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30THINTERNATIONALCOSMICRAYCONFERENCE Muons andNeutrinos 2007 8 THOMAS GAISSER 0 Bartol Research Institute, Department of Physics and Astronomy, University of Delaware, Newark, DE 0 2 19716,U.S.A. [email protected] n a J Abstract: Thispaper isthewrittenversionoftherapporteurtalkonSectionHE-2,muonsandneutri- nos,presentedatthe30thInternationalCosmicRayConference,Merida´,Yucatan,July11,2007. Topics 9 includeatmosphericmuonsandneutrinos,solarneutrinosandastrophysicalneutrinosaswellascalcula- 2 tionsandinstrumentationrelatedtothesetopics. ] h Introduction p 1.6 MINOS - o 1.5 L3+C There were 5 sections of contributed papers on r - p K model st muons and neutrinos with a total of 107 papers Nm 1.4 a distributed as shown in Table 1. The most active /+ Nm [ category is HE 2.3, astrophysical neutrinos. Par- 1.3 1 ticularlyinthisarea,therewerealsomanypapers 1.2 v presented in OG 2.5 sessions, on high-energy as- 2 trophysicalneutrinos.Iincludediscussionofthese 0.1 1 10 4 topicstotheextentnecessarytopresentacoherent 5 overviewofthefieldasofmid-year2007. Em ,0 (TeV) 4 . 1 0 Table1:Papersonmuonsandneutrinos Figure 1: Muon charge-ratio from Ref. [4]. The 8 line“πK”modelcorrespondstoafittotheratioof 0 Session Topic # µ+/µ−fromEq.1. : v HE2.1 Muonexperiments 17 i HE2.2 Observationsofsolar X &atmosphericν 16 is included in this conference in Ref. [1]. Muons r a HE2.3 Observationsof are penetrating and relatively abundant in all ter- astrophysicalν 37 restrialparticledetectors. Theyarethereforeapo- HE2.4 Theoryandsimulations 19 tentialsourceofbackground,andatthesametime HE2.5 Newexperiments they are useful for detector calibration. One use &instrumentation 18 of cosmic-ray muons is as a survey tool, some- timescalledmuontomography.Aclassicexample is the survey of the Second Pyramid of Giza and the search forhiddenchambers[2]. The statusof Atmospheric muons asimilarinvestigationofthePyramidoftheSunat Teotihuacan was presented at this conference[3]. Muonsarethegoldstandardofcosmic-rayphysics The detector is integrated and ready for installa- because they are well-measured and their physi- tion during2008in thetunnelthatgoesunderthe cal origin in the atmospheric cosmic-ray cascade pyramid. is well-understood. A summaryof measurements MUONS&NEUTRINOS An important new result presented at this confer- and Z ence is the measurement of the muon charge ra- NK+ = 0.67. (3) tiointhefardetectorofMINOS[4,6, 5]. Muons ZNK+ +ZNK− thatreachthedetectoratitsdepthof2070meters- Theratio(ZNK++ZNK−)/(ZNπ++ZNπ−)has water-equivalent(m.w.e.) haveenergiesatthesur- been keptfixed at its standardvalue. The fit uses face in therangeof1-7TeV,dependingon zenith the data of Refs [4, 10], which have data binned angle. Fig. 1 from Ref. [4] shows muon charge both in E and cosθ. The preliminary measure- µ ratio increasing in the energy range from Eµ < ment of the charge ratio in the shallow MINOS 100 GeV to Eµ > 1 TeV. The potential signif- neardetector[9](notshown)isconsistentwiththe icance of this result can be understood from the L3+C data. The largevalue of the K+/K− ratio energy-dependenceofEq.1. reflects theimportanceof forwardassociatedpro- An approximate, first-order expression for the duction (p → Λ K+) which is amplified in the muonintensityforE >100GeVis[7] spectrum-weighted moment by the steep primary µ cosmic-ray spectrum because the K+ carries on φµ± = 1φ−0(EZµN)N × (1) aTvweoragoethaersigpnaipfiecrasnftrformactMionINoOftSheabreeanmoteenweorgrtyh.y AπµZNπ± as examples of the use of cosmic-ray muons for (cid:26)1+B E cosθ/ǫ calibrating deep detectors. More than 20 mil- πµ µ π lion muonswere measured (after cuts) in the MI- + AKµZNK± , NOS far detector over a three-year period from 1+B E cosθ/ǫ (cid:27) Kµ µ K August 2003 to August 2006. One analysis uses whereZ andZ arespectrumweightedmo- the shadow of the moon to determine the angu- Nπ NK mentsforproductionofpionsandkaons,φ(E) ≡ lar resolution and absolute pointing of the far de- E dN/dE andφ0(Eµ)istheintensityofprimary tector [11]. Both the resolution and the absolute cosmic-raynucleonsevaluatedattheenergyofthe pointingare0.3◦±0.05◦. Themoon’sshadowis muon. The