Exploring the Universe with Very High Energy Neutrinos

3.4 MB·English

by  A. Kappes

NuclearPhysicsB
Proceedings
Supplement
NuclearPhysicsBProceedingsSupplement00(2015)1–10
Exploring the Universe with Very High Energy Neutrinos
A.Kappesa,fortheIceCubeCollaboration1
aErlangenCentreforAstroparticlePhysics,Friedrich-Alexander-Universita¨tErlangen-Nu¨rnberg,D-91058Erlangen,Germany
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Abstract
n
a
Withthediscoveryofahigh-energyneutrinofluxinthe0.1PeVtoPeVrangefrombeyondtheEarth’satmosphere
J
withtheIceCubedetector,neutrinoastronomyhasachievedamajorbreakthroughintheexplorationofthehigh-energy
0
3 universe.Oneofthemaingoalsistheidentificationandinvestigationofthestillmysterioussourcesofthecosmicrays
whichareobservedatEarthwithenergiesuptoseveral105 PeV.Inadditiontobeingsmoking-gunevidenceforthe

] presenceofcosmicraysinaspecificobject,neutrinosescapeevendenseenvironmentsandcanreachusfromdistant
E
placesintheuniverse,therebyprovidinguswithauniquetooltoexplorecosmicaccelerators.Thisarticlesummarizes
H
ourknowledgeabouttheobservedastrophysicalneutrinofluxandcurrentstatusofthesearchforindividualcosmic
.
h neutrinosources. Attheend,itgivesanoverviewofplansforfutureneutrinotelescopeprojects.
p
- Keywords: high-energyneutrinos,neutrinoastronomy,neutrinotelescopes,ANTARES,BAIKAL,IceCube,
o KM3NeT,cosmicneutrinoflux,point-likeneutrinosources,gamma-raybursts
r
t
s
a
[ 1. Introduction tonsorheaviernuclei[1],aclearpicturewhethersuper-

novaremnantsareindeedthemainsourcesoftheGalac-
1 ThediscoveryofcosmicraysbyVictorHessin1912
tic cosmic rays is still missing. Furthermore, the ori-
v
markedthebeginningofastroparticlephysicswhichhas
8 gin of ultra-high energy cosmic rays (UHECRs) above
9 enabled us to explore the Universe at the highest en- (cid:38) 3×1018eVremainsacompletemystery. Withtheir
7 ergies reaching up to several 105PeV. But even more unique properties, neutrinos will help in solving these
7 than 100 years later, the central question regarding the
andotherimportantastrophysicalquestions,astheirob-
0 sources that can accelerate particles to energies far be-
servation from an object or region would unambigu-
.
1 yond what is achievable with man-made accelerators ously identify it as a source of high-energy protons or
0 is mostly unanswered. First of all, cosmic rays can-
heaviernuclei.Thishasbeenoneofthemaindriversfor
5
not directly reveal their sources except maybe at the
thedevelopmentandconstructionofneutrinotelescopes
1
very highest energies as the charged particles are de-
: overthepastdecades.Atthesametime,thefactthatun-
v flected in the galactic and intergalactic magnetic fields
tilrecentlythehigh-energyneutrinoskyhasbeentotal
i
X andhencedonotpointbacktotheirorigin.High-energy terra incognita implies a high potential for unexpected
gamma-ray photons, which are produced in the inter-
r discoveries.
a action of cosmic rays with matter or photon fields, on
Thelowinteractioncrosssectionthatrendersneutri-
the other hand, can also be generated via up-scattering
nosveryvaluablefortheexplorationofthehigh-energy
oflow-energyphotonsbyacceleratedelectrons(inverse
universe, however, also makes them very hard to de-
Compton scattering). Though the gamma-ray spectra
tect. In particular, over the past years it has become
of at least two supernova remnants seem to originate clearthathugedetectorsofatleastkm3 sizeareneces-
from pion decay and hence from acceleration of pro-
sarytoeventuallyopenthisexcitingnewwindowtothe
universe(seee.g.[2,3]).
