Constraints on the neutrino emission from the Galactic Ridge with the ANTARES telescope

Abstract

A highly significant excess of high-energy astrophysical neutrinos has been reported by the IceCube Collaboration. Some features of the energy and declination distributions of IceCube events hint at a North/South asymmetry of the neutrino flux. This could be due to the presence of the bulk of our Galaxy in the Southern hemisphere. The ANTARES neutrino telescope, located in the Mediterranean Sea, has been taking data since 2007. It offers the best sensitivity to muon neutrinos produced by galactic cosmic ray interactions in this region of the sky. In this letter a search for an extended neutrino flux from the Galactic Ridge region is presented. Different models of neutrino production by cosmic ray propagation are tested. No excess of events is observed and upper limits for different neutrino flux spectral indices Gamma are set. For Gamma = 2.4 the 90% confidence level flux upper limit at 100 TeV for one neutrino flavour corresponds to phi(1f)(0) (100TeV) = 2.0 . 10(-17) GeV-1 cm(-2) s(-1) sr(-1). Under this assumption, at most two events of the IceCube cosmic candidates can originate from the Galactic Ridge. A simple power-law extrapolation of the Fermi-LAT flux to account for IceCube High Energy Starting Events is excluded at 90% confidence level.

Full text

Physics Letters B 760 (2016) 143–148
Contents lists available at ScienceDirect
Physics Letters B
www.elsevier.com/locate/physletb
Constraints on the neutrino emission from the Galactic Ridge with the
ANTARES telescope
S. Adrián-Martínez a, A. Albertb, M. André c, M. Anghinolfi ab, G. Anton d, M. Ardid a,
J.-J. Aubert e, T. Avgitas f, B. Baret f, J. Barrios-Martí g, S. Basa h, V. Bertin e, S. Biagi i,
R. Bormuth j,k, M.C. Bouwhuis j, R. Bruijn j,l, J. Brunner e, J. Busto e, A. Caponem,n,
L. Caramete o, J. Carr e, S. Celli m,n, T. Chiarusi p, M. Circella q, A. Coleiro f, R. Coniglione i,
H. Costantini e, P. Coyle e, A. Creusot f, A. Deschamps r, G. De Bonis m,n, C. Distefano i,
C. Donzaud f,s, D. Dornic e, D. Drouhin b, T. Eberl d, I. El Bojaddaini t, D. Elsässer u,
A. Enzenhöfer d, K. Fehn d, I. Felis a, L.A. Fusco p,v,∗, S. Galatà f, P. Gay w,f, S. Geißelsöder d,
K. Geyer d, V. Giordano x, A. Gleixner d, H. Glotin y,z, R. Gracia-Ruiz f, K. Graf d,
S. Hallmann d, H. van Haren aa, A.J. Heijboer j, Y. Hello r, J.J. Hernández-Rey g, J. Hößl d,
J. Hofestädt d, C. Hugon ab,ac, G. Illuminati m,n, C.W. James d, M. de Jongj,k, M. Kadler u,
O. Kalekin d, U. Katz d, D. Kießling d, A. Kouchner f,z, M. Kreter u, I. Kreykenbohm ad,
V. Kulikovskiy i,ae, C. Lachaud f, R. Lahmann d, D. Lefèvre af, E. Leonora x,ag, S. Loucatos ah,f,
M. Marcelin h, A. Margiotta p,v, A. Marinelli ai,aj, J.A. Martínez-Mora a, A. Mathieu e,
T. Michael j, P. Migliozzi ak, A. Moussa t, C. Mueller u, E. Nezri h, G.E. P˘
av˘
ala ¸so,
C. Pellegrino p,v, C. Perrina m,n, P. Piattelli i, V. Popa o, T. Pradier al, C. Racca b, G. Riccobene i,