kinematic factors are A ≈ 0.67, seenatthelevelof4 σ. Theshadowofthesunis πµ A ≈ 0.25 (including the branching ratio for alsoseen,butatasomewhatlowersignificance,in Kµ K →µν),B ≈1.07andB ≈1.13. partbecauseofbendingoftheparentcosmicrays πµ Kµ inthesolarmagneticfield. Zatsepin and Kuz’min pointed out long ago [8] thepotentialofameasurementoftheatmospheric Another paper [12] presents the analysis of sea- muon flux as a function of zenith angle for mea- sonalvariationsoftheundergroundmuonrateob- suringtherelativeimportanceofkaontopionpro- served in the MINOS far detector. The observed duction in hadronic interactions. The angular de- rateofmuonsiscorrelatedwithtemperatureby pendencearisesfromthedenominatorsofthetwo terms in Eq. 1 and the numerical values of the ∆Rµ = αT × ∆Teff , (4) critical energy parameters, ǫ ≈ 115 GeV and <Rµ > <Teff > π ǫK ≈ 850 GeV. In the TeV range, the kaon con- where Teff is an average of the temperature tributionisrelativelymoreimportantnearthever- weighted by the probabilityof meson production, tical than at large angle. Thus the angular de- whichpeaksataltitudesaround15kmfortrajecto- pendence of the muon flux is sensitive to the ra- riesnearthevertical. Thiscorrelationisexplained tio ZNK/ZNπ. The angular dependence of the intheclassicpaperofBarrettetal.[13]asaconse- muonchargeratioprovidesinformationontherel- quenceexpansionoftheatmospherewhentemper- ative importanceof kaonsand pionsseparated by atureincreases. Thecorrelationislargeformuons charge. By fitting the data of Fig. 1 to the charge with E >> ǫ where there is a competitionbe- µ π ratiocalculatedfromEq.1,theMINOSgroupfind tweendecayandre-interactionoftheparentpion. At lower energies, most pionsdecay beforeinter- Z Nπ+ = 0.55 (2) actingforanytemperature.Theobservedrate,R , µ ZNπ+ +ZNπ− shows a seasonal variation of ≈ ±2% and corre- sponds to a value of α = 0.87± 0.03, consis- T 30THINTERNATIONALCOSMICRAYCONFERENCE tent with earlier measurements by MACRO [14] steepcosmic-rayspectrum. (Anindependentcon- andAMANDA[15]. firmation of the relatively high atmospheric neutrino flux in the TeV range comes from the work of Ref. [22], which assumes the best fit oscilla- Atmospheric neutrinos tion parametersand unfoldsthe atmosphericneu- trino spectrum from the Super-K measurements.) Similar equationsto Eq.1 describethe flux of at- The implicationsfor atmosphericneutrinosof the mosphericneutrinosathighenergy. However,the MINOS measurement of the muon charge ratio kinematic factors differ in an importantway. Be- have yet to be investigated in detail, however. It causeitsmassisclosetothatofthepion,themuon should be possible to use the MINOS data to re- carries most of the energy in the π → µν de- ducetheuncertaintyinexistingcalculationsofthe cay. ThedecayK → µν ismorenearlysymmet- flux of atmospheric neutrinos in the TeV region ric. Asaresult,whilethekinematicparametersfor andabove. K → ν are nearly equal to those for K → µ, they are quite different for pions. In particular, A ≈ 0.088 as comparedto A ≈ 0.67. As a Neutrino oscillations πν πµ consequence,thedominantcontributionto neutri- noswithE >100GeVisfromkaons,andtheef- Super-Kamiokande has been restored to its full ν fectofthelargechargeratioforkaonshasastrong complementof over 11,00050 cm photomultipli- influenceontheatmosphericneutrinospectrumat ersplusanoutervetodetectorandhasbeenoper- high energy. The tendency is to harden the TeV ating since July 12, 2006 as Super-Kamiokande- neutrinospectrumandtoincreasetheratioν /ν¯ . III. Reference [26] reviews the history of Super- µ µ It is now widely accepted that the deficit of at- K, which began operation in April 1996 and an- mosphericmuon-neutrinoswithitspathlengthand nounced the discovery of oscillations of atmo- energy dependenceis the result of neutrino oscil- sphericneutrinosin1998[27]. Thephaseofoper- lations. At this conference, there were only two ationuptotheaccidentinNovember,2001isSK-I. papersconcerningcalculationofthefluxofatmo- Thedetectorwasrepairedandoperatedwithsome spheric neutrinos. Reference [16] estimates and 5000 PMTs redistributed to provide uniform but tracks the various sources of uncertainty through sparser coverage. SK-II ran for three years, start- the calculationin orderto evaluatethe systematic ing October, 2002. Preliminary results of SK-III uncertaintyinthefluxofatmosphericneutrinosas areinagreementwithSK-IandSK-II. afunctionofneutrinoenergy. Thecontributionof AseriesofSuper-Kpapersatthisconferencepre- Honda et al. [17], focuses on evaluation of rela- sented preliminaryresultsof the combinedanaly- tivelysmalleffectssuchasvariationwithsolarcy- sisofSK-I(1489days)andSK-II(804days). At- cle and the effect mountainous overburden above mosphericneutrinoresults,forexample,werepre- thedetector. Ref.