Search for Dark Matter Annihilation in the Galactic Center with IceCube-79

14 Pages·2015·1.02 MB·English

Eur. Phys. J. C manuscript No.
(will be inserted by the editor)
Search for Dark Matter Annihilation in the Galactic Center
with IceCube-79
IceCube Collaboration: M. G. Aartsen2, K. Abraham32,
M. Ackermann48, J. Adams15, J. A. Aguilar12, M. Ahlers29,
M. Ahrens39, D. Altmann23, T. Anderson45, M. Archinger30,
C. Arguelles29, T. C. Arlen45, J. Auffenberg1, X. Bai37,
S. W. Barwick26, V. Baum30, R. Bay7, J. J. Beatty17,18,
J. Becker Tjus10, K.-H. Becker47, E. Beiser29, S. BenZvi29,
5
1 P. Berghaus48, D. Berley16, E. Bernardini48, A. Bernhard32,
0 D. Z. Besson27, G. Binder8,7, D. Bindig47, M. Bissok1,a,
2 E. Blaufuss16, J. Blumenthal1, D. J. Boersma46,

n C. Bohm39, M. B¨orner20, F. Bos10, D. Bose41, S. B¨oser30,
u O. Botner46, J. Braun29, L. Brayeur13, H.-P. Bretz48,
J
A. M. Brown15, N. Buzinsky22, J. Casey5, M. Casier13,
8
E. Cheung16, D. Chirkin29, A. Christov24, B. Christy16,

] K. Clark42, L. Classen23, S. Coenders32, D. F. Cowen45,44,
E A. H. Cruz Silva48, J. Daughhetee5, J. C. Davis17,
H M. Day29, J. P. A. M. de Andr´e21, C. De Clercq13,
. H. Dembinski33, S. De Ridder25, P. Desiati29, K. D. de Vries13,
h
p G. de Wasseige13, M. de With9, T. DeYoung21,
- J. C. D´ıaz-V´elez29, J. P. Dumm39, M. Dunkman45, R. Eagan45,
o
r B. Eberhardt30, T. Ehrhardt30, B. Eichmann10, S. Euler46,
st P. A. Evenson33, O. Fadiran29, S. Fahey29, A. R. Fazely6,
a A. Fedynitch10, J. Feintzeig29, J. Felde16, K. Filimonov7,
[
C. Finley39, T. Fischer-Wasels47, S. Flis39,a, T. Fuchs20,

2 M. Glagla1, T. K. Gaisser33, R. Gaior14, J. Gallagher28,
v
L. Gerhardt8,7, K. Ghorbani29, D. Gier1, L. Gladstone29,
9
T. Glu¨senkamp48, A. Goldschmidt8, G. Golup13,
5
2 J. G. Gonzalez33, D. G´ora48, D. Grant22, P. Gretskov1,
7 J. C. Groh45, A. Groß32, C. Ha8,7, C. Haack1, A. Haj Ismail25,
0
A. Hallgren46, F. Halzen29, B. Hansmann1, K. Hanson29,
.
5 D. Hebecker9, D. Heereman12, K. Helbing47, R. Hellauer16,
0 D. Hellwig1, S. Hickford47, J. Hignight21, G. C. Hill2,
5
K. D. Hoffman16, R. Hoffmann47, K. Holzapfe32, A. Homeier11,
1
: K. Hoshina29,b, F. Huang45, M. Huber32, W. Huelsnitz16,
v
P. O. Hulth39, K. Hultqvist39, S. In41, A. Ishihara14,
i
X E. Jacobi48, G. S. Japaridze4, K. Jero29, M. Jurkovic32,
r B. Kaminsky48, A. Kappes23, T. Karg48, A. Karle29,
a
M. Kauer29,34, A. Keivani45, J. L. Kelley29, J. Kemp1,
A. Kheirandish29, J. Kiryluk40, J. Kl¨as47, S. R. Klein8,7,
G. Kohnen31, H. Kolanoski9, R. Konietz1, A. Koob1,
L. Ko¨pke30, C. Kopper22, S. Kopper47, D. J. Koskinen19,
M. Kowalski9,48, K. Krings32, G. Kroll30, M. Kroll10,
J. Kunnen13, N. Kurahashi36, T. Kuwabara14, M. Labare25,
J. L. Lanfranchi45, M. J. Larson19, M. Lesiak-Bzdak40,
M. Leuermann1, J. Leuner1, J. Lu¨nemann30, J. Madsen38,
G. Maggi13, K. B. M. Mahn21, R. Maruyama34, K. Mase14,
H. S. Matis8, R. Maunu16, F. McNally29, K. Meagher12,
2
M. Medici19, A. Meli25, T. Menne20, G. Merino29,
T. Meures12, S. Miarecki8,7, E. Middell48, E. Middlemas29,
J. Miller13, L. Mohrmann48, T. Montaruli24, R. Morse29,
R. Nahnhauer48, U. Naumann47, H. Niederhausen40,
S. C. Nowicki22, D. R. Nygren8, A. Obertacke47, A. Olivas16,
A. Omairat47, A. O’Murchadha12, T. Palczewski43, L. Paul1,
J. A. Pepper43, C. P´erez de los Heros46, C. Pfendner17,
D. Pieloth20, E. Pinat12, J. Posselt47, P. B. Price7,
G. T. Przybylski8, J. Pu¨tz1, M. Quinnan45, L. R¨adel1,
M. Rameez24, K. Rawlins3, P. Redl16, R. Reimann1,
M. Relich14, E. Resconi32, W. Rhode20, M. Richman36,
S. Richter29, B. Riedel22, S. Robertson2, M. Rongen1,
C. Rott41, T. Ruhe20, B. Ruzybayev33, D. Ryckbosch25,
