Neutrino searches with the IceCube telescope

0.69 MB·English

by  Juan A. Aguilar

Nuclear
Physics B
Proceed-
ings
NuclearPhysicsBProceedingsSupplement00(2013)1–3 Supple-
ment
Neutrino searches with the IceCube telescope.
JuanA.AguilarfortheIceCubeCollaboration.
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1 De´partementdephysiquenucle´aireetcorpusculaire,Universite´deGene´ve,CH-1211Gene`ve,Switzerland
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J
Abstract
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TheIceCubeNeutrinoObservatoryisanarrayof5,160photomultipliers(PMTs)deployedon86stringsat1.5-2.5kmdepth

withintheiceattheSouthPole. ThemaingoaloftheIceCubeexperimentisthedetectionofanastrophysicalneutrinosignal. In
]
E thiscontributionwepresenttheresultsofthepointsourceanalysisonthedatatakenfromApril2008toMay2011,whenthree
H detectorconfigurationswereoperated: the40-stringconfiguration(IC-40),the59-stringconfiguration(IC-59)andthe79-string
configuration(IC-79). Nosignificantexcessindicativeofpointsourcesofneutrinoshasbeenfound,andwepresentupperlimits
h. foranE−2muonneutrinofluxforalistofcandidatesources. Forthefirsttimetheselimitsstarttoreach10−12TeV−1cm−2s−1in
p somepartsofthesky.
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o Keywords: neutrinoastronomy,point-sources,cosmicraysorigin
r
t 1. Introduction 2. Detectorandeventselection.
s
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[ The origin of Cosmic Rays (CRs) is still one of the Oneofthemostimportantstepsinneutrinoanalysis