[17]isbasedonarevisedcalcu- sented [29] in the same format as the main SK-I lation[18]thatusesanewmodelofmesonproduc- paper[28]. Theplotsofzenithangleshowaratio tioninhadronicinteractions[19]basedoncompar- of(ν +ν¯ )/(ν +ν¯ )thatissignificantlyhigher e e µ µ isontomeasurementsofatmosphericmuons. The thanexpectationforsub-GeVneutrinosfromalldi- newHondaetal. neutrinofluxisnowclosertothe rections,andadeficitofmulti-GeVneutrinosfrom Bartol neutrino flux [16, 20] at high energy than below (∼10,000km) butconsistent with expecta- the earlier calculation [21]. In both models now, tion from above (∼15 km). The angular distribu- theratioZpK+/ZpK− islarge,consistentwiththe tionoftheelectronneutrinoshastheexpected(no interpretation of the increase in the µ+/µ− ratio oscillation)shape. Theresultsarefullyconsistent in the TeV region discussed above. The corre- with two-flavor ν ↔ ν oscillations with transi- µ τ sponding effect here is that the ratio ν/ν¯ is large tionprobability (∼ 1.7) in the TeV range). Moreover, the TeV npeourttarnincoefloufxfoisrwrealradtiavsesloychiaigtehdbpercoaduuscetioofnthoenitmhe- Pνµ↔ντ =sin2(2θ23)×sin2(cid:20)1.27δm2(EeGVe2V)Lkm(cid:21). (5) MUONS&NEUTRINOS ThedipatthefirstoscillationminimuminL/E is Table 2: Limits on supernova rates in the Milky seen[30]foratmosphericneutrinos[31].andthere Way Galaxy (events per year at 90% c.l.). The is no evidence yet for three-flavor effects such as Super-KlimitincludesSMCandLMC. non-zeroθ13. The MINOS group also presented their results Experiment Exposure Limit for neutrino oscillations using the NuMI muon- S-K[44] 2589d 0.30 neutrinobeamfromFermilab[32]. Theresults,al- LVD[45] 4919d 0.17 readypublished,[33]showadeficitofmuonneu- Baksan[46] 22yr 0.10 trinosinthefardetectorrelativetotheneardetec- toroveradistanceof735kmthatisconsistentwith theresultsoftheSuper-Katmosphericneutrinore- spectrum of these relic neutrinos peaks at a few sult. The MINOS far detector can also measure MeVandfallsquicklywithincreasingenergy[38]. νµ-inducedupwardmuonswithchargeseparation. Theprocess Although statistics are limited, they see a deficit of lower-energy neutrino-induced muons consis- ν¯e +p → n + e+ (6) tent with the Super-K oscillationparameters[34]. with its relatively large cross section is the pre- An interesting feature of neutrino-inducedmuons ferred channel for this search [39]. The convolu- inamagnetizeddetectoristhatthechargeratiocan tionofthecrosssection,whichincreaseswithen- be measured. The charge ratio is opposite to that ergy, and the spectrum of relic neutrinos may be for atmospheric muons because positive mesons (π+ andK+)decaytoµ+ andν (whichproduce abovetheexpectedbackgroundfromatmospheric µ µ−), while negative mesons give ν¯ (which pro- n¯inawindowofenergyfrom∼10to20MeV[40]. µ duceµ+). Current limits from Super-K [41] are close to the signalsexpectedfromvariousmodels,asshownin DuringthetimethatSuper-KIIoperatedwithhalf Ref. [43] at this conference. The possibility [40] the density of PMTs as compared to Super-K I, ofaddingGadoliniumtotagrecoilnucleonsfrom newreconstructionalgorithmsweredevelopedthat theprocessofEq.6wasmentionedatthisconfer- allowed sensitivity similar to that of the original enceinRef.[35]. Atestofthismethodwitha2.4 detector.Nowthatthedetectorhasbeenrestoredto litercontainerofGdCl andaradioactivesourceis 3 itsfullcomplementofPMTs,thebetteralgorithms describedinRef.