K. Roensch d, M. Saldaña a, D.F.E. Samtleben j,k, A. Sánchez-Losa g,q, M. Sanguineti ab,ac,
P. Sapienza i, J. Schnabel d, F. Schüssler ah, T. Seitz d, C. Sieger d, M. Spurio p,v,
Th. Stolarczyk ah, M. Taiuti ab,ac, A. Trovatoi, M. Tselengidou d, D. Turpin e, C. Tönnis g,
B. Vallage ah,f, C. Vallée e, V. Van Elewyck f, E. Visser j, D. Vivolo ak,am, S. Wagner d,
J. Wilms ad, J.D. Zornoza g, J. Zúñiga g
aInstitut d’Investigació per a la Gestió Integrada de les Zones Costaneres (IGIC) – Universitat Politècnica de València, C / Paranimf 1, 46730 Gandia, Spain
bGRPHE – Université de Haute Alsace – Institut universitaire de technologie de Colmar, 34 rue du Grillenbreit, BP 50568 – 68008 Colmar, France
cTechnical University of Catalonia, Laboratory of Applied Bioacoustics, Rambla Exposició, 08800 Vilanova i la Geltrú, Barcelona, Spain
dFriedrich–Alexander-Universität Erlangen–Nürnberg, Erlangen Centre for Astroparticle Physics, Erwin-Rommel-Str. 1, 91058 Erlangen, Germany
eAix-Marseille Université, CNRS/IN2P3, CPPM UMR 7346, 13288 Marseille, France
fAPC, Université Paris Diderot, CNRS/IN2P3, CEA/IRFU, Observatoire de Paris, Sorbonne Paris Cité, 75205 Paris, France
gIFIC – Instituto de Física Corpuscular, c/ Catedraático José Beltrán, 2, E-46980 Paterna, Valenc ia, Spain
hLAM – Laboratoire d’Astrophysique de Marseille, Pôle de l’Étoile Site de Château-Gombert, rue Frédéric Joliot-Curie 38, 13388 Marseille Cedex 13, France
iINFN – Laboratori Nazionali del Sud (LNS), Via S. Sofia 62, 95123 Catania, Italy
jNikhef, Science Park, Amsterdam, The Netherlands
kHuygens–Kamerlingh Onnes Laboratorium, Universiteit Leiden, The Netherlands
lUniversiteit van Amsterdam, Instituut voor Hoge-Energie Fysica, Science Park 105, 1098 XG Amsterdam, The Netherlands
mINFN – Sezione di Roma, P.le Aldo Moro 2, 00185 Roma, Italy
nDipartimento di Fisica dell’Università La Sapienza, P.le Aldo Moro 2, 00185 Roma, Italy
oInstitute for Space Science, RO-077125 Bucharest, M˘
agurele, Romania
pINFN – Sezione di Bologna, Viale Berti-Pichat 6/2, 40127 Bologna, Italy
qINFN – Sezione di Bari, Via E. Orabona 4, 70126 Bari, Italy
rGéoazur, UCA, CNRS, IRD, Observatoire de la Côte d’Azur, Sophia Antipolis, France
sUniv. Paris-Sud, 91405 Orsay Cedex, France
tUniversity Mohammed I, Laboratory of Physics of Matter and Radiations, B.P.717, Oujda 6000, Morocco
uInstitut für Theoretische Physik und Astrophysik, Universität Würzburg, Emil-Fischer Str. 31, 97074 Würzburg, Germany
vDipartimento di Fisica e Astronomia dell’Università, Viale Berti Pichat 6/2, 40127 Bologna, Italy
*Corresponding author at: INFN – Sezione di Bologna and Dipartimento di Fisica e Astronomia dell’Università di Bologna, Viale Berti-Pichat 6/2, 40127 Bologna, Italy.
E-mail address: luigiantonio.fusco@bo.infn.it (L.A. Fusco).
http://dx.doi.org/10.1016/j.physletb.2016.06.051
0370-2693/©2016 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/). Funded by
SCOAP3.

