Evidence for High-Energy Extraterrestrial Neutrinos at the IceCube Detector
M. G. Aartsen,
2
R. Abbasi,
27
Y. Abdou,
22
M. Ackermann,
42
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,
41
M. L. Benabderrahmane,
42
S. BenZvi,
27
P. Berghaus,
42
D. Berley,
16
E. Bernardini,
42
A. Bernhard,
30
D. Bertrand,
12
D. Z. Besson,
25
G. Binder,
8, 7
D. Bindig,
41
M. Bissok,
1
E. Blaufuss,
16
J. Blumenthal,
1
D. J. Boersma,
40
S. Bohaichuk,
20
C. Bohm,
34
D. Bose,
13
S. B¨oser,
11
O. Botner,
40
L. Brayeur,
13
H.-P. Bretz,
42
A. M. Brown,
15
R. Bruijn,
24
J. Brunner,
42
M. Carson,
22
J. Casey,
5
M. Casier,
13
D. Chirkin,
27
A. Christov,
21
B. Christy,
16
K. Clark,
39
F. Clevermann,
19
S. Coenders,
1
S. Cohen,
24
D. F. Cowen,
39, 38
A. H. Cruz Silva,
42
M. Danninger,
34
J. Daughhetee,
5
J. C. Davis,
17
M. Day,
27
C. De Clercq,
13
S. De Ridder,
22
P. Desiati,
27
K. D. de Vries,
13
M. de With,
9
T. DeYoung,
39
J. C. D´ıaz-V´elez,
27
M. Dunkman,
39
R. Eagan,
39
B. Eberhardt,
28
B. Eichmann,
10
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,
41
S. Flis,
34
A. Franckowiak,
11
K. Frantzen,
19
T. Fuchs,
19
T. K. Gaisser,
31
J. Gallagher,
26
L. Gerhardt,
8, 7
L. Gladstone,
27
T. Gl¨usenkamp,
42
A. Goldschmidt,
8
G. Golup,
13
J. G. Gonzalez,
31
J. A. Goodman,
16
D. G´ora,
42
D. T. Grandmont,
20
D. Grant,
20
A. Groß,
30
C. Ha,
8, 7
A. Haj Ismail,
22
P. Hallen,
1
A. Hallgren,
40
F. Halzen,
27
K. Hanson,
12
D. Heereman,
12
D. Heinen,
1
K. Helbing,
41
R. Hellauer,
16
S. Hickford,
15
G. C. Hill,
2
K. D. Hoffman,
16
R. Hoffmann,
41
A. Homeier,
11
K. Hoshina,
27
W. Huelsnitz,
16, †
P. O. Hulth,
34
K. Hultqvist,
34
S. Hussain,
31
A. Ishihara,
14
E. Jacobi,
42
J. Jacobsen,
27
K. Jagielski,
1
G. S. Japaridze,
4
K. Jero,
27
O. Jlelati,
22
B. Kaminsky,
42
A. Kappes,
9
T. Karg,
42
A. Karle,
27
J. L. Kelley,
27
J. Kiryluk,
35
J. Kl¨as,
41
S. R. Klein,
8, 7
J.-H. K¨ohne,
19
G. Kohnen,
29
H. Kolanoski,
9
L. K¨opke,
28
C. Kopper,
27, ‡
S. Kopper,
41
D. J. Koskinen,
39
M. Kowalski,
11
M. Krasberg,
27
K. Krings,
1
G. Kroll,
28
J. Kunnen,
13
N. Kurahashi,
27, ‡
T. Kuwabara,
31
M. Labare,
22
H. Landsman,
27
M. J. Larson,
37
M. Lesiak-Bzdak,
35
M. Leuermann,
1
J. Leute,
30
J. L¨unemann,
28
J. Madsen,
33
G. Maggi,
13
R. Maruyama,
27
K. Mase,
14
H. S. Matis,
8
F. McNally,
27
K. Meagher,
16
M. Merck,
27
T. Meures,
12
S. Miarecki,
8, 7
E. Middell,
42
N. Milke,
19
J. Miller,
13
L. Mohrmann,
42
T. Montaruli,
21, §
R. Morse,
27
R. Nahnhauer,
42
U. Naumann,
41
H. Niederhausen,
35
S. C. Nowicki,
20
D. R. Nygren,
8
A. Obertacke,
41
S. Odrowski,
20
A. Olivas,
16
A. O’Murchadha,
12
L. Paul,
1
J. A. Pepper,
37
C. P´erez de los Heros,
40
C. Pfendner,
17
D. Pieloth,
19
E. Pinat,
12
J. Posselt,
41
P. B. Price,
7
G. T. Przybylski,
8
L. R¨adel,
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,
36
T. Ruhe,
19
B. Ruzybayev,
31
D. Ryckbosch,
22
S. M. Saba,
10
T. Salameh,
39
H.-G. Sander,
28
M. Santander,
27
S. Sarkar,
32
K. Schatto,
28
F. Scheriau,
19