[42]. makeitpossibletolowertheenergythreshold[35]. Super-KIIIcurrentlyisoperatingwith100percent trigger efficiency down to 5 MeV, which is in the Astrophysical neutrinos(highenergy) transitionregionfrommatterdominatedtovacuum The most promising channel to use in the search oscillationsforsolarneutrinos. forastrophysicalneutrinosofhighenergy(>TeV) is neutrino-induced muons because the effective Astrophysical neutrinos (lowenergy) volume of the detector is amplified by the muon range. Theaverageenergylossrateofamuonper NeutrinosfromSN1987AintheLargeMagellanic X(g/cm2)ofmaterialtraversedis Cloud are so far the only neutrinos detected [36, dE 37] from outside the solar system. A network = −a −bE, (7) dX ofseveraldeepdetectorscontinuestomonitorthe skyforburstsofneutrinosfromnearbystellarcol- where ǫ = a/b ∼ 0.5 TeV is the char- lapses. New upper limits on the rate of stellar acteristic energy above which stochastic losses collapse in the Milky Way Galaxy based on non- (bremsstrahlungandnuclearinteractions)beginto observation of neutrino bursts are summarized in dominatetheenergyloss. Thecorrespondingaver- Table5. agemuonrangeis Itisalsopossibletosearchforadiffusefluxofrelic neutrinos from past supernova explosions. The X ≈ 1 ×ln Eµ,0+ǫ , (8) b (cid:18)Eµ,min+ǫ(cid:19) 30THINTERNATIONALCOSMICRAYCONFERENCE moEwrhµum,eomrnoeisnr.eEiTisµnh,t0ewheimastteuthhroreenosrmrhaiocunleogd.neeecnnaenerrgbgyeyosafetvtpherreoaldduketcieltociomtoneratfenordsr -2-1-1]cm s sr1100--33 HBoarnrd aat mat.m n .m +n mn–+m n–m Aa2t0Mm0A0. Nn- mD2+0A n–0-mI3 Id, uatnaf ofrldoemd eV 1100--44 Whenonlythemuonisdetected,however,thereis G onlyanaveragerelationbetweenthevisibleenergy [E 2n --55 ofthemuonandthatoftheneutrinothatproduced F 1100 it. The fraction of the neutrino energy carried by themuonvariesfromeventtoevent,andthetrack --66 1100 isonlypartiallycontained. Moreover,forE >> µ ǫtherearelargefluctuationsintheamountofvis- --77 ible energy deposited as the high-energy muon 1100 passesthroughthedetector[23]. Nevertheless,be- causetherelationbetweenenergydepositionofthe --88 1100 muon as it passes throughthe detector and its total energy is well understood (as well as the re- --99 1100 lation between the energy of the muon and that 33 33..55 44 44..55 55 55..55 66 of the neutrino that produced it), it is straightfor- log(En /GeV) ward to derive the parent neutrino spectrum from a measurement of neutrino-induced muons given Figure2: Unfoldedspectrumofatmosphericneu- sufficient statistics. From a Monte Carlo simula- trinos by AMANDA-II[25] comparedto calculation of the detector response one has to identify tionsofRefs.[20,21]. a set of measurable quantities that depend on en- ergydepositioninthedetector. Anunfoldingpro- whichhaveaharderspectrumthanneutrinosfrom cedure can then be used to reconstructthe parent decayofpionsandkaons,whicharesuppressedat spectrum. Theerroranalysismustaccountforthe highenergybecausetheparentmesonstendtoin- largefluctuationsfromeventtoevent. Aprescrip- teract rather than decay. The promptcontribution tionusingthephotondensityalongthemuontrack to the atmospheric lepton flux can be represented astheenergy-dependentobservableforreconstruc- by adding a third term to the right hand side of tionandunfoldingtheatmosphericneutrinospec- Eq.1oftheform trum in IceCube is given in Ref. [24]. This is a natural choice given the physics of muon energy A Z C N,C , lossdescribedbyEq.7. 1+B Ecosθ/ǫ C C The high-energy tail of the spectrum of atmo- where the subscript ”C” represents a charmed spheric neutrinos constitutes the background for hadron,Eistheleptonenergy(µ,e,ν orν )and searchesforneutrinosfromastrophysicalsources. µ e ǫ ∼2−9×107forarangeofcharmedhadrons Atmosphericneutrinosalsoserveasthecalibration C withsignificantleptonicbranchingratios[49]. For beam.Atmosphericneutrinosaresufficientlywell- muon neutrinos with E > ǫ = 850 GeV the understoodinthemulti-TeVrangesothatsuccess- ν K spectrumgraduallysteepensfromE−2.7 toE−3.7 fulreconstructionoftheirspectrumcanbeconsid- while the spectrumofpromptneutrinoscontinues eredasa prerequisitetoanysearchforastrophys- to reflect the primary cosmic-ray spectrum until ical neutrinos. As an example, Fig. 2 shows the E = ǫ ∼ 3×107 GeV. The crossoverenergy atmospheric neutrino spectrum derived by an un- ν C depends on the amount of charm production in foldingprocedurefromAMANDAdatatakenfrom hadronicinteractionsathighenergy(Z ),which 