IceCube: Neutrinos Associated with Cosmic Rays

IceCube: Neutrinos Associated with Cosmic Rays Francis Halzen Department of Physics, University of Wisconsin, Madison, WI 53706,USA and DESY, Zeuthen, Germany ABSTRACT After a brief review of the status of the kilometer-scale neutrino observatory IceCube, we discuss the prospect that such detectors discover the still-enigmatic sources of cosmic rays. After all, this aspiration set the scale of the instrument. While only a “smoking gun” is missing for the case that the Galactic component of the cosmic-ray spectrum originates in supernova remnants, the origin of the extragalactic component remains as inscrutable as ever. We speculate on the role of the nearby active galaxies Centaurus A and M87. 1. The First Kilometer-Scale, High-Energy Neutrino Detector: IceCube A series of first-generation experiments1) have demonstrated that high-energy neutrinos with ∼10 GeV energy and above can be detected by observing Cherenkov radiation from secondary particles produced in neutrino interactions inside large volumes of highly transparent ice or water instrumented with a lattice of photomultiplier tubes. The first second-generation detector, IceCube (see Fig.1), is under construction at the geographic South Pole2). Figure 1: The IceCube detector, consisting of IceCube and IceTop and the low-energy sub-detector DeepCore. Also shown is the first-generation AMANDA detector. arXiv:0906.3470v1 [astro-ph.HE] 18 Jun 2009 IceCube will consist of 80 km-length strings, each instrumented with 60 10-inch photomultipliers spaced by 17 m. The deepest module is located at a depth of 2.450 km so that the instrument is shielded from the large background of cosmic rays at the surface by approximately 1.5 km of ice. Strings are arranged at apexes of equilateral triangles that are 125 m on a side. The instrumented detector volume is a cubic kilometer of dark, highly transparent and sterile Antarctic ice. Radioactive background is dominated by instrumentation deployed into this natural ice. Each optical sensor consists of a glass sphere containing the photomultiplier and the electronics board that digitizes the signals locally using an on-board computer. The digitized signals are given a global time stamp with residuals accurate to less than 3 ns and are subsequently transmitted to the surface. Processors at the surface continuously collect these time-stamped signals from the optical modules; each functions independently. The digital messages are sent to a string processor and a global event trigger. They are subsequently sorted into the Cherenkov patterns emitted by secondary muon tracks that reveal the direction of the parent neutrino3). Based on data taken with 40 of the 59 strings that have already been deployed, the anticipated effective area of the completed IceCube detector is shown in Fig.2. Notice the factor 2 to 3 increase in effective area over what had been anticipated4). Figure 2: The neutrino effective area (averaged over the Northern Hemisphere) from IceCube simulation (black histogram) is compared to the convolution of the approximate muon effective area from reference5) (solid red line) that we will use in the various estimates of event rates throughout this paper. The neutrino area exceeds the design area (shown as the dashed blue line) 4) at high energy. Despite its discovery potential touching a wide range of scientific issues, construction of IceCube has been largely motivated by the possibility of opening a new window on the Universe using neutrinos as cosmic messengers. Specifically, we will revisit IceCube’s prospects to detect cosmic neutrinos associated with cosmic rays and to reveal their sources prior to the 100th anniversary of their discovery by Victor Hess in 1912. Cosmic accelerators produce particles with energies in excess of 108 TeV; we still do not know where or how6). The flux of cosmic rays observed at Earth is shown in Fig.3. The energy spectrum follows a sequence of three power laws. The first two are separated by a feature dubbed the “knee” at an energy∗of approximately 3 PeV. There is evidence that cosmic rays up to this energy are Galactic in origin. Any association with our Galaxy disappears in the vicinity of a second feature ∗We will use energy units TeV, PeV and EeV, increasing by factors of 1000 from GeV energy. log10(Ep/GeV) Ep2·dNp/dEp [GeV cm-2 s-1 sr-1] 10 -9 10 -8 10 -7 10 -6 10 -5 10 -4 10 -3 10 -2 10 -1 1 2 3 4 5 6 7 8 9 10 11 12 Figure 3: At the energies of interest here, the cosmic-ray spectrum follows a sequence of 3 power laws. The first 2 are separated by the “knee”, the 2nd and 3rd by the “ankle”. Cosmic rays beyond the ankle are a new population of particles produced in extragalactic sources. in the spectrum referred to as the “ankle”; see Fig.3. Above the ankle, the gyroradius of a proton in the Galactic magnetic field exceeds the size of the Galaxy, and we are witnessing the onset of an extragalactic component in the spectrum that extends to energies beyond 100 EeV. Direct support for this

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