1http://icecube.wisc.edu
A.Kappes/NuclearPhysicsBProceedingsSupplement00(2015)1–10 2
Figure1:SchematicviewsoftheBAIKAL(left)[4],ANTARES(middle)[5]andIceCube(right)[6]neutrinodetectors.TheEiffelTowerisshown
forscalecomparison.
2. Neutrinotelescopes (Fig.1,right),thesuccessorofAMANDA,startedwith
the aim to build the first km3-scale neutrino telescope
The idea for neutrino telescopes was first published [10]. Initsfinalconfiguration,reachedin2010,thede-
in1960byMarkov[7]. Thedetectionprincipleisbased tector consists of 5160 optical modules instrumenting
on the registration of the Cherenkov light induced by one km3 of clear glacial ice at depths between 1450m
charged particles generated in neutrino interactions in and2450matthegeographicSouthPole. Physicsdata
anopticallytransparentmediumlikeiceorwater. This taking started in 2006 with 9 installed strings. In con-
lightisrecordedwithalargenumberofphotomultipli- trast to the other detectors, the IceCube Observatory
ersarrangedinathree-dimensionalarray. Thedirection comprisesanairshowerarrayatthesurfacecalledIce-
andenergyoftheneutrinoisreconstructedusingthear- Top[6]. Itsmainpurposeistheinvestigationofcosmic
rival time of the photons (measured with nanosecond rays in the energy range between 1015eV and 1018eV
precision),themeasuredlightintensityandtheposition [11]. This article concentrates on results from the Ice-
ofthephotomultipliers. CubeandANTARESdetectorsasthesearecurrentlythe
Thetechnicalrealizationofsuchatelescopeindeep most sensitive detectors in the Southern and Northern
ocean water was pioneered by the DUMAND Collab- hemisphere,respectively.
oration between 1973 and 1995 [8] but terminated af-
ter a technical failure of the first deployed string. In Twobasiceventtopologiesinneutrinotelescopescan
the1980’s,theconstructionofdetectorsinLakeBaikal bedistinguished. Charged-currentinteractionsofmuon
[4] and in the ice at the South Pole were proposed. neutrinos produce long track-like patterns due to the
The NT200 in Lake Baikal was completed in 1998, resulting muon crossing the detector (muon channel;
instrumenting a volume of 10−4km3 with 192 optical Fig. 2, top). On the other hand, neutral-current inter-
modules on eight strings (Fig. 1, left). Three addi- actionsofallneutrinoflavors,orchargedcurrentinter-
tional strings at larger distances were added in 2005– actions of electron neutrinos, induce a more spherical
2007. The AMANDA detector at the South Pole [9] hitpatternoriginatingfromthecascadeofparticlespro-
took data from 1996 until 2009. In its final configu- ducedattheinteractionvertex(cascadechannel;Fig.2,
ration it consisted of 667 optical modules instrument- bottom). In case a charged current muon neutrino or
ing a volume of about 10−2km3. Installation of a neu- tauneutrinointeractionwithsubsequenttaudecayinto
trino telescope in the deep ocean was pursued by the amuonhappensinsidethedetectorthesetwotopologies
ANTARES,NEMOandNESTORCollaborationsinthe overlap thereby complicating the reconstruction. The
Mediterranean Sea. The ANTARES detector (Fig. 1, directionofelongatedmuontrackscanbereconstructed
middle) was eventually built off the coast of southern withsub-degreeprecisionathighenergies,significantly
France near Toulon [5] with construction lasting from betterthanthatofcascades.Attherelevantenergies,the
2002 to 2008. It comprises 885 optical modules on 12 neutrinoisapproximatelycollinearwiththemuonand,
stringsandinstrumentsavolumeof10−2km3.Datatak- hence, the muon channel is the prime channel for the
ing started early in the construction phase and is on- search for point-like sources of cosmic neutrinos. On
going. In 2005, construction of the IceCube detector the other hand, cascades deposit all of their energy in-
A.Kappes/NuclearPhysicsBProceedingsSupplement00(2015)1–10 3
Figure3:Atmosphericneutrinofluxesasafunctionofenergy.Shown
arepredictionsfortheconventionalelectronandmuonneutrinofluxes
togetherwiththerespectivemeasurements.Predictionsfortheprompt
neutrinofluxesarealsoshown. Thisplotisbasedontheplotpub-
lishedin[12]withanupdatetotheIceCubemeasurementofelectron
neutrinosdisplayedinred.
