First Observation of PeV-Energy Neutrinos with IceCube
M. G. Aartsen,
2
R. Abbasi,
27
Y. Abdou,
22
M. Ackermann,
41
J. Adams,
15
J. A. Aguilar,
21
M. Ahlers,
27
D. Altmann,
9
J. Auffenberg,
27
X. Bai,
31,
*
M. Baker,
27
S. W. Barwick,
23
V. Baum,
28
R. Bay,
7
J. J. Beatty,
17,18
S. Bechet,
12
J. Becker Tjus,
10
K.-H. Becker,
40
M. Bell,
38
M. L. Benabderrahmane,
41
S. BenZvi,
27
J. Berdermann,
41
P. Berghaus,
41
D. Berley,
16
E. Bernardini,
41
A. Bernhard,
30
D. Bertrand,
12
D. Z. Besson,
25
G. Binder,
8,7
D. Bindig,
40
M. Bissok,
1
E. Blaufuss,
16
J. Blumenthal,
1
D. J. Boersma,
39
S. Bohaichuk,
20
C. Bohm,
34
D. Bose,
13
S. Bo
¨
ser,
11
O. Botner,
39
L. Brayeur,
13
H.-P. Bretz,
41
A. M. Brown,
15
R. Bruijn,
24
J. Brunner,
41
M. Carson,
22
J. Casey,
5
M. Casier,
13
D. Chirkin,
27
A. Christov,
21
B. Christy,
16
K. Clark,
38
F. Clevermann,
19
S. Coenders,
1
S. Cohen,
24
D. F. Cowen,
38,37
A. H. Cruz Silva,
41
M. Danninger,
34
J. Daughhetee,
5
J. C. Davis,
17
C. De Clercq,
13
S. De Ridder,
22
P. Desiati,
27
M. de With,
9
T. DeYoung,
38
J. C. Dı
´
az-Ve
´
lez,
27
M. Dunkman,
38
R. Eagan,
38
B. Eberhardt,
28
J. Eisch,
27
R. W. Ellsworth,
16
S. Euler,
1
P. A. Evenson,
31
O. Fadiran,
27
A. R. Fazely,
6
A. Fedynitch,
10
J. Feintzeig,
27
T. Feusels,
22
K. Filimonov,
7
C. Finley,
34
T. Fischer-Wasels,
40
S. Flis,
34
A. Franckowiak,
11
R. Franke,
41
K. Frantzen,
19
T. Fuchs,
19
T. K. Gaisser,
31
J. Gallagher,
26
L. Gerhardt,
8,7
L. Gladstone,
27
T. Glu
¨
senkamp,
41
A. Goldschmidt,
8
G. Golup,
13
J. G. Gonzalez,
31
J. A. Goodman,
16
D. Go
´
ra,
41
D. Grant,
20
A. Groß,
30
M. Gurtner,
40
C. Ha,
8,7
A. Haj Ismail,
22
P. Hallen,
1
A. Hallgren,
39
F. Halzen,
27
K. Hanson,
12
D. Heereman,
12
D. Heinen,
1
K. Helbing,
40
R. Hellauer,
16
S. Hickford,
15
G. C. Hill,
2
K. D. Hoffman,
16
R. Hoffmann,
40
A. Homeier,
11
K. Hoshina,
27
W. Huelsnitz,
16,†
P. O. Hulth,
34
K. Hultqvist,
34
S. Hussain,
31
A. Ishihara,
14,‡
E. Jacobi,
41
J. Jacobsen,
27
K. Jagielski,
1
G. S. Japaridze,
4
K. Jero,
27
O. Jlelati,
22
B. Kaminsky,
41
A. Kappes,
9
T. Karg,
41
A. Karle,
27
J. L. Kelley,
27
J. Kiryluk,
35
F. Kislat,
41
J. Kla
¨
s,
40
S. R. Klein,
8,7
J.-H. Ko
¨
hne,
19
G. Kohnen,
29
H. Kolanoski,
9
L. Ko
¨
pke,
28
C. Kopper,
27
S. Kopper,
40
D. J. Koskinen,
38
M. Kowalski,
11
M. Krasberg,
27
K. Krings,
1
G. Kroll,
28
J. Kunnen,
13
N. Kurahashi,
27
T. Kuwabara,
31
M. Labare,
13
H. Landsman,
27
M. J. Larson,
36
M. Lesiak-Bzdak,
35
M. Leuermann,
1
J. Leute,
30
J. Lu
¨
nemann,
28
J. Madsen,
33
R. Maruyama,
27
K. Mase,
14
H. S. Matis,
8
F. McNally,
27
K. Meagher,
16
M. Merck,
27
P. Me
´
sza
´
ros,
37,38
T. Meures,
12
S. Miarecki,
8,7
E. Middell,
41
N. Milke,
19
J. Miller,
13
L. Mohrmann,
41
T. Montaruli,
21,§
R. Morse,
27
R. Nahnhauer,
41
U. Naumann,
40
H. Niederhausen,
35
S. C. Nowicki,
20
D. R. Nygren,
8
A. Obertacke,
40
S. Odrowski,
30
A. Olivas,
16
M. Olivo,
10
A. O’Murchadha,
12
L. Paul,
1
J. A. Pepper,
36
C. Pe
´
rez de los Heros,
39
C. Pfendner,
17
D. Pieloth,
19
E. Pinat,
12
N. Pirk,
41
J. Posselt,
40
P. B. Price,
7
G. T. Przybylski,
8
L. Ra