T. Schmidt,
16
M. Schmitz,
19
S. Schoenen,
1
S. Sch¨oneberg,
10
A. Sch¨onwald,
42
A. Schukraft,
1
L. Schulte,
11
O. Schulz,
30
D. Seckel,
31
Y. Sestayo,
30
S. Seunarine,
33
R. Shanidze,
42
C. Sheremata,
20
M. W. E. Smith,
39
D. Soldin,
41
G. M. Spiczak,
33
C. Spiering,
42
M. Stamatikos,
17, ¶
T. Stanev,
31
A. Stasik,
11
T. Stezelberger,
8
R. G. Stokstad,
8
A. St¨oßl,
42
E. A. Strahler,
13
R. Str¨om,
40
G. W. Sullivan,
16
H. Taavola,
40
I. Taboada,
5
A. Tamburro,
31
A. Tepe,
41
S. Ter-Antonyan,
6
G. Teˇsi´c,
39
S. Tilav,
31
P. A. Toale,
37
S. Toscano,
27
E. Unger,
10
M. Usner,
11
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
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,
37
H. Wissing,
16
M. Wolf,
34
T. R. Wood,
20
K. Woschnagg,
7
D. L. Xu,
37
X. W. Xu,
6
J. P. Yanez,
42
G. Yodh,
23
S. Yoshida,
14
P. Zarzhitsky,
37
J. Ziemann,
19
S. Zierke,
1
and M. Zoll
34
(IceCube Collaboration)
1
III. Physikalisches Institut, RWTH Aachen University, D-52056 Aachen, Germany
2
School of Chemistry & Physics, University of Adelaide, Adelaide SA, 5005 Australia
3
Dept. of Physics and Astronomy, University of Alaska Anchorage,
3211 Providence Dr., Anchorage, AK 99508, USA
4
CTSPS, Clark-Atlanta University, Atlanta, GA 30314, USA
5
School of Physics and Center for Relativistic Astrophysics,
Georgia Institute of Technology, Atlanta, GA 30332, USA
6
Dept. of Physics, Southern University, Baton Rouge, LA 70813, USA
7
Dept. of Physics, University of California, Berkeley, CA 94720, USA
8
Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA
9
Institut f¨ur Physik, Humboldt-Universit¨at zu Berlin, D-12489 Berlin, Germany
10
Fakult¨at f¨ur Physik & Astronomie, Ruhr-Universit¨at Bochum, D-44780 Bochum, Germany
11
Physikalisches Institut, Universit¨at Bonn, Nussallee 12, D-53115 Bonn, Germany
12
Universit´e Libre de Bruxelles, Science Faculty CP230, B-1050 Brussels, Belgium
13
Vrije Universiteit Brussel, Dienst ELEM, B-1050 Brussels, Belgium
arXiv:1311.5238v2 [astro-ph.HE] 16 Dec 2013
2
14
Dept. of Physics, Chiba University, Chiba 263-8522, Japan
15
Dept. of Physics and Astronomy, University of Canterbury, Private Bag 4800, Christchurch, New Zealand
16
Dept. of Physics, University of Maryland, College Park, MD 20742, USA
17
Dept. of Physics and Center for Cosmology and Astro-Particle Physics,
Ohio State University, Columbus, OH 43210, USA
18
Dept. of Astronomy, Ohio State University, Columbus, OH 43210, USA
19
Dept. of Physics, TU Dortmund University, D-44221 Dortmund, Germany
20
Dept. of Physics, University of Alberta, Edmonton, Alberta, Canada T6G 2E1
21
D´epartement de physique nucl´eaire et corpusculaire,
Universit´e de Gen`eve, CH-1211 Gen`eve, Switzerland
22
Dept. of Physics and Astronomy, University of Gent, B-9000 Gent, Belgium
23
Dept. of Physics and Astronomy, University of California, Irvine, CA 92697, USA
24
Laboratory for High Energy Physics,
´
Ecole Polytechnique F´ed´erale, CH-1015 Lausanne, Switzerland
25
