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3 Science Potential of IceCube High-energy neutrinos are unique messengers of some of the most extreme processes occurring throughout the universe. Unlike high-energy photons, high-energy neutrinos are not absorbed as they traverse the universe. Also, unlike charged particles such as protons and nuclei, neutrinos are not deflected by magnetic fields, and thus they point directly back to their sources. These unique properties of neutrinos make possible the discovery of new astrophysical systems and new physical processes through the detection of neutrinos with energies from about 1012 eV, which is as high as the current terrestrial accelerators reach, to much higher energies. The possibility of new discoveries in this very high energy range is the main motivation for building large neutrino detectors such as IceCube. IceCube could address some of the questions posed in the last chapter: What is the mysterious dark matter, and how much of it consists of neutrinos? What causes the most powerful explosions in the universe? How do supermassive black holes produce very high energy gamma rays? INTRODUCTION IceCube is an exploratory experiment in that it will search for astrophysical neutrinos in the very high energy range with much greater sensitivity than previ-
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ous efforts. It is key to realize that IceCube is a discovery instrument, more akin to a particle physics project than to a telescope—it may be that we will not see what we are looking for because there might not be any discrete sources of astrophysical neutrinos to discover. In that case, other techniques to address these science questions will certainly be needed. The discovery of very high energy neutrino sources, though, would clearly demonstrate the existence of the acceleration of hadrons (e.g., protons or nuclei) in known astrophysical discrete sources. By the same token, however, it cannot be said with certainty what IceCube will in fact detect—source flux predictions are uncertain and rely on the extrapolation from known astrophysics at lower energies. Possible sources include gamma-ray bursts, which are powerful explosions that release in seconds the same energy as a typical galaxy emits in years; active galaxies and quasars, which can be more than 1,000 times more luminous than normal galaxies and are believed to be powered by supermassive black holes at their centers; and neutron stars, which are ultracompact stellar remnants that have collapsed to densities comparable to those inside atomic nuclei. In addition, the possibility exists for new and unexpected types of sources, both astrophysical and exotic, at these energies. The lack of strong and electromagnetic interactions gives neutrinos the special ability to traverse the universe unimpeded, but it also makes them extremely hard to detect. To achieve sufficient sensitivity to detect high-energy neutrinos from distant sources, experiments on the scale of one cubic kilometer or a billion tons of detector mass are required. Such experiments use Earth as a converter and detect neutrinos as they traverse Earth from below and interact near the experiment. Neutrino interactions produce upward-going muons that generate Cerenkov light in a suitably transparent medium such as water or ice and that are recorded by an array of light sensors (photomultiplier tubes) spread throughout the volume of the detector. IceCube is to be constructed at the South Pole, making use of Antarctic ice as the detecting medium. The IceCube sensors are deployed by lowering strings of photomultiplier tubes into melted ice and allowing the strings to be frozen in place. See Figures 3.1 and 3.2 for a description of IceCube and its operation. Although IceCube is a major undertaking in a rather remote location, its design and prospects for success are bolstered by the strength of the existing polar infrastructure and, in particular, by the successful deployment and operation of AMANDA, a smaller precursor to IceCube. The properties of the South Pole ice as a detection medium are now better understood, and AMANDA has shown that the technique of detecting upward-going muons works well—it has reconstructed about 1,000 upward-going muons that are signatures of neutrinos produced in the atmosphere that lies below the horizon. IceCube will substantially improve on AMANDA’s capabilities, both through a larger detection volume and through the use of improved technology.
