusion—the "burning" of hydrogen nuclei to helium nuclei—is indeed the source of the sun's energy.

1964: Bahcall predicts number of neutrinos
John Bahcall creates the first detailed mathematical model of fusion reactions in the sun's interior. As Bahcall later notes, he has to take account of "a smorgasbord of nuclear reactions at energies where measurements are difficult." He draws upon Hans Bethe's work, including Bethe's estimate of the sun's core temperature. There are countless pitfalls in devising the model. Just a one percent error in the temperature figure alone means a 30 percent error in the predicted number of neutrinos. And the projected number is astounding: about a hundred billion solar neutrinos pass through your thumbnail every second, according to Bahcall's model.

1965-1966: Davis builds experiment
Deep in the Homestake gold mine in Lead, South Dakota, sheltered from confusing background radiation, Ray Davis oversees construction of a giant neutrino trap: a tank of cleaning fluid roughly as big as an Olympic-size swimming pool. The cleaning fluid is mostly chlorine, which occasionally turns into a radioactive isotope of argon when struck by solar neutrinos. Bahcall has calculated that roughly 10 atoms of argon will be produced each week, and Davis is confident he can extract and measure them.

1968: Davis's initial results
The much-touted experiment appears a failure. Davis announces that he has detected only about one third as many radioactive argon atoms as Bahcall predicted. Scientists call the discrepancy "The Solar Neutrino Problem." The press calls it "The Mystery of the Missing Neutrinos."

Decades of Doubt

In the two decades following their disappointing results, Davis fine-tunes his solar neutrino detector, and Bahcall refines and checks his calculations. Hundreds of other physicists, chemists, and astronomers also examine Bahcall and Davis's work. No one can find significant fault with either the apparatus or the calculations. Yet along the way there are hints of a solution to the problem:

1969: A possible explanation
Physicists Vladimir Gribov and Bruno Pontecorvo, working in the Soviet Union, suggest that Davis and Bahcall's missing neutrinos can be explained by "neutrino oscillations": perhaps, as they travel to Earth, some of the neutrinos made inside the sun oscillate, or change, into types of neutrinos that Davis's apparatus can't detect. It's been known since mid-century that different types of neutrinos exist. But few physicists take stock in Gribov and Pontecorvo's idea. According to the Standard Model, the cornerstone of modern particle physics, neutrino types are distinct and can never change one into another.

1978 and 1985: Pursuing a bold notion
Building on Gribov and Pontecorvo's radical solution, Lincoln Wolfenstein in 1978 and Stanislav Mikheyev and Alexei Smirnov in 1985 show how electron neutrinos created at the sun's core might switch quantum states as they interact with other matter in the sun and travel outward to the surface.

1985: More missing particles
In an experiment called Kamiokande, sited in the Kamioka Mozumi mine in Japan, Masatoshi Koshiba and colleagues detect far fewer atmospheric neutrinos—neutrinos produced by the collision of cosmic rays with Earth's atmosphere—than they expect to see. While atmospheric neutrinos are a different type from those produced by the sun, the so-called "atmospheric neutrino anomaly" is similar to the solar neutrino problem. Where are the missing neutrinos?

Mystery Solved

1998: Answer to riddle of atmospheric neutrinos
A scaled-up version of Kamiokande called Super-Kamiokande reports on more than 500 days of data collecting. The detector is so big that it can tell what direction atmospheric neutrinos are coming from, and it has picked up far fewer neutrinos traveling from the other side of the Earth than from the sky directly above Japan. There is evidence that many of the atmospheric neutrinos from the other side of the Earth have changed into a different type of neutrino during their journey. This confirmation of neutrino oscillation carries a profound implication: the Standard Model of particle physics must be modified.

2001-2002: Proof of solar neutrino oscillation
The Sudbury Neutrino Observatory (SNO), the first neutrino detector that can pick up all three known types of neutrinos, resolves conclusively that, in the case of the missing solar neutrinos, the neutrinos are not, in fact, missing. SNO finds that the total number of neutrinos from the sun is remarkably close to what John Bahcall predicted three decades earlier. Ray Davis's experimental work is vindicated as well, because SNO finds that only about a third of the solar neutrinos that reach Earth are still in the same state that Davis could measure. Roughly two-thirds change type—or oscillate—during the journey.

2002: Nobel Prize recognizes achievement
The Nobel Prize in Physics is awarded to Ray Davis and Masatoshi Koshiba, a leader of the Kamiokande group. The Nobel citation praises them "for pioneering contributions to astrophysics, in particular for the detection of cosmic neutrinos." The award is also a tribute to their colleagues and the many dedicated scientists whose work led to a fundamental shift in particle physics.