Dept. of Physics and Astronomy, University of Kansas, Lawrence, KS 66045, USA
26
Dept. of Astronomy, University of Wisconsin, Madison, WI 53706, USA
27
Dept. of Physics and Wisconsin IceCube Particle Astrophysics Center,
University of Wisconsin, Madison, WI 53706, USA
28
Institute of Physics, University of Mainz, Staudinger Weg 7, D-55099 Mainz, Germany
29
Universit´e de Mons, 7000 Mons, Belgium
30
T.U. Munich, D-85748 Garching, Germany
31
Bartol Research Institute and Department of Physics and Astronomy,
University of Delaware, Newark, DE 19716, USA
32
Dept. of Physics, University of Oxford, 1 Keble Road, Oxford OX1 3NP, UK
33
Dept. of Physics, University of Wisconsin, River Falls, WI 54022, USA
34
Oskar Klein Centre and Dept. of Physics, Stockholm University, SE-10691 Stockholm, Sweden
35
Department of Physics and Astronomy, Stony Brook University, Stony Brook, NY 11794-3800, USA
36
Department of Physics, Sungkyunkwan University, Suwon 440-746, Korea
37
Dept. of Physics and Astronomy, University of Alabama, Tuscaloosa, AL 35487, USA
38
Dept. of Astronomy and Astrophysics, Pennsylvania State University, University Park, PA 16802, USA
39
Dept. of Physics, Pennsylvania State University, University Park, PA 16802, USA
40
Dept. of Physics and Astronomy, Uppsala University, Box 516, S-75120 Uppsala, Sweden
41
Dept. of Physics, University of Wuppertal, D-42119 Wuppertal, Germany
42
DESY, D-15735 Zeuthen, Germany
We report on results of an all-sky search for high-energy neutrino events interacting within the
IceCube neutrino detector conducted between May 2010 and May 2012. The search follows up on
the previous detection of two PeV neutrino events, with improved sensitivity and extended energy
coverage down to approximately 30 TeV. Twenty-six additional events were observed, substantially
more than expected from atmospheric backgrounds. Combined, both searches reject a purely atmo-
spheric origin for the twenty-eight events at the 4σ level. These twenty-eight events, which include
the highest energy neutrinos ever observed, have flavors, directions, and energies inconsistent with
those expected from the atmospheric muon and neutrino backgrounds. These properties are, how-
ever, consistent with generic predictions for an additional component of extraterrestrial origin.
INTRODUCTION
High-energy neutrino observations can provide insight
into the long-standing problem of the origins and acceler-
ation mechanisms of high-energy cosmic rays. As cosmic
ray protons and nuclei are accelerated, they interact with
gas and background light to produce charged pions and
kaons which then decay, emitting neutrinos with energies
∗
Physics Department, South Dakota School of Mines and Tech-
nology, Rapid City, SD 57701, USA
†
Los Alamos National Laboratory, Los Alamos, NM 87545, USA
‡
Authors to whom correspondence should be addressed; ckop-
per@icecube.wisc.edu (C.K.); naoko@icecube.wisc.edu (N.K.);
nwhitehorn@icecube.wisc.edu (N.W.)