S. M. Saba10, L. Sabbatini29, H.-G. Sander30, A. Sandrock20,
J. Sandroos19, S. Sarkar19,35, K. Schatto30, F. Scheriau20,
M. Schimp1, T. Schmidt16, M. Schmitz20, S. Schoenen1,
S. Sch¨oneberg10, A. Sch¨onwald48, A. Schukraft1, L. Schulte11,
D. Seckel33, S. Seunarine38, R. Shanidze48, M. W. E. Smith45,
D. Soldin47, G. M. Spiczak38, C. Spiering48, M. Stahlberg1,
M. Stamatikos17,c, T. Stanev33, N. A. Stanisha45,
A. Stasik48, T. Stezelberger8, R. G. Stokstad8, A. St¨oßl48,
E. A. Strahler13, R. Str¨om46, N. L. Strotjohann48,
G. W. Sullivan16, M. Sutherland17, H. Taavola46, I. Taboada5,
S. Ter-Antonyan6, A. Terliuk48, G. Teˇsi´c45, S. Tilav33,
P. A. Toale43, M. N. Tobin29, D. Tosi29, M. Tselengidou23,
E. Unger46, M. Usner48, S. Vallecorsa24, N. van Eijndhoven13,
J. Vandenbroucke29, J. van Santen29, S. Vanheule25,
J. Veenkamp32, M. Vehring1, M. Voge11, M. Vraeghe25,
C. Walck39, M. Wallraff1, N. Wandkowsky29, Ch. Weaver29,
C. Wendt29, S. Westerhoff29, B. J. Whelan2, N. Whitehorn29,
C. Wichary1, K. Wiebe30, C. H. Wiebusch1, L. Wille29,
D. R. Williams43, H. Wissing16, M. Wolf39,a, T. R. Wood22,
K. Woschnagg7, D. L. Xu43, X. W. Xu6, Y. Xu40,
J. P. Yanez48, G. Yodh26, S. Yoshida14, P. Zarzhitsky43,
M. Zoll39
3
1III.PhysikalischesInstitut,RWTHAachenUniversity,D-52056Aachen,Germany
2SchoolofChemistry&Physics,UniversityofAdelaide,AdelaideSA,5005Australia
3Dept.ofPhysicsandAstronomy,UniversityofAlaskaAnchorage,3211ProvidenceDr.,Anchorage,AK99508,USA
4CTSPS,Clark-AtlantaUniversity,Atlanta,GA30314,USA
5SchoolofPhysicsandCenterforRelativisticAstrophysics,GeorgiaInstituteofTechnology,Atlanta,GA30332,USA
6Dept.ofPhysics,SouthernUniversity,BatonRouge,LA70813,USA
7Dept.ofPhysics,UniversityofCalifornia,Berkeley,CA94720,USA
8LawrenceBerkeleyNationalLaboratory,Berkeley,CA94720,USA
9Institutfu¨rPhysik,Humboldt-Universit¨atzuBerlin,D-12489Berlin,Germany
10Fakult¨atfu¨rPhysik&Astronomie,Ruhr-Universita¨tBochum,D-44780Bochum,Germany
11PhysikalischesInstitut,Universit¨atBonn,Nussallee12,D-53115Bonn,Germany
12Universit´eLibredeBruxelles,ScienceFacultyCP230,B-1050Brussels,Belgium
13VrijeUniversiteitBrussel,DienstELEM,B-1050Brussels,Belgium
14Dept.ofPhysics,ChibaUniversity,Chiba263-8522,Japan
15Dept.ofPhysicsandAstronomy,UniversityofCanterbury,PrivateBag4800,Christchurch,NewZealand
16Dept.ofPhysics,UniversityofMaryland,CollegePark,MD20742,USA
17Dept.ofPhysicsandCenterforCosmologyandAstro-ParticlePhysics,OhioStateUniversity,Columbus,OH43210,USA
18Dept.ofAstronomy,OhioStateUniversity,Columbus,OH43210,USA
19NielsBohrInstitute,UniversityofCopenhagen,DK-2100Copenhagen,Denmark
20Dept.ofPhysics,TUDortmundUniversity,D-44221Dortmund,Germany
21Dept.ofPhysicsandAstronomy,MichiganStateUniversity,EastLansing,MI48824,USA
22Dept.ofPhysics,UniversityofAlberta,Edmonton,Alberta,CanadaT6G2E1
23ErlangenCentreforAstroparticlePhysics,Friedrich-Alexander-Universit¨atErlangen-Nu¨rnberg,D-91058Erlangen,Germany
24D´epartementdephysiquenucl´eaireetcorpusculaire,Universit´edeGen`eve,CH-1211Gen`eve,Switzerland
25Dept.ofPhysicsandAstronomy,UniversityofGent,B-9000Gent,Belgium
26Dept.ofPhysicsandAstronomy,UniversityofCalifornia,Irvine,CA92697,USA
27Dept.ofPhysicsandAstronomy,UniversityofKansas,Lawrence,KS66045,USA
28Dept.ofAstronomy,UniversityofWisconsin,Madison,WI53706,USA
29Dept.ofPhysicsandWisconsinIceCubeParticleAstrophysicsCenter,UniversityofWisconsin,Madison,WI53706,USA
30InstituteofPhysics,UniversityofMainz,StaudingerWeg7,D-55099Mainz,Germany
31Universit´edeMons,7000Mons,Belgium
32TechnischeUniversit¨atMu¨nchen,D-85748Garching,Germany
33BartolResearchInstituteandDept.ofPhysicsandAstronomy,UniversityofDelaware,Newark,DE19716,USA
34DepartmentofPhysics,YaleUniversity,NewHaven,CT06520,USA