¨
del,
1
M. Rameez,
21
K. Rawlins,
3
P. Redl,
16
R. Reimann,
1
E. Resconi,
30
W. Rhode,
19
M. Ribordy,
24
M. Richman,
16
B. Riedel,
27
J. P. Rodrigues,
27
C. Rott,
17,∥
T. Ruhe,
19
B. Ruzybayev,
31
D. Ryckbosch,
22
S. M. Saba,
10
T. Salameh,
38
H.-G. Sander,
28
M. Santander,
27
S. Sarkar,
32
K. Schatto,
28
M. Scheel,
1
F. Scheriau,
19
T. Schmidt,
16
M. Schmitz,
19
S. Schoenen,
1
S. Scho
¨
neberg,
10
A. Scho
¨
nwald,
41
A. Schukraft,
1
L. Schulte,
11
O. Schulz,
30
D. Seckel,
31
Y. Sestayo,
30
S. Seunarine,
33
C. Sheremata,
20
M. W. E. Smith,
38
M. Soiron,
1
D. Soldin,
40
G. M. Spiczak,
33
C. Spiering,
41
M. Stamatikos,
17,¶
T. Stanev,
31
A. Stasik,
11
T. Stezelberger,
8
R. G. Stokstad,
8
A. Sto
¨
ßl,
41
E. A. Strahler,
13
R. Stro
¨
m,
39
G. W. Sullivan,
16
H. Taavola,
39
I. Taboada,
5
A. Tamburro,
31
S. Ter-Antonyan,
6
G. Tes
ˇ
ic
´
,
38
S. Tilav,
31
P. A. Toale,
36
S. Toscano,
27
M. Usner,
11
D. van der Drift,
8,7
N. van Eijndhoven,
13
A. Van Overloop,
22
J. van Santen,
27
M. Vehring,
1
M. Voge,
11
M. Vraeghe,
22
C. Walck,
34
T. Waldenmaier,
9
M. Wallraff,
1
R. Wasserman,
38
Ch. Weaver,
27
M. Wellons,
27
C. Wendt,
27
S. Westerhoff,
27
N. Whitehorn,
27
K. Wiebe,
28
C. H. Wiebusch,
1
D. R. Williams,
36
H. Wissing,
16
M. Wolf,
34
T. R. Wood,
20
K. Woschnagg,
7
C. Xu,
31
D. L. Xu,
36
X. W. Xu,
6
J. P. Yanez,
41
G. Yodh,
23
S. Yoshida,
14
P. Zarzhitsky,
36
J. Ziemann,
19
S. Zierke,
1
A. Zilles,
1
and M. Zoll
34
(IceCube Collaboration)
1
III. Physikalisches Institut, RWTH Aachen University, D-52056 Aachen, Germany
2
School of Chemistry and Physics, University of Adelaide, Adelaide South Australia 5005, Australia
3
Department of Physics and Astronomy, University of Alaska Anchorage, 3211 Providence Dr., Anchorage, Alaska 99508, USA
4
CTSPS, Clark-Atlanta University, Atlanta, Georgia 30314, USA
5
School of Physics and Center for Relativistic Astrophysics, Georgia Institute of Technology, Atlanta, Georgia 30332, USA
6
Department of Physics, Southern University, Baton Rouge, Louisiana 70813, USA
7
Department of Physics, University of California, Berkeley, California 94720, USA
8
Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA
9
Institut fu
¨
r Physik, Humboldt-Universita
¨
t zu Berlin, D-12489 Berlin, Germany
10
Fakulta
¨
tfu
¨
r Physik und Astronomie, Ruhr-Universita
¨
t Bochum, D-44780 Bochum, Germany
11
Physikalisches Institut, Universita
¨
t Bonn, Nussallee 12, D-53115 Bonn, Germany
PRL 111, 021103 (2013)
PHYSICAL REVIEW LETTERS
week ending
12 JULY 2013
0031-9007=13=111(2)=021103(7) 021103-1 Ó 2013 American Physical Society
12
Universite
´
Libre de Bruxelles, Science Faculty CP230, B-1050 Brussels, Belgium
13
Vrije Universiteit Brussel, Dienst ELEM, B-1050 Brussels, Belgium
14
Department of Physics, Chiba University, Chiba 263-8522, Japan
15
Department of Physics and Astronomy, University of Canterbury, Private Bag 4800, Christchurch, New Zealand
16
Department of Physics, University of Maryland, College Park, Maryland 20742, USA
17
Department of Physics and Center for Cosmology and Astro-Particle Physics, Ohio State University, Columbus, Ohio 43210, USA
