A number of experimental techniques are currently being deployed in an effort
to make the first detection of ultra-high energy cosmic neutrinos. To
accomplish this goal, techniques using radio and acoustic detectors are being
developed, which are optimally designed for studying neutrinos with energies in
the PeV-EeV range and above. Data from the AMANDA experiment, in contrast, has
been used to place limits on the cosmic neutrino flux at less extreme energies
(up to ~10 PeV). In this letter, we show that by adopting a different analysis
strategy, optimized for much higher energy neutrinos, the same AMANDA data can
be used to place a limit competitive with radio techniques at EeV energies. We
also discuss the sensitivity of the IceCube experiment, in various stages of
deployment, to ultra-high energy neutrinos.Comment: 4 pages, 3 figure

/Fermilab

/Wisconsin U., Madison

Halzen, Francis

Hooper, Dan

English

arXiv

A number of experimental techniques are currently being deployed in an effort to make the first detection of ultra-high energy cosmic neutrinos. To accomplish this goal, techniques using radio and acoustic detectors are being developed, which are optimally designed for studying neutrinos with energies in the PeV-EeV range and above. Data from the AMANDA experiment, in contrast, has been used to place limits on the cosmic neutrino flux at less extreme energies (up to {approx}10 PeV). In this letter, we show that by adopting a different analysis strategy, optimized for much higher energy neutrinos, the same AMANDA data can be used to place a limit competitive with radio techniques at EeV energies. We also discuss the sensitivity of the IceCube experiment, in various stages of deployment, to ultra-high energy neutrinos