35Dept.ofPhysics,UniversityofOxford,1KebleRoad,OxfordOX13NP,UK
36Dept.ofPhysics,DrexelUniversity,3141ChestnutStreet,Philadelphia,PA19104,USA
37PhysicsDepartment,SouthDakotaSchoolofMinesandTechnology,RapidCity,SD57701,USA
38Dept.ofPhysics,UniversityofWisconsin,RiverFalls,WI54022,USA
39OskarKleinCentreandDept.ofPhysics,StockholmUniversity,SE-10691Stockholm,Sweden
40Dept.ofPhysicsandAstronomy,StonyBrookUniversity,StonyBrook,NY11794-3800,USA
41Dept.ofPhysics,SungkyunkwanUniversity,Suwon440-746,Korea
42Dept.ofPhysics,UniversityofToronto,Toronto,Ontario,Canada,M5S1A7
43Dept.ofPhysicsandAstronomy,UniversityofAlabama,Tuscaloosa,AL35487,USA
44Dept.ofAstronomyandAstrophysics,PennsylvaniaStateUniversity,UniversityPark,PA16802,USA
45Dept.ofPhysics,PennsylvaniaStateUniversity,UniversityPark,PA16802,USA
46Dept.ofPhysicsandAstronomy,UppsalaUniversity,Box516,S-75120Uppsala,Sweden
47Dept.ofPhysics,UniversityofWuppertal,D-42119Wuppertal,Germany
48DESY,D-15735Zeuthen,Germany
Received:date/Accepted:date
aCorrespondingauthors:M.Bissok([email protected]),S.Flis(samuel.d.fl[email protected]),
M.Wolf([email protected])
bEarthquakeResearchInstitute,UniversityofTokyo,Bunkyo,Tokyo113-0032,Japan
cNASAGoddardSpaceFlightCenter,Greenbelt,MD20771,USA
4
Abstract TheMilkyWayisexpectedtobeembedded theobservedlarge-structuresontimescalesoftheorder
inahaloofdarkmatterparticles,withthehighestden- of the age of the universe would not have been possi-
sity in the central region, and decreasing density with ble [6–8].
thehalo-centricradius.Darkmattermightbeindirectly Though the nature of dark matter is largely un-
detectable at Earth through a flux of stable particles known, some of its properties may be deduced from
generated in dark matter annihilations and peaked in theabove-mentionedobservations.Analysesoftemper-
the direction of the Galactic Center. aturefluctuationsintheCosmicMicrowaveBackground
We present a search for an excess flux of muon (CMB)bythePlanckcollaboration[8]yieldthecurrent
(anti-) neutrinos from dark matter annihilation in the best estimate for the total content of DM in the uni-
GalacticCenterusingthecubic-kilometer-sizedIceCube verse: Ω h2 = 0.1199±0.0027, with the cold DM
CDM
neutrinodetectorattheSouthPole.There,theGalactic density parameter Ω , and h=0.673±0.012 being
CDM
Centerisalwaysseenabovethehorizon.Thus,newand the Hubble parameter divided by 100km/sMpc.
dedicated veto techniques against atmospheric muons Besidesinferencefromgravitationalinteraction,par-
are required to make the southern hemisphere accessi- ticle DM may also be detected indirectly. A weakly
ble for IceCube. interacting massive particle (WIMP) at roughly GeV-
We used 319.7 live-days of data from IceCube op- scale masses is a favorable class of DM; it naturally
erating in its 79-string configuration during 2010 and provides the right order of magnitude for the thermal
2011. No neutrino excess was found and the final re- relic abundance of DM in the early universe [9]. Ex-
sultiscompatiblewiththebackground.Wepresentup- amples of WIMPs are neutralinos in supersymmetric
per limits on the self-annihilation cross-section, (cid:104)σAv(cid:105), extensions of the Standard Model [10], or the lightest
for WIMP masses ranging from 30GeV up to 10TeV, stable excitations in Kaluza-Klein models [11].
assuming cuspy (NFW) and flat-cored (Burkert) dark
If DM decays, or (self-)annihilates, a flux of sta-
matterhaloprofiles,reachingdownto(cid:39)4·10−24cm3s−1,
blefinal-statemessengerparticles,e.g.chargedleptons,
and (cid:39) 2.6·10−23cm3s−1 for the νν channel, respec-
photons,andneutrinos,maybedetectedatEarth,mak-
tively.