18
Department of Astronomy, Ohio State University, Columbus, Ohio 43210, USA
19
Department of Physics, TU Dortmund University, D-44221 Dortmund, Germany
20
Department of Physics, University of Alberta, Edmonton, Alberta, Canada T6G 2E1
21
De
´
partement de physique nucle
´
aire et corpusculaire, Universite
´
de Gene
`
ve, CH-1211 Gene
`
ve, Switzerland
22
Department of Physics and Astronomy, University of Gent, B-9000 Gent, Belgium
23
Department of Physics and Astronomy, University of California, Irvine, California 92697, USA
24
Laboratory for High Energy Physics, E
´
cole Polytechnique Fe
´
de
´
rale, CH-1015 Lausanne, Switzerland
25
Department of Physics and Astronomy, University of Kansas, Lawrence, Kansas 66045, USA
26
Department of Astronomy, University of Wisconsin, Madison, Wisconsin 53706, USA
27
Department of Physics and Wisconsin IceCube Particle Astrophysics Center,
University of Wisconsin, Madison, Wisconsin 53706, USA
28
Institute of Physics, University of Mainz, Staudinger Weg 7, D-55099 Mainz, Germany
29
Universite
´
de Mons, 7000 Mons, Belgium
30
Technical University of Munich, D-85748 Garching, Germany
31
Bartol Research Institute and Department of Physics and Astronomy, University of Delaware, Newark, Delaware 19716, USA
32
Department of Physics, University of Oxford, 1 Keble Road, Oxford OX1 3NP, United Kingdom
33
Department of Physics, University of Wisconsin, River Falls, Wisconsin 54022, USA
34
Oskar Klein Centre and Department of Physics, Stockholm University, SE-10691 Stockholm, Sweden
35
Department of Physics and Astronomy, Stony Brook University, Stony Brook, New York 11794-3800, USA
36
Department of Physics and Astronomy, University of Alabama, Tuscaloosa, Alabama 35487, USA
37
Department of Astronomy and Astrophysics, Pennsylvania State University, University Park, Pennsylvania 16802, USA
38
Department of Physics, Pennsylvania State University, University Park, Pennsylvania 16802, USA
39
Department of Physics and Astronomy, Uppsala University, Box 516, S-75120 Uppsala, Sweden
40
Department of Physics, University of Wuppertal, D-42119 Wuppertal, Germany
41
DESY, D-15735 Zeuthen, Germany
(Received 19 April 2013; published 8 July 2013)
We report on the observation of two neutrino-induced events which have an estimated deposited energy
in the IceCube detector of 1:04 0:16 and 1:14 0:17 PeV, respectively, the highest neutrino energies
observed so far. These events are consistent with fully contained particle showers induced by neutral-
current
e;;
(
e;;
) or charged-current
e
(
e
) interactions within the IceCube detector. The events were
discovered in a search for ultrahigh energy neutrinos using data corresponding to 615.9 days effective live
time. The expected number of atmospheric background is 0:082 0:004ðstatÞ
þ0:041
0:057
ðsystÞ. The probability
of observing two or more candidate events under the atmospheric background-only hypothesis is
2:9 10
3
(2:8) taking into account the uncertainty on the expected number of background events.