144 S. Adrián-Martínez et al. / Physics Letters B 760 (2016) 143–148
wLaboratoire de Physique Corpusculaire, Clermont Univertsité, Université Blaise Pascal, CNRS/IN2P3, BP 10448, F-63000 Clermont-Ferrand, France
xINFN – Sezione di Catania, Viale Andrea Doria 6, 95125 Catania, Italy
yLSIS, Aix Marseille Université, CNRS, ENSAM, LSIS, UMR 7296, 13397 Marseille, France; Université de Toulon, CNRS, LSIS, UMR 7296, 83957 La Garde, France
zInstitut Universitaire de France, 75005 Paris, France
aa Royal Netherlands Institute for Sea Research (NIOZ), Landsdiep 4, 1797 SZ ’t Horntje (Texel), The Netherlands
ab INFN – Sezione di Genova, Via Dodecaneso 33, 16146 Genova, Italy
ac Dipartimento di Fisica dell’Università, Via Dodecaneso 33, 16146 Genova, Italy
ad Dr. Remeis-Sternwarte and ECAP, Universität Erlangen–Nürnberg, Sternwartstr. 7, 96049 Bamberg, Germany
ae Moscow State University, Skobeltsyn Institute of Nuclear Physics, Leninskie Gory, 119991 Moscow, Russia
af Mediterranean Institute of Oceanography (MIO), Aix-Marseille University, 13288, Marseille Cedex 9, France; Université du Sud Toulon-Var, 83957, La Garde
Cedex, France CNRS-INSU/IRD UM 110
ag Dipartimento di Fisica ed Astronomia dell’Università, Viale Andrea Doria 6, 95125 Catania, Italy
ah Direction des Sciences de la Matière – Institut de recherche sur les lois fondamentales de l’Univers – Service de Physique des Particules, CEA Saclay, 91191
Gif-sur-Yvette Cedex, France
ai INFN – Sezione di Pisa, Largo B. Pontecorvo 3, 56127 Pisa, Italy
aj Dipartimento di Fisica dell’Università, Largo B. Pontecorvo 3, 56127 Pisa, Italy
ak INFN – Sezione di Napoli, Via Cintia, 80126 Napoli, Italy
al Université de Strasbourg, IPHC, 23 rue du Loess, 67037 Strasbourg, France – CNRS, UMR7178, 67037 Strasbourg, France
am Dipartimento di Fisica dell’Università Federico II di Napoli, Via Cintia, 80126 Napoli, Italy
a r t i c l e i n f o a b s t r a c t
Article history:
Received 6 February 2016
Received in revised form 12 May 2016
Accepted 22 June 2016
Available online 28 June 2016
Editor: S. Dodelson
Keywords:
Neutrino telescope
Diffuse muon neutrino flux
ANTARES
A highly significant excess of high-energy astrophysical neutrinos has been reported by the IceCube
Collaboration. Some features of the energy and declination distributions of IceCube events hint at a
North/South asymmetry of the neutrino flux. This could be due to the presence of the bulk of our Galaxy
in the Southern hemisphere. The ANTARES neutrino telescope, located in the Mediterranean Sea, has
been taking data since 2007. It offers the best sensitivity to muon neutrinos produced by galactic cosmic
ray interactions in this region of the sky. In this letter a search for an extended neutrino flux from the
Galactic Ridge region is presented. Different models of neutrino production by cosmic ray propagation are
tested. No excess of events is observed and upper limits for different neutrino flux spectral indices are
set. For =2.4the 90% confidence level flux upper limit at 100 TeV for one neutrino flavour corresponds
to 1f
0(100 TeV) =2.0 ·10−17 GeV−1cm−2s−1sr−1. Under this assumption, at most two events of the
IceCube cosmic candidates can originate from the Galactic Ridge. A simple power-law extrapolation of
the Fermi-LAT flux to account for IceCube High Energy Starting Events is excluded at 90% confidence
level.
©2016 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license
(http://creativecommons.org/licenses/by/4.0/). Funded by SCOAP3.
1. Introduction
Neutrino telescopes search for high-energy (EνGeV) neutri-
nos produced by astrophysical objects. The ANTARES detector [1] is
the largest underwater neutrino telescope. Its effective area, good
angular resolution and good exposure to the southern sky has al-
lowed the detector to produce the best limits on neutrino emission
up to 100 TeV from point-like objects at negative declinations [2].
The IceCube Collaboration has reported the observation of as-
trophysical neutrinos with the High Energy Starting Events (HESE)
[3,4], and confirmed the discovery in other analyses [5,6]. The se-
lection of HESE and of the lower-energy events reported in [5] is
based on vetoing techniques [7], detecting contained events from
all directions, and the observed signal is dominantly composed
of shower-type events. Individual neutrino sources have not been
identified so far. The flux of these events is compatible with the
hypothesis of isotropy [8] and equipartition in the three neutrino
flavours [9]. The limited statistic and the poor angular resolution
make the HESE sample insensitive to a possible asymmetry from
neutrinos arising from the northern and southern sky regions.
