- Email: [email protected]
IceCube: Recent results and prospects
ARTICLE IN PRESS
Nuclear Instruments and Methods in Physics Research A 588 (2008) 92–98 www.elsevier.com/locate/nima
IceCube: Recent results and prospects T. DeYoung Department of Physics, Pennsylvania State University, University Park, PA 16802, USA For the IceCube Collaboration Available online 12 January 2008
Abstract Construction of the IceCube neutrino telescope is now more than 25% complete, with 22 strings in the ice and 26 IceTop air shower stations deployed on the surface. Initial results from data taken with nine strings in 2006 are shown, as well as results from the AMANDA array, which has been incorporated into IceCube. We also discuss the scientific potential of IceCube as it amasses 1 km3 year of data over the next few years. r 2008 Elsevier B.V. All rights reserved. PACS: 95.55.Vj; 98.70.Sa; 95.35.+d Keywords: Neutrino telescopes; Cosmic ray origins and acceleration; Dark matter
1. Introduction The IceCube Observatory, located at the South Pole, Antarctica, is the world’s largest neutrino telescope. Completion of the full km3 array is planned for 2011, but data are already being taken with the partially installed detector and the first published results indicate that the detector is operating as expected. IceCube’s primary goal is the discovery of astrophysical neutrinos in the rough energy range of 1 TeV to 100 PeV. IceCube includes an extensive air shower array on the surface, known as IceTop, which will enable a broad program of cosmic ray physics. The prototype AMANDA array, co-located with IceCube, has also been incorporated into IceCube to extend its sensitivity to events below TeV energies. After briefly describing the detector, we will survey results obtained with the AMANDA-II detector prior to integration with IceCube, and initial results from IceCube. We will then discuss the scientific prospects as IceCube is completed over the next few years. 2. The IceCube detector The IceCube design [1] consists of up to 80 strings, bundled electrical cables providing power and data E-mail address: [email protected] 0168-9002/$ - see front matter r 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2008.01.008
connections to digital optical modules (DOMs) deployed at depths of 1450–2450 m below the surface of the ice. Sixty DOMs are connected to each string, with a spacing of 17 m between modules. The strings are arranged on a triangular grid with an inter-string spacing of 125 m. IceCube also takes advantage of the solid surface above the array to support an 80-station extensive air shower array called IceTop [2]. Each IceTop station, situated above an IceCube string, consists of two light-tight tanks filled with ice and each containing two DOMs, operating at different gains in order to extend the dynamic range for signals in the tank. The DOMs [3] used by IceCube and IceTop contain 10 in. Hamamatsu photomultiplier tubes (PMTs) housed in glass pressure vessels. In addition to the PMTs, nearly all of the data acquisition electronics are embedded in the DOMs, which report fully digitized waveforms asynchronously to a software-based trigger and event builder on the surface. The electronics provide 300 megasamples per second (MSPS) sampling over a 400 ns window, plus coarser 40 MSPS sampling over a 6:4 ms window to catch late photons. The instantaneous dynamic range is approximately 103 photoelectrons (PEs) per 10 ns. Two parallel sets of digitizing electronics on each DOM operate in ‘‘ping-pong’’ mode, so that one set is active and ready to acquire while the other is read out. This design provides the advantages of autonomous DOM calibration (with timing
ARTICLE IN PRESS T. DeYoung / Nuclear Instruments and Methods in Physics Research A 588 (2008) 92–98
resolution of better than 2 ns RMS) and nearly deadtimeless operation and avoids signal degradation in transmission. The noise rate observed for DOMs in the deep array is 600–700 Hz, of which a significant fraction is non-Poissonian correlated noise due to scintillation effects in the glass pressure vessel housing the DOM. The underlying Poissonian rate of random noise hits is approximately 280 Hz, which allows us to monitor the DOM hit rates to search for low energy (MeV) neutrinos from supernova core collapse with sensitivity throughout the Milky Way and out to the Large Magellanic Cloud. The final component of IceCube is its predecessor, the AMANDA array, which has been incorporated into the IceCube detector [4]. The AMANDA optical modules are less sophisticated than the IceCube DOMs, with the data acquisition electronics on the surface and smaller 8 in. PMTs in the OMs. Roughly half of the 677 AMANDA OMs transmit their signals to the surface over optical fibers, which allows a timing accuracy of 2–3 ns, comparable to the DOMs, although with greatly reduced dynamic range. The other half are connected to the surface only by electrical cables, which stretch the pulses substantially and inhibit the separation of successive pulses, although the timing of the first pulse can still be found with an accuracy of a few ns. Although the AMANDA hardware is more primitive than the IceCube DOMs, the AMANDA array is much denser than IceCube, with typical string spacings of 30–40 m rather than IceCube’s 125 m. For relatively dim, low energy events, AMANDA thus collects considerably more information than IceCube. IceCube strings deployed in the AMANDA volume further increase the OM density, and IceCube strings surrounding AMANDA can be used as an active veto against cosmic ray muons, making the combined IceCube þ AMANDA detector considerably more effective for low energy studies than AMANDA was alone. 2.1. Construction status As of 2007, 22 IceCube strings and 26 IceTop stations have been successfully deployed [5]. The survival rate of the DOMs is 98%, consistent with expectations. Of the 1320 DOMs deployed in the deep array, only two have failed after commissioning, leading to a projected 20-year survival rate of approximately 97%. Thirteen strings were deployed in the 2006–2007 austral summer, with 14 or more additional strings planned for the 2007–2008 summer. In 2006–2007, the average time spent drilling each hole was reduced to 40 h, instilling confidence that the pace of deployments can be increased. Also in 2007, AMANDA was fully integrated into IceCube, with joint triggering and online filtering performed on the merged data recorded from both data acquisition systems.
93
3. Recent results The performance of IceCube and AMANDA has been established by the observation of atmospheric muon neutrinos, produced by the decay of p and K mesons in cosmic ray air showers in the Earth’s atmosphere. This flux is known to a precision of about 30% in the energy range to which IceCube and AMANDA are sensitive [6], above roughly 100 GeV. The atmospheric neutrino flux thus serves as a useful calibration source, although it also constitutes a background for searches for astrophysical neutrinos. The observation of atmospheric neutrinos by AMANDA was reported in Ref. [7] and updated results are available in Ref. [8]; observations with the IceCube detector are available in Ref. [9]. 3.1. AMANDA The AMANDA detector operated in its final independent configuration, known as AMANDA-II, from 2000 to 2006. Results from a search for point sources of neutrinos have been reported for the first five years of the data set, comprising 4282 candidate neutrino events with an estimated background contamination of approximately 5%, as shown in Fig. 1. No statistically significant point source of neutrinos is seen. At 90% confidence level, the upper limit placed on a reference E 2 point source flux of muon neutrinos, averaged over declination in the Northern Hemisphere, is E 2 dF=dEo5:5 108 GeV cm2 s1 . Full details of the analysis are available in Ref. [10]. A search was also performed for emission from 32 specific candidate sources of astrophysical neutrinos, chosen based on observations at various wavelengths in the electromagnetic spectrum. No statistically significant evidence for neutrino emission was found from any of the candidate sources. The candidates and results are also described in Ref. [10]. Several searches for neutralino dark matter have been performed using AMANDA. Such dark matter is expected to accumulate in gravitational potential wells such as the Earth or Sun and annihilate through various channels, producing a flux of neutrinos that might be detectable. Results from searches for such a neutrino flux from the centers of the Earth [12] and Sun [13] have been published; the limit placed on the neutralino flux from the Sun using 75°
60° 45° 30° 15°
24 h
0h
Fig. 1. Sky map produced by AMANDA-II using data from 2000 to 2004. There are 4282 neutrino candidates observed, consistent with the expected atmospheric neutrino flux.