§
also Sezione INFN, Dipartimento di Fisica, I-70126, Bari, Italy
¶
NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA
proportional to the energies of the high-energy protons
that produced them. These neutrinos can be detected on
Earth in large underground detectors by the production
of secondary leptons and hadronic showers when they
interact with the detector material. IceCube, a large-
volume Cherenkov detector [2] made of 5160 photomul-
tipliers (PMTs) at depths between 1450 and 2450 meters
in natural Antarctic ice (Fig. 1), has been designed to
detect these neutrinos at TeV-PeV energies. Recently,
the Fermi collaboration presented evidence for accelera-
tion of low energy (GeV) cosmic-ray protons in supernova
remnants [3]; neutrino observations with IceCube would
probe sources of cosmic rays at far higher energies.
A recent IceCube search for neutrinos of EeV (10
6
TeV) energy found two events at energies of 1 PeV (10
3
TeV), above what is generally expected from atmospheric
backgrounds and a possible hint of an extraterrestrial
3
Top
veto region
125 meters
90 meters
10 meters
veto region
Side
ducial volume
ducial volume
80 meters
-1450 m
-2085 m
-2165 m
-2450 m
FIG. 1. Drawing of the IceCube array. Results here are from
the complete pictured detector for 2011-2012 and from a par-
tial detector missing the dark gray strings in the bottom left
corner for the 2010-2011 season. The side view (right) shows a
cross-section of the detector indicated in the top view (left) in
blue. Events producing first light in the veto region (shaded
area) were discarded as entering tracks (usually from cosmic
ray muons entering the detector). Most background events
are nearly vertical, requiring a thick veto cap at the top of
the detector. The shaded region in the middle contains ice
of high dust concentration [1]. Because of the high degree of
light absorption in this region, near horizontal events could
have entered here without being tagged at the sides of the
detector without a dedicated tagging region.
source [4]. Although that analysis had some sensitiv-
ity to neutrino events of all flavors above 1 PeV, it was
most sensitive to ν
µ
events above 10 PeV from the region
around the horizon, above which the energy threshold in-
creased sharply to 100 PeV. As a result, it had only lim-
ited sensitivity to the type of events found, which were
typical of either ν
e
or neutral current events and at the
bottom of the detectable energy range, preventing a de-
tailed understanding of the population from which they
arose and an answer to the question of their origin.
Here we present a follow-up analysis designed to char-
acterize the flux responsible for these events by conduct-
ing an exploratory search for neutrinos at lower energies
with interaction vertices well contained within the de-
tector volume, discarding events containing muon tracks
originating outside of IceCube (Fig. 1). This event se-
lection (see Materials and Methods) allows the resulting
search to have approximately equal sensitivity to neutri-
nos of all flavors and from all directions. We obtained
nearly full efficiency for interacting neutrinos above sev-
eral hundred TeV, with some sensitivity extending to
neutrino energies as low as 30 TeV; see Fig. 7 in Ma-
terials and Methods. The data-taking period is shared
with the earlier high-energy analysis: data shown were
taken during the first season running with the completed
IceCube array (86 strings, between May 2011 and May
2012) and the preceding construction season (79 strings,
between May 2010 and May 2011), with a total combined
live time of 662 days.
-80
-60
-40
-20
0
20
40
60
80
10
2
10
3
Declination (degrees)
Deposited EM-Equivalent Energy in Detector (TeV)
Showers
Tracks
FIG. 2. Distribution of best-fit deposited energies and decli-
nations. Seven of the events contain muons (crosses) with an
angular resolution of about 1
◦
, while the remainder are either
electromagnetic or hadronic showers (filled circles) with an
energy-dependent resolution of about 15
◦
. Error bars are 68%
confidence intervals including both statistical and systematic
uncertainties. Energies shown are the energy deposited in
the detector assuming all light emission is from electromag-
netic showers. For ν
e
charged-current events this equals the
neutrino energy; otherwise it is a lower limit on the neutrino
energy. The gap in E
dep
between 300 TeV and 1 PeV does not
appear to be significant: gaps of this size or larger appear in
28% of realizations of the best-fit continuous power-law flux.
