Search for neutrinos from dark matter self-annihilations in the center of the Milky Way with 3 years PDF
Preview Search for neutrinos from dark matter self-annihilations in the center of the Milky Way with 3 years
UC Berkeley UC Berkeley Previously Published Works Title Search for neutrinos from dark matter self-annihilations in the center of the Milky Way with 3 years of IceCube/DeepCore: IceCube Collaboration Permalink https://escholarship.org/uc/item/6zm984t5 Journal European Physical Journal C, 77(9) ISSN 1434-6044 Authors Aartsen, MG Ackermann, M Adams, J et al. Publication Date 2017-09-01 DOI 10.1140/epjc/s10052-017-5213-y Peer reviewed eScholarship.org Powered by the California Digital Library University of California Eur.Phys.J.C(2017)77:627 DOI10.1140/epjc/s10052-017-5213-y RegularArticle-ExperimentalPhysics Search for neutrinos from dark matter self-annihilations in the center of the Milky Way with 3 years of IceCube/DeepCore IceCubeCollaboration M.G.Aartsen2,M.Ackermann52,J.Adams16,J.A.Aguilar12,M.Ahlers20,M.Ahrens44,I.AlSamarai25, D.Altmann24,K.Andeen33,T.Anderson49,I.Ansseau12,G.Anton24,C.Argüelles14,J.Auffenberg1,S.Axani14, H.Bagherpour16,X.Bai41,J.P.Barron23,S.W.Barwick27,V.Baum32,R.Bay8,J.J.Beatty18,19,J.BeckerTjus11, K.-H.Becker51,S.BenZvi43,D.Berley17,E.Bernardini52,D.Z.Besson28,G.Binder8,9,D.Bindig51,E.Blaufuss17, S.Blot52,C.Bohm44,M.Börner21,F.Bos11,D.Bose46,S.Böser32,O.Botner50,J.Bourbeau31,F.Bradascio52, J.Braun31,L.Brayeur13,M.Brenzke1,H.-P.Bretz52,S.Bron25,A.Burgman50,T.Carver25,J.Casey31, M.Casier13,E.Cheung17,D.Chirkin31,A.Christov25,K.Clark29,L.Classen36,S.Coenders35,G.H.Collin14, J.M.Conrad14,D.F.Cowen48,49,R.Cross43,M.Day31,J.P.A.M.deAndré22,C.DeClercq13,J.J.DeLaunay49, H.Dembinski37,S.DeRidder26,P.Desiati31,K.D.deVries13,G.deWasseige13,M.deWith10,T.DeYoung22, J.C.Díaz-Vélez31,V.diLorenzo32,H.Dujmovic46,J.P.Dumm44,M.Dunkman49,B.Eberhardt32,T.Ehrhardt32, B.Eichmann11,P.Eller49,P.A.Evenson37,S.Fahey31,A.R.Fazely7,J.Felde17,K.Filimonov8,C.Finley44, S.Flis44,A.Franckowiak52,E.Friedman17,T.Fuchs21,T.K.Gaisser37,J.Gallagher30,L.Gerhardt9, K.Ghorbani31,W.Giang23,T.Glauch1,T.Glüsenkamp24,A.Goldschmidt9,J.G.Gonzalez37,D.Grant23, Z.Griffith31,C.Haack1,A.Hallgren50,F.Halzen31,K.Hanson31,D.Hebecker10,D.Heereman12,K.Helbing51, R.Hellauer17,S.Hickford51,J.Hignight22,G.C.Hill2,K.D.Hoffman17,R.Hoffmann51,B.Hokanson-Fasig31, K.Hoshina31,b,F.Huang49,M.Huber35,K.Hultqvist44,S.In46,A.Ishihara15,E.Jacobi52,G.S.Japaridze5, M.Jeong46,K.Jero31,B.J.P.Jones4,P.Kalacynski1,W.Kang46,A.Kappes36,T.Karg52,A.Karle31,U.Katz24, M.Kauer31,A.Keivani49,J.L.Kelley31,A.Kheirandish31,J.Kim46,M.Kim15,T.Kintscher52,J.Kiryluk45, T.Kittler24,S.R.Klein8,9,G.Kohnen34,R.Koirala37,H.Kolanoski10,L.Köpke32,C.Kopper23,S.Kopper47, J.P.Koschinsky1,D.J.Koskinen20,M.Kowalski10,52,K.Krings35,M.Kroll11,G.Krückl32,J.Kunnen13, S.Kunwar52,N.Kurahashi40,T.Kuwabara15,A.Kyriacou2,M.Labare26,J.L.Lanfranchi49,M.J.Larson20, F.Lauber51,D.Lennarz22,M.Lesiak-Bzdak45,M.Leuermann1,Q.R.Liu31,L.Lu15,J.Lünemann13, W.Luszczak31,J.Madsen42,G.Maggi13,K.B.M.Mahn22,S.Mancina31,R.Maruyama38,K.Mase15,R.Maunu17, F.McNally31,K.Meagher12,M.Medici20,a,M.Meier21,T.Menne21,G.Merino31,T.Meures12,S.Miarecki8,9, J.Micallef22,G.Momenté32,T.Montaruli25,R.W.Moore23,M.Moulai14,R.Nahnhauer52,P.Nakarmi47, U.Naumann51,G.Neer22,H.Niederhausen45,S.C.Nowicki23,D.R.Nygren9,A.ObertackePollmann51, A.Olivas17,A.O’Murchadha12,T.Palczewski8,9,H.Pandya37,D.V.Pankova49,P.Peiffer32,J.A.Pepper47, C.PérezdelosHeros50,D.Pieloth21,E.Pinat12,M.Plum33,P.B.Price8,G.T.Przybylski9,C.Raab12,L.Rädel1, M.Rameez20,K.Rawlins3,R.Reimann1,B.Relethford40,M.Relich15,E.Resconi35,W.Rhode21,M.Richman40, B.Riedel23,S.Robertson2,M.Rongen1,C.Rott46,T.Ruhe21,D.Ryckbosch26,D.Rysewyk22,T.Sälzer1, S.E.SanchezHerrera23,A.Sandrock21,J.Sandroos32,S.Sarkar20,39,S.Sarkar23,K.Satalecka52,P.Schlunder21, T.Schmidt17,A.Schneider31,S.Schoenen1,S.Schöneberg11,L.Schumacher1,D.Seckel37,S.Seunarine42, D.Soldin51,M.Song17,G.M.Spiczak42,C.Spiering52,J.Stachurska52,T.Stanev37,A.Stasik52,J.Stettner1, A.Steuer32,T.Stezelberger9,R.G.Stokstad9,A.Stößl15,N.L.Strotjohann52,G.W.Sullivan17,M.Sutherland18, I.Taboada6,J.Tatar8,9,F.Tenholt11,S.Ter-Antonyan7,A.Terliuk52,G.Tešic´49,S.Tilav37,P.A.Toale47, M.N.Tobin31,S.Toscano13,D.Tosi31,M.Tselengidou24,C.F.Tung6,A.Turcati35,C.F.Turley49,B.Ty31, E.Unger50,M.Usner52,J.Vandenbroucke31,W.VanDriessche26,N.vanEijndhoven13,S.Vanheule26, J.vanSanten52,M.Vehring1,E.Vogel1,M.Vraeghe26,C.Walck44,A.Wallace2,M.Wallraff1,F.D.Wandler23, N.Wandkowsky31,A.Waza1,C.Weaver23,M.J.Weiss49,C.Wendt31,S.Westerhoff31,B.J.Whelan2, S.Wickmann1,K.Wiebe32,C.H.Wiebusch1,L.Wille31,D.R.Williams47,L.Wills40,M.Wolf31,J.Wood31, T.R.Wood23,E.Woolsey23,K.Woschnagg8,D.L.Xu31,X.W.Xu7,Y.Xu45,J.P.Yanez23,G.Yodh27,S.Yoshida15, T.Yuan31,M.Zoll44 