Muon neutrinos coming from the northern hemisphere are de-
tected as upward-going muon tracks. The IceCube Collaboration
has observed astrophysical neutrinos also in this upward-going
sample [6]. By comparing the spectral energy distribution of this
sample to that of other analyses, a difference in the shape between
the neutrino flux observed in the southern sky and the one from
the northern sky is found, with a significance of 2σ.
For the ANTARES location, in the Mediterranean Sea, the south-
ern sky is accessible via upward-going muon tracks, for which
a median angular resolution of 0.4◦for a Eμ−2neutrino energy
spectrum is achieved [10]. Individual neutrino sources have been
searched for using ANTARES data alone [2] and in combination
with IceCube events [11]. The search method uses an unbinned
maximum likelihood ratio estimation that accounts for the energy
and directional information of individual neutrino candidates. This
significantly reduces the influence of the atmospheric background
in the search for a cosmic signal.
The current ANTARES upper limit on point-like sources in the
central region of the Galaxy corresponds to a spectral energy dis-
tribution Eν2dNν/dEν∼0.9 ·10−8GeVcm−2s−1. At 1TeV, this
limit is about four times higher than the neutrino flux expected
by the only hadronic accelerator discovered so far in our Galaxy
[12]. However, as motivated in §2, the Galactic plane is considered
a guaranteed extended source of neutrinos originating from the
decay of short-lived particles induced by the interaction of cos-
mic rays (CRs) with interstellar matter. Recent computations [13,
14] suggest that the neutrino yield from this process could be at
least one order of magnitude larger than that of still unresolved
point-like sources. These conclusions have been derived by the
measurement of the flux of γ-rays by satellite and ground experi-
ments, part of which is due to hadronic mechanisms than produce
neutrinos as well. This neutrino flux is anyway too low to entirely
explain the IceCube signal observed in the Southern sky [15,16].
In this letter, a search for neutrinos (νμ+¯
νμ) on an extended
region of solid angle  =0.145 sr centred in the Galactic ori-

S. Adrián-Martínez et al. / Physics Letters B 760 (2016) 143–148 145
gin is described. Data collected by the ANTARES neutrino tele-
scope (described in §3) from 2007 to 2013 have been used. Atmo-
spheric neutrinos represent a diffuse, irreducible background for
all searches of neutrinos of astrophysical origin. The flux of atmo-
spheric neutrino at 1TeV is more than three order of magnitude
larger than the signal reported in [5]. Differently from the search of
neutrino point-like sources, the method considered here relies on
the search for an excess of neutrino-induced upward going events
in the high-energy tail of the measured spectrum. The observed
muon provides a proxy of the neutrino energy [17]. The signal is
in fact expected with harder spectral index (∝E−
ν, with studied
from 2.0 to 2.7) with respect to that of the background (∝E−3.7
ν).
A crucial point to maximise the signal is the definition of the size
of the considered region. The challenges of this work in the re-
duction of the background and the optimisation procedures based
on Monte Carlo simulations are described in §4. The results of the
analysis are presented and discussed in §5and §6.
2. Neutrinos from our Galaxy and the IceCube signal
The isotropic flux of high-energy cosmic neutrinos measured
by the IceCube Collaboration was modelled with power-laws
dNν/dEν=0E−
ν, yielding relatively soft spectral indices (>2).
The value =2is expected for neutrinos produced from primary
CRs accelerated by the simplest Fermi shock acceleration models
[19,20] and interacting near their sources [21]. The E−2.0
νspectrum
is excluded [8] in the energy range between 25 TeV and 2.8 PeV
with a significance of more than 3.8σ, assuming that the astro-
physical neutrino flux is isotropic and consisting of equal flavours
at Earth. Under the same assumptions, the best-fit spectral in-
dex is =2.50 ±0.09 and the normalisation at the energy of
100 TeV (for all three neutrino flavours, 3f) is 3f
0(100 TeV) =
6.7+1.1
−1.2·10−18 GeV−1cm−2s−1sr−1. No significant excess is found
when searching for spatial anisotropies. Muon neutrinos coming
from the northern hemisphere [6] yields a best-fit, single-flavour
flux 1f
0(100 TeV) =9.9+3.9
−3.4·10−19 GeV−1cm−2s−1sr−1and as-
suming =2. It is worth noting that this particular channel can
access neutrinos only at energies above 100 TeV because of the
more abundant atmospheric background from νμ-induced events,
while analyses including shower-like neutrino interactions have
lower energy thresholds.