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FIGURE 3.1 The IceCube detector is shown in schematic form. It will be located near the South Pole, in Antarctica. It consists of 80 strings of photomultiplier light sensors suspended more than 2 km below the surface of the ice. Each string has 60 light sensors. The 1.4-km depth is required to obtain sufficiently clear and impurity-free ice. Image courtesy of the AMANDA Collaboration, the IceCube Collaboration, and the National Science Foundation. In the global perspective, IceCube is the only cubic-kilometer-scale neutrino telescope ready for construction now. There are efforts in Europe, notably the Astronomy with a Neutrino Telescope and Abyss Environmental Research (ANTARES) project, the Neutrino Extended Submarine Telescope with Oceanographic Research (NESTOR) project, and the Italian Neutrino Mediterranean Observatory (NEMO) project, to build detectors that use the Mediterranean Sea as the detecting medium. These groups are currently building smaller detectors and plan to propose cubic-kilometer-scale experiments in the near future for approval and construction starting within the decade. The experiments planned for the South Pole and for the Mediterranean are largely complementary in nature, in terms of both their observational targets and their capabilities. By detecting upward-going muons, IceCube is sensitive to astrophysical sources in the Northern Celestial Hemisphere, whereas a Mediterranean Sea experiment would study Southern Celestial Hemisphere sources. In comparison with water, ice typically scatters more light than it absorbs. Thus ice can provide a larger effective detector volume than water for the same number of optical sensors deployed. Conversely, water experiments can achieve better angular resolution than those using ice. The lower resolution of IceCube (about 1 arc-degree) is, however, adequate for observing the expected sparse distribution of sources, which can be more precisely localized with electromagnetic telescopes in any case. Finally, the committee mentions that novel techniques using radio and acoustic detection technologies for observation of neu-
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trino interactions in ice or in natural salt deposits are currently being explored and might offer long-range possibilities for this field. Such large-scale efforts with new technologies, at even higher energies, will be much sought after as a follow-up to well-established signals from a present-day experiment like IceCube. THE SOURCES OF HIGH-ENERGY NEUTRINOS The potential sources of high-energy neutrinos can be classified into several broad categories: astrophysical point sources, diffuse cosmic backgrounds, and new physics sources. Astrophysical objects can produce neutrinos via processes involving the extreme acceleration of hadrons. These processes are similar to those that take place at particle accelerators such as Fermilab or the Stanford Linear Accelerator Center (SLAC) but under much more extreme conditions and reaching much higher energies, by factors of a thousand up to a billion. A variety of astrophysical measurements using photons or cosmic rays support the idea that there are sources of high-energy neutrinos, but their exact flux levels are uncertain. Gamma-ray telescopes on Earth and in space have shown that both galactic sources, such as pulsars and supernova remnants (produced when massive stars explode), and extragalactic sources, such as active galaxies, are capable of accelerating particles to at least 1013 eV. In addition, the charged particle cosmic-ray spectrum extends to 1020 eV and beyond. Particles accelerated to such extreme energies will unavoidably produce ultrahigh-energy neutrinos. Based on these high-energy cosmic-ray and gamma-ray studies, the detector scale of IceCube (1 km3) is the minimum size that offers a reasonable chance of detecting neutrino emission from known sources. Understanding the processes that lead to such powerful accelerators and deciphering the mystery of the cosmic rays are the prime motivations for exploring the high-energy neutrino universe. Astrophysical Point Sources of Neutrinos The leading candidates for high-energy neutrino point sources are extragalactic objects such as active galactic nuclei (AGN), powered by supermassive black holes, and gamma-ray bursts (GRBs), whose origin is not yet understood (see Figure 3.3). Galactic phenomena such as pulsars and supernovae are also possible sources. Given typical neutrino-producing models for cosmic accelerators (i.e., hadronic models), IceCube’s sensitivity restricts its observations to only the most powerful or closest sources. For example, for a steady emitter located at cosmological distances, the source must have the power equivalent of 1013 solar luminosities to generate 10 neutrino events per year in a cubic-kilometer-scale detector. Some AGN can maintain such high luminosities for relatively long periods of
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FIGURE 3.2 (a) An aerial view of the Amundsen-Scott South Pole research station, where IceCube will be located. The station sits on a 2,700-m thick plateau of snow-covered ice. Image courtesy of USAF MSgt. David McCarthy and the National Science Foundation, (b) Teams of scientists and engineers use a hot-water drill to bore holes deep into the ice. Strings of light sensors are deployed in the holes, (c) Lowering one of the strings of light sensors; the glass capsule encases the photomultiplier tube. Once the water in the bore-hole refreezes, it will be impossible to access the sensors. Images courtesy of the AMANDA Collaboration, the IceCube Collaboration, and the National Science Foundation.
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FIGURE 3.3 (a) An image from a computer simulation showing IceCube’s principle of operation. A neutrino incident from below scatters in the ice, creating a muon. The high-energy muon leaves behind a trail of Cerenkov light whose direction and intensity are monitored by the strings of light sensors. The different colors in the figure represent the different times of arrival of the light signal, (b) IceCube will detect muons generated by cosmic neutrinos as they traverse and interact in Earth. Two candidate sources of extragalactic cosmic neutrinos are pictured here, NGC 4261 on the left and Hydra A on the right, (a) Image courtesy of the AMANDA Collaboration, the IceCube Collaboration, and the National Science Foundation, (b) Image of Earth courtesy of the Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California; image of NGC 4261 courtesy of Greg Taylor/NRAO; image of Hydra A courtesy of NASA/Jeffe, Ford, Ferrarese, Van Den Bosch, O’Connell, and NRAO.