2000-2003[25]. N,C is highly uncertain, particularly in the fragmenta- An important remaining uncertainty in the atmo- tionregion. Foramodelwithasignificantcontri- spheric neutrino flux at high energy is the level bution of intrinsic charm [50] the neutrinos from of the contributionfrompromptneutrinos. These charm decay become the dominantcomponentof are neutrinos from the decay of charmed hadrons atmospheric ν for E > 100 TeV [51]. For the µ ν MUONS&NEUTRINOS -1]r 10-4 AMANDA-II 2000 atms. n m data (prelim.) s 1 Barr et al. atms. n + prompt atms. n -s 2 Honda et al. atms. n + prompt atms. n -m 10-5 Max uncertainty in atms. n c V Frejus e G MACRO dE [ 10-6 AMANDA B-10 1997 n m diffuse N/ AMANDA-II 2000 Cascades (all-flavor / 3)* d 2 AMANDA B-10 1997 UHE (all-flavor / 3)* E 10-7 Baikal 1998 - 2002 ( all-flavor / 3 )* this analysis RICE 1999-2005 ( all-flavor / 3 )* AMANDA-II 2000 unfolding (prelim.) 10-8 AMANDA-II 2000-3 n m limit W&B limit/2 (transparent sources) Full IceCube 1 yr * assumes a 1:1:1 flavor ratio at Earth 10-9 3 4 5 6 7 8 9 log [E (GeV)] 10 n Figure3:LimitonthediffusefluxofastrophysicalneutrinosfromAMANDA-II[48]. much steeper spectrum of ν the crossover of the iceandrockbelowthedetector. Thisistheenergy e charmcomponentisaround3TeV. region and the signature for which existing large Diffusesearch. High-energyastrophysicalneutri- detectorsindeepwateroriceareoptimized. Such nos are expectedto be producedby interactionof a searchislimited to neutrinoswith Eν < 1 PeV high-energyacceleratedparticleswithgasorelec- because the Earth absorbs neutrinos with higher tromagneticradiationinornearthesources. Gen- energy. erally the particle beams, andhencethe produced Inthe PeVenergyregionandabovea diffusesig- neutrinos, are expected to have a harder energy nal would be dominated by events near the hori- spectrumthanthebackgroundatmosphericneutri- zon, where the target length is maximized with- nos.Astandardbenchmarkforhigh-energyneutri- out absorbing the neutrinos. Here one is gener- nosofextra-galacticoriginistheWaxman-Bahcall ally lookingfor eventscharacterizedby large and limit [47], which assumes an E−2 differential concentrated depositions of energy, either radiat- spectrum. The normalization of the Waxman- ing ν -induced muons or cascades from interac- µ Bahcall limit for ν +ν¯ at Earth after account- tions of ν or ν in or near the detector. Fig. 3 µ µ e τ ing for oscillations is E2dN /dE < 2.2×10−8 includes the limit from the Baikal experiment in ν GeVcm−2s−1sr−1. Toexceedthislevelwouldre- this energy range. The ”all-flavor” limit is di- quire the existence of cosmic acceleratorsopaque vided by three on the plot to make it comparable to the particles they accelerate. The limit might with the limits on ν alone. A preliminary value µ also be relaxed to some extent at lower energy in of 2.4 × 10−7 GeVcm−2s−1sr−1 was presented thecaseofsteepersourcespectra. at this conference for the upper limit of all neu- Figure 3 shows current limits on a diffuse flux trinoflavorsfromAMANDAinthe”UHE”energy of neutrinoswith anE−2differentialenergyspec- range 105 < Eν < 109 GeV [52]. This is ap- trum from AMANDA. The figure illustrates sev- proximatelyafactorofthreebelowtheBaikallimit eral points. The search labeled ”this analysis” and only slightly higher than the limit from up- looksforneutrino-inducedmuonsgeneratedinthe 30THINTERNATIONALCOSMICRAYCONFERENCE wardmuons[48]intheenergyrangearound1PeV are Active Galactic Nuclei (AGNs), Gamma-ray wherebothlimitsapply. Bursts (GRBs) and supernova remnants and ac- The ”diffuse” limit on any particular modelspec- tive compact objects in our galaxy. Since it trum is obtain by convolving the spectrum with is still not known which gamma-ray sources are the detector response. In the case of the diffuse hadronic, identification neutrinosfrom sources of limits for an E−2 spectrum, the plotted limit is a gamma rays (and/or electromagnetic radiation in horizontal line on a plot of E2dN/dE extending other wavelengths) is the central goal of high en- over the energy region that gives 90% of the sig- ergyneutrinoastronomy. nal. Ingeneral,aseparatelimitmustbecalculated As an example, it is interesting to consider likely foreachassumedmodelspectrum. Ref.