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arXiv:astro-ph/0605103v2   7 Jul 2006AMANDA Observations Constrain the Ultra-High Energy Neutrino FluxFrancis Halzen1 and Dan Hooper21Department of Physics, University of Wisconsin, Madison, WI2Particle Astrophysics Center, Fermilab, Batavia, ILA number of experimental techniques are currently being deployed in an effort to make the firstdetection of ultra-high energy cosmic neutrinos. To accomplish this goal, techniques using radio andacoustic detectors are being developed, which are optimally designed for studying neutrinos withenergies in the PeV-EeV range and above. Data from the AMANDA experiment, in contrast, hasbeen used to place limits on the cosmic neutrino flux at less extreme energies (up to ∼10 PeV). Inthis letter, we show that by adopting a different analysis strategy, optimized for much higher energyneutrinos, the same AMANDA data can be used to place a limit competitive with radio techniquesat EeV energies. We also discuss the sensitivity of the IceCube experiment, in various stages ofdeployment, to ultra-high energy neutrinos.PACS numbers: 95.55.Vj, 95.85.Ry, 98.70.Sa; MADPH-06-1465; FERMILAB-PUB-06-097-AAs ultra-high energy protons and nuclei propagate overcosmological distances, they interact with the cosmic mi-crowave and infra-red backgrounds, producing pions [1].These pions decay, generating neutrinos with typical en-ergies in the range of 107 to 1010 GeV. These ultra-highenergy particles, known as cosmogenic or GZK neutrinos,have not yet been observed.A wide range of experimental efforts are currently un-derway to detect ultra-high energy neutrinos [2]. Theseinclude experiments using radio antennas such as RICEunder the Antartic ice of the South Pole [3], and theballoon mission ANITA (and its predecessor ANITA-lite) [4]. Techniques for observing ultra-high energy neu-trinos using acoustic detectors are also being explored,although the prospects for this technology are not yetwell understood [5]. Cosmic ray experiments, such asthe Pierre Auger Observatory [6], are also capable of de-tecting ultra-high energy neutrinos by observing slighlyupgoing showers produced by Earth-skimming tau neu-trinos or deeply penetrating quasi-horizontal showers [7].Each of these experimental techniques is specificallysuited to studying particles (ie. neutrinos or cosmic rays)at extremely high energies. The effective volumes ofRICE, AUGER and ANITA become appreciable onlyabove roughly ∼107 GeV, ∼108 GeV and ∼109 GeV,respectively. Acoustic techniques are likely to haveenergy thresholds comparable to or higher than theseother methods. In contrast, high-energy neutrino tele-scopes using optical detectors, such as AMANDA [8],ANTARES [9], IceCube [10], and KM3 [11], while alsocapable of observing ultra-high energy neutrinos [12], aredesigned to be sensitive to neutrino induced cascadeswith energies as low as a few TeV.The strongest limit on contained neutrino-inducedcascades in the ∼PeV energy region has been pub-lished by the AMANDA collaboration [8]. This limit(E2νdNν/dEν < 8.6 × 10−7 GeV s−1 sr−1 cm−2, at the90% confidence level, in the 50 TeV to 6 PeV energyrange) was arrived at after making a number of exper-imental cuts on the data to remove a large fraction ofthe background from atmospheric neutrinos and muons.Furthermore, the analysis was optimized for a spectrumfalling with dNν/dEν ∝ E−2ν.This approach is less than ideal for searching for cos-mogenic neutrinos, which appear as isolated spectacularevents of much greater energy (∼10 EeV). At these en-ergies, the showers generated in an experiment such asAMANDA are very large (approximate radii of ∼ 300 me-ters for a 107 GeV shower and ∼ 500 meters for a 1011GeV shower; considerably larger than the instrumentedwidth of AMANDA). Such enormous volume events areexpected to have a negligibly small background due tothe rapidly falling spectrum of atmospheric neutrinos andmuons. The information contained in these events is somuch superior to that of average AMANDA events, thatthroughout this study we assume that showers and muonscan be efficiently seperated. Given that the backgroundcan be effectively eliminated, the optimal analysis strat-egy for studying this class of events is very different fromthat suited for neutrinos with TeV-PeV energies.The largest shower reconstructed in the AMANDA ex-periment has an energy close to 100 TeV, occupying a vol-ume that triggered fewer than 300 of its 677 modules [13].Motivated by the fact that no larger events have beenobserved, we have considered an alternative cut designedto retain as many potential ultra-high energy events aspossible. This cut is to simply consider only events inwhich 300 or more of the modules trigger while omittingthe restrictive cuts of Ref. [8]. Our simulation adoptsa simple geometric approach in which spherical showersare overlayed against the AMANDA geometry (roughlya cylinder with height and diameter of 600 and 200 me-ters, respectively) [14]. This method reflects the scienceinvolved and can be considered a good approximation forsuch extremely large events, even in