ing DM experimentally accessible by indirect searches
Keywords Dark Matter · Galactic Center · Indirect (e.g. [12–14]). The neutrino is an attractive messen-
Search · Neutrinos · IceCube · DeepCore ger particle because it propagates without absorption,
and neutrino vacuum oscillations do not alter the en-
PACS 95.35.+d · 98.70.Sa · 98.35.-a
ergy and direction information. Further, no fore- or
background from astrophysical objects has been iden-
1 Introduction tified yet. Regions of increased DM density, like mas-
sivecelestialobjects,dwarfgalaxies,galactichalos,and
The first clear evidence for the existence of an invisible the Galactic Center, provide targets to search for an
mass component in the universe was Zwicky’s observa- increased flux of neutrinos. Due to its proximity, the
tion of the dynamics of the Coma galaxy cluster [1]. Galactic Center is expected to yield the highest flux of
Subsequently, a broad range of cosmological and astro- annihilation products [15]. While most of these sources
physical observations supported the existence of this would appear as (nearly) point-like sources in the sky,
darkmatter(DM)onvariousscales,fromgalaxycluster theGalacticCenterisanextendedsource,andasignal
scales down to galactic scales. Measurements of galac- from the Milky Way halo would lead to a large-scale
tic velocity profiles hint at invisible mass distributed anisotropy in neutrino arrival directions [16, 17]. With
beyond the visible disks [2]. Galaxy cluster dynamics its 4π acceptance, the IceCube neutrino detector [18],
exhibit a similar behavior [3]. is well-suited for DM searches from all of the above-
Further evidence for the existence of dark matter mentioned sources.
can be found in galaxy cluster mergers like the Bullet In this paper we present the results from a search
Cluster [4, 5]. Following a collision, the interstellar and for a neutrino signal from DM self-annihilation in the
intergalactic gas components as seen in X-ray obser- Galactic Center, targeting DM masses ranging from
vations are spatially separated from the reconstructed 30 GeV to 10 TeV. Due to the wide range of event
mass distribution. Such a separation strongly disfavors topologies associated with neutrinos from this energy
theories of modified gravity. range,twoeventselectionsaremotivatedandpresented.
According to the current understanding of the for- Oneeventselectionfocusesonthelow-massregionrang-
mationandevolutionoflarge-scalestructures,cold(non- ing from 30GeV to 100GeV, accessible through the
relativistic), or warm dark matter is preferred over hot low-energy in-fill array DeepCore (DC) [19], with the
(relativistic) dark matter. Otherwise, the formation of surrounding parts of IceCube used as veto. The sec-
5
ondeventselectionfocusesonthemassrange100GeV– Table 1 DM Halo parameters used in this analysis. Taken
1TeV, but extends up to 10TeV. For this selection a from[26].
largerpartoftheIceCubedetectorisdefinedasfiducial
Parameter NFW Burkert
volume.Throughoutthispaperwedenotethelow-mass
(α, β, γ, δ) (1, 3, 1, 0) (2, 3, 1, 1)
event selection as LE and the high-mass selection as
HE. ρ0 [107M(cid:12)/kpc3] 1.40+−20..9903 4.13−+61..26
rs [kpc] 16.1+−177.8.0 9.26−+54..62
ρ [GeV/cm3] 0.471+0.048 0.487+0.075
local −0.061 −0.088
2 Dark Matter Halos 3 Neutrino Signal from Dark Matter
Annihilation
DMhalosareconsideredtobegravitationallyself-bound
overdensities of DM particles, formed through hierar- The flux of final state particles from annihilating dark
chical merging of proto-halos from primordial density matter depends on the DM mass density squared, inte-
fluctuations [20]. There is a tension between halo pro- grated along the line-of-sight (l.o.s.) through the DM
filefitstoDMoverdensitiesinN–bodysimulations,and halo, and is given by the J -factor. Following e.g. [13,
a
fits to observational data (the cusp-core problem) [21]. 15], the J -factor is
a
N–body simulations seem to favor cuspy halos, while
observations of e.g. dwarf spheroidal galaxies imply a l(cid:90)max (cid:16)(cid:112) (cid:17)
J (Ψ)= dl ρ2 R2 −2lR cosΨ +l2 . (2)
rather flat central core region. However, the inner part a DM SC SC
ofthehaloprofilemaydependonthehosthalomass[22]. 0
Simple models of DM halos describe the density by Here,thedensityprofilealongthel.o.s.isparametrized
a smooth spherically-symmetric function of the halo- foragivenanglebetweenthel.o.s.andthedirectionof
centric radius, with the maximal density at the cen- the center of the galaxy, Ψ. The parameters are the
ter, and a decreasing density with increasing radius. radius of the solar circle, R ≈8.5 kpc, and the max-
SC
One parametrization of such density profiles is given imal distance from the observer along the l.o.s., l .
max
by (modified from [23]) The latter is
(cid:113)
ρ (r)= ρ0 , (1) lmax = RM2W−RS2Csin2Ψ +RSCcosΨ, (3)
DM (cid:16) (cid:17)γ (cid:16) (cid:16) (cid:17)α(cid:17)(β−γ)/α
δ+ r · 1+ r with the assumed radius of the Milky Way, R ≈
rs rs 50 kpc. Typically, radii larger than the scale radMiuWs do
notcontributesignificantlytothetotalvalueofJ .Fig-
a
with the shape parameters α, β, γ, the scale radius r , ure1(toppanel)showsJ fortheNFW(solidline)and
s a
andthemassdensitynormalizationρ ,whichisusually Burkert (dashed line) profiles.
