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Astronomy and astrophysics with neutrinos

Francis Halzen and Spencer R. Klein

Traversing cosmological distances without bending or energy loss, high-energy neutrinos are messengers from extreme astrophysical environments.

Francis Halzen is a Hilldale Professor in the physics department at the University of Wisconsin–Madison. Spencer Klein is a senior scientist at Lawrence Berkeley National Laboratory in California. Both are members of the IceCube neutrino-telescope collaboration.

The idea of the neutrino was put forward in 1930 by Wolfgang Pauli in a desperate attempt to preserve energy conservation in nuclear beta decay. Soon Enrico Fermi ex- ploited the idea to create a theory of the weak interactions that laid the foundation for the now-standard model of par- ticle physics. Because of the putative neutrino’s small inter- action cross section, Pauli dubbed it “the particle that cannot be detected.” But in 1956 Frederick Reines proved him wrong. Reines realized that one could compensate for the tiny cross section with a large detector and a copious neu- trino source. After toying with the idea of an atomic bomb as the source, Reines settled on a nuclear reactor and discovered the neutrino with a detector that would look familiar today: a 200-liter liquid scintillator target monitored by photo- multiplier tubes.1

Reines has said that the idea of the neutrino as an astro- nomical messenger came to him immediately after its dis- covery. Now, half a century later, two first-generation high- energy neutrino telescopes, the Lake Baikal telescope in Siberia and AMANDA at the South Pole, are operating. And a third, ANTARES, off France’s Côte d’Azur, is nearing com- pletion. They transform fresh water, ice, and seawater, re- spectively, into particle detectors. A second-generation ex- periment, IceCube, which will encompass a cubic kilometer of Antarctic ice, is halfway toward completion. Those exper- iments are designed to search the neutrino sky beyond the Sun, possibly to the edge of the Universe.

Although those projects are the focus of this article, neu- trino astronomy predates them: Physicists have “seen” the Sun and a 1987 supernova in neutrinos. Both observations were of tremendous importance. The former showed that neutrinos have a tiny nonzero mass, which opened the first small crack in the standard model, and the latter confirmed that supernovae are indeed nuclear explosions.

Cosmic messengers and local backgrounds

Figure 1 shows the neutrino energy spectrum at Earth’s sur- face. It spans an enormous energy range, from microwave en- ergies (10–4 eV) up to the highest cosmic-ray energies (1020 eV). The lowest-energy neutrinos in the present cosmos were pro- duced in the Big Bang; they’ve been losing energy ever since in the cosmic Hubble expansion. The energies are so low that those neutrinos cannot be detected by present technology. But

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10–7 10–17

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10–6 10–1 104

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Active galactic nuclei

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NEUTRINO ENERGY (eV)

Figure 1. The cosmic neutrino spectrum. A low- energy background left over from the Big Bang is believed to suffuse the cosmos. Neutrinos from the Sun confirmed the fusion processes that provided its heat, and they yielded the first evidence of neutrino flavor oscillation. Neutrinos have also been detected from the nearby supernova explosion 1987A. Much of the spectrum of atmospheric neutrinos from cos- mic-ray air showers has been measured by the Fréjus underground detector (orange data points)

in France and the AMANDA detector (blue dots) deeply embedded in ice near the South Pole. Not yet observed are neutrinos expected from cosmological point sources such as gamma-ray bursts and active galactic nuclei. The most energetic neutrinos are ex- pected from the decay of pions created in collisions between cosmic-microwave-background photons and cosmic-ray protons with energies above 4 × 1019 eV (the Greisen-Zatsepin-Kuzmin threshold).

© 2008 American Institute of Physics, S-0031-9228-0805-010-2 May 2008 Physics Today 29

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Atmospheric

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NEUTRINO FLUX [(GeV sr s)–1 cm–2]

Image | Astronomy and astrophysics with neutrinos



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