The separate fit of the fluxes from the northern and southern
hemispheres [8] indicates a preference (although with small sta-
tistical significance) for a harder spectrum in the northern hemi-
sphere. Moreover, some authors have observed that events are con-
centrated near the Galactic Centre and Galactic Plane regions in a
way that seems inconsistent with an isotropic neutrino distribution
[22,23]. Such a difference between the northern and southern skies
could potentially stem from the presence of a softer contribution to
the neutrino flux from the Galaxy in the southern hemisphere [24].
The isotropic distribution of extragalactic sources (such as ac-
tive galactic nuclei or γ-ray bursts) presumably dominates the sig-
nal from the northern hemisphere. Models generally predict that
neutrinos from these sources will be generated via photo-hadronic
interactions of high-energy protons with low-energy photons of
the background. These models are characterised by relatively high-
energy thresholds (due to charged pion production) and disfavour
a soft neutrino spectrum [25,26]. Other extragalactic sources, such
as starburst galaxies [27], are expected to produce neutrinos pri-
marily by proton–proton (or nuclei) interactions and subsequent
decay of secondary charged mesons (mainly pions). In this case,
the emission has a spectral index close to that of the parent
hadrons and a lower energy threshold [28]. Since in p–p interac-
tions the number of charged pions is approximately twice that of
neutral pions (which decay to a pair of γ), the neutrino flux can be
constrained from the observed γ-ray flux. Due to the high density
of matter in the central part of the Galactic Plane, a neutrino signal
coming from this part of the sky, mostly located in the southern
hemisphere, is expected to follow this emission scenario.
Fermi-LAT data provide the best measurement of the diffuse
γ-ray flux in the Galactic Plane up to ∼100 GeV [29]. Given
certain model assumptions, the fraction of this flux attributed to
hadronic processes can be estimated, allowing the derivation of
the neutrino yield from CR propagation. Models with a constant
diffusion coefficient of CR in our Galaxy predict a much lower and
softer neutrino spectrum (2.7) [30,31] than that measured by
IceCube.
New predictions for the neutrino production due to CR propa-
gation have been presented recently. The authors of [13] start with
the observation that conventional models of Galactic CR propa-
gation cannot explain the large γ-ray flux measured by Milagro
[32] from the inner Galactic Plane region and by H.E.S.S. [33]
from the Galactic Ridge region. To reconcile Fermi-LAT, Milagro
and H.E.S.S. data, they have developed a phenomenological model
characterised by radially-dependent CR transport properties, which
predicts a neutrino spectral index in the range 2.4–2.5. In [14],
a sizeable neutrino flux is expected to be produced by the inter-
action of fresh CRs, which are hadrons supplied by young acceler-
ators and contained by the local magnetic field, with the ambient
matter. The authors of [34] note that IceCube observes 3 events
in the Eν>100 TeV energy range with arrival direction compati-
ble with a Galactic Ridge origin (|| <30◦, |b| <4◦). Furthermore,
the corresponding neutrino flux matches the high-energy power-
law extrapolation of the spectrum of diffuse γ-ray emission from
the Galactic Ridge as observed by Fermi-LAT. This motivates the hy-
pothesis that these IceCube neutrino events and Fermi-LAT γ-ray
flux are both produced in interactions of CRs with the interstel-
lar medium in the inner Galactic region. All these models predict
an enhancement of the neutrino flux coming from a limited region
close to the Galactic Centre.
3. The ANTARES detector and dataset
The ANTARES underwater neutrino telescope [1] is located
40 km off the southern coast of France in the Mediterranean Sea
(42◦48N, 6◦10E). It consists of a three-dimensional array of
10-inch photomultiplier tubes (PMTs). Neutrino detection is based
on the observation of Cherenkov light induced in the medium by
relativistic charged particles. Some of the emitted photons produce
signals in the PMTs (“hits”). The position, time and collected charge
of the hits are used to infer the direction and energy of the inci-
dent neutrino.
The study presented here focuses on track-like events, asso-
ciated with CC interactions of muon neutrinos. The muon direc-
tion is correlated with that of the incoming neutrino, and a sub-
degree angular resolution on the neutrino arrival direction can be
achieved by means of a maximum likelihood fit [10].
