Field of Science

What mad pursuit...

These days we are all excited about the Higgs boson, but as Frank Close reminds us in his lucid and comprehensive yet succinct book, the real heroic efforts in particle physics of the twentieth century were in pursuing and hunting down the elusive neutrino. The neutrino is copiously produced by solar processes and every second billions of neutrinos astonishingly pass through our bodies, yet the particle has no charge and for a long time was postulated to have no mass, which made its detection difficult to put it mildly.

Close documents the initial theoretical efforts by Wolfgang Pauli, Enrico Fermi and others to explain atomic processes like beta decay by invoking the neutrino. But the real heroes in the story are the experimentalists who spent their entire careers and gambled their scientific lives in dogged pursuit of this ghost particle. It was Bruno Pontecorvo, a protege of Fermi who realized that one could set up chlorine tanks near nuclear reactors to detect the existence of neutrinos. Pontecorvo also proposed other creative and theoretical ideas to capture and analyze neutrinos. He certainly deserved and would probably have won a Nobel Prize had he lived long enough and not defected to the Soviet Union. After Pontecorvo, the great modern heroes of the neutrino story are Raymond Davis and John Bahcall who spent their lives making heroic efforts to nail down the identity of Fermi's "little neutral one". Davis read Pontecorvo's paper in the early 50s and decided to set up an ambitious experiment with a chlorine tank several kilometers underground in an abandoned mine. The location was necessary to shield out other radiation from cosmic rays and capture only neutrinos, which being massless can travel virtually unimpeded through the earth. At the same time their lack of charge and mass makes their interaction with matter very rare and fleeting. Bahcall was a theoretical wizard who provided increasingly accurate estimates of the rate of capture. Half a century of almost obsessive work by the two men won Davis a Nobel Prize in physics, which he should have shared with Bahcall.

The story also has amusing side-lines, such as when a group of physicists called a nearby nuclear power station to correct their calculations for antineutrinos produced by the reactor. Not knowing what an antineutrino was, the reactor personnel assumed that the particle was harmful and that the physicists were environmentalists, and they tried to assure the scientists that "no antineutrinos were being produced" which would have been impossible and violated some fundamental laws of physics. One of the most intriguing discussions in the book documents the resolution of the so-called "solar neutrino problem". The generation of neutrinos in the processes that produce solar energy had been described by Hans Bethe and others. But the actual rate of detection turned out to be far less than the theoretical postulated rate. Something was missing and this caused a lot of angst for several decades. Bahcall and Davis gambled their entire careers on this paradox. A lot of creative, Nobel Prize caliber work by many scientists involving the decay of other novel particles like muons and pions finally revealed that the neutrinos emitted by the sun were actually changing their identities between two "flavors" called electron and muon neutrinos. This process was termed neutrino oscillation. The underground detectors could detect only one flavor of neutrino, explaining the discrepancy between theory and experiment. It was one of particle physics's resounding triumphs and revealed among other things that neutrinos have a vanishingly small but finite mass.

The tremendous work with neutrinos in the 20th century has led to the flourishing of a branch of astronomy called "neutrino astronomy" in the 21st. The study of the types, numbers, directions and flavors of neutrinos can shed valuable light on astrophysical processes taking place inside exotic objects like supernovas millions of light years away. Some of the facilities set up to detect neutrinos involve football field sized underground detectors filled with hundreds of tons of material located in some of the most extreme environments on the planet like the South Pole in order to avoid interference from other sources. Neutrino astronomy has turned physicists into intrepid explorers traveling to the far reaches of the planet. Their work is ensuring that we now have an additional window into the workings of the farthest and deepest reaches of the cosmos. But as Close excitingly documents in this slim volume, the foundation for all these exciting developments was laid by the theoreticians and experimentalists who participated in some of the most exciting races and pursuits of particle physics during the twentieth century. It's a story that's as rousing as any in science.

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