Figure2:Neutrinointeractionsignaturesinaneutrinotelescope(here,
twoeventsfromIceCubedata).Thesizeofthecoloredspheresispro-
portionaltothenumberofdetectedphotonsinanopticalmoduleand
thecolorindicatesthearrivaltimeofthefirstphotononthatmod-
investigate very interesting physics topics like for ex-
ule,withredbeingearlyandbluelatetimes. Top:Track-likepattern
fromamuonoriginatingfromachargedcurrentinteractionofamuon ample neutrino oscillations [13, 14]. Figure 3 shows
neutrino. Bottom: cascade-like pattern from e.g. a neutral current the atmospheric muon and electron neutrino fluxes as
neutrinointeraction.
measured by several experiments, together with the-
oretical predictions. The conventional neutrino flux
side the detector and therefore allow for a much better [15] originates from the decay of kaons and pions
reconstructionoftheirenergywitharesolutionofupto produced in cosmic-ray interactions in the atmosphere
10%athighenergies. (π±,K± → µ± + νµ) and the subsequent muon decay
(µ± → e±+ν +ν ). Electronneutrinosarelessabun-
µ e
dantthanmuonneutrinosastheyareonlygeneratedin
3. Atmosphericmuonandneutrinofluxes muondecays. Theν /ν ratiodecreaseswithincreasing
e µ
energy as an increasing number of muons reaches the
The main background for the detection of cosmic detectorbeforedecaying.Atenergiesabove(cid:38)100TeV,
neutrinos originates from muons and neutrinos pro- electronneutrinosfromthedecayofK andK consti-
L S
ducedininteractionsofcosmicrayswithEarth’satmo- tute a significant fraction of the total electron neutrino
sphere. Atmosphericmuonsarecommonlysuppressed flux [16]. Measurements of the conventional neutrino
bylookingthroughtheEarthtotheoppositehemisphere fluxescoverabroadenergyrangeuptoenergiesofsev-
using the Earth as an absorber. However, for energies eral100TeVandarewelldescribedbytheoreticalpre-
above∼100TeVtheEarthstartstobecomeopaquealso dictions.Thepromptneutrinofluxstemsfromthedecay
for neutrinos which renders the upward direction the ofcharmedparticlesgeneratedintheveryearlyphaseof
onlypartoftheskyfromwhichneutrinoswithEeVen- theairshowerdevelopment. Astheseparticlesarevery
ergiesandabovecanbeobserved. Fortunately,atthese short-livedtheydecaywithoutinteractionandtherefore
energies the atmospheric muon background is negligi- yield a harder spectrum compared to the conventional
ble. neutrinoflux,withaboutequalfluxesofmuonandelec-
Atmosphericneutrinosareatoughbackgroundforall tron neutrinos. Currently, there exists no measurement
searchesforcosmicneutrinos. Ontheotherhand,they ofthispromptfluxanditsnormalizationhaslargetheo-
areinvaluableasacalibrationsourceandalsoallowto reticaluncertainties[17].
A.Kappes/NuclearPhysicsBProceedingsSupplement00(2015)1–10 4
Figure4:IceCubepre-trialsignificanceskymapinequatorialcoordi-
nates. TheblacklineindicatestheGalacticplane,andtheblackplus
signindicatestheGalacticCenter.Themostsignificantfluctuationin Figure6: SensitivityoftheIceCubedetectorinmuonneutrinostoa
eachhemisphereisindicatedwithasquaremarker.Takenfrom[19]. point-likesourceswithanE−2spectrumasfunctionofdeclinationfor
differentenergyranges.