123 627 Page 2 of 11 Eur.Phys.J.C(2017)77 :627 1III.PhysikalischesInstitut,RWTHAachenUniversity,52056Aachen,Germany 2DepartmentofPhysics,UniversityofAdelaide,Adelaide5005,Australia 3DepartmentofPhysicsandAstronomy,UniversityofAlaskaAnchorage,3211ProvidenceDr.,Anchorage,AK99508,USA 4DepartmentofPhysics,UniversityofTexasatArlington,502YatesSt.,ScienceHallRm108,Box19059,Arlington,TX76019,USA 5CTSPS,Clark-AtlantaUniversity,Atlanta,GA30314,USA 6SchoolofPhysicsandCenterforRelativisticAstrophysics,GeorgiaInstituteofTechnology,Atlanta,GAGA30332,USA 7DepartmentofPhysics,SouthernUniversity,BatonRouge,LA70813,USA 8DepartmentofPhysics,UniversityofCalifornia,Berkeley,CA94720,USA 9LawrenceBerkeleyNationalLaboratory,Berkeley,CA94720,USA 10InstitutfürPhysik,Humboldt-UniversitätzuBerlin,12489Berlin,Germany 11FakultätfürPhysik&Astronomie,Ruhr-UniversitätBochum,44780Bochum,Germany 12UniversitéLibredeBruxelles,ScienceFacultyCP230,1050Brussels,Belgium 13VrijeUniversiteitBrussel(VUB),DienstELEM,1050Brussels,Belgium 14DepartmentofPhysics,MassachusettsInstituteofTechnology,Cambridge,MA02139,USA 15Department ofPhysicsandInstituteforGlobalProminentResearch,ChibaUniversity,Chiba263-8522,Japan 16DepartmentofPhysicsandAstronomy,UniversityofCanterbury,PrivateBag4800,Christchurch,NewZealand 17DepartmentofPhysics,UniversityofMaryland,CollegePark,MD20742,USA 18DepartmentofPhysicsandCenterforCosmologyandAstro-ParticlePhysics,OhioStateUniversity,Columbus,OH43210,USA 19DepartmentofAstronomy,OhioStateUniversity,Columbus,OH43210,USA 20NielsBohrInstitute,UniversityofCopenhagen,2100Copenhagen,Denmark 21DepartmentofPhysics,TUDortmundUniversity,44221Dortmund,Germany 22DepartmentofPhysicsandAstronomy,MichiganStateUniversity,EastLansing,MI48824,USA 23DepartmentofPhysics,UniversityofAlberta,Edmonton,ABT6G2E1,Canada 24ErlangenCentreforAstroparticlePhysics,Friedrich-Alexander-UniversitätErlangen-Nürnberg,91058Erlangen,Germany 25Départementdephysiquenucléaireetcorpusculaire,UniversitédeGenève,1211Geneva,Switzerland 26DepartmentofPhysicsandAstronomy,UniversityofGent,9000Gent,Belgium 27DepartmentofPhysicsandAstronomy,UniversityofCalifornia,Irvine,CA92697,USA 28DepartmentofPhysicsandAstronomy,UniversityofKansas,Lawrence,KS66045,USA 29SNOLAB,1039RegionalRoad24,CreightonMine9,Lively,ONP3Y1N2,Canada 30DepartmentofAstronomy,UniversityofWisconsin,Madison,WI53706,USA 31DepartmentofPhysicsandWisconsinIceCubeParticleAstrophysicsCenter,UniversityofWisconsin,Madison,WI53706,USA 32InstituteofPhysics,UniversityofMainz,StaudingerWeg7,55099Mainz,Germany 33DepartmentofPhysics,MarquetteUniversity,Milwaukee,WI53201,USA 34UniversitédeMons,7000Mons,Belgium 35Physik-Department,TechnischeUniversitätMünchen,85748Garching,Germany 36InstitutfürKernphysik,WestfälischeWilhelms-UniversitätMünster,48149Münster,Germany 37BartolResearchInstituteandDepartmentofPhysicsandAstronomy,UniversityofDelaware,Newark,DE19716,USA 38DepartmentofPhysics,YaleUniversity,NewHaven,CT06520,USA 39DepartmentofPhysics,UniversityofOxford,1KebleRoad,OxfordOX13NP,UK 40DepartmentofPhysics,DrexelUniversity,3141ChestnutStreet,Philadelphia,PA19104,USA 41PhysicsDepartment,SouthDakotaSchoolofMinesandTechnology,RapidCity,SD57701,USA 42DepartmentofPhysics,UniversityofWisconsin,RiverFalls,WI54022,USA 43DepartmentofPhysicsandAstronomy,UniversityofRochester,Rochester,NY14627,USA 44OskarKleinCentreandDepartmentofPhysics,StockholmUniversity,10691Stockholm,Sweden 45DepartmentofPhysicsandAstronomy,StonyBrookUniversity,StonyBrook,NY11794-3800,USA 46DepartmentofPhysics,SungkyunkwanUniversity,Suwon440-746,Korea 47DepartmentofPhysicsandAstronomy,UniversityofAlabama,Tuscaloosa,AL35487,USA 48DepartmentofAstronomyandAstrophysics,PennsylvaniaStateUniversity,UniversityPark,PA16802,USA 49DepartmentofPhysics,PennsylvaniaStateUniversity,UniversityPark,PA16802,USA 50DepartmentofPhysicsandAstronomy,UppsalaUniversity,Box516,75120Uppsala,Sweden 51DepartmentofPhysics,UniversityofWuppertal,42119Wuppertal,Germany 52DESY,15735Zeuthen,Germany Received:23May2017/Accepted:9September2017/Publishedonline:20September2017 ©TheAuthor(s)2017.Thisarticleisanopenaccesspublication 123 Eur.Phys.J.C(2017)77 :627 Page 3 of 11 627 Abstract We present a search for a neutrino signal from directionaldistributionofdarkmatterintheMilkyWay.The darkmatterself-annihilationsintheMilkyWayusingtheIce- background is uniform in right ascension and is estimated CubeNeutrinoObservatory(IceCube).In1005daysofdata fromexperimentaldata.Ashapelikelihoodanalysisonthe we found no significant excess of neutrinos over the back- reconstructedneutrinodirectionisusedtoestimatethefrac- ground