Data collected from May 2007 to December 2013 constitute the
data sample for the present analysis, with an effective total life-
time of 1622 days. High quality data runs, defined according to
environmental and data taking conditions, have been selected for
this work (analogously to [2]). A detailed Monte Carlo simulation
is available for each data acquisition run [35,36].
4. The search method
An enhancement of the neutrino diffuse emission from a re-
gion of the sky covering a small solid angle can be searched for

146 S. Adrián-Martínez et al. / Physics Letters B 760 (2016) 143–148
by comparing the number of events coming from the region (on-
zone) to that of regions with no expected signal and the same
acceptance to the background (off-zones). To enhance the harder
signal over the background of atmospheric neutrinos, a cut select-
ing mainly high-energy events is defined. This approach has al-
ready been used to search for neutrino candidates from the region
of the Fermi Bubbles [37]. Optimising this method requires: 1) an
efficient suppression of atmospheric events; 2) the optimisation of
the size of the search region and 3) the subsequent definition of
background-only regions, each having the same exposure as that of
the signal region. The analysis uses Monte Carlo simulations only
in the optimisation of the event selection; this avoids biases in the
estimation of the signal and background and reduces systematic
effects. Monte Carlo data sets are produced simulating real data
acquisition conditions, taking into account the actual detection ef-
ficiency of the apparatus.
The signal is assumed to be a power-law diffuse flux with
arbitrary normalisation and spectral indices varying from =
2.0to2.7. Motivated by the IceCube best fit and models of neu-
trino production from CR propagation, the event selection criteria
have been optimised in order to achieve the best sensitivity for a
signal with spectral index =2.4. They are identical to those ob-
tained for =2.5. The optimal cuts are found using the Model
Rejection Factor (MRF) minimisation technique [38].
The background component due to mis-reconstructed atmo-
spheric muons, which mimick upgoing neutrino events, has been
simulated using the MUPAGE program [39]. This background is
suppressed by cuts on quality parameters of upgoing reconstructed
tracks: , which is related to the maximum likelihood of the
fit, and β, which estimates the angular error. The distributions
of and βfor atmospheric neutrinos, atmospheric muons and
data are reported in [10]. It is found that the cut  >−5.0 and
β<0.5◦optimises the MRF and suppresses the contamination
from wrongly reconstructed atmospheric muons in the upgoing
sample to the level of 1%.
The remaining background consists of atmospheric neutrinos
[17]. The conventional component, coming from the decay of pi-
ons and kaons, has been modelled according to [40] while the flux
from [41] has been used for the prompt component, expected from
charmed hadron decays. This component is reduced by imposing
a cut on the estimated energy of the events, limiting the event
sample to the energy where the harder cosmic flux is expected
to emerge above the atmospheric background. For this analysis,
the energy estimator EANN [18], derived from an artificial neural
network algorithm, is used. The standard deviation of the variable
log10(EANN /Etrue), where Etrue is the Monte Carlo true energy of
the muon, is almost constant at ∼0.4over the considered energy
range. The MRF optimisation results in Ecut
ANN =10 TeV as the best
cut value. Above Ecut
ANN, only 6% of the selected atmospheric neutri-
nos survive while 40% of the signal (for =2.4) passes the cut.
Assuming a direct connection between the emission of γ-rays
and neutrinos from pion decay in hadronic mechanisms [42], the
γ-ray flux measured by Fermi-LAT [29] is used to estimate the
flux of Galactic neutrinos. Though this diffuse emission is extended
over the whole Galactic Plane, it is much brighter in the very
central region; including non-central regions of the plane in this
search would mostly increase the atmospheric background. The
MRF method is used to determine the optimal search region for
each spectral index. For a signal spectrum with =2.4, the signal
region is represented by the rectangle (enclosing the Galactic Cen-
tre) in galactic coordinates with longitude || <40◦and latitude
|b| <3◦. This corresponds to a solid angle of  =0.145 sr. Mod-
ifications to the longitudinal size of the signal region do not sig-
nificantly reduce the resulting sensitivity, while the latitude bound
Fig. 1. Aitoff projection in galactic coordinates of the signal (black) and background
(red) regions, representing the considered Galactic Plane region and off-zones of
the analysis. Also shown are the Fermi Bubbles (grey) as in [43]. The signal region,
delimited by || <40◦, |b| <3◦covers a solid angle of 0.145 sr. (For interpretation
of the references to colour in this figure legend, the reader is referred to the web
version of this article.)
has a larger effect – about 10% worsening per degree of increased
size.