RESULTS
In the two-year dataset, 28 events with in-detector
deposited energies between 30 and 1200 TeV were ob-
served (Fig. 2, Table I) on an expected background of
10.6
+5.0
−3.6
events from atmospheric muons and neutrinos;
see Materials and Methods. The two most energetic
of these were the previously reported PeV events [4].
Seven events contained clearly identifiable muon tracks,
whereas the remaining twenty-one were shower-like, con-
sistent with neutrino interactions other than ν
µ
charged-
current. Four of the low energy track-like events started
near the detector boundary and are downgoing, consis-
tent with the properties of the expected 6.0 ± 3.4 back-
ground atmospheric muons, as measured from a control
sample of penetrating muons in data. One of these—
the only such event in the sample—had hits in the Ice-
Top surface air shower array compatible with its arrival
time and direction in IceCube (event 28). The points at
which the remaining events were first observed were uni-
formly distributed throughout the detector (Fig. 3). This
is consistent with expectations for neutrino events and in-
consistent with backgrounds from penetrating muons or
with detector artifacts, which would have been expected
to trace the locations of either the fiducial volume bound-
ary or the positions of the instrumentation.
As part of our blind analysis, we tested a pre-defined
fixed atmospheric-only neutrino flux model [6] includ-
ing a benchmark charm component [7], reevaluated using
4
-400
-200
0
200
400
0
200
2
300
2
400
2
500
2
Vertical Position (m)
r
2
(m
2
)
Dust Layer
FIG. 3. Coordinates of the first detected light from each
event in the final sample. Penetrating muon events are first
detected predominantly at the detector boundaries (top and
right sides) where they first make light after crossing the veto
layer. Neutrino events should interact uniformly throughout
the approximately cylindrical detector volume, forming a uni-
form distribution in (r
2
, z) with the exception of interactions
in the less-transparent ice region marked “dust layer”, which
is treated as part of the detector boundary for purposes of
our event selection. The observed events are consistent with
a uniform distribution.
current measurements of the cosmic-ray spectrum in this
energy range [8, 9]. This adds an additional 1.5 charm
neutrinos to our mean background estimate and predicts
on average 6.1 (π/K and charm) background neutrinos
on top of the 6.0 ± 3.4 background muon events. Signifi-
cance was evaluated based on the number of events, the
total collected photomultiplier charge of each, and the
events’ reconstructed energies and directions (see Mate-
rials and Methods). Our procedure does not allow us to
separately incorporate uncertainties on the various back-
ground components. To nevertheless obtain an indica-
tion of the range of possible significances we have calcu-
lated values relative to background-only hypotheses with
charm at the level called “standard” in [7] as a bench-
mark flux as well as at the level of our current 90% CL ex-
perimental bounds [9] (corresponding to 3.8 times “stan-
dard”). To prevent possible confirmation bias, we split
the data set into two samples. For the 26 new events
reported here, using the benchmark flux, we obtain a
significance of 3.3σ (one-sided). Combined by Fisher’s
method with the 2.8σ observation of the earlier analy-
sis where the two highest energy events were originally
reported [4], and which uses the same benchmark atmo-
spheric neutrino flux model, we obtain a final significance
for the entire data set of 28 events of 4.1σ. The same
calculation performed a posteriori on all 28 events gives
4.8σ. These two final significances would be reduced to
3.6σ and 4.5σ, respectively, using charm at the level of
our current 90% CL experimental bound.