ARTICLE IN PRESS T. DeYoung / Nuclear Instruments and Methods in Physics Research A 588 (2008) 92–98
94
data from the AMANDA-II configuration are shown in Fig. 2. In addition to searches for individual sources of neutrinos, AMANDA observations have been used to place a limit on a possible diffuse flux of neutrinos from populations of unresolved distant sources. This diffuse flux is distinguished from the roughly isotropic foreground of atmospheric neutrinos due to the harder spectra expected for most astrophysical sources. A limit of E 2 dF=dEo7:4 108 GeV cm2 s1 sr1 is placed on the diffuse muon neutrino flux in the energy range from 16 TeV to 2.5 PeV at 90% confidence level [8]. Limits on several specific proposed diffuse spectra are also given in Ref. [8].
events, shown in Fig. 3, suggests that there is residual background contamination at the level of approximately 10% from cosmic ray muons in the declination band near the horizon, but for more steeply upgoing directions the sample is consistent with the expected flux of atmospheric neutrinos. The geometry of the nine-string detector also presents a challenge for the detector simulation software. Fig. 4 shows the very strong effect of the highly asymmetric string layout on the observed data: the detector is considerably more sensitive to muons traveling along the long axis of the
3.2. IceCube The IceCube array is expanding rapidly as construction proceeds. In the 2006 austral winter, the detector consisted of nine strings. The total instrumented volume of this stage of the detector was comparable to that of AMANDA-II, although the density of instrumentation is lower. The total integrated exposure of the detector is thus considerably lower than that of the full AMANDA data set, but still sufficient to observe atmospheric neutrinos. As with AMANDA, the relatively well-known flux of atmospheric muon neutrinos serves as a useful calibration source for IceCube, as described in Ref. [9]. A total of 234 neutrino candidate events were identified in the first 137.4 days of lifetime. The zenith angle distribution of those
AMANDA-II 2001 SI
XENON10 2007 SUPER-K 1996-2001
Muon flux from the Sun (km-2 yr-1)
105
BAKSAN 1978-95
Fig. 3. Zenith angle distribution of the 234 neutrino candidates found in the 2006 IceCube nine-string data set. Except for some residual cosmic ray muon events near the horizon, the sample is consistent with expectations for the atmospheric muon neutrino flux. The band of predictions for the atmospheric neutrino flux includes both theoretical uncertainties in the flux and systematic uncertainties in the modeling of the detector response. The error bars on the data are purely statistical.
MACRO 1989-98
104
103
102
10
1 10
103 102 Neutralino Mass (GeV)
104
Fig. 2. Limits at 90% confidence level placed on the flux of neutrinos from neutralino annihilation in the center of the Sun, as a function of the hypothetical neutralino mass. A hard (Wþ W ) annihilation channel is assumed, and the flux is that above a muon energy threshold of 1 GeV. Dots indicate supersymmetric parameters probed by the XENON-10 limits [11], and crosses indicate regions of parameter space not constrained by those limits.
Fig. 4. Azimuth angle distribution of the 234 neutrino candidates found in the 2006 IceCube nine-string data set. The inset shows the irregular surface layout of the strings. The strong modulation of the detector response is due to the string geometry, with the highest sensitivity to tracks oriented with the long axis of the detector, approximately 45–2251. The Monte Carlo simulation reproduces this effect in detail.
ARTICLE IN PRESS T. DeYoung / Nuclear Instruments and Methods in Physics Research A 588 (2008) 92–98
detector than those cutting across that axis. This effect is correctly reproduced in our simulation of the detector. As the remainder of the IceCube strings are installed, of course, the detector geometry will become more regular and these asymmetries in its response will disappear. 4. Scientific prospects The integrated exposure of IceCube will grow rapidly as construction progresses. Fig. 5 shows the projected deployment schedule and the anticipated exposure in km3 year available during the construction phase. One km3 year of data, approximately an order of magnitude more than collected over the full lifetime of AMANDA-II, will already have been collected in 2009. Four km3 year of data will be available after the first year of operations with the complete detector. This substantial increase in exposure will produce considerable gains in sensitivity for the types of analyses described in Section 3. For diffuse fluxes, for example, the sensitivity expected from one year of operations with the complete detector is E 2 dF=dE8 109 GeV cm2 s1 sr1 [14], an order of magnitude better than the current AMANDA-II limit and well below the Waxman–Bahcall limit (assuming flavor equality due to oscillations). In addition to the effective volume of the detector, IceCube will provide a substantial improvement in the precision of event reconstruction. Median angular resolution better than 0.81 is expected for muons above 1 TeV 80
5 km3-yr
Strings
4.5
70
4 60
km3-yr
3 2.5 2
50 40 30
# Deployed Strings
3.5
1.5 20 1 0.5 0 2005 2006 2007 2008 2009 2010 2011 2012 Date
10 0
Fig. 5. Projected string deployment schedule and expected integrated exposure of the IceCube detector during the construction phase of the project. One km3 year of data is expected by 2009, two years before construction of the full array is completed.