0
determined from the assumed local DM density, ρ , ThefinaldifferentialneutrinofluxfromDMannihi-
local
in our solar system. We introduced the parameter δ to lation, dφ /dE, depends on the neutrino energy spec-
ν
allowforacentralcoreifsetto1,whileδ =0describes trumoftheactualannihilationchannel.Thedifferential
a cuspy halo profile. neutrino flux is
The parametrization of Eq. (1) is a combination of
dφ (cid:104)σ v(cid:105) 1 dN
power laws, where e.g. the power-law index γ describe dEν = A2 4πm2 dEν Ja(Ψ), (4)
theinnerslope,whileαandβ describetheouterslope. χ
The halo-centric distance of this transition region de- with (cid:104)σ v(cid:105) being the thermally averaged product of
A
pends on the scale radius rs. self-annihilation cross-section, σA and WIMP velocity
In view of the unresolved cusp-core problem, we v.Further,m istheWIMPmass,dN /dE istheneu-
χ ν
presentresultsfortwohalodensityprofiles.Thewidely trinoenergyspectrumperannihilatingWIMPpair,and
usedNavarro-Frenk-White(NFW)profilerepresentscuspy J is the DM abundance along the l.o.s..
a
halos[24],andischosenforcomparabilityamongdiffer- We consider several benchmark annihilation chan-
ent experimental results. The Burkert profile is chosen nels with 100% branching ratios (χχ → b¯b, W+W−,
asrepresentativeofflat-coredprofiles[25].Basedonob- µ+µ−,τ+τ−,νν¯)forthecalculationoftheneutrinoen-
servation, the latter profile is currently favored for the ergy spectrum dN /dE. The resulting spectra bracket
ν
MilkyWay[26].Table1showstheparametervaluesfor the realistic annihilation neutrino energy spectra with
the two models used in this work. amixtureofdifferentannihilationbranchingratios.We
6
generated a neutrino energy spectrum from annihilat-
]
ing DM for a particular WIMP mass and annihilation 1r−
channel using the PYTHIA8 (version 8.175) software c s104
p
k
package [27]. Our PYTHIA8 simulation was set up to
6−
simulateagenericresonancewithanenergyoftwicethe m102
c

WIMP mass forming only the final state particle pair 2V
(i.e.bb,W+W−,etc.)inquestion.Subsequentprocesses Ge
like hadronization and decay were simulated using the [Ψ)100 NBuFrWkert
default PYTHIA8 implementations. The generic reso- J(a 10-1 100 101 102
nance ensures an isotropic decay of weak bosons, e.g. Ψ [◦]
in the W+W− channel. Thus, the spin of the annihi-
102
lating WIMPs is not considered and we don’t assume V]
e
a specific WIMP model like the lightest neutralino de- [G100
scribed by supersymmetric models [9]. If the WIMP 2E
is indeed the lightest neutralino, the spin (1/2) of the dE)10-2
WIMP would affect the generation of the neutrino en- N/ b¯b
ergy spectra. The spin of such a WIMP would lead to (d10-4 Wµ++µW−−
fully transversely polarized W-bosons in the final state
0.0 0.2 0.4 0.6 0.8 1.0
of the annihilation process, thus altering the neutrino E/m
χ
energyspectrum[28].Thedifferencesinthedifferential
Fig. 1 Top:Line-of-sightintegralJ (Ψ)oftheNFW(solid)
a
neutrino yield compared to the isotropically decaying
andBurkert(dashed)DMprofileusingparametersasgivenin
W bosonsisabout±40%.Examplesoftheneutrinoen- Tab. 1. Bottom: Example of DM annihilation spectra gener-
ergy spectra used here are shown in the bottom panel atedwithPYTHIA8[27]foraWIMPmassofmχ=500GeV.
Three annihilation channels are shown: b¯b (solid), W+W−
of Fig. 1 for the bb, W+W−, and µ+µ− annihilation
(dashed),and µ+µ− (dotted).
channels.
Ingeneral,neutrinosaresubjecttoneutrinooscilla-
tionsonthewayfromtheGalacticCentertotheEarth.
Duetotheverylongbaseline,weassumearelativeneu- tor strings with 60 digital optical modules (DOMs) on
trino flavor ratio of 1:1:1 at Earth. eachstring[32,33]weredeployed.EachDOMcontains
Thesesimulationshavethedetectorresponsefolded a 25cm Hamamatsu photomultiplier tube (PMT) and
in and are generated using the ANIS event genera- on-board electronics to readout and digitize the sig-
tor [29] modelling the neutrino-nucleon charged and nal from the PMT [34]. In December 2010, the Ice-
neutralcurrentinteractionsviaCTEQ5[30]partondis- Cube detector construction was completed. The final
tributions for neutrino and anti-neutrino interactions. IceCube detector consists of 86 strings. A schematic
Finally, the neutrino energy distributions are used layoutofthedetectorisshowninFig.2.The79strings
to weight generic simulated neutrino data to DM anni- used in this analysis are marked by green and yellow
hilation signal. markers within the outer shaded grey area. The square
markers denote the additional strings constituting the
completed IceCube detector. Of the 79 strings used
4 The IceCube Neutrino Observatory
in this analysis, 73 strings (green) have a horizontal
spacing of 125m and a vertical spacing of 17m be-
TheIceCubeNeutrinoObservatory,situatedatthegeo-
tween DOMs. The six remaining strings (yellow) are
graphicSouthPole,consistsofanin-icedetectorarray,
located near the central string of IceCube. The DOMs
IceCube, and a surface air shower detector array, Ice-
on these strings are equipped with PMTs with a 30%
Top[31],dedicatedtoneutrinoandcosmicrayresearch,
increasedquantumefficiency.Togetherwiththeirnear-
respectively. IceCube [32] is installed in the glacial ice
est IceCube strings, these strings constitute the inner
at depths between 1450m and 2450m below the sur-
detector, DeepCore [19] (for IceCube-79). The vertical
face, instrumenting a total volume of one cubic kilo-
distancebetweenDOMsisreducedto7m(10m)forthe
meter. IceCube detects neutrinos by optical detection
bottom 50 (upper 10) DOMs. The horizontal distance
of Cherenkov radiation induced by secondary charged
between strings in DeepCore is less than 75m.