Dep. Energy Time Decl. R.A. Med. Angular Event
ID (TeV) (MJD) (deg.) (deg.) Error (deg.) Type
1 47.6
+6.5
−5.4
55351 −1.8 35.2 16.3 Shower
2 117
+15
−15
55351 −28.0 282.6 25.4 Shower
3 78.7
+10.8
−8.7
55451 −31.2 127.9 . 1.4 Track
4 165
+20
−15
55477 −51.2 169.5 7.1 Shower
5 71.4
+9.0
−9.0
55513 −0.4 110.6 . 1.2 Track
6 28.4
+2.7
−2.5
55568 −27.2 133.9 9.8 Shower
7 34.3
+3.5
−4.3
55571 −45.1 15.6 24.1 Shower
8 32.6
+10.3
−11.1
55609 −21.2 182.4 . 1.3 Track
9 63.2
+7.1
−8.0
55686 33.6 151.3 16.5 Shower
10 97.2
+10.4
−12.4
55695 −29.4 5.0 8.1 Shower
11 88.4
+12.5
−10.7
55715 −8.9 155.3 16.7 Shower
12 104
+13
−13
55739 −52.8 296.1 9.8 Shower
13 253
+26
−22
55756 40.3 67.9 . 1.2 Track
14 1041
+132
−144
55783 −27.9 265.6 13.2 Shower
15 57.5
+8.3
−7.8
55783 −49.7 287.3 19.7 Shower
16 30.6
+3.6
−3.5
55799 −22.6 192.1 19.4 Shower
17 200
+27
−27
55800 14.5 247.4 11.6 Shower
18 31.5
+4.6
−3.3
55924 −24.8 345.6 . 1.3 Track
19 71.5
+7.0
−7.2
55926 −59.7 76.9 9.7 Shower
20 1141
+143
−133
55929 −67.2 38.3 10.7 Shower
21 30.2
+3.5
−3.3
55937 −24.0 9.0 20.9 Shower
22 220
+21
−24
55942 −22.1 293.7 12.1 Shower
23 82.2
+8.6
−8.4
55950 −13.2 208.7 . 1.9 Track
24 30.5
+3.2
−2.6
55951 −15.1 282.2 15.5 Shower
25 33.5
+4.9
−5.0
55967 −14.5 286.0 46.3 Shower
26 210
+29
−26
55979 22.7 143.4 11.8 Shower
27 60.2
+5.6
−5.6
56009 −12.6 121.7 6.6 Shower
28 46.1
+5.7
−4.4
56049 −71.5 164.8 . 1.3 Track
TABLE I. Properties of the 28 events. Shown are the de-
posited electromagnetic-equivalent energy (the energy de-
posited by the events in IceCube assuming all light was made
in electromagnetic showers) as well as the arrival time and
direction of each event and its topology (track or shower-
like). The energy shown is equal to the neutrino energy for
ν
e
charged-current events, within experimental uncertainties,
and is otherwise a lower limit on the neutrino energy due to
exiting muons or neutrinos. Errors on energy and the angle
include both statistical and systematic effects. Systematic
uncertainties on directions for shower-like events were deter-
mined on an individual basis; track systematic uncertainties
here are equal to 1
◦
, which is an upper limit from studies of
the cosmic ray shadow of the moon [5].
DISCUSSION
Although there is some uncertainty in the expected
atmospheric background rates, in particular for the con-
tribution from charmed meson decays, the energy spec-
trum, zenith distribution, and shower to muon track ratio
of the observed events strongly constrain the possibility
that our events are entirely of atmospheric origin. Al-
most all of the observed excess is in showers rather than
5
10
-1
10
0
10
1
10
2
Events per 662 Days
10
2
10
3
Deposited EM-Equivalent Energy in Detector (TeV)
Background Atmospheric Muon Flux
Bkg. Atmospheric Neutrinos (
π
/K)
Background Stat. and Syst. Uncertainties
Atmospheric Neutrinos (Benchmark Charm Flux)
Atmospheric Neutrinos (90% CL Charm Limit)
Signal+Bkg. Best-Fit Astrophysical
E
−
2
Spectrum
Data
0
2
4
6
8
10
Events per 662 Days
1.0 0.5 0.0 0.5 1.0
sin(Declination)
Northern Sky (upgoing)Southern Sky (downgoing)
Background Atmospheric Muon Flux
Bkg. Atmospheric Neutrinos (
π
/K)
Background Stat. and Syst. Uncertainties
Atmospheric Neutrinos (Benchmark Charm Flux)
Atmospheric Neutrinos (90% CL Charm Limit)
Signal+Bkg. Best-Fit Astrophysical
E
−
2
Spectrum
Data
FIG. 4. Distributions of the deposited energies and declination angles of the observed events compared to model predictions.