95
[14] from nearly all directions; this expectation is likely pessimistic, as it is based on techniques developed for AMANDA which do not make use of the full capabilities of the IceCube hardware. Because the sensitivity to point sources of neutrinos improves with angular resolution due to reduced background in the source search bin, this constitutes an additional source of improved sensitivity. For point sources, an average 90% upper limit on E 2 spectra of E 2 dN=dE5:5 109 GeV cm2 s1 would be expected with one year of data from the complete IceCube detector [14], an increase of an order of magnitude in sensitivity over the current AMANDA-II limit based on five years of operation. Furthermore, the DOMs used in IceCube record the total light detected with considerably higher precision than AMANDA OMs. This leads to an improved energy resolution for both muons and cascades in the detector. For muons, an energy resolution of about 0.3 in log10 E m is expected above 104 GeV [15]. For lower energy muons, stochastic energy loss mechanisms such as bremsstrahlung are less significant and the light emitted per unit track length is approximately constant, so that other methods for muon energy reconstruction will be required (for example based on the measured length of the track). 4.1. IceTop IceTop will be a 1 km2 EAS array on the surface above the deep IceCube array. IceTop will be uniquely capable of measuring the composition of cosmic rays in the region of the knee due to its ability to independently measure the electromagnetic and muonic content of air showers by comparing the response of the surface array to that of the deep array. Generally speaking, heavier primaries produce more muons per unit primary energy than do lighter primaries. Fig. 6 shows one of the several parameters with sensitivity to composition: the charge recorded by in-ice DOMs as a function of distance from the shower axis. Heavy primaries with multiple muons penetrating deep into the ice produce more light in the detector, as expected [16]. 4.2. The combined IceCube-AMANDA detector As mentioned in Section 2, the existing AMANDA detector was converted in 2007 into a subcomponent of IceCube, with joint online triggering and on-site filtering based on data from the combined detector. Although AMANDA’s hardware is somewhat primitive compared to IceCube, its dense spacing should provide improved sensitivity to low energy neutrinos. For example, Fig. 7 shows the increase in the number of low energy atmospheric muon neutrinos triggering the detector and passing the online filters [4]. With a complete analysis of these events, now in progress, we expect a significant increase in IceCube’s sensitivity to neutrinos at sub-TeV energies due to the inclusion of AMANDA. Sensitivity to neutrinos at
ARTICLE IN PRESS 96
T. DeYoung / Nuclear Instruments and Methods in Physics Research A 588 (2008) 92–98
these energies is important for a number of physics topics, including dark matter searches, observation of neutrino oscillations, and searches for neutrino emission from Galactic sources. Based on their observed TeV gamma ray emission, most galactic neutrino source candidates are expected to have softer neutrino spectra and/or spectral cutoffs at lower energies than is predicted for extragalactic sources [17–20]. Sensitivity to lower energy neutrinos is clearly important for observing such sources. In addition, many sources are located in the region of the Galactic center [21], which is in the southern sky. A densely instrumented region may permit the identification of (partially) contained events, in
which the neutrino-nucleon vertex is located within the detector. Cutting on such events would reject the cosmic ray muon background, leaving only the irreducible atmospheric neutrino background to astrophysical neutrinos, and permit a search for neutrino emitters from the Southern Hemisphere, such as the TeV gamma ray sources observed by HESS. The cost of such a strategy would be reduced sensitivity to higher energy neutrinos, for which the effective volume is increased by the long muon range; however, for muons with ranges less than the detector length scale (i.e., below roughly E m 200 GeV), nearly all muons will be produced within the detector anyway. For searches for WIMP dark matter, increased sensitivity to low energy neutrinos corresponds to sensitivity to WIMPs of lower mass. Fig. 8 compares the effective volume of the combined detector to that of IceCube alone, for WIMP annihilations in the Sun through hard or soft channels [22]. For low mass WIMPs, an order of magnitude improvement is seen. 4.3. Neutrino flavor sensitivity
Fig. 6. Number of photoelectrons detected by DOMs in the deep array as a function of distance from the shower axis, for proton and iron air showers with comparable size as measured by IceTop.
We expect IceCube to be sensitive to neutrinos of all flavors over a wide range of energies. This represents a significant step forward over AMANDA, in which only muon neutrinos were identified, with upper limits placed on the flux of neutrinos of other flavors. At the lowest energies, nt and ne interactions are indistinguishable from each other, but it should be possible to distinguish either from nm events. We are presently working on methods to reconstruct vertical muons from atmospheric muon neutrinos with energies down to perhaps 25 GeV, and to identify small atmospheric electron neutrino showers at comparable energies. This raises the possibility of measuring the first dip in the nm survival probability due to neutrino oscillations at 28 GeV (for Dm2 ¼ 2:4 103 eV2 [23]) and searching for deviations
Atm. ν rate [1/200 d]
2400 2200
Combined
2000
IceCube only
1800 1600 1400 1200 1000 800 600 400 200 0
0.5
1
1.5
2
2.5 3 log (E/GeV)
3.5
4
4.5
5
Fig. 7. Number of atmospheric muon neutrino events passing the online filters for 200 days lifetime in 2007. The inclusion of AMANDA in the trigger and filter decisions provides a significant increase in the event rate at low energies.
ARTICLE IN PRESS T. DeYoung / Nuclear Instruments and Methods in Physics Research A 588 (2008) 92–98
Fig. 8. Effective volume for neutrinos from solar WIMP decays, integrated over possible neutrino spectra. Solid lines correspond to a spectrum typical of hard annihilation channels, and dashed lines to soft annihilation channels. Squares give the effective volumes for the 2007 IceCube configuration including AMANDA, and open circles without AMANDA. For low WIMP masses or soft decay channels, AMANDA improves the effective volume significantly.
from the expectation under the standard oscillation hypothesis. At higher energies, it will become possible to separate electron and tau neutrinos and obtain full flavor discrimination. At 100 TeV, a tau lepton produced in a CC nt interaction will have a decay length of about 5 m. Scattering of light in the ice will cause such an event to look identical to an electron neutrino event once the light has traveled several scattering lengths, but DOMs located very close to the event should observe two successive pulses of light, the first from the nt N interaction vertex and the second from the nt decay. At energies above the PeV scale, the physical separation between the two showers will be large enough that the showers will be fully distinct, producing the classic ‘‘double-bang’’ signature of a tau event [24]. At still higher energies, taus may be identifiable via the ‘‘lollipop’’ signature [25] or by observation of tau leptons decaying in flight to muons [26]. Standard production mechanisms for astrophysical neutrinos rely on the decay of charged pions or kaons, leading to a flavor ratio at the source of ne : nm : nt ¼ 1 : 2 : 0. Standard vacuum neutrino oscillations will convert this ratio to 1 : 1 : 1 over astrophysical distances, so tau neutrinos are expected in comparable numbers to other flavors. However, the rate of tau neutrino production in atmospheric air showers is significantly lower than for electron and muon neutrinos, so identification as a tau neutrino would be a very strong indicator of astrophysical origin, even for a single event.
97