leptons which are produced in neutrino interactions in
the surrounding ice or the nearby bedrock. These two densely-instrumented parts (see Fig. 2)
Construction of the IceCube detector started in the are separated by a region with significantly reduced
Austral summer of 2004. In January 2010, 79 detec- scattering and absorption lengths for Cherenkov pho-
7
tons due to dust particles. It is located at a depth of
IceCube-79 DeepCore IceCube-86
about 2050m.
600
For muon-neutrino events, the neutrino arrival di-
rections are inferred from the muon arrival direction.
Thelatterisreconstructedusingalikelihoodapproach,
based on the arrival times of photons at DOMs [35].
300
Thus,agoodunderstandingoftheabsorptionandscat-
tering of photons is necessary for direction reconstruc-
tion. The clean glacial ice is a natural medium, built
up over tens of thousands of years. Thus, the optical m]
properties exhibit a variation over the 1km depth of Y [ 0
the instrumented volume. A detailed description of the
optical ice properties is given in [36].
-300
5 Event Selection
Thesignalforthisanalysisismuonsproducedincharged-
-600
currentneutrinointeractions.Thesemuonsproducetrack-
-1400
like event signatures in the detector, which allow for a
reconstruction of the arrival direction.
At the South Pole, the Galactic Center is always -1600
29◦ above the horizon. Thus, neutrinos from the direc-
tion of the Galactic Center will appear as down-going
events in IceCube. The backgrounds for this analysis -1800
are therefore down-going atmospheric muons and, at m]
a lower rate, muons produced by neutrinos originating pth [
e
from cosmic-ray showers in the atmosphere. The over- D-2000
whelming majority of the 2500 Hz trigger rate is due
toatmosphericmuons.Theatmosphericneutrinoback-
ground contributes to the trigger rate at the 1mHz- -2200
level. This event class is an irreducible background,
since in the energy range of interest for this analysis
theaccompanyingmuoncomponentoftheatmospheric -2400
shower is absorbed in the ice sheet above the detector.
-600 -300 0 300 600
Theapproachadoptedheretoreducethebackground X [m]
is to consider only neutrinos which interact within the
Fig. 2 A footprint (top) and side view (bottom) of Ice-
detector, and reject the background of penetrating (in- Cubeindetectorcoordinates.Thegreencirclesmarkregular
coming) muons. In order to select events which appear IceCube DOMs with 17m vertical spacing, while the yellow
diamondsmarkDeepCoreDOMswithabouthalftheregular
to start within the detector we developed several com-
vertical DOM spacing and higher quantum efficiency PMTs.
plementary veto techniques, exploiting differences in
The black squares mark DOMs which are part of the final
timing and topology of background and signal events. 86-string configuration, but are not present in the here used
This work is based on two independently developed 79-string configuration of IceCube. The purple shaded and
red shaded areas illustrate the fiducial volumes used by the
event selections, referred to as low-energy (LE) and
LEandHEevent selections,respectively.
high-energy (HE) selections or samples. There are two
reasonsforsuchenergy-specificoptimization.First,the
efficiencyofthesevetoesdecreasesrapidlywithdecreas- areverysimilar.First,aninitialonlineselectionisper-
ingeventenergy,becauselow-energymuonsareableto formed at the South Pole. Second, a set of cuts on the
traverse several string-layers without being detected. eventquality,topology,andarrivaldirectionsisapplied.
Second,thelikelihoodfunction(describedinSection6) Third, a boosted decision tree (BDT) is used to re-
does not use the event energy. move remaining background events, using the TMVA
Though the individual event selections differ, the software package [37]. The BDT is trained on a rep-
general selection techniques and the analysis pipelines resentative signal assumption for each sample. Finally,
8
a likelihood analysis is performed, exploiting the dif- assumed for BDT training are events that are events
ferent distributions of arrival directions of background which start in the fiducial volume, and originate in an-
events and events originating in dark matter annihi- nihilations of 600GeV WIMPs to W+W−-pairs in the
lations in the Galactic Center. The differences between NFW halo. The search window around the Galactic
thetwoeventselectionsarehighlightedinthefollowing Center is given by ±15◦ in both declination and right
sections. ascension.
ThisanalysisusesdatacollectedwithIceCubeinits The HE sample data rate at the analysis level is
79-stringconfigurationbetweenMay31,2010andMay 10mHz.
13, 2011 with a total live-time of 319.7 days of sta-
ble high-quality data. The LE sample contains 35,538
6 Analysis Method and Sensitivity
events, and the HE sample contains 293,043 events.
4,706 events appear in both samples; about 13% of the
A maximum likelihood analysis is performed indepen-
LEeventsareintheHEsample,andabout1.6%ofthe
dently on each event selection for a number of different
HE events are in the LE sample.
WIMP masses ranging from 30GeV to 10TeV, assum-
inga 100%branchingratiofor eachtested annihilation
channel.
5.1 Low-Energy Event Selection
Considering the large number of events in the two
final samples, a binned likelihood method was chosen.