of neutrinos produced in atmospheric air showers tionofeventspossiblyoriginatingfromthetargetedsignal. fromcosmicrayinteractions.Wederiveupperlimitsonthe From the signal fraction a limit on the signal flux is calcu- velocityaveragedproductofthedarkmatterself-annihilation latedandthecorrespondingvalueof(cid:2)σ v(cid:3)canbedetermined A crosssectionandtherelativevelocityofthedarkmatterpar- foranycombinationofWIMPmassandWIMPannihilation ticles(cid:2)σ v(cid:3).Upperlimitsaresetfordarkmatterparticlecan- channeltoneutrinos. A didatemassesrangingfrom10GeVupto1TeVwhilecon- This search focuses on charged-current muon neutrinos sidering annihilation through multiple channels. This work because their directions can be accurately reconstructed. setsthemoststringentlimitonaneutrinosignalfromdark However, other neutrino flavors and events from neutral- matter with mass between 10 and 100GeV, with a limit of currentneutrinointeractionarealsopresentinthefinalselec- 1.18·10−23cm3s−1for100GeVdarkmatterparticlesself- tion(ensuringthemostinclusivelimits). annihilatingviaτ+τ− toneutrinos(assumingtheNavarro– Frenk–Whitedarkmatterhaloprofile). 2 IceCubeNeutrinoObservatory 1 Introduction IceCubedetectsCherenkovlightfromchargedparticlesmov- ingthroughonecubickilometerofverytransparenticeunder- With the increasingly strong indications of the existence neaththeSouthPole[18,19].Thearrayconsistsof78verti- of extended halos of dark matter surrounding galaxies and calstringsinahexagonalgridwith60digitalopticalmod- galaxyclusters[1],thereismuchinterestwithintheparticle ules(DOMs)[20]spacedevenly oneachstringevery17m physicscommunitytodeterminethenatureandpropertiesof between 1450 and 2450m below the surface. The spacing darkmatter[2].Thefrequentlyconsideredhypothesisisthat between these nominal strings is approximately 125m (as dark matter consists of stable massive particles interacting shown by the black dots in Fig. 1). In addition there are feebly with Standard Model particles. The density of dark eightstringsinthecentralarea(reddotsinFig.1)withthe matterparticlestodayisdeterminedbythe‘freeze-out’[3– DOMsmoredenselyspacedconstitutingtheinfillIceCube/ 6] in the early universe when the thermal equilibrium can DeepCore[21]. no longer be sustained as the universe expands and cools ThefiducialvolumeusedinthisworkisdefinedbyDOMs down.Thisworkfocusesonagenericcandidateparticlefor located2140–2420mbelowthesurfacesituatedonthemost darkmatterreferredtoasaweaklyinteractingmassivepar- centralstrings(indicatedwithasolidblueregioninFig.1). ticle (WIMP) [7–10], though this search is sensitive to any TherestofIceCubeisusedasavetovolumetorejectincom- self-annihilatingdarkmatterparticlewithacouplingtothe ingandthrough-goingatmosphericmuons. StandardModelresultinginafluxofneutrinos.Thesource The strings outside the DeepCore sub-detector volume consideredistheMilkyWaygalaxy,whichisembeddedina (indicatedwithabluelineinFig.1)areonlyusedintheinitial sphericalhaloofdarkmatter[11–15].Foragivenhaloden- filteringoftriggereddata,andarechosentobeshieldedby sityprofile,thetotalamountofdarkmatterinthelineofsight threerowsofDOMsfromtheedgeofthearray. fromEarthcanbedetermined[16]. IfWIMPscanself-annihilateintoStandardModelparti- clesandthedarkmatterdensityissufficientlyhigh,anexcess 3 Signalexpectation ofneutrinosandphotonsshouldbeobservedfrompartsofthe skywithalargeamountofdarkmatter,abovethebackground ForWIMPsself-annihilatingtovariousStandardModelpar- ofmuonsandneutrinosproducedintheEarth’satmosphere. ticles(leptons,quarks,orbosons),thedecaychainofthepar- Althoughphotonsproducedinsuchannihilationsarefareas- ticleswillultimatelyproduceleptonsandphotons.Depend- iertodetect,itisstillofinteresttoconsiderscenarioswhere ing on the WIMP mass (mDM) and annihilation channel, a onlyneutrinosareproduced[17]. number of neutrinos will be produced in the decay chain, The targeted neutrino signal is estimated from a dataset propagatetoEarth,andcanbedetectedinneutrinoobserva- of simulated neutrino events reweighted to the energy and tories. Using PYTHIA [22,23], a generic resonance with twice theWIMPmassisforcedtodecaythroughoneoftheparticle ae-mail:[email protected] bEarthquakeResearchInstitute,UniversityofTokyo,Bunkyo,Tokyo pairs(annihilationchannels)consideredandtheenergyspec- 113-0032,Japan. traoftheresultingneutrinosarerecordedforallthreeneu- 123 627 Page 4 of 11 Eur.Phys.J.C(2017)77 :627 Fig. 2 EnergyspectrumofmuonneutrinosatEarthproducedinthe annihilationandsubsequentdecayofvariousStandardModelparticles createdintheannihilationofa100GeVWIMP.Thelinespectrumof theνν¯-channelismodeledbyaGaussianwithawidthof5%ofmDM sity along the line of sight (los) through the dark matter halo in