Confirmation of the expected 1 : 1 : 1 flavor ratio is also extremely interesting. Deviations from this ratio can probe the environment of the neutrino source; for example, strong magnetic fields would lead to ‘‘muon damping’’, suppressing the flux of electron neutrinos at a given energy and leading to a detected flavor ratio between 1 : 1 : 1 and 4 : 7 : 7, depending on the amount of damping [27,28]. On the other hand, a source emitting a large flux of neutrons, either due to photodisintegration of accelerated heavy nuclei [29] or to extremely strong magnetic fields [30] associated with the source, would give a neutrino flux enriched in electron antineutrinos due to neutron decay. This would shift the detected flavor ratio from 1 : 1 : 1 to 5 : 2 : 2 in the limit of a pure n¯ e beam. Deviations of the flavor ratio could also indicate new particle physics. For example, neutrino decay would lead to a flavor ratio of 6 : 1 : 1 in the normal neutrino mass hierarchy, or 0 : 1 : 1 with an inverted hierarchy. No known astrophysical mechanisms can produce these flavor ratios at the detector [31]. 5. Extensions of IceCube We are currently investigating the feasibility of extending the energy range over which IceCube is sensitive to neutrinos. At energies below the TeV scale, IceCube uses the AMANDA detector to provide a densely instrumented region with improved sensitivity to relatively dim low energy events. However, AMANDA uses more primitive equipment than IceCube, and it is located near the edge of IceCube and in the top half of the array. A densely instrumented region using modern DOMs and located in the bottom center of the IceCube volume would both be more sensitive to low energy events and be able to use outer IceCube strings as an active veto against cosmic ray muons. A new IceCube Deep Core array built along such lines would significantly increase the sensitivity of IceCube at energies of a few tens of GeV to a few hundred GeV. This would greatly improve IceCube’s ability to search for WIMPs, measure neutrino oscillations, and increase sensitivity to neutrinos from the southern sky at low energies through improved identification of neutrinonucleon vertices within the detector. At higher energies, we are looking into extensions of IceCube using radio or acoustic technology. To detect neutrino fluxes around the EeV scale, such as the ‘‘guaranteed’’ GZK neutrino flux, a fiducial volume of approximately 100 km3 is required, significantly larger than IceCube. Optical Cherenkov technology would not allow the construction of such an array at reasonable cost, due to relatively short attenuation length of optical light. However, the absorption length of radio and acoustic waves in the ice may be as long as 1 km, so a sparse array of the required scale might be financially feasible. Prototype radio and acoustic devices have been deployed in IceCube drill holes and data from the devices is being analyzed. The prototype radio array consists of three
ARTICLE IN PRESS 98
T. DeYoung / Nuclear Instruments and Methods in Physics Research A 588 (2008) 92–98
clusters with four dipoles each, co-deployed with three IceCube strings. One cluster was deployed several hundred meters below the surface, and the other two at about 1.4 km below the surface [32]. The acoustic prototype array consists of three clusters of seven stages each at depths of 80–400 m below the surface [33]. Measurements of noise levels, triggering, attenuation length, and refractive index are now underway. 6. Conclusions With 22 strings already installed and operational and up to 18 more planned for installation this austral summer, IceCube is already the world’s largest neutrino telescope. Initial results from IceCube indicate that the detector is operating as expected. As the detector grows, it will permit us to address a number of exciting physics topics. In addition to increased sensitivity to muon neutrinos, IceCube’s larger scale and more sophisticated hardware will allow us to explore new types of physics analysis. Acknowledgments We acknowledge the support from the following agencies: National Science Foundation-Office of Polar Program, National Science Foundation-Physics Division, University of Wisconsin Alumni Research Foundation, Department of Energy, and National Energy Research Scientific Computing Center (supported by the Office of Energy Research of the Department of Energy), the NSF-supported TeraGrid system at the San Diego Supercomputer Center (SDSC), and the National Center for Supercomputing Applications (NCSA); Swedish Research Council, Swedish Polar Research Secretariat, and Knut and Alice Wallenberg Foundation, Sweden; German Ministry for Education and Research, Deutsche Forschungsgemeinschaft (DFG), Germany; Fund for Scientific Research (FNRS-FWO), Flanders Institute to encourage scientific and technological research in industry (IWT), Belgian Federal Office for Scientific, Technical and Cultural affairs (OSTC); the Netherlands Organisation for Scientific Research (NWO); M. Ribordy acknowledges the support of the SNF (Switzerland); A. Kappes and J.D. Zornoza acknowledge the support of the EU Marie Curie OIF Program. References [1] J. Ahrens, et al., IceCube Collaboration, IceCube preliminary design document hhttp://icecube.wisc.edui. [2] T. Gaisser, IceCube Collaboration, et al., in: Proceedings of the 30th International Cosmic Ray Conference, 2007 [arXiv:astro-ph/ 0711.0353]. [3] K. Hanson, O. Tarasova, IceCube Collaboration, Nucl. Instr. and Meth. A 567 (2006) 214.
[4] A. Gross, IceCube Collaboration, et al., in: Proceedings of the 30th International Cosmic Ray Conference, 2007 [arXiv:astro-ph/ 0711.0353]. [5] A. Karle, IceCube Collaboration, et al., in: Proceedings of the 30th International Cosmic Ray Conference, 2007 [arXiv:astro-ph/ 0711.0353]. [6] G.D. Barr, T.K. Gaisser, S. Robbins, T. Stanev, Phys. Rev. D 74 (2006) 094009 [arXiv:astro-ph/0611266]. [7] E. Andres, et al., Nature 410 (2001) 441. [8] A. Achterberg, IceCube Collaboration, et al., Phys. Rev. D 76 (2007) 042008 [arXiv:astro-ph/0705.1315]. [9] A. Achterberg, IceCube Collaboration, et al., Phys. Rev. D 76 (2007) 027101 [arXiv:astro-ph/0705.1781]. [10] A. Achterberg, IceCube Collaboration, et al., Phys. Rev. D 75 (2007) 102001 [arXiv:astro-ph/0611063]. [11] J. Angle, et al., XENON Collaboration [arXiv:astro-ph/0706.0039]. [12] A. Achterberg, AMANDA Collaboration, et al., Astropart. Phys. 26 (2006) 129. [13] M. Ackermann, AMANDA Collaboration, et al., Astropart. Phys. 24 (2006) 459 [arXiv:astro-ph/0508518]. [14] J. Ahrens, IceCube Collaboration, et al., Astropart. Phys. 20 (2004) 507 [arXiv:astro-ph/0305196]. [15] J. Zornoza, D. Chirkin, IceCube Collaboration, et al., in: Proceedings of the 30th International Cosmic Ray Conference, 2007 [arXiv:astro-ph/ 0711.0353]. [16] C. Song, P. Niessen, K. Rawlins, IceCube Collaboration, et al., in: Proceedings of the 30th International Cosmic Ray Conference, 2007 [arXiv:astro-ph/0711.0353]. [17] M.D. Kistler, J.F. Beacom, Phys. Rev. D 74 (2006) 063007 [arXiv:astro-ph/0607082]. [18] A. Kappes, J. Hinton, C. Stegmann, F.A. Aharonian, Astrophys. J. 656 (2007) 870 [arXiv:astro-ph/0607286]. [19] F. Halzen, Astrophys. Space Sci. 309 (2007) 407 [arXiv:astro-ph/ 0611915]. [20] F. Vissani, Astropart. Phys. 26 (2006) 310 [arXiv:astro-ph/0607249]. [21] F. Aharonian, HESS Collaboration, et al., Astrophys. J. 636 (2006) 777 [arXiv:astro-ph/0510397]. [22] G. Wikstro¨m, IceCube Collaboration, et al., in: Proceedings of the 30th International Cosmic Ray Conference, 2007 [arXiv:astro-ph/ 0711.0353]. [23] MINOS Collaboration [arXiv:hep-ex/0708.1495]. [24] J.G. Learned, S. Pakvasa, Astropart. Phys. 3 (1995) 267 [arXiv:hep-ph/ 9405296]. [25] J.F. Beacom, N.F. Bell, D. Hooper, S. Pakvasa, T.J. Weiler, Phys. Rev. D 68 (2003) 093005 [J.F. Beacom, N.F. Bell, D. Hooper, S. Pakvasa, T.J. Weiler, Erratum, Phys. Rev. D 72 (2005) 019901] [arXiv:hep-ph/0307025]. [26] T. DeYoung, S. Razzaque, D.F. Cowen, Astropart. Phys. 27 (2007) 238 [arXiv:astro-ph/0608486]. [27] J.P. Rachen, P. Meszaros, Phys. Rev. D 58 (1998) 123005 [arXiv:astro-ph/9802280]. [28] T. Kashti, E. Waxman, Phys. Rev. Lett. 95 (2005) 181101 [arXiv:astro-ph/0507599]. [29] L.A. Anchordoqui, H. Goldberg, F. Halzen, T.J. Weiler, Phys. Lett. B 593 (2004) 42 [arXiv:astro-ph/0311002]. [30] S. Razzaque, private communication. [31] J.F. Beacom, N.F. Bell, D. Hooper, S. Pakvasa, T.J. Weiler, Phys. Rev. Lett. 90 (2003) 181301 [arXiv:hep-ph/0211305]. [32] H. Landsmann, et al., IceCube Collaboration, L. Ruckmann, G.S. Varner, Proceedings of the 30th International Cosmic Ray Conference, 2007 [arXiv:astro-ph/0711.0353]. [33] S. Bo¨ser, et al. [arXiv:astro-ph/0708.2089].
