Task 2.8 RENDERING PRACTICE
Preparation for rendering: Read the text / Find clue words / Make up a plan / Retell the text in your own words.
High-energy Neutrino Astronomy
High-energy neutrinos, with energies much larger than 100 MeV, must be emitted as a by-product of collisions of charged cosmic rays with matter; in fact, only neutrinos provide incontrovertible evidence for hadronic acceleration. Since they can escape much denser celestial environments than light, they can be tracers of processes which stay hidden to traditional astronomy. At the same time, however, their extremely low reaction probability makes their detection extraordinarily difficult.
First ideas to search for cosmic neutrinos other than those from the Sun date back to the late fifties. In 1960, K. Greisen proposed a 3000 ton underground Cherenkov detector to record neutrinos emitted by the Crab nebula. He was flanked by F. Reines, who realized, however, that “the cosmic neutrino flux cannot be usefully predicted” – i.e. that a mass of 3000 tons may be far too small. In the same year, M. Markov made his groundbreaking proposal “to install detectors deep in a lake or in the sea to determine the direction of charged particles with the help of Cherenkov radiation”. In the decades since then it was realized that high-energy neutrino astronomy requires detectors of a cubic kilometer or larger that can indeed only be implemented in open media. Actually, the first project of that size, IceCube at the South Pole, has just been completed. Two others, KM3NeT in the Mediterranean Sea and GVD in Lake Baikal, are in their preparatory phases. No doubt, we are entering an exciting era of opportunity.
Already now, neutrino astronomy is reality in the low-energy sector, where the detection of neutrinos from the Sun and the supernova SN 1987A was honoured by the 2002 Nobel Prize for physics. No practicable idea exists on how to measure the neutrinos of the 1,9 K neutrino counterpart to the cosmic microwave background. At higher energies, neutrinos from the Sun, from SN 1987A, from reactors and from the interior of the Earth have already been detected, as have so-called “atmospheric neutrinos” created in cosmic ray interactions in the Earth’s atmosphere. Still awaiting detection are high-energy cosmic neutrinos from extraterrestrial sources such as active galactic nuclei (AGN) or from interactions of ultra energetic protons with the cosmic microwave background. These cosmic neutrinos will hopefully be detected by neutrino telescopes in the next decade, even though predictions for their fluxes are uncertain by orders of magnitude in many cases.
The development of high-energy neutrino astronomy is reflected in a series of reviews spanning the period 1995 to 2009. The neutrino telescopes discussed here focus on energies beyond a few GeV. First searches for such neutrinos were made in the 1960s in the Kolar Gold Field mine in India and in the East Rand mine in South Africa. In the 1980s, the spectrum of atmospheric muon neutrinos was measured with a detector in the Fréjus tunnel between France and Italy, and a first limit on the diffuse flux of extra-terrestrial TeV neutrinos was set. Over the following decades, the evolution of underground neutrino detectors culminated in two experiments with an area of about 1000 m2 each: MACRO in the Gran Sasso Underground Laboratory in Italy and Super-Kamiokande in the Japanese Kamioka mine. MACRO collected more than thousand atmospheric neutrinos over six years of data taking. Super-Kamiokande, with an even larger data sample, is still in operation. The atmospheric neutrino results from these detectors have demonstrated that neutrinos oscillate between their flavour states νµ and ντ, additionally to the νe oscillations observed for solar neutrinos.
The first-generation detectors in water and ice have beaten the largest underground detectors by a factor of about 30 with respect to their sensitivity to high-energy neutrinos. The second-stage detectors on the cubic-kilometer scale will yield another factor of 30. Compared to detectors underground we therefore enter a “factor-1000 era”. Arguably, this factor is not a guarantee for discoveries. On the other hand it rarely happened in astronomy that improvements of more than an order of magnitude (in sensitivity or in angular or time resolution) came along without discovering new, unexpected phenomena. “Nothing is guaranteed, but history is on our side”: In some years we will know whether we indeed have entered an era of discovery or not.