These two events could be a first indication of an astrophysical neutrino flux; the moderate significance,
however, does not permit a definitive conclusion at this time.
DOI: 10.1103/PhysRevLett.111.021103 PACS numbers: 95.85.Ry, 95.55.Vj, 98.70.Sa
Astrophysical neutrinos are key probes of the high-
energy universe. Because of their unique properties,
neutrinos escape even dense regions, are undeflected in
galactic or extragalactic magnetic fields, and traverse the
photon-filled universe unhindered. Thus, neutrinos provide
direct information about the dynamics and interiors of
cosmological objects of the high redshift universe like
gamma-ray bursts and active galactic nuclei. Neutrinos at
energies above several hundred TeV are particularly inter-
esting as the atmospheric background in this region is very
low and a few astrophysical neutrinos can be significant.
This Letter reports on the observation of two high-energy
particle shower events discovered in a search for ultrahigh
energy neutrinos above about 1 PeV using the IceCube
detector.
IceCube [1] detects and reconstructs neutrinos by
recording Cherenkov photons emitted from secondary
charged particles produced in neutral-current (NC) or
charged-current (CC) interactions of the neutrinos in the
2800 m thick glacial ice at the geographic South Pole.
IceCube was built between 2005 and 2010. It consists of
an array of 5160 optical sensors [digital optical modules,
(DOMs)] on 86 strings at depths between 1450 and 2450 m
that instrument a volume of 1km
3
of ice. Eight of the 86
strings belong to the DeepCore subarray [2], a more
densely instrumented volume in the bottom center of the
PRL 111, 021103 (2013)
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021103-2
detector. Each DOM consists of a 10’’ photomultiplier tube
[3] in a spherical glass pressure vessel. Events are recorded
as a series of pulses (waveform) in each DOM [4] where
two basic neutrino event signatures are distinguished: a
tracklike light pattern originating from neutrino-induced
muons (tracks) and a spherical light pattern produced by
hadronic or electromagnetic particle showers (cascades).
The analysis selects neutrino candidates calorimetrically
using the total number of observed photoelectrons in
each event (NPE) [4] as a proxy of the deposited energy
[5], thus, retaining both bright tracks and cascades.
Backgrounds come from muons and neutrinos generated
in interactions of cosmic rays in the atmosphere. Because
of their steeply falling energy spectra, little background is
expected in the signal region above 1 PeV. The zenith angle
distribution of atmospheric muons peaks in the downward-
going direction and sharply decreases towards the horizon
with a cutoff at a zenith angle of cos 0:15 due to
absorption in Earth. The atmospheric neutrino distributions
have a weaker zenith-angle dependence. The analysis
rejects downward-going atmospheric muons by employing
event reconstructions based on a track hypothesis in com-
bination with a higher NPE selection criterion in the
downward-going region. All remaining events above the
combined NPE threshold are considered to be signal can-
didates independent of their topological properties.
Data were collected between May 2010 and May 2012,
an effective live time of 615.9 days excluding 54.2 days
used for the optimization of the analysis. From May 2010
to May 2011, DOMs on 79 strings (IC79) were operational
(285.8 days live time with 33.4 days excluded). This period
was immediately followed by the first year data taking with
the full 86-string (IC86) detector that lasted until May 2012
(330.1 days live time with 20.8 days excluded). The IC86
configuration is shown in Fig. 1. Events are triggered when
eight or more DOMs record signals in local coincidences
which occur when a nearest or next-to-nearest DOM on the
same string triggers within 1 s [4].
The data are filtered at the South Pole with a condition
NPE 1000, and then sent to a northern computer farm
via satellite. In order to avoid biases, we performed a blind
analysis and only 10% of the data were used to develop
the analysis. Photon arrival times are extracted from each
waveform and stored as ‘‘hits.’’ To remove hits from
coincident noise, a two-staged cleaning based on the spa-
tial separation and the time interval between hits is applied.