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time (i.e., weeks, months). Transient objects, such as GRBs, can reach much higher peak power levels (higher by a factor of 100,000), but over shorter periods of time, such as tens of seconds. Certain types of AGN, called blazars, are copious sources of high-energy gamma radiation. A blazar is thought to be an AGN with a relativistic jet powered by matter falling into a black hole (with a mass about a billion times the mass of the Sun) nearly aligned with the direction to Earth. Detection of neutrinos at 1012 eV to 1015 eV energies from these sources can determine if jets are powerful accelerators of hadrons (i.e., protons and nuclei). If relativistic hadrons were accelerated with power comparable to that of the observed gamma rays, then detectable fluxes of neutrinos would be produced in the jet through pion production by nuclear and photohadronic interactions. If inelastic nuclear interactions are important, the neutrino spectrum is expected to reflect the spectrum of the relativistic particles from 109 eV to the highest energies. The electromagnetic gamma radiation from AGN has been studied by satellite- and ground-based telescopes. IceCube can probe these objects using neutrinos, extending the energy range by a factor of several hundred or more. The Energetic Gamma-Ray Experiment (EGRET) satellite detected almost 100 AGN at gamma-ray energies up to 1010 eV, while ground-based telescopes such as Whipple detected 5 AGN at energies up to 1013 eV. Many more AGN will be detected with the next generation of ground-based and satellite instruments. Neutrino fluxes from known gamma-ray-emitting AGN are detectable by IceCube, under two assumptions. First, a substantial fraction (e.g., 50 percent) of the power in the high-energy beam in the AGN jet must go into the acceleration of hadrons. Second, the energy spectrum of the neutrinos produced by the source must be relatively flat (i.e., it must extend to very high energies, decreasing slowly, at most as the inverse energy squared). Gamma-ray bursts represent another important potential source of high-energy neutrinos. Current models of GRBs involve the dissipation of the kinetic energy of a relativistically expanding fireball, caused by some explosive event, possibly the collapse of a massive star or the coalescence of two compact objects. The shocks resulting from this dissipation can accelerate particles to very high energies (gamma rays up to 1010 eV energies have been detected from GRBs). In most GRB models, the observed MeV gamma rays, as well as the recently discovered lower-energy afterglows (x ray, optical, radio), are attributed to emission from shock-accelerated electrons in magnetic fields. Under certain model assumptions, the neutrino emission from GRBs should be detectable by IceCube. With these assumptions, one derives an estimate of approximately 10 neutrino events per year at energies of 1014 eV in IceCube, detected from an ensemble of GRBs in the Northern Celestial Hemisphere. Gamma-ray bursts produced in the
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collapse of massive stellar progenitors could lead to about 10 neutrino events per individual burst, detected a few times a year in IceCube at energies of 1012 eV. Because these events would be correlated in position and in time with the gamma-ray bursts themselves, they would be largely background free, since the background outside this position and time window can be rejected. If detected, these neutrinos would help unveil the mysterious progenitors of GRBs by testing the gamma-ray emission model and the shock acceleration physics. Although AGN and GRBs are the likeliest high-energy neutrino source candidates, a variety of other astrophysical objects could be sources of neutrinos. As a very wide field telescope, IceCube could search for neutrino emission from most of the Northern Celestial Hemisphere. Among potential sources, young pulsars, which are highly magnetized, rapidly rotating neutron stars, are known to accelerate electrons and are likely to accelerate hadrons. Microquasars, smaller versions of AGN located in our galaxy, also show jets that may be observable with neutrinos, if the jets are sites of hadronic acceleration. Microquasar jets are associated with accreting stellar-mass black holes or neutron stars. Finally, supernovae—the very bright explosions of massive stars—and the remnants of supernova explosions are also likely to be sites of hadronic acceleration and neutrino emission. Diffuse Astrophysical Sources of Neutrinos Cosmic neutrinos coming from point sources in the sky are more easily detected than those neutrinos from diffuse sources because of the atmospheric neutrino background. Atmospheric neutrinos are constantly being produced from all directions in the sky at the low-energy range of IceCube’s reach. The exact level of this atmospheric neutrino background depends on the unknown forward production of neutrinos from charm quark decays. On the one hand, if this production channel is very efficient, the flux of atmospheric neutrinos as well as the flux of astrophysical neutrino point sources will increase accordingly. On the other hand, if the charm production of neutrinos is suppressed, astrophysical diffuse backgrounds could be correspondingly easier to resolve. Another potential source of background, however, is charm decay into high-energy muons at sufficient energies. The impact of this effect is not yet well understood, but it could influence IceCube’s abilities at the highest energies. The observation of cosmic rays with energies in excess of 1020 eV reaching Earth isotropically from all directions indicates that hadrons are accelerated to ultrahigh energies in extragalactic sources. IceCube can help determine the origin of these highest-energy cosmic rays by observing neutrinos generated at the same acceleration sites. A number of ultrahigh-energy astrophysical accelerators, proposed to explain the origin of the highest-energy cosmic rays, could be detectable