10-6 MACRO upper limits (2300 d)
IceCube sensitivity (1373 d, preliminary)
-1]s 10-7 IAcNeCTAubReE Sup speenrs liitmiviittsy ((11337338 dd,) preliminary) 4.1. Searchesforpoint-likesources
-2 m 10-8 ANTARES upper limits (1338 d) Searches for point-like sources have the advantage
c
V 10-9 that for a particular source they observe only a small
e
T
x [ 10-10 portionofthesky(typicalangularresolutionsformuon
flu× 10-11 neutrinos are around 0.5◦) thereby reducing the back-
2E 10-12 ground of atmospheric neutrinos considerably. Up
to now, IceCube has observed about 180,000 up-
going muon neutrinos in 1373 days of livetime [19],
-1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1
sin(δ) ANTARES in a similar time about 5,500 [20]. None
ofthetwoexperimentshasfoundasignificantdeviation
Figure5:Sensitivities(solidlines)andupperlimits(symbols)ofvari- from the background, neither in searches on the whole
ousexperimentsinmyonneutrinosforpoint-likesourcesasafunction skynorforselectedsources.
ofdeclinationassuminganunbrokenE−2spectrum.Datatakenfrom
Thisleadstoupperlimitsonthemuonneutrinoflux
[19,20,21].
from point-like sources which are plotted in Fig. 5 as
functionofdeclinationforanassumedE−2sourcespec-
trum.ThecomparisontotheupperlimitsfromMACRO
4. Theclassicalpictureofneutrinoastronomy
[21]illustratesthehugeimprovementinsensitivityofa
factor 1000over thepast 15years. Currently, IceCube
isthemostsensitiveoperatingneutrinotelescope.Inthe
Comparedtoatmosphericmuonsandneutrinos,cos- northernsky,itssensitivitystartstoapproachthediscov-
mic neutrinos are expected to have a harder spectrum eryregionbelow∼10−12TeVcm−2s−1whereaccording
followingapower-lawwithanindexofabout−2asin- tocalculations[2,3]fluxesfrom(Galactic)sourcesare
ferredfromshockacceleration(seee.g.[18]andrefer- expected. For sources in the southern sky, ANTARES
encestherein)inobjectslikesupernovaremnantsorjets located in the Mediterranean Sea is currently the most
of active galactic nuclei. Therefore, cosmic neutrinos sensitive detector [20, 22], in particular in the TeV to
are expected to dominate over the atmospheric back- PeVrange,wherethesensitivityofIceCuberapidlyde-
ground above a certain energy threshold. In order to teriorates due to the huge background of atmospheric
lowerthisenergythresholdintotheTeVtoPeVrange, muons (Fig. 6). This energy region is in particular
where in particular Galactic sources are anticipated to interesting for Galactic sources which are expected to
produceneutrinos,”classical”neutrinoastronomyuses emit neutrinos in this energy range. However, with its
theEarthasshieldagainsttheoverwhelmingfluxofat- ∼ 100 times smaller volume, the overall sensitivity of
mosphericmuons. ANTARESismuchlowerthanthatofIceCube.
A.Kappes/NuclearPhysicsBProceedingsSupplement00(2015)1–10 5
rameters are still poorly determined or completely un-
IceCubePreliminary
100 known, leading to significant uncertainties in the pre-
10−7 Ahlersetal. dictedfluxes. Thissituationisimprovedwithso-called
Waxman-Bahcall 90
1sr)− 80 nelesu,ttrhoenneesucatrpineomfloudxelissocfloGsRelBysti[e2d3,to24th].eIonbtsheersveemdoedx--
21s−−10−8 70 L(%) tragalacticcosmic-rayspectrum. Becauseofthisdirect
m C link,thesemodelsaremorerobustagainstassumptions
c 60 n
GeV usio on unmeasured parameters. IceCube has searched for
2εεΦ()(νbb10−9 90% 68% 345000 Excl csthuoecrrcmeelosasdtiesolons.sfaForifg[un2re6eu]t7prisrnhoovoswidwsinittghhesoeigvxnecrilfiu5sc0iao0nntblcuimornsitststa,rnawidnitcthsoomoun-t
pares it to predictions from two neutron-escape mod-
104 105 5100%6 107 20 els. Thoughnotallmodelshavebeenfullyexcludedyet
Neutrinobreakenergyεb(GeV) withintheiruncertainties,thepersistentnullresultssig-
nificantlyquestiontheroleofGRBsasmajorsourcesof
Figure7:Compatibilityofneutrinofluxpredictionsbasedoncosmic theUHECRs.