Data from the DeepCore strings are discarded to main-
tain uniformity across the detector volume. To reject
downward-going atmospheric muon background, only
events with at least 300 hits and NPE 3200 are retained.
To further reduce this background, the directions of the
remaining events are reconstructed with a track hypothesis,
and a stricter NPE criterion for downward-going tracks is
applied [see Fig. 2 and Eq. (1)]: for IC79, a log-likelihood
fit is performed [6] and an event selection based on a fit
quality parameter is applied to remove events which con-
tain muons from independent air showers. For IC86, a
robust regression technique [7,8] is utilized to remove
hits that have a timing significantly different from
what is expected from the bulk of the photons from a
muon track. Afterwards, the direction of the particle is
FIG. 1 (color online). Surface view of the full IceCube detec-
tor layout. Filled marks represent the positions of the IceCube
strings. Red marks in the central region are the DeepCore strings.
Squares represent the strings that did not exist in the IC79
configuration. Open circles are the positions of the closest strings
to the observed two cascade events. Stars are their reconstructed
vertex positions.
θ
cos
-1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1
NPE
10
log
3.5
4
4.5
5
5.5
6
6.5
7
7.5
-6
10
-5
10
-4
10
-3
10
-2
10
-1
10
1
10
(a)
θ
cos
-1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1
NPE
10
log
3.5
4
4.5
5
5.5
6
6.5
7
7.5
-6
10
-5
10
-4
10
-3
10
-2
10
-1
10
1
10
(b)
θ
cos
-1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1
NPE
10
log
3.5
4
4.5
5
5.5
6
6.5
7
7.5
-6
10
-5
10
-4
10
-3
10
-2
10
-1
10
1
10
(c)
FIG. 2 (color online). Distribution of NPE and reconstructed zenith angle for (a) the IC79 experimental test sample, (b) the total
background, and (c) cosmogenic signal neutrino [11]. The colors show event numbers per live time of 33.4 days. The solid lines
represent the final selection criteria for IC79.
PRL 111, 021103 (2013)
PHYSICAL REVIEW LETTERS
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021103-3
reconstructed with a basic algorithm that assumes a plane
wave of photons traveling along the direction of the muon,
‘‘LineFit’’ [5]. Both algorithms reconstruct muon tracks
with a zenith angle resolution of 1
or better.
Cascade events which pass the initial hit and NPE
selection criteria are considered signal events and, there-
fore, should be affected as little as possible by the event
rejections just described. As they resemble pointlike light
sources, the reconstruction behavior of the two algorithms
is indeed quite different finding nearly arbitrary zenith
angles, albeit with a tendency toward upward-going and
horizontal directions for the log-likelihood fit and LineFit,
respectively. Since, for these directions, the NPE threshold
value is lower than for downward-going events [see Fig. 2
and Eq. (1)], such events are retained in the final sample
even if they would be rejected on account of their true
direction.
The NPE threshold values for the two samples were
separately optimized based on the simulations to maximize
the signal [9,10] from the cosmogenic neutrino model [11].
Figure 2 shows the event distributions for the simulations
and the experimental IC79 test sample (a live time of 33.4
days). The solid lines in Fig. 2 represent the final selection
criteria for IC79 where events above the lines constitute the
final sample. The final selection criteria for the IC86
sample are
log
10
NPE
8
>
<
>
:
4:8 cos<0:075
4:8 þ 1:6
ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
1
1:0cos
0:925
2
s
cos 0:075:
(1)
The resulting neutrino effective areas, the equivalent area
at Earth’s surface in which neutrinos are detected with
100% efficiency, averaged over the two-year period from
May 2010 to May 2012 taking into account the different
detector configurations, is shown in Fig. 3. The analysis
starts to be sensitive in the energy region around 1 PeV
with its sensitivity rapidly increasing with energy. The
effective area is larger for
e
than
or
below
10 PeV showing the sensitivity of the present analysis to
cascade events in this energy region.