the Milky Way. Although there remain uncertain- ties about the dark matter density profile [25], a spherical profileisassumedwithoneoftwostandardradialdistribu- Fig. 1 ThehorizontalpositionofthedeployedstringsintheIceCube tions: Navarro–Frenk–White (NFW) [13] and Burkert [14] coordinate system. The blue line shows the strings constituting the withparametervaluesfrom[26].Theresultingrateofdark DeepCore subdetector, strings outside of this region are used in the matter self-annihilations along the line of sight is strongly initialevent rejection.Thefiducial volume used inthefinal analysis dependentontheassumedhalodensity,withthelargestdis- isindicatedwiththesolidblueregionconsistingofbothnominaland densestrings crepanciesnearthecenteroftheMilkywaywheretheden- sityislargest.Becauseofthelargeuncertaintyonthemodel parametersthedarkmatterhalomodelconstitutesthelargest trinoflavors.ThisworkconsidersWIMPswithmassesfrom systematicuncertainty. 10 to 1000GeV self-annihilating through either b-quarks The resulting differential flux of signal neutrinos pro- (bb¯),W-bosons(W+W−),muons(μ+μ−),ortaus(τ+τ−) duced by WIMP self-annihilation in the dark matter halo to neutrinos. Annihilation directly to neutrinos (νν¯) is also oftheMilkyWayfromasolidangleofthesky,(cid:5)(cid:6),isgiven considered.InFig.2theenergyspectrum,dN/dE,ofmuon as neutrinosfromapairof100GeVWIMPsispresentedforthe (cid:2) annihilationchannelsconsideredinthisanalysis.Theenergy d(cid:7)((cid:5)(cid:6))= (cid:2)σAv(cid:3) dN ρ2(r(l,(cid:5)(cid:6)))dl, (1) spectrumisshownafterapplyinglongbaselineoscillations dE 4π ·2m2 dE DM los (determinedfromparametersin[24]). For the W+W−-channel only WIMP masses above the wherethe4π arisesfromasphericallysymmetricannihila- mass of the W boson are probed. The energy spectrum of tion,l isthelineofsightthroughthedarkmatterhalowith the νν¯-channel is dominated by the line at m , which is densityprofileρ(r)asafunctionofradiusr,andthefactor DM modeledwithaGaussiandistributionwithawidthof5%of of1/2andthesquaredWIMPmassandhalodensityprofile m .Thiswidthprovidesthepossibilitytousethesamesim- arise from the fact that two WIMPs are needed in order to DM ulateddataset,whilestillbeingconsistentwithalinespec- annihilate. trumaftersmearingbytheeventreconstruction.Forthesig- A sample of neutrino events of each flavor is generated nalfromtheνν¯-channelaflavorratioproducedatthesource with energies between 1 and 1000GeV using GENIE [27] of(νe :νμ :ντ)=(1:1:1)isused(thoughthemostcon- and weighted to the targeted flux of Eq. (1) according to servativelimitsarefoundforaflavorratioof(1 : 0 : 0)at theirflavor,energy,andarrivaldirectionforeachcombina- sourceresultingin10–15%weakerlimits).Theresultswill tionofm ,annihilationchannelanddarkmatterhaloden- DM bepresentedwitha100%branchingratioforeachannihila- sityprofile.Thisneutrinosampleprovidesthedistributionof tionchannelconsidered. thetargetedsignalthatisusedintheshapelikelihoodanaly- TherateofWIMPself-annihilationseeninagivensolid sistodeterminethefractionofpossiblesignaleventsinthe angle is determined from the integrated dark matter den- experimentaldata. 123 Eur.Phys.J.C(2017)77 :627 Page 5 of 11 627 4 Backgroundestimation bution,whichisnotnecessarilythecaseforgalacticneutri- nos.Butattheenergiesconsidered,bothareexpectedtobe Thebackgroundconsistsofneutrinoswithotherastrophys- morethanthreeordersofmagnitudebelowthebackground icalorigin,atmosphericneutrinos,andatmosphericmuons. ofatmosphericneutrinos. At the energies considered, the event sample is dominated byatmosphericneutrinosandmuonsproducedincosmicray inducedairshowers.Thecosmicrayfluxisisotropicinright 5 Eventselection ascension,sotheatmosphericbackgroundcanbeestimated fromexperimentaldatabyrandomizingthearrivaltimesof The event selection was optimized for the signal of muon eachevent.SinceIceCubehasauniformexposurethiscor- neutrinos from 100GeV WIMPs self-annihilating through responds to randomizing the right ascension values, which + − the W W -channel (benchmark channel) and is applied hasshowninapreviousanalysistobeanunbiasedapproach eventwiseontheexperimentaldataandthesimulatedevent toestimatethebackground[28]. samples.Theaimistoselecthighqualityneutrinoinduced The largest expected background contribution is from muons,signifiedbyelongated eventtopologies(referredto down-goingatmosphericmuons.ThisisbecauseIceCubeis astracks)startinginsideIceCube/DeepCore. locatedattheSouthPole,sothecenteroftheMilkyWay(cor- The neutrino induced muons need to be distinguished respondingtothedirectionwiththestrongestsignal)willbe fromthemuonsproducedintheatmosphere.Allatmospheric abovethehorizon,wheretherewillalsobethehighestrate muons detected in IceCube