Zenith angle entries for data (right) are the best-fit zenith position for each of the 28 events; a small number of events (Table I)
have zenith uncertainties larger than the bin widths in this figure. Energies plotted (left) are reconstructed in-detector visible
energies, which are lower limits on the neutrino energy. Note that deposited energy spectra are always harder than the spectrum
of the neutrinos that produced them due to the neutrino cross-section increasing with energy. The expected rate of atmospheric
neutrinos is shown in blue, with atmospheric muons in red. The green line shows our benchmark atmospheric neutrino flux (see
text), the magenta line the experimental 90% bound. Due to lack of statistics from data far above our cut threshold, the shape
of the distributions from muons in this figure has been determined using Monte Carlo simulations with total rate normalized
to the estimate obtained from our in-data control sample. Combined statistical and systematic uncertainties on the sum of
backgrounds are indicated with a hatched area. The gray line shows the best-fit E
−2
astrophysical spectrum with a per-flavor
normalization (1:1:1) of E
2
Φ
ν
(E) = 1.2 · 10
−8
GeV cm
−2
s
−1
sr
−1
.
muon tracks, ruling out an increase in penetrating muon
background to the level required. Atmospheric neutrinos
are a poor fit to the data for a variety of reasons. The
observed events are much higher in energy, with a harder
spectrum (Fig. 4) than expected from an extrapolation of
the well-measured π/K atmospheric background at lower
energies [9–11]: nine had reconstructed deposited ener-
gies above 100 TeV, with two events above 1 PeV, rela-
tive to an expected background from π/K atmospheric
neutrinos of approximately 1 event above 100 TeV. Rais-
ing the normalization of this flux both violates previous
limits and, due to ν
µ
bias in π and K decay, predicts
too many muon tracks in our data (2/3 tracks vs. 1/4
observed).
Another possibility is that the high-energy events re-
sult from charmed meson production in air showers
[7, 12]. These produce higher energy events with equal
parts ν
e
and ν
µ
, matching our observed muon track frac-
tion reasonably well. However, our event rates are sub-
stantially higher than even optimistic models [12] and
the energy spectrum from charm production is too soft
to explain the data. More importantly, increasing charm
production to the level required to explain our observa-
tions violates existing experimental bounds [9]. As atmo-
spheric neutrinos produced by any mechanism are made
in cosmic ray air showers, downgoing atmospheric neu-
trinos from the southern sky will in general be accompa-
nied into IceCube by muons produced in the same par-
ent air shower. These accompanying muons will trigger
our muon veto, removing the majority of these events
from the sample and biasing atmospheric neutrinos to
the northern hemisphere. The majority of our events,
however, arrive from the south. This places a strong
model-independent constraint on any atmospheric neu-
trino production mechanism as an explanation for our
data.
By comparison, a neutrino flux produced in extrater-
restrial sources would, like our data, be heavily biased
toward showers because neutrino oscillations over as-
tronomical baselines tend to equalize neutrino flavors
[13, 14]. An equal-flavor E
−2
neutrino flux, for exam-
ple, would be expected to produce only 1/5 track events
(see Materials and Methods). The observed zenith distri-
bution is also typical of such a flux: as a result of absorp-
tion in the Earth above tens of TeV energy, most events
(approximately 60%, depending on the energy spectrum)
from even an isotropic high-energy extraterrestrial pop-
ulation would be expected to appear in the Southern
Hemisphere. Although the zenith distribution is well ex-
plained (Fig. 4) by an isotropic flux, a slight southern
excess remains, which could be explained either as a sta-
tistical fluctuation or by a source population that is either