[52]gives sources of high-energy neutrinos in our local ausefultableofmodelsandsensitivitiesthatspec- galaxy. In the Northern hemisphere, visible from ifies which models are inconsistent at 90% confi- Antarctica, the Cygnus region is of particular in- dencelevelwith the presentAMANDAUHE dif- terest [56]. A systematic survey of the sensitiv- fuse limit. Several early models of neutrino pro- ity ofa futurekilometer-cubeneutrinodetectorin ductioninAGNareruledout,including,forexam- theMediterraneantopotentialgalacticsourcesvis- ple Ref. [53], while others(e.g. [54, 55]) are still ible from the North was given in Ref. [57]. (A viable. more detailed discussion of the analysis is given InthecaseofsearchesforUHEneutrinoswithop- inRef.[59].) Table3summarizesresultsforthose ticaldetectorsthesignalwouldbecharacterizedby H.E.S.S.sourceswithspectraforwhichabreaken- a large amount of light in the detector. Since the ergy (where the spectrum steepens) has been de- events will be from above and from the sides, an termined. The corresponding neutrino spectrum important background is from bundles of muons would steepen at a somewhat lower energy than generatedbyhigh-energycosmicrayscascadesin the observed steepening of the gamma-ray spec- the atmosphere. Showingthat this physicalback- trumandaboutafactorof40lowerthantheenergy ground is well-understood (for example by com- atwhichtheparentprotonspectrumsteepens. As- paringsimulationswith dataatvariouscutlevels) sumptionsofthecalculationarethattheobserved isneededtodemonstrateunderstandingofthede- gamma-rayspectrumisentirelyhadronicinorigin, tectorresponse. Afeatureusedtodiscriminatebe- producedbyinteractionofanacceleratedspectrum tween signal and background in Ref. [52] is the of protonswith gasin or near the source and that numberofopticalmoduleswithmultiplehitsver- thereisnoabsorptionofgamma-raysinthesource. sus single hits. Muon bundles from cosmic-ray Aratioatproductionofνe : νµ : ντ = 1 : 2 : 0 cascadestendtoproducelesslightperparticle,and isassumedwithaflavorratioatEarthof1 : 1 : 1. thesourceofthelightissomewhatdiffuseascom- Theneutrinoeffectiveareaofakm3detectorinthe paredtotheintenseandconcentratedburstoflight Mediterranean for the νµ + ν¯µ channel is calcu- fromasingleparticlewithenergyinthePeVrange latedinsomedetailtoobtaintheexpectednumber orhigher.Thesignalwouldproducemoremultiple ofeventsforsource(Nsrc)andbackground(Natm) hits. byconvolutionwiththespectrumofthesourceand withtheatmosphericneutrinospectrum.However, Point sources. Particularly luminous and/or efficiencies for event selection and reconstruction nearby sources of neutrinos should eventually arenotaccountedfor. emergeabovethediffuseatmosphericbackground. Likely candidates are the subset of gamma-ray TheresultssummarizedinTable3nicelyillustrate sources in which the gamma-raysare hadronic in some important features of point source searches origin, from decay of neutral pions produced in withkilometer-scaleneutrinotelescopes. Thesec- interactions of accelerated protons and nuclei in ondandthirdcolumnsofthetablegivethediame- or near the sources. The kinematic relation be- ter of each gamma-ray source and the fraction of tween π0 → γγ and π+ → µ+ν provides a the time it is below the horizon. Typically, the µ close connection between neutrinos and gamma- sourcesizesexceedtheresolutionofH.E.S.S.and rays if the photons are not significantly absorbed are comparableto or larger than the resolution of in the sources. Examples of potential sources the neutrino telescope. If the characteristic neu- MUONS&NEUTRINOS E >1TeV E >5TeV ν ν Sourcename Dia(◦) Vis ǫν (TeV) Nsrc Natm Nsrc Natm A:RXJ0852.0-4622 2.0 0.83 1.19 11 104 4.2 21 A:RXJ1713.7-3946 1.3 0.74 1.25 11 41 4.6 8.2 B:LS5039(INFC) 0.1 0.57 1.01 0.5 2.5 0.2 0.5 C:HESSJ1303-631 0.3 1.0 0.21 1.6 11 0.3 2.1 D:VelaX 0.8 0.81 0.84 16 23 10 4.6 D:HESSJ1825-137 0.5 0.57 4.24 8 9.3 3.7 1.8 D:CrabNebula <0.1 0.39 1.72 5.8 5.2 1.9 1.1 Table3: TeVgalacticγ-raysourcesfromtheH.E.S.S.catalog[58]withcorrespondingneutrinoratescal- culatedfor5yearsoperationofKM3NeT(1km3instrumentedvolume)[59]. Traditional searches use a bin size around the source optimized for the source size and point spread function of the detector. In Ref. [57, 59], for example, the bin size is 1.6× σ2 +σ2 . PSF src Unbinnedlikelihoodproceduresthaptimprovesen- sitivitybyusingenergy-dependenceandtimeclus- tering (as well as direction) are discussed for ANTARESinRef.