penetrate through the veto vol- from atmospheric muons. Therefore the goal of the initial ume.Thecorrespondinghits(reconstructedpulsesfromone event selection is to reduce the rate of atmospheric muons. ormoredetectedphotons)canthereforebeusedtoidentify Theoverallanalysisisverifiedusingasimulationofatmo- andremovethesethrough-goingtracks. sphericmuonsgeneratedwith CORSIKA[29]comparedto The event selection is a multi-step background rejection theexperimental data.Therateofsimulatedbackground is procedure that reduces the atmospheric muons by seven within5%oftheexperimentaldata(seeTable1). ordersofmagnitude. The other significant background contribution is atmo- ThefirststepistocleantheDOMhitstoremovenoiseso sphericneutrinos.TheyarriveatIceCubefromalldirections thattheprecisionofthereconstructionisnotdegraded.Next, and cannot be distinguished from extraterrestrial neutrinos eventswithmorethanonehitinthevolumeoutsidetheDeep- event-by-event. However, from the full statisticalensemble Core sub-detector volume causally connected to a charge the distributions can be distinguished by their energy and weightedcenterofgravityinthefiducialvolumewithinapre- arrival direction. Simulated GENIE neutrino datasets are definedtimewindowanddistanceareremoved.Thisfilters usedforestimatingthefractionofatmosphericneutrinosin outatmosphericmuonswithverybasiceventinformation. thefinalselectionoftheexperimentaldata,usingtheatmo- Byrequiringmorethantenhitsdistributedonatleastfour sphericneutrinofluxmodeldescribedin[30].Thesimulated stringsnearlyallnoise-onlyeventsareremoved.Inaddition, atmosphericneutrinosdonotimpacttheresult,asthecom- this requirement ensures that the events can be well recon- binedbackgroundisestimatedfromexperimentaldata. structed. The three first hits in the event are required to be The extra-galactic neutrino background can be distin- in the fiducial volume, as that is more likely to indicate a guishedfromtheWIMPneutrinosignalbythearrivaldistri- startingeventandthusreducetherateofpenetratingatmo- Table1 Eventratesforthevariouscomponentsexpectedintheexper- mentaldataisbasedonsimulation.Theatmosphericmuonsratesare imentaldatagiveninmHz,andthesignalneutrinosarepresentedas basedontheGaisserH3aenergyspectrum[34].Theatmosphericneu- percentageoftheeventsatfilteredlevelforthebenchmarksignal(anni- trinosratesarebasedonneutrinooscillationparametersin[35].Dueto hilationofa100GeVWIMPtoW+W−).Everythingbuttheexperi- vanishingratesathigherlevelstherateofatmosphericντ arenotlisted Dataset DeepCorefilteredtriggerdata Qualitycuts Atm.bkgd.rejection Pre-BDTlinearcuts BDT Experimentaldata ∼15×103 655.0 36.73 3.59 0.27 Atmos.μ(H3a) ∼9.5×103 656.9 37.88 3.53 0.19 Atmos.νμ 6.49 2.14 0.319 0.199 0.07 Atmos.νe 2.06 0.43 0.043 0.027 0.01 Noise-onlyevents ∼6.6×103 0.1 0 0 0 Signalνμ 100% 70.48% 14.67% 9.29% 6.20% Signalνe 100% 81.31% 10.94% 6.94% 4.96% Signalντ 100% 80.61% 10.63% 7.29% 5.88% 123 627 Page 6 of 11 Eur.Phys.J.C(2017)77 :627 spheric muons. The events are reconstructed to preliminar- ilyestimatethedirectionandinteractionpointofthecandi- dateneutrino-inducedmuon.Theeventswithapreliminary zenithangleforthearrivaldirectionofzen > zen +20◦ GC or zen < zen −10◦ are rejected, where zen denotes GC GC the zenith of the Galactic center. The cut is asymmetric because the atmospheric muon background is increasingly larger towards a zenith of zero (i.e. the southern celestial pole). A containment cut is used to keep only events that haveareconstructedinteractionvertexwithinacylinderwith a radius corresponding to the analysis volume depicted on Fig.1.Inadditioncutsareappliedontrackquality[31]. Fig. 3 Resolutionoftheazimuthalandzenithdirectionofνμ inthe Byconsideringthehitsinthevetovolumethatarecleaned eventsample,shownasafunctionofenergy,comparedtothekinematic away(aspossiblenoise),clustersaredeterminedforhitsthat openingangle are within 250m and 1000ns from each other and are reg- istered earlier than the first quantile of cleaned hits. These clustersarerequiredtohavefewerthanthreehits,aslarger reconstructing the event with a finite track (expected from clusters are generally observed more often for penetrating a neutrino induced starting muon) compared to an infinite atmosphericmuons. track (expected for a through-going atmospheric muon) is ◦ Aconewitha20 openingangleaimedtowardsthearrival used.Anadditionalvetotechniquetracesbackinthedirec- directionisusedtocheckforhitsintheuncleanedhitseries tionofarrivalfromtheinteractionvertextolookforcharge within 1µs of the interaction. At most one hit is allowed, on DOMs that would identify the event as a through-going sinceeventsstartingwithinthefiducialvolumeshouldhave muonmisidentifiedasastartingevent.Boththenumberof