unresolvedquestionsofmodernphysics. Thedetection istoremovethelargeatmosphericmuonbackground.A
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ofapointsourceofneutrinoscouldnotonlyprovethe significantpartofthisreductionisachievedattheSouth
v
4 hadronicaccelerationmodelsresponsibleforCRemis- Polebyrejectingpoorlyreconstructedup-goingevents
0 sion but also identify the sources of these CRs. The fromthenorthernskyandselectinghighenergydown-
5 IceCubeneutrinotelescopeislocatedattheSouthPole goingmuonsinthesouthernsky. However,theseonline
6 aimingatthedetectionofastrophysicalneutrinosorig- filters are not enough to obtain an event selection suit-
.
1 inating in the same environment as CRs. An array of ableforneutrinoanalysis. Furtherprocessingisneeded
0 5,160photomultipliers(PMTs)hasbeendeployedinthe including the addition of more CPU consuming track
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antarcticiceinordertodetecttheCherenkovlightemit- reconstructionsandalgorithms. Inthisanalysisweuse
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tedbythesecondarymuonproducedinmuonneutrino threedifferentgeometriesoftheIceCubedetectorover
:
v interaction in the vicinity of the detector. The energy thelastthreeyearsandthereforetheeventselectionsfor
Xi and direction of the secondary muon is inferred by the thesethreeperiodsaredifferent.
amountoflightcollectedbythePMTsandthegeomet- The event selection in IC-40 data was done by cut-
r
a ricalpropertiesoftheCherenkovemission. Thisdirec- tingonobservableswithdiscriminationpowerbetween
tion is, on average, very close to the direction of the signal (generally an E−2 neutrino signal) and back-
parent neutrino, specially for E > 10 TeV, and there- ground (atmospheric neutrinos and muons). The de-
ν
fore the IceCube detector can work as a neutrino tele- tailsofthiseventselectioncanbefoundinRef.[1]. A
scope. In Sec. 2 of these proceedings we describe the multi-variate classification algorithm was used for the
threedatasamplesusedinthisanalysis. Thelikelihood event selections in IC-59 and IC-79. A Boosted Deci-
methodusedforthesearchofpointneutrinosourcesis sion Tree (BDT) [2] classifies events as signal-like or
explained in Sec. 3 and the results are given in Sec. 4. background-like based on a large number of detector
ConclusionsaregiveninSec.5. observables. For the northern sky 12 and 17 variables
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using simulation and scrambled real data, respectively.
Sincethreegeometriesandeventselectionswerecom-
bined, thedensitydistributionoftheith eventinthe jth
samplewithenergyE anddistancetothesource|x−x |,
i i s
isgivenby:
Pij(|xi−xs|,Ei,γ,nsj)= NnsjjSij+1− NnsjjBij, (1)
where j represents the three different event selections,
IC-40, IC-59 and IC-79. The total number of events
in the jth data set, Nj, satisfies the condition N =
tot
NIC40 + NIC59 + NIC79, where N is the total number
tot
of events in the three samples. The signal is consid-
ered to follow the uniform hypothesis among data sets
and therefore the spectral index meets the condition of
Figure1.MuonneutrinoeffectiveareasatfinalcutlevelfortheIC-79 γ = γ . The fitted number of signal events nj in each
j s
periodforsixdeclinationbands(solid-angle-averaged)asafunction
samplearealsofixedrelativetoeachother,accordingto
oftheneutrinoenergy.
thesignalhypothesistestedandtheresultingfractionof
Conf. Year t [days] SMT[Hz] R totalsignaleventsexpectedineachsample. Fortheall-
live ν
40 2008 376 1100 40/day skysurvey,thelikelihoodisevaluatedineachdirection
59 2009 348 1900 120/day intheskyinstepsof0.1◦×0.1◦centeredattheposition
79 2010 316 2300 180/day of the source xs. The significance of any excess found
inthenorthernskyiscorrectedfortrialsbydetermining
Table1.EventratesfordifferentIceCubeconfigurations.Firstcolumn thefractionofscrambleddatasetswhichcontainanex-
isthenumberofstringsintheconfiguration.Theyearofoperationis cessofequalorhighersignificancealsointhenorthern
showninthesecondcolumn. Thelive-timeindaysisgiveninthe
part of the sky. Likewise, the most significant excess
thirdcolumn. Therateattriggerlevel(SimpleMajorityTrigger)is
showninthefourthcolumnwhilethelastcolumnshowsthenumber found in the southern sky is reported and the post-trial
ofup-goingevents(mainlyatmosphericneutrinos)afterthefinalcut. significanceiscalculatedbycomparingitwiththefrac-
tion of scrambled data sets with an excess of equal or
highersignificanceinthatpartofthesky.
were used in the BDT training for the IC-59 and IC-
79periods respectively. Thesouthernskyin theIC-59
4. Results
datasetwas,however,filteredusingacutontheenergy
estimatorasafunctionofthezenithangletoseparatethe The results of the all-sky scan are shown in the pre-
large amount of down-going atmospheric muons from trialsignificancemapinFigure2. Themostsignificant
a hypothetical neutrino signal with a harder spectrum. deviation in the northern sky has a pre-trial p-value of
In the IC-79 dataset a combination of a BDT cut and 1.96×10−5 andislocatedat34.25◦ r.a. and2.75◦ dec.
a zenith dependent energy cut was applied. Figure 1 Similarly,themostsignificantdeviationinthesouthern
shows the muon neutrino effective area of the IC-79 skyhasapre-trial p-valueof8.97×10−5andislocated
sample for different declination bands as a function of at 219.25◦ r.a. and −38.75◦ dec. The post-trial proba-
theneutrinoenergy. bilitiescalculatedasthefractionofscrambledskymaps
Thedifferentdataratesforthethreeperiodsatdiffer- with at least one spot with an equal or higher signifi-
entlevelscanbeseeninTable1. canceforeachregionoftheskycorrespondto57%and
98%forthenorthernandthesouthernspotsrespectively
andthereforebothexcessesarewellcompatiblewiththe
3. Likelihoodmethodforpointsourcesearch
backgroundhypothesis.
Inthissearchanunbinnedmaximumlikelihoodratio Figure3showstheE−2muonneutrinofluxupperlim-
test [3] was used in order to calculate the significance its calculated at 90% C.L. based on the classical (fre-
of an excess of neutrinos above the background for a quentist)approach[4]foreachofthesourcesonacan-
given direction in the sky. The signal (S) and back- didatelist. Alsoshownisthemedianupperlimit(sen-
ground (B) probability density functions are modeled sitivity) for each declination for a live-time of 1,040
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Figure2.Significanceskymapinequatorialcoordinates(J2000)oftheall-skypointsourcescanforthecombinedIC79+IC59+IC40datasample.
Thedashedlineindicatesthegalacticplane.
5. Conclusions
We presented the results of the point source analy-
sis using the data-sets corresponding to three different
IceCubeconfigurations. Thecombineddatahasatotal
live-time of 1,040 days. The search for neutrino point
sources found no evidence of a neutrino signal. Both
the north and the south hot spots have post-trial prob-
abilitiescompatiblewithbackgroundfluctuations. The
muon neutrino upper limits presented here are a factor
∼ 3.5 better than the latest published upper limits by
IceCube[1]andarethemostcompetitiveneutrinolim-
itstodateoverthemajorityofthesky.
References
Figure3.Muonneutrinoflux90%C.L.upperlimitsandsensitivities
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foraE−2spectrum.PublishedlimitsofANTARES[5]areshown.
[2] S.S.Kerthietal.,NeuralComp.13(2001)637.
[3] J.Braunetal.,Astropart.Phys.,33(2010)175.
[4] J.Neyman.Phi.Tran.oftheRoyalSoc.ofLondonSerA,236
(1937)333-380.
days and the discovery potential flux for a 5σ confi- [5] S.Adria´n-Mart´ınezetal.[ANTARESColl.],SubmittedtoAs-
dencelevel. The90%C.L.muonneutrinoupperlimits trop.J.,(2012)arXiv:1207.3105
andsensitivitiesfromANTARES[5]correspondtothe
analysisof813daysofdata.
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