the absence of a fulland detailed detector simulation. The physics of neutrinointeractions and detection at these high energies has beenestablished and confirmed though the calibration of theAMANDA experiment.In Fig. 1, we compare the effective volume (for elec-tron neutrinos) of the AMANDA experiment, as calcu-lated using the cuts adopted in the cascade analysis opti-2FIG. 1: The effective volume of AMANDA to electron neu-trinos as found using the cuts from the AMANDA cascadeanalysis [8] and using the >300 module cut adopted in thisstudy.FIG. 2: The current experimental limits on the diffuse fluxof high and ultra-high energy neutrinos (summed over flavorsin the ratio φνe : φνµ : φντ = 1 : 1 : 1, after oscillations areconsidered). The AMANDA cascade, RICE and ANITA-litelimits are found in Refs. [8], [3] and [15], respectively. TheAMANDA and ANITA-lite limits are shown at the 90% confi-dence level, while the RICE limit shown is at 95% confidence.For comparison, we also show the neutrino spectrum pre-dicted from the propagation of ultra-high energy cosmic rayprotons (the cosmogenic flux) [16] and the Waxman-Bahcallbound [17]. The solid and dotted curves correspond to cosmo-genic neutrino fluxes calculated assuming normal (∝ (z+1)3)and strong (∝ (z + 1)4) source evolution, respectively.FIG. 3: The effective volumes (water equivalent) of variouscurrent and near future ultra-high energy neutrino detectors.The volume of the AMANDA experiment has been calculatedusing the >300 module cut described in the text. A similarapproach was taken for IceCube, requiring a number of mod-ules be triggered equivalent to a volume of 300 modules inAMANDA (the module spacing in IceCube is greater than inAMANDA). The upper and lower RICE curves (dots) corre-spond to hadronic and electromagnetic showers, respectively.The ANITA, ANITA-lite and Auger curves are shown for com-parison with AMANDA, IceCube and RICE in terms of 2πsteridians equivalent. In the case of ANITA and ANITA-lite,the volumes have been scaled by a factor of 10/365 to accountfor the 10 day duration of each balloon mission. See text formore details.mized for ∼PeV energies [8], to the effective volume usingour >300 module cut. Our cut would remove far fewerof the signal events. Whereas it may be debatable howwell background rejection at energies below ∼PeV wouldwork, we expect the sample to have very low backgroundfrom atmospheric neutrinos at GZK energies.So far, 6 years of AMANDA data has been analyzed(with approximately 200 live days per year). Assumingthat no events with more than 300 triggered modules willbe found in this sample, it can be used to place a limit onthe flux of ultra-high energy cosmic neutrinos. In Fig. 2,we plot this limit and compare it to the current limit ofthe RICE [3] and ANITA-lite [15] experiments and theAMANDA cascade analysis limit [8]. From this figure, itis clear that AMANDA, even though capable of studyingneutrinos of lower energies, is highly competitive as anultra-high energy neutrino detector. In the 108-1010 GeVrange, the AMANDA limit we find here is comparable tothe limit placed by RICE [3]. Also shown in Fig. 2 arethe predictions for the cosmogenic neutrino flux gener-ated in the propagation of ultra-high energy protons forthe cases of normal (solid) and strong (dots) source evo-lution [16]. The Waxman-Bahcall bound is also shownfor comparison (dot-dash) [17]. By noticing that these3benchmark neutrino flux models are not far below thecurrent limits of AMANDA and RICE, we assess thatas these experiments continue to collect data, they maydetect the first ultra-high energy neutrino event at anytime.Over the next few years, a number of experiments willbecome considerably more sensitive to neutrinos withultra-high energies. The first ANITA flight is scheduledto take place later this year, and the Pierre Auger Ob-servatory continues to be constructed, hopefully to becompleted by 2007. The first 9 (out of 80) strings ofIceCube modules are currently operating, with another12-14 scheduled to be deployed in the 2006-2007 season.Its full cubic kilometer instrumented volume should becompleted around 2011. Future deployment of IceCubestrings will also be accompanied by additional radio an-ntennas, further extending the volume of RICE.In Fig. 3, we compare the effective volumes (2π stere-dians, water equivalent) of several current and futureultra-high energy neutrino experiments. The effectivevolumes shown for AMANDA and IceCube have beencalculated using the same >300 module cut as describedearlier in this letter (in the case of IceCube, we haveused a volume cut equivalent to that of 300 modules inAMANDA. The actual number of modules required ismodified by the string spacing being greater in IceCubethan in AMANDA). By IceCube (2007), we refer to aconfiguration of 23 IceCube strings, as they may be ar-ranged following the 2006-2007 deployment season. ByIceCube (2011), we refer to the completed IceCube con-figuration with all 80 strings.The effective volume of RICE shown in Fig. 3 (shownas a dotted line) seperates into two lines above ∼ 108GeV. The higher and lower curves