zero hits within the cone, but one accidental noise hit is hitsandthetotalchargeidentifiedbythevetoareusedinthe allowed.Duetothehighrateofatmosphericmuonsversus BDT. possiblesignalneutrinos,thereisaclassofbackgroundmuon The events are selected based on the BDT score, opti- eventswheresparsehitsinthevetovolumeareremoveddur- mized for the best sensitivity to the benchmark signal of a + − ingthehitcleaning.Theuncleanedhitsinacylinderwitha 100GeVWIMPannihilatingthroughW W .Thesamecut radius of 250m pointed towards the arrival direction start- valueisusedacrossmultipleWIMPmassesandannihilation ing behind the interaction vertex, are used to calculate the channels. likelihood value for the reconstructed track. A high likeli- Themedianresolutioninazimuthalangleispresentedin hoodvalueindicatesthatthetrackprobablyoriginatedfrom Fig. 3 as a function of true neutrino energy. Because the apenetratingmuon,forwhichthehitsdepositedintheveto azimuthalanglemapsdirectlytorightascension,itprovides volumeareerroneouslycleanedaway. thedominatingseparationbetweensignalandbackground.A Attheenergiesconsideredinthisanalysis,thereconstruc- comparisonofthreecombinationsofWIMPmassandanni- tionmusttakeintoaccountboththehadroniccascadeandthe hilation channel is presented in Fig. 4, illustrating a better muonproducedinatypicalmuonneutrinochargedcurrent resolution for cases where the neutrino spectrum continues interaction. With the experimental data event rate reduced tohigherenergies. by six orders of magnitude from 2kHz to 3.7mHz by the Thefinaleventselectionresultsinadatarateof0.27mHz, cuts described, a more specific event reconstruction can be correspondingtoareductionby7ordersofmagnitudefrom run. This low energy specialized event reconstruction fits the initial triggering of the data, while retaining 6% of the all relevant parameters (direction, interaction vertex, muon benchmark signal of muon neutrinos. No cuts have been track length, and hadronic cascade energy) simultaneously incorporatedtoexplicitlyremovenon-muonneutrinoflavors. andtakesintoaccountbothDOMsthatdidanddidnotdetect Inthefinaleventsamplethenon-muonneutrinosofthetar- any light. In order to thoroughly sample the complex like- getedsignalarepresentwithacombinedratecomparableto lihood space of the full 8-dimensional parameter space the thatofmuonneutrinos.Usingthe GENIEneutrinosimula- BayesiansamplinginferencetoolMultiNest[32]isused. tionweightedtotheatmosphericfluxmodel,itisestimated Thefinalstepoftheeventselectionisamultivariateanaly- thatatmosphericneutrinosconstituteonequarterofthefinal sisusingaBoostedDecisionTree(BDT)[33].Firstofall,the experimental data. A summary of the event selection rates directionandvertexinformationfromthespecialisedevent andsignalefficiencyisgiveninTable1. reconstructionareusedalongwiththenumberofhitsina10 In Fig. 5 the effective area at the final level is presented degreeopeninganglevetocone,updatedwiththespecialised for the individual neutrino flavors combining both neutral- eventreconstruction.Further,thedifferenceinlikelihoodin andcharged-currentneutrinointeractions. 123 Eur.Phys.J.C(2017)77 :627 Page 7 of 11 627 Fig. 4 Cumulativedistributionoftheresolutionoftheazimuthaldirec- tion of νμ in the final event sample, for various WIMP masses and annihilationchannels Fig. 6 Eventdistributioninrightascension(RA)relativetothegalactic center(GC)ofdata,scrambledsignal,andtargetedsignalfora100GeV WIMPannihilationtoneutrinosthroughtheW+W−-channel(shown forasingledeclinationbin) the estimated background distribution which is constructed fromtheexperimentaldata. TheexperimentaldatascrambledinRA(assignedaran- dom RA value for each event) consist of a component of scrambledbackgroundandpotentialsignal(alsoscrambled): PDF =(1−μ)PDF +μPDF , (2) scr.data scr.bkg scr.sig where μ ∈ [0,1] parametrizes the fraction of signal in the totalsample. FromEq.2thebackground PDF canbeestimatedfrom theexperimentaldata(bysubtractingthescrambledsignal) underthehypothesisthatthebackgroundisuniforminRA Fig. 5 Effectiveareaoffinaleventsampleforthethreeneutrinoflavors andhenceinvariantunderscrambling. withbothcharged-andneutral-currentinteractionscombined The total fraction of events within a specific bin i ∈ [bin ,bin ]iscalculatedasafunctionofthesignalfrac- min max tionas 6 Analysismethod f(i|μ)=μPDF (i)+(1−μ)PDF (i). (3) sig scr.bkg. Thefinaleventsampleisfilledinto2Dhistogramswithbins covering the range [0,2π] rad in right ascension (RA) and InFig.6anexampleoftherelevantPDFsispresentedover [−1,1]radindeclination(Dec)usingthereconstructedval- thefullrangeinrightascensionforasinglebinindeclination uesfromthespecialisedeventreconstruction.Thebinwidth (dec∈[−1/3,−2/3])wherethelargestdifferencebetween is chosen to be 0.4 and 0.63 radians for RA and declina- signalandbackgroundisexpected.Sincethebackgroundis tion,respectively,basedontheresolutionoftheeventrecon- uniform in right ascension and the signal is peaked around struction. In order to ensure a consistent analysis the same the position of the center of the Milky Way, it is in right binwidthischosenforthecombinationofWIMPmassand ascensionthatthedifferencebetweensignalandbackground annihilation channel that exhibits the worst resolution. The can be found. Figure 6 also illustrates the difference in the 2Ddistributionsconstitutetheprobabilitydensityfunctions targetedsignalbetweentheNFWandBurkertmodelsofthe (PDFs)usedintheshapelikelihoodanalysisdescribedbelow. darkmatterhalodensityprofile. The shape of the 2D distribution of experimental data pro- Witha2Dbinnedshapelikelihoodanalysis,thedataPDF duces the data PDF which is compared to the expectation iscomparedtotheexpectationfromthebackgroundPDFand fromtheweightedsignaldistributions(orsignalPDF)and the signal PDF, for multiple combinations of WIMP mass, 123 627 Page 8 of 11 Eur.Phys.J.C(2017)77 :627 annihilation channel, and halo profile. This way the most The optical properties of the ice in IceCube have been probablesignalfractionisdeterminedfromtheexperimental modelledandshowanabsorptionandscatteringlengththat data.Thelikelihoodiscalculatedbycomparingthenumber varywithdepth,generallybecomingmoreclearinthedeeper ofobservedeventsintheindividualbinsn (i),assuminga regions of IceCube. For the experimental data there will obs Poissonuncertaintyonthenumberofeventsexpected,deter- always be a discrepancy between the ice the photons are minedfromthetotalnumberofeventsfilledinthehistogram propagatingthrough,andtheice[37]assumedintherecon- ntotal and f(i|μ)calculatedinEq.3.Thisresultsinthefol- struction (as the complicated structure of the real ice can obs lowingformulationofthelikelihoodfunctionL(μ): not be perfectly modeled). This is also the case in simula- tion, where the latest iteration of the ice model is used in bi(cid:3)nmax (cid:4) (cid:5) (cid:6) the Monte Carlo event simulation, but because of its com- L(μ)= Poisson nobs(i)(cid:5)ntoobtsalf(i|μ) . (4) plexity, cannot currently be used for reconstruction. While i=binmin estimatingtheimpactofusingadifferenticemodelforevent reconstructionthanusedinthephotonpropagationsimula- Usingthelikelihoodanalysis,thebestestimateofthesig- tion,itadditionallyaccounts forthefactthattheicemodel nal fraction can be found by minimizing −logL, and if it in simulation is different from that used in simulation. The isconsistentwithzerothe90%confidenceintervalisdeter- effect is calculated using a variant Monte Carlo simulation minedapplyingtheFeldman-Cousinsapproach[36]toesti- with a different ice model used for the photon propagation matetheupperlimitonthesignalfractionμ90%.Usingthe (thesameasusedintheeventreconstruction).Thisresultsin simulatedsignalneutrinosthesignalfractioncanberelated a5–15%(dependingonWIMPmass,10%forthebenchmark to(cid:2)σAv(cid:3).Theexpectedlimiton(cid:2)σAv(cid:3)intheabsenceofsignal channel)improvementinsensitivityon(cid:2)σAv(cid:3),comparedto iscalculatedfrom10,000pseudoexperimentssampledfrom thebaselinesimulation. thebackground-onlyPDF,fromwhichthemedianvalueof The ice in the drill hole columns has different optical theresulting90%upperlimitsisquotedasthesensitivity. propertiesfromthebulkice.Thescatteringlengthisgreatly reducedduetothepresenceofimpurities.Oneeffectofthis column is to increase the detection probability for down- 7 Systematicuncertainties going photons. Since the DOMs are facing downwards, no down-goingphotonswouldbeobservedwithoutscattering. Thestatisticaluncertaintyduetothelimitednumberofevents Thecolumniceistreatedashavingamuchshortergeo- inthesimulateddatasetsisinsignificantcomparedtothesys- metrical scattering length: 50cm as a baseline [37], imple- tematicuncertainties,asthesimulationholds20timesmore mented in simulation as photons approach the DOMs. The eventsthanintheexperimentaldata,aftercuts.However,all uncertaintyonthescatteringlengthiscoveredbyincluding systematicuncertaintiesareeffectivelynegligiblecompared variationsof30and100cm.Thisvariationresultsina25– totheastrophysicaluncertaintiesassociatedwiththeparam- 30%reductionor5–10%improvementofthesensitivityon etersofthedarkmatterhalomodels. (cid:2)σ v(cid:3) respectively (depending on WIMP mass, 25 and 8% A Thebiggestsystematicuncertaintyarisesfromthemod- forthebenchmarkchannel). ellingoftheicepropertiesandtheuncertaintyontheoptical The photon detection efficiency of the DOMs (combin- efficiencyoftheDOMs,whichincreasewithlowerneutrino