Nuclear Instruments and Methods in Physics Research A 588 (2008) 92–98 www.elsevier.com/locate/nima
IceCube: Recent results and prospects T. DeYoung Department of Physics, Pennsylvania State University, University Park, PA 16802, USA For the IceCube Collaboration Available online 12 January 2008
Abstract Construction of the IceCube neutrino telescope is now more than 25% complete, with 22 strings in the ice and 26 IceTop air shower stations deployed on the surface. Initial results from data taken with nine strings in 2006 are shown, as well as results from the AMANDA array, which has been incorporated into IceCube. We also discuss the scientific potential of IceCube as it amasses 1 km3 year of data over the next few years. r 2008 Elsevier B.V. All rights reserved. PACS: 95.55.Vj; 98.70.Sa; 95.35.+d Keywords: Neutrino telescopes; Cosmic ray origins and acceleration; Dark matter
1. Introduction The IceCube Observatory, located at the South Pole, Antarctica, is the world’s largest neutrino telescope. Completion of the full km3 array is planned for 2011, but data are already being taken with the partially installed detector and the first published results indicate that the detector is operating as expected. IceCube’s primary goal is the discovery of astrophysical neutrinos in the rough energy range of 1 TeV to 100 PeV. IceCube includes an extensive air shower array on the surface, known as IceTop, which will enable a broad program of cosmic ray physics. The prototype AMANDA array, co-located with IceCube, has also been incorporated into IceCube to extend its sensitivity to events below TeV energies. After briefly describing the detector, we will survey results obtained with the AMANDA-II detector prior to integration with IceCube, and initial results from IceCube. We will then discuss the scientific prospects as IceCube is completed over the next few years. 2. The IceCube detector The IceCube design [1] consists of up to 80 strings, bundled electrical cables providing power and data E-mail address: [email protected] 0168-9002/$ - see front matter r 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2008.01.008
connections to digital optical modules (DOMs) deployed at depths of 1450–2450 m below the surface of the ice. Sixty DOMs are connected to each string, with a spacing of 17 m between modules. The strings are arranged on a triangular grid with an inter-string spacing of 125 m. IceCube also takes advantage of the solid surface above the array to support an 80-station extensive air shower array called IceTop [2]. Each IceTop station, situated above an IceCube string, consists of two light-tight tanks filled with ice and each containing two DOMs, operating at different gains in order to extend the dynamic range for signals in the tank. The DOMs [3] used by IceCube and IceTop contain 10 in. Hamamatsu photomultiplier tubes (PMTs) housed in glass pressure vessels. In addition to the PMTs, nearly all of the data acquisition electronics are embedded in the DOMs, which report fully digitized waveforms asynchronously to a software-based trigger and event builder on the surface. The electronics provide 300 megasamples per second (MSPS) sampling over a 400 ns window, plus coarser 40 MSPS sampling over a 6:4 ms window to catch late photons. The instantaneous dynamic range is approximately 103 photoelectrons (PEs) per 10 ns. Two parallel sets of digitizing electronics on each DOM operate in ‘‘ping-pong’’ mode, so that one set is active and ready to acquire while the other is read out. This design provides the advantages of autonomous DOM calibration (with timing
ARTICLE IN PRESS T. DeYoung / Nuclear Instruments and Methods in Physics Research A 588 (2008) 92–98
resolution of better than 2 ns RMS) and nearly deadtimeless operation and avoids signal degradation in transmission. The noise rate observed for DOMs in the deep array is 600–700 Hz, of which a significant fraction is non-Poissonian correlated noise due to scintillation effects in the glass pressure vessel housing the DOM. The underlying Poissonian rate of random noise hits is approximately 280 Hz, which allows us to monitor the DOM hit rates to search for low energy (MeV) neutrinos from supernova core collapse with sensitivity throughout the Milky Way and out to the Large Magellanic Cloud. The final component of IceCube is its predecessor, the AMANDA array, which has been incorporated into the IceCube detector [4]. The AMANDA optical modules are less sophisticated than the IceCube DOMs, with the data acquisition electronics on the surface and smaller 8 in. PMTs in the OMs. Roughly half of the 677 AMANDA OMs transmit their signals to the surface over optical fibers, which allows a timing accuracy of 2–3 ns, comparable to the DOMs, although with greatly reduced dynamic range. The other half are connected to the surface only by electrical cables, which stretch the pulses substantially and inhibit the separation of successive pulses, although the timing of the first pulse can still be found with an accuracy of a few ns. Although the AMANDA hardware is more primitive than the IceCube DOMs, the AMANDA array is much denser than IceCube, with typical string spacings of 30–40 m rather than IceCube’s 125 m. For relatively dim, low energy events, AMANDA thus collects considerably more information than IceCube. IceCube strings deployed in the AMANDA volume further increase the OM density, and IceCube strings surrounding AMANDA can be used as an active veto against cosmic ray muons, making the combined IceCube þ AMANDA detector considerably more effective for low energy studies than AMANDA was alone. 2.1. Construction status As of 2007, 22 IceCube strings and 26 IceTop stations have been successfully deployed [5]. The survival rate of the DOMs is 98%, consistent with expectations. Of the 1320 DOMs deployed in the deep array, only two have failed after commissioning, leading to a projected 20-year survival rate of approximately 97%. Thirteen strings were deployed in the 2006–2007 austral summer, with 14 or more additional strings planned for the 2007–2008 summer. In 2006–2007, the average time spent drilling each hole was reduced to 40 h, instilling confidence that the pace of deployments can be increased. Also in 2007, AMANDA was fully integrated into IceCube, with joint triggering and online filtering performed on the merged data recorded from both data acquisition systems.
93
3. Recent results The performance of IceCube and AMANDA has been established by the observation of atmospheric muon neutrinos, produced by the decay of p and K mesons in cosmic ray air showers in the Earth’s atmosphere. This flux is known to a precision of about 30% in the energy range to which IceCube and AMANDA are sensitive [6], above roughly 100 GeV. The atmospheric neutrino flux thus serves as a useful calibration source, although it also constitutes a background for searches for astrophysical neutrinos. The observation of atmospheric neutrinos by AMANDA was reported in Ref. [7] and updated results are available in Ref. [8]; observations with the IceCube detector are available in Ref. [9]. 3.1. AMANDA The AMANDA detector operated in its final independent configuration, known as AMANDA-II, from 2000 to 2006. Results from a search for point sources of neutrinos have been reported for the first five years of the data set, comprising 4282 candidate neutrino events with an estimated background contamination of approximately 5%, as shown in Fig. 1. No statistically significant point source of neutrinos is seen. At 90% confidence level, the upper limit placed on a reference E 2 point source flux of muon neutrinos, averaged over declination in the Northern Hemisphere, is E 2 dF=dEo5:5 108 GeV cm2 s1 . Full details of the analysis are available in Ref. [10]. A search was also performed for emission from 32 specific candidate sources of astrophysical neutrinos, chosen based on observations at various wavelengths in the electromagnetic spectrum. No statistically significant evidence for neutrino emission was found from any of the candidate sources. The candidates and results are also described in Ref. [10]. Several searches for neutralino dark matter have been performed using AMANDA. Such dark matter is expected to accumulate in gravitational potential wells such as the Earth or Sun and annihilate through various channels, producing a flux of neutrinos that might be detectable. Results from searches for such a neutrino flux from the centers of the Earth [12] and Sun [13] have been published; the limit placed on the neutralino flux from the Sun using 75°
60° 45° 30° 15°
24 h
0h
Fig. 1. Sky map produced by AMANDA-II using data from 2000 to 2004. There are 4282 neutrino candidates observed, consistent with the expected atmospheric neutrino flux.