TheLEeventselectionconsiderseventsfromtheDeep-
To reduce the number of bins, event arrival directions
Core online-filter [19], and is optimized for low-mass
were only considered in a search window around the
WIMPsbelow100GeV,andthususesthebottompart
GalacticCenter.Thesearchwindowshapeandsizedif-
ofthedensely-instrumentedDeepCoresub-arrayasfidu-
fer slightly between the two event samples as defined
cial volume. The remaining instrumented IceCube vol-
in the two previous sections 5.1 and 5.2 as well as the
ume as well as the two bottom DOM layers are usedas
background estimations. Due to these differences the
a veto. The fiducial volume is illustrated in Fig. 2, and
likelihood analysis performed on the LE and HE selec-
corresponds to roughly 27Mton of ice.
tions is not identical. The LE likelihood has the more
The LE selection cuts are based on experience from
complicated form and is defined as a function of signal
the IceCube-79 Solar WIMP analysis [38], which used
fraction, ξ, in the following way:
DeepCoreforthefirsttimeinlow-massWIMPsearches.
The signal used for BDT training are events that are (cid:18)N(cid:19) (cid:89)n
L(ξ)= pn(1−p)N−n f(X ,|ξ) (5)
fully contained in the fiducial volume, and originate in n i
annihilationsof65GeVWIMPstob¯b-pairsintheNFW i=1
where the binomial factor in front of the product ac-
halo.ThesearchwindowfortheLEanalysisextendsto
±30◦ inrightascension(α)withrespecttotheGalactic counts for the probability of observing n events in the
search window given N total events in the event selec-
Center, while the declination (δ) width is asymmetric
and extends from −39◦ to −9◦. tion, and the shape term, i.e the direction, X =(δ,α),
of the events is accounted for by the term f(X ,|ξ).
The LE sample data rate at the analysis level is i
ThebinomialprobabilitywaschoseninfavorofaPois-
1.4mHz.
son probability since the search window covers a non
negligible fraction of the declination band. The proba-
5.2 High-Energy Event Selection bilityofaneventtofallinthesearchwindowisdefined
as p=π ξ+π (1−ξ),where π andπ aretheprob-
s bg s bg
The IceCube array has a trigger energy threshold of ability for a signal or a background event, respectively,
about (cid:39) 100GeV. Therefore, the search for WIMPs to fall in the search window. Note that the signal orig-
in the mass range above a few hundred GeV benefits inating in a dark matter halo is an extreme case of an
from the large volume of IceCube in addition to Deep- extended source as it is present in the whole sky. Any
Coreatthecostofadecreasingvetoefficiency.TheHE background estimation based on data will be contami-
selection considers events from the dedicated Galactic natedbysignal.π isdeterminedfromtherelationbe-
bg
Center online-filter and the DeepCore online-filter. tween the size of the search window and the size of the
The veto for the HE event selection is defined by background estimation region, while π is determined
s
the upper 12 DOM layers and the two outer string lay- from simulation.
ers, which roughly corresponds to 200m and 125m of The directional probability density function (pdf),
instrumented distance, respectively. The fiducial vol- f(X|ξ), in Eq. (5) is constructed from binned expec-
ume for the HE selection is shown in Fig. 2. The signal tations of event directions for background and signal.
9
Figure 3 shows examples of these binned expectations. 16h 20h 0◦ 4h 8h 12h
ThesoftwarepackageHEAL-Pix[39]wasusedtoensure
equal-areabinsonthesphereforthesetwo-dimensional −30◦
pdfs. To determine the signal pdfs, IceCube neutrino
simulations were used, weighting events according to −60◦
Eq. (4) and the corresponding DM annihilation spec-
trum.Backgroundpdfswerecreated byscramblingthe 16h 20h 0◦ 4h 8h 12h
right ascension of experimental data events in the final
event selections. −30◦
Thesamereasoningregardingsignalcontamination,
as stated above, applies to the directional pdf, f(X|ξ). −60◦
In effect, the expected arrival directions of background
events will depend on the signal strength. This needs 10-1 100
p.d.f. value per sr
to be accounted for in the directional pdf, f(X|ξ) in
the likelihood, as well as in the background simulation Fig.3 Exampleskymapsofthebackground(top)andsignal
(bottom) pdfs in equatorial coordinates for the LE event se-
during the construction of confidence intervals. The di-
lection, for 100GeV WIMPs annihilating into W+W−. The
rectional pdf is defined as
LEsearchwindowismarkedwithadashedline.Forillustra-
tion the search window for the HE event selection is marked
f(X|ξ)=wf (X)+ withadash-dottedline.
s
(1−w)[(1+w)f (X)−wf (X)] (6)
bg sc
where f and f are the signal and background di-
s bg
rectional pdfs, respectively, f is a pdf describing the WIMPmassandannihilationchannel.Thus,thecross-
sc
signal scrambled in right ascension. The signal fraction over point in the WIMP mass between the two event
inside the search window is samples depends on the DM halo model and WIMP
annihilation channel. This procedure circumvents the
π ξ
w = s . (7) necessity to deal with the small overlap of both sam-
π ξ+π (1−ξ)
s bg ples. Figure 4 illustrates how the two event samples
A different approach is used for the HE analysis. contribute to the best sensitivity at different WIMP
The background estimation is performed on off-source masses,inthiscaseforWIMPannihilationtoνν¯.Each
data, excluding all events within ±30◦ of the Galac- sample has a WIMP mass range where it outperforms
tic Center. Therefore, any signal contamination of the the other.
