Sky & Telescope August 1985 [antikvár]

Neutrino Astronomy: A New Window on the UniverseEric B. Norman, Lawrence Berkeley LaboratoryIN A VAST CAVERN carved out several kilometers beneath Earth's surface sits a huge, darkened reservoir of water. After months of blackness, a few brief flashes of light gleam in the liquid and are recorded by an array of electronic detectors. Scientists monitoring the experiment check their instruments, confirm the events, and consult with colleagues at other, similar underground "observatories" around the world. Upon learning of such detections, astronomers point their telescopes at a particular area of the sky and soon confirm the occurrence of one of nature's most awesome events a supernova explosion!Although such a chain of events is still only a hope for the future, it is not a forlorn one. But how could flashes of light in an underground tank of water signal the violent death of a star?The connection is provided by one of nature's most elusive particles, the neutrino, whose reactions can cause such flashes. Many neutrinos should be produced in supernova explosions and other astronomical environments. Because they interact so feebly with matter, neutrinos could literally pass through light-years of rock without significant attenuation. If we could "see" these particles we could learn a great deal about previously obscured regions of space, such as the interiors of stars. Fortunately, a number of neutrino "telescopes" are nowoperating throughout the world, using techniques developed for research in nuclear and particle physics. While no supernova has yet been detected in this way, the growing field is beginning to provide new insights into the nature of the universe.PHYSICS OF NEUTRINOSNeutrinos, unlike photons of visible light, are objects to which our eyes do not respond. This helps explain why their existence was not discovered until fairly recently. They were first proposed by Wolf gang Pauli in 1930 to explain the type of radioactivity known as beta decay. In this process a neutron changes into a proton and an electron. The proton stays in the nucleus of the atom while the electron (the beta particle) is ejected. If these were the only particles produced, conservation of energy would demand that the emitted electrons should have a specific energy.However, as can be seen from the figure on the next page, the electrons actually have all possible energies from zero up to the value predicted by the conservation law. Pauli suggested that the missing en-ergy was carried off by ai ed object the neutrino, tion also conserves linear mentum.) Experimental i that this particle have charge, intrinsic spin like tron, and a small or zeroother, undetect-(This explana-ind angular mo-vidence requires zero electrical that of an elec-rest mass.Because they are so penetrating, neutrinos were not directly detected until more than 20 years after Pauli's proposal. Eventually, Frederick Reines, Clyde L. Cowan, and their collaborators observed anti-neutrinos produced in a nuclear reactor. Since then, neutrino-induced reactions have become a well-established branch of particle-physics research. In fact, beams of such particles are now routinely available at a number of laboratories. The illustration below shows an example of a high-energy neutrino-induced reaction as photographed in a bubble chamber.Experiments have shown that there are at least three different "flavors" of neutrinos: electron, muon, and tau. Just what distinguishes these types is still not well understood, but the evidence suggests that a neutrino retains its identity.However, recent attempts to combine the weak and strong nuclear forces along with electromagnetism into one grand unified theory have led to the ideas that neutrinos may occasionally change from one type to another. If they, in fact, have some mass, they could provide the so-called missing mass required to slow universal expansion and close the universe (see page 22 of the July issue and page 396 of November, 1984). For these reasons there is currently a great deal of interest among both astronomers and physicists concerning the properties of neutrinos.