ARTICLE IN PRESS T. DeYoung / Nuclear Instruments and Methods in Physics Research A 588 (2008) 92–98
94
data from the AMANDA-II configuration are shown in Fig. 2. In addition to searches for individual sources of neutrinos, AMANDA observations have been used to place a limit on a possible diffuse flux of neutrinos from populations of unresolved distant sources. This diffuse flux is distinguished from the roughly isotropic foreground of atmospheric neutrinos due to the harder spectra expected for most astrophysical sources. A limit of E 2 dF=dEo7:4 108 GeV cm2 s1 sr1 is placed on the diffuse muon neutrino flux in the energy range from 16 TeV to 2.5 PeV at 90% confidence level [8]. Limits on several specific proposed diffuse spectra are also given in Ref. [8].
events, shown in Fig. 3, suggests that there is residual background contamination at the level of approximately 10% from cosmic ray muons in the declination band near the horizon, but for more steeply upgoing directions the sample is consistent with the expected flux of atmospheric neutrinos. The geometry of the nine-string detector also presents a challenge for the detector simulation software. Fig. 4 shows the very strong effect of the highly asymmetric string layout on the observed data: the detector is considerably more sensitive to muons traveling along the long axis of the
3.2. IceCube The IceCube array is expanding rapidly as construction proceeds. In the 2006 austral winter, the detector consisted of nine strings. The total instrumented volume of this stage of the detector was comparable to that of AMANDA-II, although the density of instrumentation is lower. The total integrated exposure of the detector is thus considerably lower than that of the full AMANDA data set, but still sufficient to observe atmospheric neutrinos. As with AMANDA, the relatively well-known flux of atmospheric muon neutrinos serves as a useful calibration source for IceCube, as described in Ref. [9]. A total of 234 neutrino candidate events were identified in the first 137.4 days of lifetime. The zenith angle distribution of those
AMANDA-II 2001 SI
XENON10 2007 SUPER-K 1996-2001
Muon flux from the Sun (km-2 yr-1)
105
BAKSAN 1978-95
Fig. 3. Zenith angle distribution of the 234 neutrino candidates found in the 2006 IceCube nine-string data set. Except for some residual cosmic ray muon events near the horizon, the sample is consistent with expectations for the atmospheric muon neutrino flux. The band of predictions for the atmospheric neutrino flux includes both theoretical uncertainties in the flux and systematic uncertainties in the modeling of the detector response. The error bars on the data are purely statistical.
MACRO 1989-98
104
103
102
10
1 10
103 102 Neutralino Mass (GeV)
104
Fig. 2. Limits at 90% confidence level placed on the flux of neutrinos from neutralino annihilation in the center of the Sun, as a function of the hypothetical neutralino mass. A hard (Wþ W ) annihilation channel is assumed, and the flux is that above a muon energy threshold of 1 GeV. Dots indicate supersymmetric parameters probed by the XENON-10 limits [11], and crosses indicate regions of parameter space not constrained by those limits.
Fig. 4. Azimuth angle distribution of the 234 neutrino candidates found in the 2006 IceCube nine-string data set. The inset shows the irregular surface layout of the strings. The strong modulation of the detector response is due to the string geometry, with the highest sensitivity to tracks oriented with the long axis of the detector, approximately 45–2251. The Monte Carlo simulation reproduces this effect in detail.
ARTICLE IN PRESS T. DeYoung / Nuclear Instruments and Methods in Physics Research A 588 (2008) 92–98
detector than those cutting across that axis. This effect is correctly reproduced in our simulation of the detector. As the remainder of the IceCube strings are installed, of course, the detector geometry will become more regular and these asymmetries in its response will disappear. 4. Scientific prospects The integrated exposure of IceCube will grow rapidly as construction progresses. Fig. 5 shows the projected deployment schedule and the anticipated exposure in km3 year available during the construction phase. One km3 year of data, approximately an order of magnitude more than collected over the full lifetime of AMANDA-II, will already have been collected in 2009. Four km3 year of data will be available after the first year of operations with the complete detector. This substantial increase in exposure will produce considerable gains in sensitivity for the types of analyses described in Section 3. For diffuse fluxes, for example, the sensitivity expected from one year of operations with the complete detector is E 2 dF=dE8 109 GeV cm2 s1 sr1 [14], an order of magnitude better than the current AMANDA-II limit and well below the Waxman–Bahcall limit (assuming flavor equality due to oscillations). In addition to the effective volume of the detector, IceCube will provide a substantial improvement in the precision of event reconstruction. Median angular resolution better than 0.81 is expected for muons above 1 TeV 80
5 km3-yr
Strings
4.5
70
4 60
km3-yr
3 2.5 2
50 40 30
# Deployed Strings
3.5
1.5 20 1 0.5 0 2005 2006 2007 2008 2009 2010 2011 2012 Date
10 0
Fig. 5. Projected string deployment schedule and expected integrated exposure of the IceCube detector during the construction phase of the project. One km3 year of data is expected by 2009, two years before construction of the full array is completed.