The expected numbers of background events in the final
sample for the 615.9 day live time from atmospheric muons
and neutrinos from decays of pions and kaons are 0:038
0:004ðstatÞ
þ0:021
0:038
ðsystÞ and 0:012 0:001ðstatÞ
þ0:010
0:007
ðsystÞ,
respectively. Compared to previous analyses, the utilized
atmospheric neutrino flux models [12] accommodate an
improved parametrization of the primary cosmic ray spec-
trum and composition which accounts now for the ‘‘knee’’
in the cosmic ray spectrum. Adding prompt atmospheric
neutrinos from decays of charmed mesons assuming
the model in [13] with the improved cosmic ray spectrum
modeling, the total number of background events
increases to 0:082 0:004ðstatÞ
þ0:041
0:057
ðsystÞ. Theoretical
uncertainties in our baseline charmed-meson model [13]
which uses perturbative-QCD calculations are included in
the background estimation. Potential nonperturbative con-
tributions, such as intrinsic charm in nuclei [14] or from the
gluon density at small x, could lead to significantly larger
cross sections and, hence, higher prompt neutrino fluxes.
Preliminary IceCube limits on the prompt flux at 90% C.L.
are a factor of 3.8 higher than the baseline model [15].
The main systematic uncertainties on the backgrounds
are from the measurement of NPE and from uncertainties
in the cosmic ray flux. They are estimated by varying the
associated parameters in the simulation. The two dominant
sources of experimental uncertainties are the absolute
DOM sensitivity and the optical properties of the ice which
contribute with (þ 43%, 26%) and (þ 0%, 42%),
respectively. Uncertainties in the cosmic ray flux models
are dominated by the primary composition (þ 0%, 37%)
and the flux normalization (þ 19%, 26%). The theoreti-
cal uncertainty in the neutrino production from charm
decay [13] relative to the total background is (þ 13%,
16%). The systematic uncertainties are assumed to be
evenly distributed in the estimated allowed range and are
summed in quadrature.
The atmospheric muon and neutrino background events
are simulated independently. However, at higher energies,
events induced by downward-going atmospheric neutrinos
should also contain a significant amount of atmospheric
muons produced in the same air shower as the neutrino
[16]. Since these events are reconstructed as downward-
going, they are more likely to be rejected with the higher
NPE threshold in this region. Thus, the number of simu-
lated atmospheric neutrino background events is likely
overestimated here.
After unblinding 615.9 days of data, we observe two
events that pass all the selection criteria. The hypothesis
that the two events are fully explained by atmospheric
/GeV
ν
E
10
log
5678910
]
2
neutrino effective area [m
0.01
0.1
1
10
100
1000
10000
e
ν
µ
ν
τ
ν
FIG. 3 (color online). The average neutrino effective area for a
4 isotropic flux, 615.9 days live time, and the IC79 and IC86
string configurations. Exposure of the sample used in this
analysis is obtained by multiplying the effective area with the
live time and 4 solid angle. The sharp peak for
e
is the
Glashow resonance [24].
PRL 111, 021103 (2013)
PHYSICAL REVIEW LETTERS
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021103-4
background including the baseline prompt atmospheric
neutrino flux [13] has a p value of 2:9 10
3
(2:8).
This value includes the uncertainties on the expected num-
ber of background events by marginalizing over a flat error
distribution. While the prompt component has large theo-
retical uncertainties, obtaining two or more events with a
probability of 10% would require a prompt flux that is
about 15 times higher than the central value of our
perturbative-QCD model. This contradicts our preliminary
upper limit on the prompt flux [15]. Using an extreme
prompt flux at the level of this upper limit, which covers
a potential unknown contribution from intrinsic charm
[17], yields a significance of 2:3.
The two events are shown in Fig. 4. They are from the
IC86 sample, but would have also passed the selection
criteria of the IC79 sample. Their spherical photon distri-
butions are consistent with the pattern of Cherenkov
photons from particle cascades induced by neutrino inter-
actions within the IceCube detector. There are no indica-
tions for photons from incoming or outgoing muon or tau
tracks. Hence, these events are most likely induced by
either CC interactions of
e
or NC interactions of
e
,
,
or
. CC interactions of
induce tau leptons with mean
decay lengths of about 50 m at these energies [18]. The
primary neutrino interaction and the secondary tau decay
initiate separate cascades which, in a fraction of such
events, lead to an observable double-peak structure in the
recorded waveforms. The two events do not show a sig-
nificant indication of such a signature. Figure 5 shows the
final-selection NPE distributions for the experimental data,
signal models, and background simulations. The two
events are near the NPE threshold of the analysis and are
consistent with a previous upper limit by IceCube [9]onan
unbroken E
2
flux, while a flux corresponding to this upper
limit predicts about 10 events above the NPE cut. The
cosmogenic neutrino model [ 11] predicts an event rate of
about 2 events in the corresponding live time but at sig-
nificantly higher energies.