rayproductioninGRBswithobservations.Thehorizontalaxisshows
theenergyofthefirstbreakinthestandarddouble-brokenpowerlaw
neutrino spectrum ofGRBs in the fireball model, which is propor- 5. Neutrinosfromabove–thepowerofveto
tionaltothesquareoftheLorentzfactoroftheejectedmaterial. The
verticalaxisdisplaysthenormalizationoftheneutrinofluxwhichis
For a long time, the field of view of neutrino tele-
relatedtotheacceleratedprotonflux.Theareabelowthelinesarethe
allowedvaluesintheneutrinofluxversusneutrinobreakenergyplane scopeswasthoughttoberestrictedtotheoppositehemi-
atthecorrespondingconfidencelevel.Modelpredictions[23,24]with sphere for the observation of cosmic neutrinos in the
estimateduncertaintiesareshownaspoints. TeVrange.However,neutrinosinteractinginsidethein-
strumented volume can be separated from atmospheric
muons,whichenterthedetectorfromtheoutside,byre-
4.2. Neutrinosfromgamma-raybursts
quiringthattheouterlayersofthedetectordonotcon-
ApartfromActiveGalacticNuclei(AGNs),Gamma-
tain any correlated signals. As atmospheric neutrinos
Ray Bursts (GRBs) are currently the only good candi-
are always produced together with muons in the atmo-
dates for the sources of the ultra-high energy cosmic
sphere,thistechniquecanevenbeusedtosuppressthe
rays above 1019eV. The fact that GRBs are transient
fluxofatmosphericneutrinosasfirstsuggestedin[28].
phenomenawithdurationsfrombelowasecondtosev-
Dependingonthesizeofthevetoregion,thistechnique
eralhundredseconds,allowsforanothersignificantre-
allows for lowering the energy threshold for observa-
ductionoftheatmosphericbackground.
tions of down-going cosmic neutrinos significantly at
ThecurrentlyleadingmodelforGRBsisthefireball
theexpenseofrestrictingtheeffectivevolumeforneu-
model [25] with the energy source being the rapid ac-
trinointeractionstothenon-vetovolumeofthedetector.
cretion of a large mass onto a black hole formed af-
The latter could be avoided by using a detector at the
ter the collapse of a super-massive star. In this model,
surfacelikeIceToptorejectatmosphericshowers. With
a highly relativistic outflow (fireball) dissipates its en-
itscurrentextension,however,thesolidanglecoverage
ergy via synchrotron or inverse Compton radiation of
of IceTop is rather limited and up to now it has only
electrons accelerated in internal shock fronts. This ra-
beenusedtocheckforthepresenceofanairshowerin
diation in the keV–MeV range is observed as a broken
coincidencewithanIceCubeeventinhindsight.
power-lawspectrumingammarays.Inadditiontoelec-
trons, protons are thought to be accelerated which in-
teract with the keV–MeV photons via pγ → π+,π0. 6. DiscoveryofcosmicneutrinoswithIceCube
The decaying charged pions produce neutrinos of en-
ergyO(1014eV). Becauseofthe∆+resonance,theneu- Thevetomethoddescribedabovewasdevelopedand
trino spectrum mirrors the broken power-law shape of applied to search for cosmic neutrinos of all flavors in
the gamma-ray spectrum with the break lying in the the IceCube detector. In two years of data, 28 events
100TeVrange. Thecoolingofthepionsathigherener- wereobservedoveranexpectedbackgroundof10.6+5.0
−3.6
giesinmagneticfieldsintroducesasecondbreakinthe yielding a significance of 4.2σ [31]. Adding another
PeVrange. year of data provided 9 additional events and raised
Despite being the leading model, several of its pa- therejectionofthebackground-onlyhypothesisto5.7σ
A.Kappes/NuclearPhysicsBProceedingsSupplement00(2015)1–10 6
Figure8:High-energyneutrinosasobservedbyIceCubein3yearsofdata.Left:Numberofmeasuredeventsandpredictedfluxesasfunctionof
depositedenergyequivalenttoanelectromagneticshower. Right: Distributionofdataandpredictionsasafunctionofdeclinationfordeposited
energies>60TeV(samecolorcodeasleft).Takenfrom[27].
23 37 9
31 17 16 8 11 26
36
29 35 4 6 27 5
11830◦ 1 34 Norther3n SHoeu3tmihseprhn erHeemisphere215822422 112415 2300 19 3 -180◦
21 10 7 Galactic
0 TS=2log(L/L0) 11.3
Figure9: ArrivaldirectionsoftheeventsinGalacticcoordinatesof
high-energyneutrinosobservedbyIceCube. Shower-likeeventsare
markedwith+(resolution∼ 15◦)andthosecontainingmuontracks
with×(resolution(cid:46)1◦).TheColorscaleindicatesthecompatibility
withthebackground-onlyhypothesisforapoint-likesource. Nosig-
nificantclusteringwasobservedwiththemostsignificantfluctuation Figure 10: ANTARES exclusion limit at 90% CL. for a neutrino
havingap-valueof84%.Takenfrom[27]. sourceneartheGalacticCenterassumingwidthsbetween0◦ (point
source)and3◦. Thebluehorizontallinerepresentsthesourceflux
predictedin[29].Takenfrom[22].
[27]. Threeofthealtogether37eventsdepositedmore
than 1PeV of energy inside the detector 2. Figure 8
(left) shows the distribution of observed and expected (Fig.8,right).
events as a function of deposited energy. A clear ex- A combined fit of a conventional, prompt and E−2
cessathighenergiescanbeobservedwhichcanbeac- astrophysical neutrino flux to the data between 60TeV
counted for with an astrophysical component. In the and 3PeV yields a best-fit astrophysical normalization
northernsky,thefluxofcosmicneutrinosissuppressed of E2φ = (0.95±0.3)×10−8GeVcm−2sr−1; the best-
due to Earth absorption whereas in the southern sky it fit slope is E−2.3±0.3 with a normalization of E2φ =
stays constant and is clearly separated from the atmo- 1.5×10−8(E/100TeV)−0.3GeVcm−2sr−1[27].Theob-
spheric neutrino flux which is suppressed by the veto servedeventsarecompatiblewithanisotropicneutrino
fluxwithaflavorratio(ν :ν :ν )of(1:1:1)asexpected
e µ τ
fromfullmixingduetoneutrinooscillations. Thissug-
2Note, that this energy corresponds to the amount of light de- geststhattheobservedcosmicneutrinofluxistoalarge
positedbyanelectromagneticcascadeofthatenergy,andinparticular
extent of extra-galactic origin with possibly a compo-
doesnotaccountfortheenergycarriedawaybytheneutrinoinaNC
interactionorbythemuonwhenitleavesthedetector. nentfromtheMilkyWay’shalo.
A.Kappes/NuclearPhysicsBProceedingsSupplement00(2015)1–10 7
Figure11: Atmosphericandastrophysicalneutrinofluxesmeasured Figure 12: Artist’s view of a high-energy (blue volume) and low-
byIceCubetogetherwithcorrespondingpredictions.Takenfrom[30]. energy (green volume) extension of the IceCube neutrino detector.
ThecurrentIceCubedetectorisshowninred.
Searchesforadirectional(Fig.9)ortemporalcluster-
ingoftheeventsaswellascorrelationswiththeGalac-
tic plane yield negative results [27]. Though statisti-
cally not significant, a cluster of seven events near the
Galactic Center inspired flux calculations for a poten-
tialassociatedsource[29]. TheANTARESCollabora-
tiontestedthishypothesisandexcludestheexistenceof
suchasourcewithanextensionofupto0.5◦withmore
than90%CL[22](Fig.10).
In the meantime, cosmic neutrinos have also been
Figure13: Artist’sviewofasurfacevetofortheIceCubeneutrino
seenwiththeIceCubedetectorinup-goingmuonneutri- detector.
nos: intwoyearsofdataananalysisthatsearchedfora
diffusecosmicneutrinofluxobservedanexcessof3.9σ
sor experiments or on upgrading their detectors to the
(preliminary)athighenergiesoverthepredictedatmo-
gigatonscale.
spheric background. The best-fit astrophysical compo- In order to coordinate these efforts, an umbrella or-
nent(greenhorizontalbandinFig.11)iscompatiblein
ganization, the Global Neutrino Network (GNN), was
slope and normalization with the cosmic neutrino flux
founded in 2013 [32]. It includes all current neutrino
discussedabove.
telescope projects ANTARES, BAIKAL, IceCube and
KM3NeTandaimsatdevelopingacoherentstrategyfor
7. Futurestrategies thefield. Thiscoverso.a.tounderstandhowthediffer-
entdetectorscomplementeachotherinthebestway,the
Currently,onlytheIceCubedetectorprovidesthere- coordinationofalertandmulti-messengerpolicies, de-
quired volume to investigate the observed cosmic neu- velopmentofstandardsfordataexchange,cross-checks
trino flux. Though the full potential of reconstruction ofresultsaswellastheorganizationofmeetingsforex-
andselectionmethodshasnotbeenfullyexploitedyet, changeofexpertise.
it is foreseeable that even after 10 years of operation,
IceCube will only have gathered about 90 astrophysi- 7.1. NextgenerationIceCube
calmuonneutrinosabove100TeV,100cascadesabove The IceCube Collaboration is currently planning an
60TeV and 10 events above 1PeV. For a detailed in- extensionoftheexistingdetectorwithabout120addi-
vestigation of the properties of the astrophysical flux tionalstrings,eachequippedwithupto96opticalmod-
and in particular the search for individual sources of ules. AnartistconceptionofsuchanextendedIceCube
neutrinoemission,significantlymoreeventswillbere- detectorisdepictedinFig.12. Intensivestudiesforop-
quired. Accordingly,thecollaborationsofallthreerun- timizing this future detector are underway. 80 of these
ning neutrino telescopes are either working on succes- strings are foreseen for a high-energy extension with a
A.Kappes/NuclearPhysicsBProceedingsSupplement00(2015)1–10 8
7.2. KM3NeT
The KM3NeT Collaboration, which comprises the
expertise of all previous Mediterranean neutrino tele-
scopeprojects(ANTARES,NEMO,NESTOR),intends
tobuildamulti-km3 neutrinotelescopeintheMediter-
ranean Sea [37]. For the optical module, it uses an
innovative design [38] with31 small 3” photomultipli-
ersinsteadofasinglelarge(8–10”)photomultiplieras
adopted in current neutrino telescopes. Among oth-
ers, this configuration yields three times the sensitive
area of an optical module with a single 10” photomul-
tiplier, provides intrinsic directional sensitivity and al-
lows for improved photon counting. The full detector
Figure14: IncreaseinmuonneutrinoeventsforthecurrentIceCube
detectorcomparedtopurelyup-goingmuonneutrinosasafunctionof will contain over 12,000 of these sensors distributed
theradiusofa100%efficientsurfacevetoforvariousenergythresh- over about 600 vertical lines arranged in 6 so-called
olds.Fortheneutrinospectrumthebest-fitcosmicneutrinofluxwith
building blocks of 115 lines each. It is planned to in-
anindexof−2.3andnohigh-energycutoffwasassumed.Takenfrom
stall these building blocks at sites near France, Italy
[33].
and Greece. A single building block will allow to re-
construct the direction of muon neutrinos with reso-
widehorizontalspacingofupto300m,yieldinginstru- lutions down to 0.1◦ at high energies. Furthermore,
mentedvolumesbetween5and10km3andangularres- studies show that a precision of 3◦ in the direction re-
olutions down to 0.1◦ with an energy threshold in the constructionofcascadescanbereachedat10TeV,and
some10TeVrange. better than 2◦ above 100TeV. This would for the first
The remaining 40 strings are planned to be installed time allow to also use the cascade channel in searches
inaverydenseconfigurationcalledPINGUinthecenter forpoint-likeneutrinosources,therebysignificantlyin-
ofthecurrentlyexistingdetectortoallowfortherecon- creasingthenumberofeventsforagivenflux.
structionofneutrinosintheGeVrange. Themaingoal A first phase of KM3NeT with 31 strings has been
ofPINGUisthemeasurementoftheneutrinomasshier- funded with 31MAC and will be installed until the end
archyexploitingmattereffectsintheoscillationpattern of2016togetherwith8towerscontaining”traditional”
ofatmosphericneutrinos(fordetailssee[34]). optical modules with one large PMT per module. The
Inadditiontothein-iceextensions,thepotentialofan main purpose of this first phase is the demonstration
extended IceTop-like detector for vetoing atmospheric of the functionality of the detector design but it will
muons and neutrinos at the surface via the registration alsoprovideimprovedperformanceovertheANTARES
of the associated air shower is being investigated (see detector. Afterwards, the construction of two building
Fig. 13). Such a surface veto has the virtue that the blocksisenvisaged(additionalcosts50–60MAC). Such
wholeicevolumebetweenthedeepdetectorandthesur- aconfigurationwouldhaveaboutthesameinstrumented
facewouldbeavailableforneutrinointeractionsthereby volume as the current IceCube detector with the main
significantly enlarging the effective volume. In order purpose to verify and further investigate the observed
tocoverareasonablylargesolidangle,suchadetector cosmicneutrinoflux. Figure15(left)displaysaprelim-
would have to extend horizontally significantly further inary estimate of the sensitivity in tracks and cascades
than the detector in the deep ice. The energy thresh- to this flux. With the full detector in the final phase
old is determined by the density of detection elements (additional costs 140–160MAC), KM3NeT will instru-
whichcanbeverysimplecomparedtothecurrentIce- ment between 3 and 6km3 of water and be sensitive
Top detectors as only basic signal information is re- to Galactic neutrino sources like the supernova rem-
quired. A first preliminary study concerning the gain nantRXJ1713.7-3946orthepulsarwindnebulaVelaX
in signal muon neutrinos with such a surface veto for (Fig. 15, right). The KM3NeT Collaboration currently
thecurrentIceCubedetectorisdepictedinFig.14. For also investigates the performance of a densely instru-
arealisticenergythresholdof100TeVandanextension mented detector called ORCA consisting of one build-
radius of 5km, an increase of about 75% compared to ing block. As with PINGU, the main goal is the mea-
up-goingmuonneutrinoscanbeexpected. surementoftheneutrinomasshierarchy.
A.Kappes/NuclearPhysicsBProceedingsSupplement00(2015)1–10 9
KM3NeT Phase-1.5 (detector with 2 building blocks) - Preliminary
)σ 7
e (
c 6
n
a
c
nifi 5
g
Si
4
muon tracks θzenith>80°
3
cascades
2
1
flux per flavour 1.2 10-8 E-2 exp(-E/3 PeV) GeV-1 sr-1 s-1 cm-2
0
0 1 2 3 4 5
N
year
Figure15:Left:SensitivityofKM3NeTwithtwobuildingblock(115lineseach)tothecosmicneutrinofluxdiscoveredbyIceCubeasafunction
ofobservationtimeformuontracksandcascades. Takenfrom[35]. Right: SensitivityofKM3NeTwithsixbuildingblockstothesupernova
remnantRXJ1713.7-3946andthepulsarwindnebulaVelaXasafunctionofobservationtimeassumingthattheobservedgamma-rayfluxfully
originatesfromπ0decay.Takenfrom[36].
7.3. Baikal in full swing and, pending funding, could be realized
within 10–15 years. With these new detectors we will
TheBaikalCollaborationplansthestepwiseinstalla-
thenhopefullybeabletofullyopenthisnewandexcit-
tion of a km3-scale array in Lake Baikal dubbed GVD
ingwindowtotheuniverse.
(GigatonVolumeDetector)[39]. Itwillconsistofclus-
tersof8stringswithupto48opticalmodulesperstring.
Phase 1 of the project envisages 12 clusters. In 2008- 9. Acknowledgements
2013, the basic elements were tested and engineering
strings operated. Meanwhile, the first cluster is de- The author gratefully acknowledges the support by
ployed to a large part and completion is planned for the German Ministry for Education and Research
April2015. (BMBF),grantnumbers05A11KHBand05A14WE3.
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