[64],forAMANDAinRef.[65] and for IceCube in Ref. [66]. Ref. [65] finds Figure 4: Preliminary sky map from Ref. [65] an improvement in sensitivity and discovery po- showing log (p) for an unbinned point source tential for AMANDA-II of 30% compared to the 10 searchwithAMANDA-IIin2005. binnedsearch. TheANTARESanalysis[64]finds a greater improvement, up to a factor of two or moreinsomecases. Figure4illustratesthekindof trinobreakenergyis≥1TeVorhigher,thesignal confidencelevelmapthatresultsfromanunbinned to backgroundratioimprovesathigherenergy,so pointsourcesearch. theabilitytomeasureasignalrelatedtoenergywill Variable sources. For sources known to be vari- beimportant. Forneutrinosourcesthatsteepenin able in electromagnetic radiation, e.g. in X- or theTeVregion,thesignal/backgroundimprovesby γ-radiation,the significanceof a smallnumberof aboutafactoroftwoifthethresholdcanberaised neutrinos from the direction of that source could from1to5TeV. begreateriftheyoccuratthesametimeasflares. Expected rates are low, and signal/background is Assessingtheextrasignificancedependsontheex- lessthanorcomparabletounitydependingonthe tent to which the pattern of flaring is understood. size of the source. Techniques such as “source Ref. [62] proposes a method in which directions stacking” will therefore be important [60] to im- tosourcessuchasspecificAGNknowntobevari- prove the significance. Similar conclusionsabout able in electromagneticradiation are searched for signal/backgroundforsuchgalacticsourcecanbe time clusters of neutrinos on various time scales. inferredfromRef.[61]. Hadronicmodelsaredis- When a significant fluctuation above background favored for several of the types of sources listed. is found, a check is made to see if the source is Prime candidates are A: shell-type SNRs and C: in a high state in electromagnetic radiation at the TeV γ-ray sources with no counterparts at other sametime. Ifnot(oriftheEMdataarenotavail- wavelengths. Pulsar windnebulae(D) andbinary able)atime-clusteringalgorithmiscomparedwith systems(B)aremoreoftenexplainedwithelectro- a large set of Monte Carlo data samples from the magneticmodels,althoughhadronicmodelsexist. selected set of sources to look for an excess over background. A similarlymotivatedapproach[63] 30THINTERNATIONALCOSMICRAYCONFERENCE looks for correlations on various time scales using known propertiesof the atmospheric neutrino ) backgroundaspartoftheanalysis. sr Cascades n e+n e+n m +n m +n t+n t . Themoststraightforwardapproachtomakinguse s of variability in electromagnetic radiation from a 2.m 10-6 potential neutrino source is to look for a correla- c Murase, Nagataki, 2006 V/ Razaqqe et al., 2003 tion in the historical record between, for exam- e 10-7 Waxman, 2003 ple, flares from blazars and times of neutrinos G Mezsaros, Waxman, 2001 ( from the direction of the same sources. For co- E incidencewithagamma-raytelescopewithnearly Fdd10-8 2 E continuouscoverageofalargepartofthesky(e.g. Milagro or Tibet) this is a good approach [67]. up-going muon 10-9 For telescopes with a limited field of view, however (such as VERITAS, MAGIC, H.E.S.S.), the telescope will most likely be looking elsewhere 10-10 104 105 106 107 108 109 10101011 whena neutrinosignaloccurs. If, as islikely, the E (GeV) neutrino events are not significantly above atmo- n spheric background on their own, then no signal can be claimed. One way to address this asym- Figure 5: AMANDA limits on neutrinos from metryistosendanalertwhenapre-specifiedcon- GRBs[69]. Seetextfordiscussion. dition is satisfied by the neutrino detector, which is continuouslysensitive to the hemispherebelow hundred bursts reported by BATSE and IPN3 be- the detector. The gamma-ray telescope can then tween 1997 and 2003. No neutrinos were ob- slew to the selected source and see if it is flar- served. Fig. 5 shows a comparisonof limits with ing. Atestofsuchaneutrino-triggered”Targetof models. Thethreelineslabeled“up-goingmuon” Opportunity”(ToO)arrangementforapre-selected are limits for the models with the corresponding set of sources was reported at this conference for shapes. Thus the model of Ref. [72] is ruled out AMANDAandMAGIC[68]. andthemodelofRef.