Nuclear Physics B
Proceedings
Supplement
Nuclear Physics B Proceedings Supplement 00 (2012) 1–6

Ultra-high energy neutrinos with IceCube
Aya Ishiharaa,ba for the IceCube collaborationb Faculty of Science, Chiba University, 1-33, Yayoi-cho, Inage-ku, Chiba-shi, Chiba, 263-8522 Japan

Abstract
We report on a search for ultra-high energy neutrinos with energy greater than 106 GeV using the data taken with
the IceCube detector at the South Pole. The data was collected between June 2010 and May 2011 when 90% of the
IceCube detector was in operation and from May 2011 to May 2012 which corresponds to the first physics run withthe fully completed IceCube detector. Two signal neutrino candidate events are observed in the sample of 670.1 daysof livetime over expected background rates of 0.06 events. These events are consistent with the cascade-like events
induced by νe charged current or νe,µ,τ neutral current interaction within the IceCube detector volume. Preliminary
p-values for a background-only hypothesis are 1.6x10−3 (2.9 σ) without a prompt atmospheric neutrino contribution
and 1.5x10−2 (2.2 σ) with a default perturbative QCD-based prompt neutrino contribution.
Keywords: Astrophysical neutrinos, Ultra-high energy cosmic-rays, the GZK effect

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1. Introduction
Neutrinos are important probes for exploring the highenergy universe. Because of their unique nature, neutrinos are undeflected in the galactic or extra-galacticmagnetic fields and unabsorbed in the photon-filled universe, and thus they may be the missing link between
TeV gamma-rays and EeV cosmic-rays. Photons, which
are the conventional messengers from the universe, arehighly attenuated by cosmic microwave background in
this energy range. Therefore, cosmic neutrinos at PeV
energies or higher are of particular importance for thecomplete survey of the ultra-high energy universe.
Cosmic neutrinos are expected to be produced byinteractions of high energy hadronic beams from cosmic accelerators with surrounding photons and/or matter. However, the neutrino flux produced in each sourceis highly uncertain. A way of detecting neutrinos frompossible weak sources is the diffuse neutrino search.
This is because superposition of fluxes from all the
weak astrophysical neutrino sources could compose a
detectable signal. Still, the expected astrophysical neutrino flux is so small that a huge target is required inorder for neutrinos to interact and produce secondary

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charged particles that can be detected through their
Cherenkov emissions. With IceCube [1], the first cubickilometer scale neutrino detector is realized.
IceCube construction started in 2005 and was completed in December 2010. IceCube had been in operation with the partial detector configurations since 2005
until the beginning of the full configuration run. IceCube uses 2,800 m thick glacial ice as a Cherenkov
medium. Cherenkov photons emitted from relativistic
charged particles are detected by an array of Digital Optical Modules (DOMs) that amplify and digitally sample the high-speed pulses [3] from the enclosed photomultiplier tube (PMT) [2]. The cable assemblies, often
called strings, were lowered into holes drilled to a depthof 2450 m. The DOMs occupy the bottom 1000 m atintervals of 17 m where the glacial ice is transparent. In
addition to the large scale array of optical sensors, theinner dense array called DeepCore was also deployed.
Additional DOMs frozen into tanks located at the surface near the top of each hole constitute an air shower
array called IceTop. IceTop provides us with the capability to study the atmospheric muon background reliably. The strings and tanks are arranged in a hexagonalAya Ishihara for the IceCube collaboration / Nuclear Physics B Proceedings Supplement 00 (2012) 1–6
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lattice pattern with a spacing of approximately 125 m. 97
The array comprises 5160 DOMs on 86 strings of which 98
8 strings corresponds to DeepCore, and 324 modules in 99
the surface array. The DeepCore array was excluded 100
in the current analysis. From June 2010 to May 2011, 101
79 out of 86 strings were in operation. This run was 102
followed by the initial year of data taken with the Ice- 103
Cube full 86 string configuration that continued until 104
May 2012. In this paper, results from the diffuse ultra- 105high energy (UHE) neutrino searches at PeV energies 106
and higher with the 79 and full 86 string configurations 107
of IceCube with data taken between 2010 and 2012 in 108
total of 670.1 days of effective livetime are presented. 109
11060