95
[14] from nearly all directions; this expectation is likely pessimistic, as it is based on techniques developed for AMANDA which do not make use of the full capabilities of the IceCube hardware. Because the sensitivity to point sources of neutrinos improves with angular resolution due to reduced background in the source search bin, this constitutes an additional source of improved sensitivity. For point sources, an average 90% upper limit on E 2 spectra of E 2 dN=dE5:5 109 GeV cm2 s1 would be expected with one year of data from the complete IceCube detector [14], an increase of an order of magnitude in sensitivity over the current AMANDA-II limit based on five years of operation. Furthermore, the DOMs used in IceCube record the total light detected with considerably higher precision than AMANDA OMs. This leads to an improved energy resolution for both muons and cascades in the detector. For muons, an energy resolution of about 0.3 in log10 E m is expected above 104 GeV [15]. For lower energy muons, stochastic energy loss mechanisms such as bremsstrahlung are less significant and the light emitted per unit track length is approximately constant, so that other methods for muon energy reconstruction will be required (for example based on the measured length of the track). 4.1. IceTop IceTop will be a 1 km2 EAS array on the surface above the deep IceCube array. IceTop will be uniquely capable of measuring the composition of cosmic rays in the region of the knee due to its ability to independently measure the electromagnetic and muonic content of air showers by comparing the response of the surface array to that of the deep array. Generally speaking, heavier primaries produce more muons per unit primary energy than do lighter primaries. Fig. 6 shows one of the several parameters with sensitivity to composition: the charge recorded by in-ice DOMs as a function of distance from the shower axis. Heavy primaries with multiple muons penetrating deep into the ice produce more light in the detector, as expected [16]. 4.2. The combined IceCube-AMANDA detector As mentioned in Section 2, the existing AMANDA detector was converted in 2007 into a subcomponent of IceCube, with joint online triggering and on-site filtering based on data from the combined detector. Although AMANDA’s hardware is somewhat primitive compared to IceCube, its dense spacing should provide improved sensitivity to low energy neutrinos. For example, Fig. 7 shows the increase in the number of low energy atmospheric muon neutrinos triggering the detector and passing the online filters [4]. With a complete analysis of these events, now in progress, we expect a significant increase in IceCube’s sensitivity to neutrinos at sub-TeV energies due to the inclusion of AMANDA. Sensitivity to neutrinos at
ARTICLE IN PRESS 96
T. DeYoung / Nuclear Instruments and Methods in Physics Research A 588 (2008) 92–98
these energies is important for a number of physics topics, including dark matter searches, observation of neutrino oscillations, and searches for neutrino emission from Galactic sources. Based on their observed TeV gamma ray emission, most galactic neutrino source candidates are expected to have softer neutrino spectra and/or spectral cutoffs at lower energies than is predicted for extragalactic sources [17–20]. Sensitivity to lower energy neutrinos is clearly important for observing such sources. In addition, many sources are located in the region of the Galactic center [21], which is in the southern sky. A densely instrumented region may permit the identification of (partially) contained events, in
which the neutrino-nucleon vertex is located within the detector. Cutting on such events would reject the cosmic ray muon background, leaving only the irreducible atmospheric neutrino background to astrophysical neutrinos, and permit a search for neutrino emitters from the Southern Hemisphere, such as the TeV gamma ray sources observed by HESS. The cost of such a strategy would be reduced sensitivity to higher energy neutrinos, for which the effective volume is increased by the long muon range; however, for muons with ranges less than the detector length scale (i.e., below roughly E m 200 GeV), nearly all muons will be produced within the detector anyway. For searches for WIMP dark matter, increased sensitivity to low energy neutrinos corresponds to sensitivity to WIMPs of lower mass. Fig. 8 compares the effective volume of the combined detector to that of IceCube alone, for WIMP annihilations in the Sun through hard or soft channels [22]. For low mass WIMPs, an order of magnitude improvement is seen. 4.3. Neutrino flavor sensitivity
Fig. 6. Number of photoelectrons detected by DOMs in the deep array as a function of distance from the shower axis, for proton and iron air showers with comparable size as measured by IceTop.
We expect IceCube to be sensitive to neutrinos of all flavors over a wide range of energies. This represents a significant step forward over AMANDA, in which only muon neutrinos were identified, with upper limits placed on the flux of neutrinos of other flavors. At the lowest energies, nt and ne interactions are indistinguishable from each other, but it should be possible to distinguish either from nm events. We are presently working on methods to reconstruct vertical muons from atmospheric muon neutrinos with energies down to perhaps 25 GeV, and to identify small atmospheric electron neutrino showers at comparable energies. This raises the possibility of measuring the first dip in the nm survival probability due to neutrino oscillations at 28 GeV (for Dm2 ¼ 2:4 103 eV2 [23]) and searching for deviations
Atm. ν rate [1/200 d]
2400 2200
Combined
2000
IceCube only
1800 1600 1400 1200 1000 800 600 400 200 0
0.5
1
1.5
2
2.5 3 log (E/GeV)
3.5
4
4.5
5
Fig. 7. Number of atmospheric muon neutrino events passing the online filters for 200 days lifetime in 2007. The inclusion of AMANDA in the trigger and filter decisions provides a significant increase in the event rate at low energies.
ARTICLE IN PRESS T. DeYoung / Nuclear Instruments and Methods in Physics Research A 588 (2008) 92–98
Fig. 8. Effective volume for neutrinos from solar WIMP decays, integrated over possible neutrino spectra. Solid lines correspond to a spectrum typical of hard annihilation channels, and dashed lines to soft annihilation channels. Squares give the effective volumes for the 2007 IceCube configuration including AMANDA, and open circles without AMANDA. For low WIMP masses or soft decay channels, AMANDA improves the effective volume significantly.
from the expectation under the standard oscillation hypothesis. At higher energies, it will become possible to separate electron and tau neutrinos and obtain full flavor discrimination. At 100 TeV, a tau lepton produced in a CC nt interaction will have a decay length of about 5 m. Scattering of light in the ice will cause such an event to look identical to an electron neutrino event once the light has traveled several scattering lengths, but DOMs located very close to the event should observe two successive pulses of light, the first from the nt N interaction vertex and the second from the nt decay. At energies above the PeV scale, the physical separation between the two showers will be large enough that the showers will be fully distinct, producing the classic ‘‘double-bang’’ signature of a tau event [24]. At still higher energies, taus may be identifiable via the ‘‘lollipop’’ signature [25] or by observation of tau leptons decaying in flight to muons [26]. Standard production mechanisms for astrophysical neutrinos rely on the decay of charged pions or kaons, leading to a flavor ratio at the source of ne : nm : nt ¼ 1 : 2 : 0. Standard vacuum neutrino oscillations will convert this ratio to 1 : 1 : 1 over astrophysical distances, so tau neutrinos are expected in comparable numbers to other flavors. However, the rate of tau neutrino production in atmospheric air showers is significantly lower than for electron and muon neutrinos, so identification as a tau neutrino would be a very strong indicator of astrophysical origin, even for a single event.
97