Maximum-likelihood methods are used to reconstruct
the two events. The likelihood is the product of the Poisson
probabilities to observe the recorded number of
photoelectrons in a given time interval and DOM for a
cascade hypothesis which depends on the interaction ver-
tex, deposited energy and direction. Here, the time of
the first hit mainly determines the vertex position and the
recorded NPE plays a dominant role in estimating the
deposited energy. The hit information used in the recon-
struction is extracted from an unfolding procedure of the
waveforms. The open circles in Fig. 1 indicate the strings
closest to the reconstructed vertex positions. The recon-
structed deposited energies of the two cascades are 1.04
and 1.14 PeV, respectively, with combined statistical and
systematic uncertainties of 15% each. The errors on the
deposited energies are obtained by simulating cascade
events in the vicinity of the reconstructed energies and
vertices. The study is specifically performed on each event
and the larger of the two event uncertainties is cited for
both events. Thus, the error associated with the two events
differs from that of other cascade events observed in
IceCube [19]. Since there is no absolute energy standard
with adequate precision at these energies, the energy scale
is derived from simulations based on measured ice
properties and photomultiplier tube efficiencies which
are assured by measurements of atmospheric muons.
The main sources of systematic uncertainty on the
FIG. 4 (color online). The two observed events from
(a) August 2011 and (b) January 2012. Each sphere represents
a DOM. Colors represent the arrival times of the photons where
red indicates early and blue late times. The size of the spheres is
a measure for the recorded number of photoelectrons.
NPE
10
log
4.5 5 5.5 6 6.5 7 7.5
Number of events
-5
10
-4
10
-3
10
-2
10
-1
10
1
10
2
10
3
10
data
-1
s
-2
cm
-1
GeV sr
-8
= 3.6x10
φ
2
E
Yoshida
ν
cosmogenic
Ahlers
ν
cosmogenic
sum of atmospheric background
µ
atmospheric
conventional
ν
atmospheric
prompt
ν
atmospheric
FIG. 5 (color online). NPE distributions for 615.9 days of live
time at final selection level. The black points are the experimen-
tal data. The error bars on the data show the Feldman-Cousins
68% confidence interval [25]. The solid blue line marks the sum
of the atmospheric muon (dashed blue), conventional atmos-
pheric neutrino (dotted light green) and the baseline prompt
atmospheric neutrino (dotted-dashed green) background. The
error bars on the line and the shaded blue region are the
statistical and systematic uncertainties, respectively. The red
line represents the cosmogenic neutrino model [11]. The shaded
region is the allowed level of the cosmogenic flux by Ahlers
et al. [26]. The orange line represents an E
2
power-law flux up
to an energy of 10
9
GeV with an all-flavor normalization of
E
2
e
þ
þ
¼ 3:6 10
8
GeV sr
1
s
1
cm
2
, which is the in-
tegral upper limit obtained in a previous search in a similar
energy range [9]. The signal fluxes are summed over all neutrino
flavors, assuming a flavor ratio of
e
:
:
¼ 1:1:1.
PRL 111, 021103 (2013)
PHYSICAL REVIEW LETTERS
week ending
12 JULY 2013
021103-5
![FIG. 2 (color online). Distribution of NPE and reconstructed zenith angle for (a) the IC79 experimental test sample, (b) the total background, and (c) cosmogenic signal neutrino [11]. The colors show event numbers per live time of 33.4 days. The solid lines represent the final selection criteria for IC79.](/figures/fig-2-color-online-distribution-of-npe-and-reconstructed-28yxjfrw.webp)
![FIG. 3 (color online). The average neutrino effective area for a 4 isotropic flux, 615.9 days live time, and the IC79 and IC86 string configurations. Exposure of the sample used in this analysis is obtained by multiplying the effective area with the live time and 4 solid angle. The sharp peak for e is the Glashow resonance [24].](/figures/fig-3-color-online-the-average-neutrino-effective-area-for-a-3fjkxxbk.webp)