2. Analysis

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2.1. Signals and backgrounds
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Possible sources of astronomical neutrinos at PeV 114
energies and higher include potential accelerators of 115
cosmic-rays above 3 x 1015 eV (the “knee” of the 116
cosmic-ray spectra), such as AGNs or GRBs. These 117
cosmic-ray source models and the resultant neutrino 118
fluxes have large uncertainties. Still, many models pre- 119dict signal neutrino spectra which follow a power-law. 120
The spectral slope of the fluxes is predicted to be harder 121
for cosmic neutrinos than that of background atmo- 122
spheric muons and neutrinos.
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In the energy region above 1 PeV, in addition to neu- 124
trinos directly propagating from each source, there are 125
highly expected secondary produced neutrinos in the 126
propagation of the cosmic-rays. These “cosmogenic” 127
neutrinos are produced in interactions of the highest 128
energy cosmic-rays with the cosmic-microwave back- 129
ground (CMB) and higher energy (infra-red, optical, 130
and ultra violet) background [8, 9]. They may provide 131
us a direct evidence of the highest energy cosmic-ray 132
sources. This is because the spectral shapes and flux 133
levels are sensitive to the redshift dependence of the 134
cosmic-ray source distributions and cosmic-ray primary 135
compositions [10]. The undeflected and unabsorbed na- 136
ture of the neutrino propagation also give us an insight 137
into their origin. The cosmic-ray fluxes and the CMB 138
density are well measured, and thus secondary neutri- 139
nos are guaranteed to exist due to well known photo- 140pion production mechanisms. The cosmogenic neutri- 141
nos are distributed in a higher energy region than possible background events. This makes the cosmogenic 142
neutrino signal events less uncertain to be distinguished 143
from background.
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Most background events are induced by particles gen- 145
erated in the interaction of cosmic-rays in the atmo- 146
sphere. These atmospheric interactions produce two 147

2

types of background particles. One of these is muons
with energies high enough to reach to at least the depthof 1.5 km in ice before losing all of their energy [11],and the other is atmospheric neutrinos. There is a great
advantage in searches for neutrinos at PeV energies andhigher also from the perspective of background reduction. Since the conventional atmospheric neutrino andmuon spectra are proportional to E −3.7 or steeper, a verysmall number of background events is expected in theenergy region.
In addition to those energy dependence, the atmospheric muon distribution has a strong zenith angle
dependence. Because of the difference in the propagation distance, atmospheric muons peaks at vertical downward-going direction and sharply decrease to
the horizontal direction. Atmospheric neutrinos havea weaker zenith angle dependence due to the interaction probability of neutrinos in the Earth. Because theneutrino-nucleon cross-section increases with energy,
the Earth is transparent to neutrinos from all directions
only up to PeV energies. At higher energies, the neutrino mean free paths become significantly smaller thanthe Earth radius. At PeV energies and above, a neutrino
distribution has a peak near the horizon.
When we refer to the atmospheric neutrinos, we assume the conventional atmospheric neutrinos unless explicitly written. The conventional atmospheric neutrinos come from decays of cosmic-ray induced pionand kaons in the atmosphere. At PeV energies andabove, neutrinos from decays of charmed mesons are
expected to dominate over the conventional atmospheric
neutrinos, however. This contribution is called prompt
atmospheric neutrinos. The energy at which prompt
neutrinos begin to dominate is expected to be around3x105 GeV depending on models. The prompt atmospheric neutrino spectrum has a slope index of approximately -2.7, harder than that of conventional atmospheric neutrinos. The prompt atmospheric neutrino
models involve highly uncertain charm meson production cross-section. While the perturbative-QCD calculations [4] predict prompt neutrinos with theoretical uncertainty of a factor of two, non-perturbative part of
QCD in the charm production involves a larger uncertainty. We have not observed clear evidence for prompt
contributions in the atmospheric neutrinos so far [1].
2.2. Event selections
Above PeV energies, the primary variable to discriminate signal from background is the energy of theparticles. Because the amount of energy deposited by
the neutrino-induced charged particles in the detector
and observed as Cherenkov photons is highly correlatedAya Ishihara for the IceCube collaboration / Nuclear Physics B Proceedings Supplement 00 (2012) 1–6
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Figure 1: Event number distributions on the plane of NPE and reconstructed zenith angle in 498.4 hours are shown for the experimental 10% test
sample from 2011-2012 (left panel), the total background (middle panel), and signal cosmogenic neutrino model [13] Monte Carlo simulations(right panel). The colors indicate event numbers per bin. The signal distributions are adding all three flavors of neutrinos. The background
distributions include atmospheric muons from the CORSIKA package with SIBYLL high energy interaction model and conventional atmospheric
neutrinos. The solid lines in each panel indicate the final selection criteria.