Off-zones are defined as fixed regions in equatorial coordinates,
which have identical size and shape as the signal region and are
not overlapping with it or each other. In local coordinates, off-
zones span the same fraction of the sky as the on-zone, but with
some fixed delay in time, i.e. they differ only in right ascension.
They are shifted in the sky to avoid any overlap with the Fermi
Bubble regions [43], so that none of the possible signal events from
these areas enters into the background estimation. The maximum
number of independent off-zone regions is 9. The signal and back-
ground regions in galactic coordinates are shown in Fig. 1. Data
from the signal region were blinded until the event selection pro-
cedure was completely defined. Off-zones can also be used to test
the agreement between data and Monte Carlo.
After the optimisation procedure, considering a signal flux with
an energy spectrum with =2.4(2.5) the expected limit at 90%
confidence level (c.l.) for the considered data sample corresponds
to 1f
0(1GeV) =2.0 (6.0) ·10−5GeV−1cm−2s−1sr−1. For the nor-
malisation at a different energy E, the fluxes must be multiplied by
the factor E
1GeV
−. For all flavours, the normalisation must be
multiplied by a factor three under the assumption of a cosmic flux
in flavour equipartition (νe:νμ:ντ=1 :1 :1). The energy range
between 3 and 300 TeV contains the central 90% of the expected
detected signal.
5. Results
After the unblinding of the entire data sample, 3.7 events sur-
viving cuts are observed on average in the off-zone regions, while
two are detected from the Galactic Plane region. In the evaluation
of the upper limit, our method is sensitive only to signals in ex-
cess of the off-zones, i.e. any isotropic flux is treated as background
for this purpose. The isotropic neutrino flux of astrophysical origin
as measured by IceCube would produce 0.2 events equivalently in
each off-zone and in the on-zone region. The distributions of the
number of selected events in the on-zone and off-zone regions as
a function of the reconstructed energy are reported in Fig. 2.
A smaller number of events is observed in the signal region
than the expected background, and the Feldman and Cousins 90%
c.l. upper bound [44] is computed. For =2.4the correspond-
ing flux 1f
0(1GeV) =1.5 ·10−5GeV−1cm−2s−1sr−1. However,
adopting the same conservative approach as for the limits from
selected point-like sources [2] in the case of an underfluctuation,
the 90% c.l. upper limit on the signal flux is set to the value of the

S. Adrián-Martínez et al. / Physics Letters B 760 (2016) 143–148 147
Fig. 2. Distribution of the reconstructed energy EANN of upgoing muons in the
Galactic Plane (black crosses) and average of the off-zone regions (red histogram).
The grey line shows the energy selection cut applied in the procedure.
Fig. 3. ANTARES upper limits (black) derived for the Galactic Plane region for dif-
ferent signal spectral indices , compared to the flux required to produce from
2 to 6 IceCube HESE in the signal region (red dashed lines). Selection cuts have
been optimised for =2.4and2.5. The limits for softer and harder spectral in-
dices are thus derived with non-optimal criteria. The values of the normalisation
factor 1f
0(100 TeV)are reported on the right y-axis.
ANTARES sensitivity. One limit for each considered spectral index
is obtained.
The 90% c.l. upper limits on 1f
0(1GeV)are reported in Fig. 3
for particular values of . For each value of , the one-flavour neu-
trino flux from the considered region necessary to produce from
2 to 6 HESE is also reported. The curves are computed on the
basis of the effective areas reported in [3] according to the pre-
scription of [24]. All fluxes above the horizontal black lines are
excluded at 90% c.l. by ANTARES observation. For instance, a flux
with spectral index =2.5 that produces 3 or more HESE in the
signal region of  =0.145 sr is excluded. For the conventional
CR propagation scenario, the 90% c.l. upper limit for =2.7is
1f
0(1GeV) =7.5 ·10−4GeV−1cm−2s−1sr−1.