correspond to the ef-fective volumes for hadronic and electromagnetic show-ers, respectively [3]. For the effective volumes shown forthe ANITA [18], ANITA-lite and Pierre Auger experi-ments, we have implicitly included the solid angle andtime windows of these experiments for more convienentcomparison. The volumes shown are equivalent to a 2πsolid angle field of view and, for ANITA and ANITA-lite,a factor of 10/365 was included to compare the exposureof a ten day flight to a year of operation by AMANDA,IceCube, RICE or Auger. For the Pierre Auger Observa-tory, we show the approximate equivalent effective vol-ume for Earth-skimming tau neutrino induced cascades.The exact shape of this contour is complicated by theeffects of tau absorption and regeneration in the Earth.For more details, see Ref. [19].In table I, we estimate the event rates in each of theseexperiments, for the case of the cosmogenic neutrino flux(n = 3) [16]. With the current integrated exposure,AMANDA and RICE each expect 0.1-0.2 events fromsuch a flux. By the end of 2008, they will collectivelyexpect about ∼0.7 events. By 2011, IceCube alone willhave accumulated the exposure needed to observe ∼1.2events, although the precise value will depend on the con-figuration of strings in various intermediate stages of de-ployment.1 Each ANITA flight is expected to observe ofthe order of 1 cosmogenic neutrino event, although thisrate depends critically on the precise energy threshold ofthe experiment. The Pierre Auger Observatory, currentlyunder construction at its Southern cite in Argentina, isexpected to be completed by 2007. They also anticipateof the order of 1 cosmogenic neutrino event per year,generated by Earth-skimming tau neutrinos[19].The standard cosmogenic neutrino flux we have usedas our benchmark scenario may plausibly underestimatethe rates observed in these experiments. If the sourceevolution scales with (z +1)4 rather than a homogeneousdistribution scaling as (z + 1)3, the flux of neutrinos willbe larger, especially below ∼ 1010 GeV [16, 20]. Withsuch a flux, rates for AMANDA and IceCube will belarger by a factor of ∼3 than those given in table I. RICEand Auger will benefit somewhat less from strong sourceevolution, as they are most sensitive at higher energieswhere the effects of strong source evolution are less pro-nounced. ANITA’s rates will be increased only slightlyas a result of strong source evolution.On the other hand, the cosmogenic neutrino flux couldbe smaller if the observed ultra-high energy cosmic raysare generated in local sources, or if they consist of heavyor intermediate mass nuclei rather than protons [21].Experiments beyond those discussed thus far in thisletter will likely be capable of detecting far larger num-bers of ultra-high energy neutrinos. A northern PierreAuger Observatory site, currently in the planning phase,could potentially be expanded to a surface area severaltimes larger than its southern counterpart. Extensionsof IceCube, optimized for ultra-high energies, have alsobeen proposed [14]. Very large acoustic and radio arraysare also promising [5]. The possibility of using naturalrock salt, rather than water or ice, as a medium for ultra-high energy neutrino detection is also being explored [22].In summary, although it is also capable of observingneutrino induced cascades with energies as low as a fewTeV, we demonstrate that AMANDA is also very com-petative as an ultra-high energy experiment. Assumingthat no very large volume cascades are present withinthe six years of analyzed AMANDA data, we calculate alimit on the ultra-high energy neutrino flux. This limitis comparable to that published by the RICE collabo-ration, and is considerably stronger than the limit fromANITA-lite.As the IceCube experiment continues to be deployed,the exposure of the AMANDA/IceCube program is ac-cumulating at an increasing rate. At the end of 2008,∼0.4 events will be expected at AMANDA/IceCube, forthe case of a standard cosmogenic neutrino flux. By the1 The rates for AMANDA and IceCube shown in the table arefor cascade events alone and do not include the rate of muonsproduced in charged current νµ interactions. The rate of muonsis expected to be similar to that of cascades [14].4Event Rate Current Exposure 2008 Exposure 2011 ExposureAMANDA (300 hits) 0.044 yr−1 3.3 yrs, 0.17 events NA NAIceCube, 2007 (300 hits equiv.) 0.16 yr−1 NA 0.4 events NAIceCube, 2011 (300 hits equiv.) 0.49 yr−1 NA NA 1.2 eventsRICE ∼ 0.07 yr−1 2.3 yrs, 0.1-0.2 events 0.2-0.3 events 0.3-0.4 eventsANITA-lite 0.009 per flight [15] 1 flight, 0.009 events NA NAANITA ∼ 1 per flight NA 1 flight, ∼ 1 event 3 flights, ∼ 3 eventsPierre Auger Observatory 1.3 yr−1 [19] NA ∼ 2 events ∼ 5 eventsTABLE I: Estimated event rates in various ultra-high energy neutrino experiments assuming the standard cosmogenic neutrinoflux of Ref. [16]. Projected exposures have assumed 200 live days per year for AMANDA/IceCube and RICE. 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AMANDA Observations Constrain the Ultra-High Energy Neutrino Flux

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