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with their energy, the extremely-high energy neutrino 180
signal stands out against the atmospheric muon and neutrino background due to the much higher light depo- 181
sition [7]. The total number of photo-electrons (NPE)
recorded in a time interval of 11 µs of an event is used
182
as the main distinctive feature to separate signal from183
background after a lower level background reduction re184
questing the number of hit DOMs greater than 300.
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Then as the second step, we reconstruct the direc- 186tion of in-coming particles. Since the background atmo- 187
spheric muons are concentrated in the direction of verti- 188
cally downward-going while expected neutrino signals 189
above PeV energies can only enter the IceCube detector 190
from directions above slightly below the horizontal ge- 191
ometries, the final selection criterion was obtained as a 192
reconstructed direction dependent NPE threshold value. 193
From May 31, 2010 to May 12, 2011, IceCube with 194
79 strings was in operation, and the first year IceCube 195
full configuration with 86 strings took data from May 196
13, 2011 to May 15, 2012. The corresponding effec- 197
tive livetimes for 2010-2011 and 2011-2012 runs are 198319.18 days and 350.91 days respectively excluding the 199
period of detector calibration and unstable operation. 200
Selection criteria are obtained by optimizing in each set 201
of samples with different detector configurations using 202
slightly different zenith angle reconstruction algorithms 203
to minimize the model discovery factor [5, 6]. Presented 204
here as an example is the selection criteria for the full 205
string configuration run. The criteria for the 2010-2011 206
data sample is quite similar but with slightly different 207
values. The final selection criteria for 2011-2012 sam- 208
ple are events with NPE exceeding the threshold values 209

as:



 4.8
q
log10 NPE ≥ 2
 4.8+1.6 1−( cosθ−1.0
0.925 )cosθ ≤ 0.075
otherwise.

(1)

In Fig. 1, the event number distributions of the Monte
Carlo simulations and experimental data for 498.4 hours
are shown. The period corresponds to the livetime of a
10% test sample for the IceCube 2011-2012 configuration for examinations of the Monte Carlo simulations
and detector responses. Following the blind analysis
strategy, the signal selection criteria is obtained fully
based on the Monte Carlo simulations of backgroundand signal events. The solid lines in Fig. 1 indicates the
final criteria (Eq. 1) for the sample.
The effective neutrino area of the analysis averaged
over the approximately two years period with the different IceCube detector configurations from June 2010
to May 2012 is shown in Fig. 2. The effective areas are given for each neutrino flavor, averaged over
4π solid angle. An equal flux of neutrinos and antineutrinos is assumed. The effective areas are approximately two times larger than for the 40 string configuration of data sampling period from 2008-2009 [5].
The sharp peaked structure for electron neutrino effective area corresponds to the Glashow resonance [12].
Below 5 PeV, the effective area of electron neutrinos
exceeds that of muon or tau neutrinos. This reflects thefact that muon and tau tracks from muon/tau neutrinos
partially deposit their energies into the detector volume
while the electromagnetic (EM) cascades from electron
neutrinos deposit 100% of their energy. While tracks
have a longer propagation length, they do not satisfy theAya Ishihara for the IceCube collaboration / Nuclear Physics B Proceedings Supplement 00 (2012) 1–6

Preliminary

10-5

E2 φ(E) [GeV cm-2 sec-1 sr-1]

neutrino effective area [m2]10000

PAO(2009) ντ limit x3

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4ANITA-II(2010)

RICE(2012)

Preliminary

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IC40 ( 333.5 days)
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10-8-8νeνµ1

Cosmogenic ν models

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ντ

0.1
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IceCube2012
May31/2010-May15/2012 (670.1 days)

Engel et al.
Kotera et al. (FRII)
Ahlers et al. (max)

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10-10

Ahlers et al. (best)
Yoshida et al.

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log ν energy/GeV

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log (Energy/GeV)

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Figure 2: The IceCube neutrino effective area averaged over
670.1 days with different string configurations. Exposure of the sample used in this analysis is obtained by multiplying the effective areawith the 670.1 days of livetime and 4π solid angle. The sharp peaked
structure at 6.3 PeV for electron neutrino is due to the Glashow resonance [12].