Fig. 4 shows the computed ANTARES 90% c.l. upper limit for the
neutrino emission in the region || <40◦and |b| <3◦assuming a
=2.4neutrino flux. The limit on 3f
0assuming flavour equipar-
tition is reported, along with expectations from models. The simple
extrapolation [34] to IceCube energies of the diffuse γ-ray flux
measured by Fermi-LAT [29] is excluded at 90% confidence level,
assuming flavour equipartition. Models (KRAγ, Fig. 4) that consider
a harder CR spectrum in the inner Galaxy, and the hardening of the
CR spectrum measured by PAMELA and AMS-02 [13], yield a neu-
trino flux (at 100 TeV) of a factor of two to three lower. Models
Fig. 4. ANTARES upper limit (magenta line) on the neutrino flux integrated over
the solid angle  =0.145 sr corresponding to the Galactic Plane region || <40◦,
|b| <3◦. Our limit is compared to expectations as computed in [13], assuming a
CR cut-off at 5 ×107GeV, both with (KRAγ) and without (KRA) spectral hardening.
The neutrino flux (dot-dashed line) extrapolated from the Fermi-LAT diffuse γflux
(purple circles) adapted from [34] up to IceCube energies is shown. The implied
flux from the three events from the IceCube 3 years sample [4] is shown as black
triangles. The solid black line shows the all-sky average neutrino intensity from the
IceCube global fit analysis in the energy range 25 TeV–2.8 PeV [8] integrated over
.
not including the CR hardening (KRA, Fig. 4) yield neutrino fluxes
one order of magnitude smaller than that of the extrapolation from
Fermi-LAT.
6. Conclusions and outlook
An enhanced neutrino production from the central part of the
Galactic Plane has been searched for using track-like events ob-
served by the ANTARES telescope from 2007 to 2013. No excess
of events has been observed, and limits on the contribution from
this possible source to the astrophysical neutrino signal observed
by IceCube have been set as a function of spectral index. For a
neutrino flux ∝E−2.5we exclude at 90% c.l. that 3 or more events
from the 3 year IceCube HESE sample are originating from this
region. The extrapolation of the Fermi-LAT γ-ray measurement to
the IceCube neutrino flux in the Galactic Plane area has also been
constrained.
Data taking of the ANTARES neutrino telescope will continue
at least up to the end of 2016, increasing the νμstatistics avail-
able for this analysis. In addition, a new reconstruction procedure
for showering events has been developed, with an angular resolu-
tion of 3–4 degrees in the TeV–PeV range [45], which can be used
to enhance the sensitivity for point-like sources and diffuse emis-
sion from small regions of the sky. Preliminary results indicate that
using reconstructed cascades, the sensitivity to point sources with
=2spectrum improves by about 30%. This suggests that at the
end of data taking the sensitivity of ANTARES will reach a level
close to the prediction of the model that includes a CR spectral
hardening (KRAγ) [13].

148 S. Adrián-Martínez et al. / Physics Letters B 760 (2016) 143–148
Acknowledgements
We are indebted to D. Gaggero, D. Grasso, A. Urbano and
M. Valli for the useful discussion, comments and suggestions. The
authors acknowledge the financial support of the funding agen-
cies: Centre National de la Recherche Scientifique (CNRS), Com-
missariat à l’Énergie Atomique et aux Énergies Alternatives (CEA),
Commission Européenne (FEDER fund and Marie Curie Program),
Institut Universitaire de France (IUF), IdEx program and UnivEarthS
Labex program at Sorbonne Paris Cité (ANR-10-LABX-0023 and
ANR-11-IDEX-0005-02), Région Île-de-France (DIM-ACAV), Région
Alsace (contrat CPER), Région Provence-Alpes-Côte d’Azur, Départe-
ment du Var and Ville de La Seyne-sur-Mer, France; Bundesmin-
isterium für Bildung und Forschung (BMBF), Germany; Istituto
Nazionale di Fisica Nucleare (INFN), Italy; Stichting voor Funda-
menteel Onderzoek der Materie (FOM), Nederlandse Organisatie
voor Wetenschappelijk Onderzoek (NWO), the Netherlands; Coun-
cil of the President of the Russian Federation for young scientists
and leading scientific schools supporting grants, Russia; National
Authority for Scientific Research (ANCS), Romania; Ministerio de
Economía y Competitividad (MINECO), Prometeo and Grisolía pro-
grams of Generalitat Valenciana and MultiDark, Spain; Agence de
l’Oriental and CNRST, Morocco. We also acknowledge the technical
support of IFREMER, AIM and Foselev Marine for the sea operation
and the CC-IN2P3 for the computing facilities.
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