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by IceCube. Optically thin sources of protons at energies greater than 1019 eV are constrained to have neutrino fluxes that lie below the Waxman-Bahcall (or W-B) limit. This limit is based on the consideration that the energy input into neutrinos cannot exceed the observed cosmic-ray flux at high energies. IceCube can reach fluxes down to one-tenth of the W-B limit. In addition, there may be “hidden” sources that exceed this bound for neutrinos. In such sources, the high-energy hadrons are prevented from leaving, but the neutrinos escape. Whatever the source of the ultrahigh-energy cosmic rays, they are likely to produce a flux of high-energy neutrinos. In addition, the ultrahigh-energy cosmic rays can themselves produce neutrinos as they propagate through and interact with the remnant radiation from the big bang, the cosmic microwave background (CMB) radiation. The interactions between the CMB and protons with energies of 1020 eV and above produce pions, which subsequently decay, generating neutrinos. The flux of these photopion neutrinos rises around 1017 eV, where it is marginally accessible to IceCube at the level of one event per year. In the ultrahigh-energy range, IceCube is complementary to cosmic-ray experiments such as the Auger project, where the energy threshold for neutrinos starts around 1018 eV. Signatures of New Physics In addition to launching neutrino astronomy at the very high energies, IceCube has the potential to discover new interactions and new or possibly exotic relics from the early universe. Among the early universe relics that IceCube can study are the likeliest form of dark-matter particles, called neutralinos, whose collective gravitational force appears to dominate that of the ordinary visible matter in galaxies. Neutralinos may be indirectly detected by high-energy neutrino telescopes (e.g., IceCube) through their annihilations in the Sun. These searches are complementary in several ways to the direct searches that are discussed in the next section. IceCube will be sensitive to heavy neutralinos because the expected number of events does not depend sensitively on the neutralino mass, while the event rate falls linearly in direct detection experiments. It should also be noted that IceCube is sensitive to spin-dependent neutralino interactions on nuclei since neutralinos interact with protons in the Sun before they annihilate at the center, while the direct detection experiments are mostly sensitive to spin-independent neutralinos’ interaction with heavy nuclei. Therefore, the two types of experiments are sensitive to different parts of the parameter space of neutralinos. It must be borne in mind, however, that the neutralino-detection capabilities of IceCube are in some ways more speculative and more limited (i.e., they require certain assumptions and sample a smaller area of the parameter space).
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Some other proposed relics of the early universe, such as topological defects, can be copious emitters of neutrinos along with gamma rays and cosmic rays. Gamma rays and cosmic rays from these sources become severely depleted when propagating across the universe, while neutrinos reach Earth unimpeded. Finally, the detection of high-energy neutrinos from a known astrophysical source can be used to test the assumption of special relativity that photons and neutrinos have the same limiting speed, as well as the weak equivalence principle, according to which photons and neutrinos should suffer the same time delay as they pass through the gravitational potential of galaxies. Other departures from the Standard Model predictions, such as new physics at scales of beyond 1012 eV—the highest energies currently available from terrestrial accelerators—might also be inferred by studying the neutrino cross section on hadrons at energies well above 1012 eV. ICECUBE IN AN INTERNATIONAL CONTEXT IceCube is not the only option for a high-energy neutrino telescope. As mentioned, there are alternative technologies including the use of water (instead of ice) as the detector medium as well as techniques using radio or acoustic detectors still under development. As established above, a so-called gigaton detector is required to answer some of the key science questions. Will IceCube be unique in its abilities to address the questions? The jury is still out as to whether ice or water is a better detector and signal-transmission medium. (Detectors in ice suffer from scattering losses higher than those underwater, but ice is generally more transparent and possesses lower backgrounds, for example, from radioactive potassium-40 and bioluminescent marine life.) An expert panel of the International Union of Pure and Applied Physics (IUPAP) recently endorsed an underwater cubic-kilometer-scale follow-up to the NESTOR Mediterranean project, but no concrete proposals have been submitted as there is significant remaining research and development to determine the best design. As such, then, IceCube is unique in its stage of development, in its employment of ice as the detector medium, and in its location in the Southern Hemisphere. The IceCube project involves scientists from institutions in the United States, Belgium, Germany, Japan, Sweden, the United Kingdom, and Venezuela and so in itself is an international effort. Plans call for the detector to be built in stages toward the full cubic-kilometer volume, over a 5- or 6-year period. Unlike many large-scale experiments, IceCube will be operational during the construction period. Currently, IceCube has a head start on its competitors, so its timely deployment will give it a lead in the exploration of this new window onto astrophysics.
Representative terms from entire chapter: