Neutrino Telescopes

0.55 MB·

by  Juan Jose Hernandez-Rey

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1 Neutrino Telescopes
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J. J. Hern´andez–Rey

n IFIC - Instituto de F´ısica Corpuscular,
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Universitat de Val`encia–C.S.I.C., Apdo. 22085, E-46071Valencia, Spain
J

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The observation of high energy cosmic neutrinos can shed light on the astrophysical sites and mechanisms

] involvedintheacceleration ofprotonsandnucleitothehighenergies observedat Earthbycosmicraydetectors.
E More generally, high energy neutrinos can be a key instrument in the multimessenger study of the high energy
H sky. Severalneutrinotelescopesofdifferentsizesandcapabilitiesarepresentlytakingdataandprojectstofurther
increase their size and sensitivity are underway.
.
h In this contribution we review the present status of neutrino telescopes based on the cherenkov ligh detection
p
technique,their recent results and theplans to increase their sensitivity.
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o
r
t
s
a
1. Introduction chargedpionsproducedintheinteractionofthose
[
hadrons with the surrounding matter or radia-

1 The detection of high energy cosmic neutrinos tion, being therefore a smoking gun of hadronic
v is a long and much sought-after scientific goal.
acceleration mechanisms.
2 In the low energy domain (few MeV to several
The observation of neutrinos in a cherenkov
8
GeV) the observation of extraterrestrial and at-
5 neutrinotelescopeisbasedonthedetectionofthe
mospheric neutrinos gave rise to the discovery of
0 muons producedby the neutrino chargedcurrent
1. neutrino oscillations and to the -arguably- first interactionswiththemattersurroundingthetele-
experimental test of our models of supernova ex-
0 scopebymeansofthecherenkovlightinducedby
1 plosion. In the high energy regime (several GeV the muons when crossing the detector medium,
1 to EeV), the advantages of neutrinos as cosmic
natural ice or water. The telescope consists in
: messengers are well known, their importance to
v a three dimensional array of light sensors, usu-
i provide information on the particle acceleration ally photomultipliers, that record the position
X
mechanisms in astrophysical objects is also ac-
and time of the emitted cherenkov photons and
r knowledged and experimental methods to detect
a therefore enable the reconstruction of the muon
themexistand havebeentechnologicallyproven.
track. To avoid the huge background of muons
Therefore, at present there is a struggle to reach
produced in cosmic rays showers, the telescopes
therequiredsensitivitiestodetectthem,thehope
look at the other side of the Earth, using it as
being that nature is generous enough to provide
a shield. The increase in the range of muons at
fluxes at a level observable with our present or
high energies (form kilometres to several kilome-
soon-to-come neutrino telescopes.
tres) together with the increase with energy of
Letusbrieflysummarizetheadvantagesofneu-
the neutrino cross section give rise to an approx-
trinos as cosmic messengers. They are neutral
imately exponential increase of the effective ar-
particles,theyarenotdeflectedbymagneticfields
eas of these devices in the GeV to PeV energy
and therefore they point back to their sources.
range. Above a few TeV, the telescopes can de-
They are weakly interacting and thus they can
termine the direction of the incoming neutrinos
escape fromverydense astrophysicalobjects and ◦
withangularresolutionsbetterthan1 ,hencethe
cantravellong distances without being absorbed
name “telescope”. At energies above the PeV,
by matter or backgroundradiation. Moreover,in
the Earth becomes opaque to neutrinos, but the
cosmic sites where hadrons are accelerated it is
atmospheric muon flux decreases dramatically so
likelythatneutrinosaregeneratedinthedecayof
thattheneutrinotelescopescanlookfordowngo-
1
2
ing neutrinos in that energy regime. Other neu-
trino flavourscanbe observedthroughthe detec-
tionofhadronicorelectromagneticshowersor,in
the case of tau neutrinos, via the observation of
its interaction and the subsequent decay of the
produced tau lepton.
2. IceCube
IceCube is at present the largest high energy
neutrino telescope. It is under construction at
the South Poleand it will consistin its final con- Figure 1. Skymap obtained with 40-string Ice-
figuration of 80 + 6 strings [1]. The 80 regu- Cube. The black dots are the directionns of the
lar strings contain 60 optical modules uniformly selected events, the colour grade represents the
distributed between 1450 and 2450 m under the pre-trial p-values.
ice surface. The 6 remaining strings form the
so-called Deep Core on which the optical mod-
ules are located between 1760 and 2450 m, thus
providing a denser distribution to yield a lower thedirectionandestimatedenergyofthetracksis
energy threshold. The optical modules contain used. Theskymapoftheselectedeventstogether
25 cm PMTs read by waveform data acquisition with the p-values obtained for eachdirection can
electronics. Inaddition,theobservatoryalsocon- be seenin figure1. The highestsignificance(a p-
tains a surface detector, IceTop, consisting in 80 value of 5.2 × 10−6) is observed at a declination
◦ ◦
pairsoftanksontopofthe strings,filledwithice of 113.75 and a right ascension of 15.15 . Us-
andequippedwithopticalmodules. The comple- ing scrambled data sets it is estimated that such
tion of the telescope is expected to take place by a significance or higher anywhere in the sky is
thebeginningof2011. Atpresent(2010)thedec- expected to take place 18% of the time. This
tector contains 79 of the 86 strings and 73 of the post-trial p-value indicates that this fluctuation
80 IceTop tank pairs. is compatible with the background.
A variety of studies have been carried out by From the absence of a signal, upper limits on
the IceCube collaboration during the construc- the neutrino flux from point sources can be es-
tion phase. In this article we briefly summarize tablishedassuminganE−2 ν spectrumfromthe
µ
some of the most recent results. source. The open blue squares in figure 2 show
The data taken by IceCube in the 40 string the 90% C.L. upper limit for a candidate list of
configuration in 2008 and 2009 corresponds to a selected sources [2]. The sensitivities, i.e. the
livetimeof375.5days. Thefinalsamplecontains expected average upper limits for a given direc-
almost 40 thousand events. Approximately one tion, for IceCube 86 strings and ANTARES (cf.
thirdoftheeventsareupgoingandtheremaining section 4) are also shown. The current IceCube
twothirdsdowngoing. Theupgoingevents,origi- sensitivityisthreetimesbetterthantheprevious
natinginthenorthernhemisphere,aremainlyat- 22-string sensitivity [3].
mospheric neutrinos in the ten to a few hundred Neutrino telescopes can indirectly search for
TeV energy range, while the downgoing events darkmatterparticleslookingforhighenergyneu-
are high energy atmospheric muons above a few trinos coming from the Sun. Weakly interacting
PeV, where the atmospheric muon energy spec- massiveparticles (WIMPs)canaccumulategrav-
trum is softer than that expected for a cosmic itationally at the centre of astrophysical objects
signal. Therefore, both sky hemispheres can be like the Sun, the Earth or the Galactic Centre,
analysed in the search for cosmic neutrinos. To and annihilate producing neutrinos. IceCube is
this end, an unbinned likelihood ratio based on competitive with direct search experiments for
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22 strings 275.7 d
40 strings disc. pot. 375.5 d
40 strings sens. 375.5 d
10-9 40 strings list 90% UL
-2-1] sm 8A6N TstAriRngEsS p prerel.l .s 3e6n5s .d
c
V 10-10
e
T
E [
d
N/
d10-11
2 E
10-12-1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1
sin(δ)
Figure 2. Upper limits (90% C.L.) and sensitiv-
ities. The blue line shows the current IceCube
Figure 3. Neutralino-proton spin dependent
40 string upper limit and the squares the upper
cross-section as a function of neutralino mass.
limits for a list of pre-selected sources [2]. The
The solid and dashed blue lines give the upper
previous22-stringIceCube22upper limit[3]and
limits set by IceCube 22-string for soft (via b¯b)
the future 86-string configuration sensitivity are
and hard (W+W−) annihilation channels. Also
also shown.
given are the limits set by direct search exper-
iments. The prediction for IceCube plus Deep
Core after 5 years of data taking is also shown
WIMPswithmostlyspin-dependentinteractions. (bottom dotted line).
As can be seen in figure 3, the upper limit set
by IceCube 20-string [4] is the most stringent for
masses above 250 GeV provided the annihilation
proceeds through the hard channels (W+W−). eight strings, 72 m long, each with 24 pair of op-
Also shown are the limits set by AMANDA, Ice- tical modules and is running since 1998. The ex-
Cube’sforerunner,andthepredictionforthesen- tension NT200+ contains three additional outer
sitivity reached by the full IceCube, including strings, 140 m long with 12 optical modules each
Deep Core, after five years of observation of the and is working since 2005. In addition, in 2008
Sun. As canbe seen, it is more thanone orderof a km3-prototype string with new technology was
magnitude better and covers a mass region from installed. The most stringent upper limit pro-
50 GeV to severaltens of TeV. vided by NT200 for a diffuse astrophysical flux
of ν + ν + ν assuming an E−2 spectrum is
e µ τ
E2Φ=2.9 ×10−7 GeV cm−2 s−1 in the 20 TeV
3. Neutrino telescopes in lake and sea wa-
to 20 PeV energy range [7].
ter
The NESTOR project started in 1989. Its site
The first attempt to use sea water as the isnearPylosintheGreekIoniancoastatadepth
mediuminaneutrinotelescopewasDUMAND[5, of around 4000 m. The designed tower consists
6]. Althought the project was discontinued in of 12 titanium stars with six arms of 16 m ra-
1996 and only a line was deployed (out of the dius with a pair of optical modules at the end of
nine which DUMAND-II was planned to have) each arm adding up to a total of 144 photomul-
this initiative was the seedfor subsequentcosmic tipliers. A first prototype consisting of one floor
neutrino telescopes in water. with a5 mradiusstarandholding twelveoptical
The BAIKAL neutrino telescope NT200+ is modules was deployed in 2003. Using more than
deployed in Lake Baikal, Russia, 3.6 km from 5 million triggers the zenith angle distribution
shore at a depth of 1.1 km. NT200 consists of of atmospheric muons and their flux was mea-
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sured [8]. The NESTOR initiative is now part of ANTARES consists of12 lines anchoredto the
the KM3NeT consortium (see section 5). sea bed. Each line contains 25 storeys. The low-
The NEMO project started in 1998. The se- est storey is 100 m above the sea bed and the
lected site is 80 km from the coast of Capo vertical distance between consecutive storeys is
Passero in Sicily at a depth of 3500 m, but a 14.5 m. The total line length is 480 m. Each
shallower site (2031 m) closer to the shore (25 storey has a triplet of OMs and an electronics
km from Catania) is also used for tests. The module. The OMs contain a 10-inchphotomulti-
◦
NEMO concept for the detector unit is a tower plierlooking45 downwards. Thehorizontalsep-
of 16 storeys separated vertically by 40 m with aration between lines is 60-80 m. The first line
a total tower length of 750 m. The storeys con- was deployed in 2006, the detector was operated
sist of 20 m arms with two OMs at each end. As with 5-lines during several months in 2007 and
part of NEMO Phase-1 a prototype tower was was fully deployed in 2008. Taking advantage of
deployed at the end of 2006. A small sample of thepossibilityofdetectormaintenanceofferedby
downgoingmuonswasanalyzedanditszenithan- water,theANTAREScollaborationhasrecovered
gle distribution measured [9]. The tower suffered andrepairedsomeofthelinesandfixedproblems
some technical problems and stopped working at in some of the detector’s interlink cables.
the beginning of 2007. The NEMO collaboration In figure 4, the distribution of the angle of el-
is feeding the wider KM3NeT consortium with evation above the horizon is shown for the data
new ideas and technical solutions (see section 5). taken in 2007 (5-line configuration) and 2008 (9,
The most advanced initiative in the Mediter- 10 and 12 lines) corresponding to a live time of
raneanSea, ANTARES, is dealth with in the fol- 341 days. The total number of upgoing recon-
lowing section. structedeventsis 1062. The MonteCarlopredic-
tions for atmospheric muons and neutrinos are
also shown.
4. ANTARES
ANTARESislocatedintheMediterraneansea,
40 km off the French coast near Toulon.
-1 r] Frejus MACRO
s
ENumber of eventsle1111v00004567ation dMMMaCCCta taaottmmta..l µν 2-2-1 EdN/dE [GeV cms1100--76 AmandBaa-IiIk al NT-20(0M (Pe+R)/2 )/3 A(me+anda-)I/3I UHE
103 ANTARES- 2007-09
102 (W&B)/2
-8
10 10103 104 105 106 107 108
E(GeV)
1 −80 −60 −40 −20 0 20 40 60 80
θ
Figure 5. ANTARES 90% C.L. upper limit for
an E−2 high energy ν + ν¯ diffuse flux. Up-
Figure 4. Elevation angle distribution for µ µ
per limits obtainedbyotherexperimentsarealso
ANTARESevents. Blackpointsarethedata,the
shown [11].
red and blue lines are the Monte Carlo predic-
tions for atmospheric muons and neutrinos. The
green band indicates the total predicted atmo-
spheric background together with its systematic Using the data collected by ANTARES during
uncertainty. theperiodfromdecember2007todecember2009,
5
a total live time of 334 days with different detec- 5. KM3NeT
torconfigurations(9,10and12lines)asearchfor
The ANTARES, NESTOR and NEMO col-
adiffusefluxofastrophysicalmuonneutrinoswas
laborations have joined efforts in the KM3NeT
performed[10]. Fromthecompatibilityoftheob-
consortium to build a km3-size neutrino tele-
servednumber of events with the expected back-
ground and assuming an E−2 flux spectrum for scope in the Mediterranean Sea [14]. The design
the signal,a90%C.L.upper limit onthe ν +ν¯ and preparatory phases of the project have been
µ µ
diffuse flux of E2Φ=5.3 ×10−8 GeV cm−2 s−1 in fundedbytheEuropeanCommission. Attheend
of the design phase, a Conceptual Design Report
the energy range 20 TeV to 2.5 PeV is obtained,
that describes the goals of the telescope and the
as shown in figure 5
options for its technical implementation was de-
A first search for neutrino point sources has
livered and a preliminary versionof its Technical
beeen performed using the 5-line configuration
Design Report was released in 2010 [15]. The
data, corresponding to a live time of 140 days.
minimum requirements for the telescope are an
Binned and unbinned methods were used in the
effective volume of several km3, sensitive to all
search [12]. Figure 6 shows the 90% C.L. upper
neutrinoflavours,withanangularresolutionbet-
limits obtained for a list of 25 candidate sources
◦
assuming an E−2 spectrum. Also shown is the terthan0.1 formuonneutrinosabove1TeVand
atargetenergyrange1-100TeVforpointsources.
sensitivity for ANTARES 12-line after one year
of data taking.
-1]s10-6 ANTARES 5-Lines (140 days)
-2 m MACRO Source List (2299.5 days)
c Super-K Source List (2623 days)
V 10-7 AMANDA-II Source List (1387 days)
Te IC22 Source List (275.7 days)
E [ ANTARES Sensitivity 12L (1 year)
d
% / 10-8
Φ90νµ
d
2 E10-9
10-10
Figure 7. A self-deployable tower proposed for
10-11
KM3NeT.Theopticalmodulesineachstoreyare
-80 -60 -40 -20 0 20 40 60 80 hold by bars. Each bar is perpendicular to those
declination [°]
in the nearby storeys. The tower can be densely
packedforstorageandtransportationandunfurls
itself when it reaches the seabed.
Figure 6. Muon neutrino flux upper limit at
90%C.L. obtained by the ANTARES 5-line tele-
scope for a list 25 selected candidate sources
(blue points) and expected sensitivity for the full Awidevarietyofstudieshavebeencarriedout
ANTARES 12-line after one year of data taking. to explore the technical options to build and de-
Alsoshownaretheupperlimitsobtainedbyother ploy such an instrument with an overall cost be-
experiments [13]. low 250 Meuros. Among the different solutions
6
proposed let us mention as an example the con- 7. A. Avrovin et al., Proceedings of the 31st
ceptofunfurlabletower(seefigure7). Thisdetec- ICRC, arXiv:0909.5562.
tor unit can be folded in a densely packed struc- 8. G. Aggouras et al., Astrop. Phys. 23 (2005)
ture suitable for storage, transportation and de- 377.
ployment and it unfurls itself once it reaches the G. Aggouras et al., Nucle. Phys B (Proc.
seabed. Thisenablesthedeploymentofseveralof Suppl.) (2006) 279.
theseunitsduringthesameseaoperation. Afirst 9. Carla Distefano, Proceedings of CRIS 2008,
test of the concept was succesfully carried out in arXiv:0901.1252.
2010 with a tower of reduced length. 10. J.A. Aguilar et al., arXiv:1011.3772, to be
KM3NeT is also meant to be a multidis- published in Phys.Lett. B.
ciplinary infrastructure that will house instru- 11. W. Rhode et al., Astropart. Phys. 4 (1996)
mentation relevant for seismology, radioactivity, 217; M. Ambrosio et al., Astropart. Phys 19
geomagnetism, oceanography, geochemistry and (2003) 1; A. Achterberg et al., Phys. Rev
otherseasciencies,involvingthereforealargesci- D76(2007)042008;A.Avrovinetal.,Astron.
entific community. KM3NeT is included in the Lett.35(2009)651;M.Ackermannetal.,As-
roadmapoftheEuropeanStrategicForumonRe- trop. Journal 675 (2008) 1014.
search Infrastructures, is mentioned in the AS- 12. J.A.AguilarandJ.J.Hernandez-Rey,Astrop.
TRONET roadmap and was selected by the AS- Phys. 29 (2008) 117.
PERA ERA-net as one of the sevenastroparticle 13. S.Desaietal.,Astrop.Phys.29(2008)42;E.
physics infrastructures to be promoted [16]. Thrane et al., Astrophys. J. 704 (2009) 503;
M. Ambrosio et al., Astrophys. J. 546 (2001)
1038;R.Abbasietal,Astrophys.J.701(2009)
6. Acknowledgements
47; R. Abbasi et al., Phys. ReV. D79 (2009)
I would like to thank the organizers to in- 062001.
vite me to NOW2010 in Otranto. I greatly en- 14. http://www.km3net.org/
joyedthe nice scientificatmosphereandthe won- 15. P.Bagleyetal.,KM3NeTConceptualDesign
derful environment. This work has been par- Report, ISBN 978-90-6488-031-5.
tially supported by the following funding agen- http://www.km3net.org/CDR; P. Bagley et
ciesandgrants: theSpanishMinisteriodeCiencia al.,KM3NeTTechnicalDesignReport,ISBN
y Tecnolog´ıa, MICINN, FPA2099-13983-C02-01, 978-90-6488-033-9.
ACI2009-1020 (KM3NeT) and Consolider Mul- http://www.km3net.org/TDR.
tiDark CSD2009-00064; Generalitat Valenciana, 16. ESFRI Roadmap:
Prometeo/2009/026; Commission of the Euro- http://cordis.europa.eu/esfri/roadmap.htm;
peanCommunities,KM3NeTPreparatoryPhase, ASPERA Roadmap:
grant agreement 212525. www.aspera-eu.org/images/stories/roadmap/
aspera roadmap.pdf;
ASTRONET: www.astronet-eu.org/.
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