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selection criteria (Eq. 1) in the energy region, in contrast
to the EM cascades induced within detector.
The quasi-differential model-independent IceCube
sensitivity on neutrino fluxes using two years of IceCube data is shown in Fig. 3. The sensitivity is calculated as in [20] requiring 2.44 events in a bin size of
one energy decade. The 2.44 is the Feldman-Cousins
up-fluctuation 90% CL limit event number for null ex- 233
pected events. The IceCube sensitivity to the neutrino 234
fluxes in the region between 1 PeV to 100 EeV from 235
previous study [5] is approximately a factor of four im- 236
proved. The factor of four improvement from previous 237
IceCube results is due to approximately a factor of two 238
longer livetime and a factor of two gain in the effective 239
area, while we are able to keep the similar background 240
event rates with the improved reconstruction algorithms 241
242
and Monte Carlo simulations.
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3. Results

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After unblinding of the full data sample with 246
670.1 days of effective livetime, we observe two signal 247
candidate neutrino events that pass all the selection cri- 248
teria. One event is from August 8th, 2011 with the NPE 249
value of 7.0x104 p.e. and 312 DOMs recorded photon 250

Figure 3: The IceCube differential sensitivity to all flavor neutrino
flux by the 2010-2012 ultra-high energy neutrino analysis with an effective livetime of 670.1 days (red solid line). The systematic errors
are not included. Several model predictions are shown for comparison: Engel et al. (ΩΛ = 0.7) [14], Kotera et al. (FRII) [15], Ahlers etal. (best and max) [16], Yoshida et al. ((m, Zmax ) = (4,4)) [13]. Model
fluxes are summed over all neutrino flavors, assuming standard neutrino oscillations. The model independent differential upper limits byother experiments are also shown for Auger (PAO) [17], RICE [18],
ANITA [19], and the previous IceCube result (IC40) [5]. Limits fromother experiments are converted to the all flavor limit assuming standard neutrino oscillation and a 90% quasi-differential limit per one
energy decade when necessary.

signal. The other event is from Jan 3rd, 2012 with the
NPE value of 9.6x104 p.e. and 354 DOMs recorded
photon signal.
The expected background event rate of atmosphericmuons and conventional atmospheric neutrinos is 0.057
events in the period. The background with prompt atmospheric neutrinos is 0.190 events assuming the standard prompt neutrino flux from Ref. [4]. Expected total
background numbers and each background contribution
are presented in Table 1. The preliminary p-value for the
background only hypothesis is calculated to be 1.6x10−3(σ = 2.9) and for the background with the nominal number of prompt atmospheric neutrinos [4] is 1.5x10−2 (σ= 2.2).
An event display of the event from August 2011 isshown in Fig. 4. Both events have a characteristic spherical photon distribution. These distributions are consistent with Cherenkov photons from particle cascades in-Aya Ishihara for the IceCube collaboration / Nuclear Physics B Proceedings Supplement 00 (2012) 1–6

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Number of events per bin (670.1days)

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Figure 4: An event display of the one of the two observed events. Eachspheres represent a DOM. Colors indicate the arrival timing of thephoton (red = earliest, blue= latest). The size of the sphere indicatesthe measured photon in each DOM.

IceCube experimental
Atmospheric νAtmospheric µ
Atmospheric ν prompt Enberg et al.
Total background
GZK ν Yoshida et al.
GZK ν Ahlers et al.
GZK ν Kotera et al.
E2 φ( ν e+ ν µ + ν τ)=3.6x10 -8 [GeVsr-1cm -2 sec -1], Emax =109 GeV

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10-1
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10

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duced by neutrino interactions within the IceCube detector without outgoing muon or tau tracks. It is likelythat these events are induced by either electron neutrinos of any interaction or neutral current interaction ofmuon or tau neutrinos. The preliminary total reconstructed energy deposits of these two cascade events are
1.1 and 1.3 PeV with the reconstruction uncertaintiesof ∼35% including statistical uncertainty and systematic uncertainties associated with the ice properties andthe absolute DOM sensitivity. These energy deposits
correspond to the incoming neutrino energy if each cascade is the result of a charged current interaction of an 271
electron neutrino where 100% of the neutrino energy is 272
deposited near the interaction vertex. Including the hy- 273
pothesis of neutral current interaction of three flavors of 274
neutrinos, the statistical error, and the systematic error, 275
the 90% most probable neutrino energies of two events 276
at the earth surface correspond to 780 TeV-5.6 PeV and 277
890 TeV-8.5 PeV, respectively, assuming that the sur- 278
face neutrino flux follows an E −2 power law.
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Background
Contributions
Atmospheric muons
Conventional atmospheric neutrinos
Total backgroundTotal backgroundwith prompt ν [4]Event Rates
in 670 days
0.036
0.021
0.057