Confirmation of the expected 1 : 1 : 1 flavor ratio is also extremely interesting. Deviations from this ratio can probe the environment of the neutrino source; for example, strong magnetic fields would lead to ‘‘muon damping’’, suppressing the flux of electron neutrinos at a given energy and leading to a detected flavor ratio between 1 : 1 : 1 and 4 : 7 : 7, depending on the amount of damping [27,28]. On the other hand, a source emitting a large flux of neutrons, either due to photodisintegration of accelerated heavy nuclei [29] or to extremely strong magnetic fields [30] associated with the source, would give a neutrino flux enriched in electron antineutrinos due to neutron decay. This would shift the detected flavor ratio from 1 : 1 : 1 to 5 : 2 : 2 in the limit of a pure n¯ e beam. Deviations of the flavor ratio could also indicate new particle physics. For example, neutrino decay would lead to a flavor ratio of 6 : 1 : 1 in the normal neutrino mass hierarchy, or 0 : 1 : 1 with an inverted hierarchy. No known astrophysical mechanisms can produce these flavor ratios at the detector [31]. 5. Extensions of IceCube We are currently investigating the feasibility of extending the energy range over which IceCube is sensitive to neutrinos. At energies below the TeV scale, IceCube uses the AMANDA detector to provide a densely instrumented region with improved sensitivity to relatively dim low energy events. However, AMANDA uses more primitive equipment than IceCube, and it is located near the edge of IceCube and in the top half of the array. A densely instrumented region using modern DOMs and located in the bottom center of the IceCube volume would both be more sensitive to low energy events and be able to use outer IceCube strings as an active veto against cosmic ray muons. A new IceCube Deep Core array built along such lines would significantly increase the sensitivity of IceCube at energies of a few tens of GeV to a few hundred GeV. This would greatly improve IceCube’s ability to search for WIMPs, measure neutrino oscillations, and increase sensitivity to neutrinos from the southern sky at low energies through improved identification of neutrinonucleon vertices within the detector. At higher energies, we are looking into extensions of IceCube using radio or acoustic technology. To detect neutrino fluxes around the EeV scale, such as the ‘‘guaranteed’’ GZK neutrino flux, a fiducial volume of approximately 100 km3 is required, significantly larger than IceCube. Optical Cherenkov technology would not allow the construction of such an array at reasonable cost, due to relatively short attenuation length of optical light. However, the absorption length of radio and acoustic waves in the ice may be as long as 1 km, so a sparse array of the required scale might be financially feasible. Prototype radio and acoustic devices have been deployed in IceCube drill holes and data from the devices is being analyzed. The prototype radio array consists of three
ARTICLE IN PRESS 98
T. DeYoung / Nuclear Instruments and Methods in Physics Research A 588 (2008) 92–98
clusters with four dipoles each, co-deployed with three IceCube strings. One cluster was deployed several hundred meters below the surface, and the other two at about 1.4 km below the surface [32]. The acoustic prototype array consists of three clusters of seven stages each at depths of 80–400 m below the surface [33]. Measurements of noise levels, triggering, attenuation length, and refractive index are now underway. 6. Conclusions With 22 strings already installed and operational and up to 18 more planned for installation this austral summer, IceCube is already the world’s largest neutrino telescope. Initial results from IceCube indicate that the detector is operating as expected. As the detector grows, it will permit us to address a number of exciting physics topics. In addition to increased sensitivity to muon neutrinos, IceCube’s larger scale and more sophisticated hardware will allow us to explore new types of physics analysis. Acknowledgments We acknowledge the support from the following agencies: National Science Foundation-Office of Polar Program, National Science Foundation-Physics Division, University of Wisconsin Alumni Research Foundation, Department of Energy, and National Energy Research Scientific Computing Center (supported by the Office of Energy Research of the Department of Energy), the NSF-supported TeraGrid system at the San Diego Supercomputer Center (SDSC), and the National Center for Supercomputing Applications (NCSA); Swedish Research Council, Swedish Polar Research Secretariat, and Knut and Alice Wallenberg Foundation, Sweden; German Ministry for Education and Research, Deutsche Forschungsgemeinschaft (DFG), Germany; Fund for Scientific Research (FNRS-FWO), Flanders Institute to encourage scientific and technological research in industry (IWT), Belgian Federal Office for Scientific, Technical and Cultural affairs (OSTC); the Netherlands Organisation for Scientific Research (NWO); M. Ribordy acknowledges the support of the SNF (Switzerland); A. Kappes and J.D. Zornoza acknowledge the support of the EU Marie Curie OIF Program. References [1] J. Ahrens, et al., IceCube Collaboration, IceCube preliminary design document hhttp://icecube.wisc.edui. [2] T. Gaisser, IceCube Collaboration, et al., in: Proceedings of the 30th International Cosmic Ray Conference, 2007 [arXiv:astro-ph/ 0711.0353]. [3] K. Hanson, O. Tarasova, IceCube Collaboration, Nucl. Instr. and Meth. A 567 (2006) 214.
[4] A. Gross, IceCube Collaboration, et al., in: Proceedings of the 30th International Cosmic Ray Conference, 2007 [arXiv:astro-ph/ 0711.0353]. [5] A. Karle, IceCube Collaboration, et al., in: Proceedings of the 30th International Cosmic Ray Conference, 2007 [arXiv:astro-ph/ 0711.0353]. [6] G.D. Barr, T.K. Gaisser, S. Robbins, T. Stanev, Phys. Rev. D 74 (2006) 094009 [arXiv:astro-ph/0611266]. [7] E. Andres, et al., Nature 410 (2001) 441. [8] A. Achterberg, IceCube Collaboration, et al., Phys. Rev. D 76 (2007) 042008 [arXiv:astro-ph/0705.1315]. [9] A. Achterberg, IceCube Collaboration, et al., Phys. Rev. D 76 (2007) 027101 [arXiv:astro-ph/0705.1781]. [10] A. Achterberg, IceCube Collaboration, et al., Phys. Rev. D 75 (2007) 102001 [arXiv:astro-ph/0611063]. [11] J. Angle, et al., XENON Collaboration [arXiv:astro-ph/0706.0039]. [12] A. Achterberg, AMANDA Collaboration, et al., Astropart. Phys. 26 (2006) 129. [13] M. Ackermann, AMANDA Collaboration, et al., Astropart. Phys. 24 (2006) 459 [arXiv:astro-ph/0508518]. [14] J. Ahrens, IceCube Collaboration, et al., Astropart. Phys. 20 (2004) 507 [arXiv:astro-ph/0305196]. [15] J. Zornoza, D. Chirkin, IceCube Collaboration, et al., in: Proceedings of the 30th International Cosmic Ray Conference, 2007 [arXiv:astro-ph/ 0711.0353]. [16] C. Song, P. Niessen, K. Rawlins, IceCube Collaboration, et al., in: Proceedings of the 30th International Cosmic Ray Conference, 2007 [arXiv:astro-ph/0711.0353]. [17] M.D. Kistler, J.F. Beacom, Phys. Rev. D 74 (2006) 063007 [arXiv:astro-ph/0607082]. [18] A. Kappes, J. Hinton, C. Stegmann, F.A. Aharonian, Astrophys. J. 656 (2007) 870 [arXiv:astro-ph/0607286]. [19] F. Halzen, Astrophys. Space Sci. 309 (2007) 407 [arXiv:astro-ph/ 0611915]. [20] F. Vissani, Astropart. Phys. 26 (2006) 310 [arXiv:astro-ph/0607249]. [21] F. Aharonian, HESS Collaboration, et al., Astrophys. J. 636 (2006) 777 [arXiv:astro-ph/0510397]. [22] G. Wikstro¨m, IceCube Collaboration, et al., in: Proceedings of the 30th International Cosmic Ray Conference, 2007 [arXiv:astro-ph/ 0711.0353]. [23] MINOS Collaboration [arXiv:hep-ex/0708.1495]. [24] J.G. Learned, S. Pakvasa, Astropart. Phys. 3 (1995) 267 [arXiv:hep-ph/ 9405296]. [25] J.F. Beacom, N.F. Bell, D. Hooper, S. Pakvasa, T.J. Weiler, Phys. Rev. D 68 (2003) 093005 [J.F. Beacom, N.F. Bell, D. Hooper, S. Pakvasa, T.J. Weiler, Erratum, Phys. Rev. D 72 (2005) 019901] [arXiv:hep-ph/0307025]. [26] T. DeYoung, S. Razzaque, D.F. Cowen, Astropart. Phys. 27 (2007) 238 [arXiv:astro-ph/0608486]. [27] J.P. Rachen, P. Meszaros, Phys. Rev. D 58 (1998) 123005 [arXiv:astro-ph/9802280]. [28] T. Kashti, E. Waxman, Phys. Rev. Lett. 95 (2005) 181101 [arXiv:astro-ph/0507599]. [29] L.A. Anchordoqui, H. Goldberg, F. Halzen, T.J. Weiler, Phys. Lett. B 593 (2004) 42 [arXiv:astro-ph/0311002]. [30] S. Razzaque, private communication. [31] J.F. Beacom, N.F. Bell, D. Hooper, S. Pakvasa, T.J. Weiler, Phys. Rev. Lett. 90 (2003) 181301 [arXiv:hep-ph/0211305]. [32] H. Landsmann, et al., IceCube Collaboration, L. Ruckmann, G.S. Varner, Proceedings of the 30th International Cosmic Ray Conference, 2007 [arXiv:astro-ph/0711.0353]. [33] S. Bo¨ser, et al. [arXiv:astro-ph/0708.2089].