backgroundestimateisignored,andthelikelihoodfunc-
tion from Eq. (5) simplifies to: Figure 5 shows the neutrino effective area for the
two event selections. Even though the effective area for
L(ns)= (nbgn+!ns)ne−(nbg+ns)(cid:89)n f(Xi,nbg|ns), (8) tehveenLtEseelevcetniotns,eltehcetisoennsiistisvmitayllteor tthheannutmhabterofotfhseigHnaEl
i
eventsforlow-massWIMPsisbetter,ascanbeseenin
where ns is the number of signal events and nbg is the Fig. 4. The reasons are the larger on-source region for
expected number of background events in the search the LE event selection, and a lower background event
windowwhichisgivenbythenumberofoff-sourceevents rate due to higher veto efficiency.
multiplied by the ratio of the on-source and off-source
regionsizes.Thedirectionalpdfconsequentlybecomes: In order to avoid confirmation bias throughout the
n n development of the analysis, blindness with respect to
f(X,n |n )= s f (X)+ bg f (X) (9)
bg s n +n s n +n bg theright-ascensioninformationwasimposedbyscram-
bg s bg s
blingtheright-ascensioninformationoftheexperimen-
All confidence intervals are constructed using the taldata.Onlythedeclinationinformationoftheevents
prescriptionbyFeldmanandCousins[40].Thesensitiv- was used for cut development.
ity is defined as the median upper limit on the number
of signal events at 90% confidence level. Following the optimization of the event selections,
The final limits for each WIMP mass and annihi- meaning all cuts are fixed, and the choice of the event
lation channel are obtained from the sample (LE or selectionforeachchannel,halo,andWIMPmass(based
HE)whichgivesthebestsensitivity(w/osystematics), onbestsensitivity),therightascensioninformationwas
i.e the lowest median upper limit, for the particular unblinded.
10
– Astrophysical uncertainties (choice of halo model,
-20 LE event selection model-specific parameter)
10
HE event selection
The former are incorporated into the calculated lim-
Combined Best Sensitivity
its, while the latter are studied to estimate model un-
χχ νν¯
] → certainties. Both classes are discussed in the following
1
s−10-21 sections.

3
m
c
[ 7.1 Detector Systematics

fi
v
A -22
σ10 The uncertainties in the signal efficiency are mainly
›
governed by the uncertainties in the optical efficiency
of the DOMs and the optical properties of the glacial
NFW
ice,manifestedintheabsorptionandscatteringlength.
10-23 Todeterminetheeffectson(cid:104)σAv(cid:105)duetothementioned
1 2 3 4 uncertainties,theeventselectionsandanalysiswereap-
10 10 10 10
m [GeV] pliedtosetsofsimulateddatawheretheopticalproper-
χ
tiesoftheDOMsandtheicewerechanged.Theoptical
Fig. 4 Median upper limits, i.e. sensitivities, at 90% C.L. efficiency of the DOMs was varied by ±10%. The same
(w/o systematics) for the two event selections assuming
was done for the absorption and scattering lengths of
WIMPannihilationtoνν¯fortheNFWDMprofile.Theblack
the ice. The resulting uncertainties on (cid:104)σ v(cid:105) generally
solidlineshowsthecombinedbestsensitivityforthispartic- A
ular annihilation channel and DM profile, considering both lie in the range 10% – 20% except for the lowest neu-
eventselections.At200GeVtheHEselectionyieldsaslightly trino energies where they reach up to ≈ 70%. This is
bettersensitivityand isthusused here.
due to threshold effects where events with just enough
hit DOMs to trigger the detector would fail to do so
with increased absorption and scattering or decreased
LE event selection
HE event selection optical efficiency.
10-2
Theabove-describedsystematicuncertaintiesarein-
] cluded into the limits by degrading the baseline results
2m by the relative variation of the detector uncertainties
[
a with respect to the baseline, as stated above.
e
ar10-4
e
v
ti 7.2 Astrophysical Uncertainties
c
e
Eff The astrophysical uncertainty is studied by using two
10-6 different halo profiles, and also by varying the parame-
tersρ andr withintheuncertaintiesstatedin[26],
local s
and summarized in Tab. 1. Figure 6 compares the sen-
101 102 103 104 sitivity for WIMPs annihilating to νν¯-pairs for both
E [GeV]
ν profiles. The bands depict the variation of the sensi-
tivity within each profile that arises from varying the
Fig. 5 Neutrino effective areas as a function of energy for
the two event selections. The effective areas for the LE and profile parameters within the given uncertainty.
HEselectionareshownassolid,anddottedlines,respectively. The relative variation of (cid:104)σ v(cid:105) due to the halo pro-
A
AlthoughtheHEeffectiveareaisbiggerthantheLEeffective
file parameter uncertainties was estimated to be 60% –
areaatlowenergies,thehigherbackgroundcontaminationat
100% for the LE likelihood analysis and 60% – 200%
low energiesintheHEselectionmakesit lessefficient.
for the HE likelihood analysis, and is shown in Fig. 6.
However,werefrainfromincludinguncertaintiesonthe
7 Discussion of Uncertainties halo profile parameters into the limits.
The uncertainties relevant for this analysis can be cat-
8 Results
egorized into two classes:
– Detectorsystematicuncertaintiesimpactingthesig- Table 2 shows the number of events for the two un-
nal efficiency blinded event selections. The quantities n and n
obs bg