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0.190

Table 1: Preliminary number of background events in 670.1 days ofeffective livetime. Systematic uncertainties are not included. The
magnitude of these uncertainties can be larger than the best-fit background expectation listed in the table.

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Figure 5: Event distributions from 2010-2012 sample passing the final selection criteria as function of log10 NPE. Black marks indicate
experimental data. Purple, red and orange solid lines indicate cosmogenic neutrino models with different assumed parameters. Greenline indicates a power-law flux which follows Eν−2 up to Eν = 109 GeVwith the three flavor neutrino flux level of E 2 φνe +νµ +ντ = 3.6x108 GeV
sr−1 sec−1 cm−2 corresponding to the obtained upper limit from theprevious IceCube result in the similar energy range [5]. Filled magenta area is prompt atmospheric neutrino distribution and filled blue
area is conventional atmospheric muon and neutrino background sum.

Figure 5 shows event distributions from two years ofdata passing the final selection criteria as a function oflog10 NPE for experimental data, signal and backgroundMonte Carlo simulations. Signal models include threecosmogenic neutrino models with different cosmic-ray
source distributions or spectra. Among the three cosmogenic neutrino models, the distribution from Ref. [15],with an assumption that cosmic-ray spectra at sources
extend to lower energy than the other models, shows abetter agreement compared to the other models, which
produce neutrinos from interaction of UV/optical photons. One of the signal models is an E −2 power-law
neutrino flux which represent the contribution from direct neutrino production in cosmic-ray sources. The
flux level corresponds to the upper limit obtained by
the 2008-2009 IceCube data [5]. Table 2 presents the
expected numbers of signal events from reference cosmogenic neutrino models and experimentally observed
numbers after the final selection. The numbers are given
for the full energy region and the region above 100 PeV.
Most of the cosmogenic neutrinos are neutrinos with energies above 100 PeV. We would like to note that while

Aya Ishihara for the IceCube collaboration / Nuclear Physics B Proceedings Supplement 00 (2012) 1–6

Cosmogenic ν
Signal Models
Yoshida et al. [13]Kotera et al. (FRII) [15]Ahlers et al. (maximal) [16]Ahlers et al. (the best fit) [16]

All
2.1
4.1
3.2
1.6

Event Rates
Eν ≥100 PeV
2.0
2.6
3.0
1.5

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331
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333

IceCube Experimental
May 31, 2010 - May 15, 2012

Event Rates
All Eν ≥ 100 PeV
2
0

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Table 2: Preliminary number of signal events from Monte Carlo simulations and experimentally observed events in 670.1 days of effective
livetime at the final level of the analysis. Numbers are given for all theenergy range and for the primary neutrino energy larger than 100 PeV.
Errors are not included.

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the cosmogenic neutrino models in the table predict 346
1.6∼4.1 events in 670.1 days of an effective livetime, 347
348
which is consistent as total rates with the experimental 349
observation of two neutrino events, some of the mod- 350
els can be constrained with null observation of events 351
352
above 100 PeV.
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354

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4. Summary316

We presented the results of the analysis using the
2010-2012 data by the 79-string and 86-string IceCube
detector to search for the ultra-high energy neutrinos
with energies exceeding 106 GeV. The sensitivity of theanalysis is improved by approximately a factor of four
from the previous analysis [5]. Not including systematic uncertainties, the observation of 2 neutrino events
corresponds to a 2.9 sigma excess above the event rates
expected from atmospheric muons and conventional atmospheric neutrinos and 2.2 sigma excess with prompt
atmospheric neutrinos. It is however possible that thesignificance of the excess may be reduced by the systematic uncertainties in the background estimate. The
level of expected flux from the observation is consistent
with the upper limit obtained by the previous UHE neutrino search [5], which is already below the WaxmanBahcall limit [21].

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