ingtheeffectofthequantumefficiencyofthePMT,photon energies,andthereforeforlowerWIMPmasses.Thepreci- absorption by the cables in the ice, and other subdominant sionofthedetectorgeometryandtimingaresohighthatthe hardwareelements)isdeterminedto10%accuracy.Increas- associatedsystematicuncertaintyisnegligibleandtherefore ingordecreasingtheDOMefficiencyinthesimulationcor- notincludedinthisstudy. responds to a 5–40% (depending on WIMP mass, 15% for Theeffectofexperimentalsystematicuncertaintiesonthe thebenchmarkchannel)effectthatsymmetricallyimproves finalsensitivityisestimatedusingMonteCarlosimulations orreducesthesensitivityon(cid:2)σ v(cid:3). A of neutrinos with uncertainty values varied by ±1σ from The systematic uncertainties are considered to be inde- the values used in the baseline sets. Each of the datasets pendentandthe±variationthatresultsinthelargestuncer- withvariationsisrunthroughtheeventselectionandanaly- taintyforeachsystematicuncertaintyisaddedinquadrature sis,providingadifferentvalueforthesensitivityon(cid:2)σ v(cid:3). toformthetotalsystematicuncertainty.Theseareincluded A Thedifferencebetweenthebaselineandthevariationwillbe inthefinalresultbyscalingupthelimitswiththetotalsys- quoted as the systematic uncertainty on (cid:2)σ v(cid:3), for each of tematicuncertainty. A thevariations.Thesystematicuncertaintiesaredependenton Thedominanttheoreticalsystematicuncertaintyisrelated theneutrinoenergy,andhenceonthetargetedWIMPmass. tofittedparametersofthedarkmatterhaloprofiles.Consid- Since the background is estimated from experimental data, eringthe1σ variationonbothparametersfortheindividual thevariationsareappliedtothesignalsimulationonly. modelsresultina150–200%uncertaintyonthesensitivityon 123 Eur.Phys.J.C(2017)77 :627 Page 9 of 11 627 Fig. 7 The final limits without systematic uncertainties (solid line), annihilatingthroughtheW+W−channeltoneutrinosassumingaNFW compared to the sensitivity (dashed line). Showing the 1σ (green (Burkert)haloprofileontheleft(right)plot band)and2σ (yellowband)statisticaluncertaintyfordarkmatterself- (cid:2)σAv(cid:3).Sincethiseffectistheory-dependent,andmaychange Table2 Upperlimitsontheself-annihilationcrosssectionassuming as dark matter halo models evolve, it is not included in the theNFWhaloprofile totalsystematicuncertainty.Instead,theresultsarepresented mdm (cid:2)σAv(cid:3)[10−23cm3s−1]forNFWprofile forbothdarkmatterhalomodels. (GeV) bb¯ W+W− μ+μ− τ+τ− νν¯ 10 53.4·103 – 25.1 33.4 1.46 20 269 – 3.43 4.25 0.40 8 Results 30 89.1 – 1.75 2.10 0.32 40 56.9 – 1.39 1.69 0.33 Afterthefinaleventselection,22,632eventswereobserved 50 38.7 – 1.22 1.46 0.25 in 1005 days of IceCube data. The data are presented in 100 20.6 3.29 1.03 1.18 0.42 Fig. 6 illustrating that the data are compatible with the 200 16.2 4.49 1.44 1.53 0.87 background-onlyhypothesis.Sincenosignificantexcesshas 300 15.7 5.89 2.13 2.18 1.86 beenobserved,anupperlimiton(cid:2)σ v(cid:3)isdetermined.Fig- A 400 16.4 7.28 2.94 2.84 2.88 ure 7 shows the 90% confidence upper limits (solid black + − 500 17.3 8.40 3.71 3.37 4.38 line) for the W W -annihilation channel for the two dark 1000 22.8 14.7 9.57 7.66 26.2 matterhaloprofiles.Thecoloredbandsrepresenttherange of expected outcomes of this measurement with no signal present. The result is very near the median sensitivity, and Table3 Upperlimitsontheself-annihilationcrosssectionassuming thuscompatiblewiththebackground-onlyhypothesis,which theBurkerthaloprofile isthecaseacrossallannihilationchannels. mdm (cid:2)σAv(cid:3)[10−23cm3s−1]forBurkertprofile Tables2and3showthefinalupperlimitson(cid:2)σAv(cid:3)forall (GeV) bb¯ W+W− μ+μ− τ+τ− νν¯ annihilationchannelsandWIMPmassesconsideredinthis analysisafteraccountingforthesystematicuncertainties. 10 132·103 – 47.12 64.35 3.22 IceCubehaspreviouslysearchedforaneutrinosignalfrom 20 578 – 9.67 12.9 1.35 annihilatingdarkmatterinthecenteroftheMilkyWay,using 30 230 – 5.81 7.47 1.16 acombinedeventselectionatlowandhighenergies.Thelow 40 164 – 4.88 6.17 1.35 energyselectionobservedanunderfluctuationthatresultedin 50 119 – 4.50 5.75 1.31 anenhancedlimiton(cid:2)σ v(cid:3),whilethehighenergyselection 100 74.2 15.6 4.96 5.92 2.15 A gaveaccesstohigherenergies.Thisanalysisimprovesonthe 200 67.3 22.7 7.39 8.04 4.79 previousresultatmostoftheenergiesconsidered.Inorderto 300 69.9 29.3 10.7 11.2 8.41 comparethisworktopreviousresults,Fig.8showstheupper 400 73.3 35.8 14.8 14.5 14.9 limitson(cid:2)σAv(cid:3)fortheτ+τ−annihilationchannelandNFW 500 79.7 42.5 19.2 18.1 24.5 halo profile of this work to previous results from IceCube 1000 110 76.3 52.3 42.4 187 andotherindirectdarkmatterdetectionexperiments.Itcan 123
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