[73]ismarginallyincompat- Anotherpossibilityistodefineanalertasagroup ible with the limit. A 3σ upper limit is set at 1.3 neutrinos from the same direction within a pre- timesthelevelpredictedinthemodelofWaxman selectedtimewindowforanydirectioninthesky. andBahcall[74]. WithIceCubethesensitivityfor SuchapossibilityisdescribedinRef.[69]whereit detectionofneutrinosfromGRBswillrapidlyim- isproposedtosendanalerttoopticalcamerasthat prove. IceCube is operating now with 22 strings can quickly point to the direction defined by the and is expected to have 36 to 40 strings in oper- groupof neutrinos. Inthisway itmightbepossi- ation by the time GLAST turns on in 2008. As- bletodiscovertheonsetofanopticalsupernovaor sumingGLASTwillobservesome200GRBs per a GRB afterglow, which could elevate the signif- yearoverthewholesky,itisestimated[69]thatob- icance of the neutrino observation from a chance servationof 70burstsin the Northernhemisphere coincidenceofseveralatmosphericneutrinostoan without associated neutrinos would be in conflict identifiedastrophysicalneutrinoevent. with the model of Ref. [74] at the σ level. This Gamma-ray bursts. Neutrinos associated with level of sensitivity shouldbe possible with an ex- gamma-ray bursts would have both a time tag posureequivalenttooneyearoffullIceCube. and a location which would make the detection of even a small number of such neutrinossignifi- cant. Limitsonneutrinosassociatedwithgamma- Cosmogenicneutrinos rayburstsusingAMANDAhasbeenpublishedre- cently[70,71]andpresentedatthisconferencein There is now a growing consensus that the pri- Ref. [69]. The most sensitive search was in the marycosmic-rayspectrumbecomessteeperabove νµ-induced muon channel [70], which used 400 5 × 1019 eV [75, 76, 77]. This is generally at- MUONS&NEUTRINOS 1]10-3 limitsarebasedonsearchesforatypicalhorizontal -1 sr GLUE’04 (e, m , t ) FORTE’04 (e, m , t) showersasdiscussedinthenextsection. -2- sm10-4 FshoorwunnifionrmFiigty.,6limaistssufmroinmgoatnheerqeuxaplermimixetnutrseaoref c the three neutrino flavors at Earth. Limits from eV 10-5 the optical detectors, AMANDA [82, 48] and G Baikal [83], are at lower energy. Limits from ra- [ E) dio detectors include RICE [84] in the ice at the 2 f(E10-6 AMANDA (e, m , t ) RICE’05A (Ne,I Tm A, t- l)ite (e, m , t ) SdiooutdhePteoclteo,rAloNoIkTiAng-Lfitoer[n8e5u],trainboaslloinotner-bacotrinnegrian- Baikal (e, m , t ) 10-7 Auger (t) the Antarctic ice sheet, and FORTE [86], search- AMANDA ( m ) ing for radio pulses from neutrino interactions in the Greenland ice mass. GLUE [87] looks for 10-8 GZK, each flavor microwave signals of neutrino interactions in the Moon. 1014 1016 1018 1020 1022 1024 1026 Neutrino Energy [eV] There are several calculations of the spectrum of cosmogenic neutrinos, which vary depending on assumptions about the spectrum and cosmologi- Figure 6: Figure from Ref. [80] showing upper calevolutionofthecosmic-raysources. Theband limitsfromvariousexperimentsassumingν :ν : e µ showninFig.6isarangebasedoncalculationsby ν = 1 : 1 : 1 at the detector. See text for a τ two groups[88, 89]. The calculation of Ref. [89] discussionofthisfigure. was discussed at this conference in [90] for two different models of the primary cosmic-ray com- position.Theresultinthehigh-energypeakregion tributed to the ”GZK” effect [78, 79] of energy loss as particles interact with photons of the mi- (Eν ∼ 1018 eV)isratherindependentofthecom- position. crowavebackgroundradiationduringpropagation fromsourcesatcosmologicaldistances. Alackof high energyparticles could, however,also be due Neutrino detectors and techniques to a lack of sources capable of accelerating particles to energies of ∼ 1020 eV. In any case, the I conclude with a summary of the status of large numberandspectrumofcosmogenicneutrinosisa neutrino detectorswhich have the primaryaim of keytotheoriginofthehighestenergycosmicrays. finding high energy (≥TeV) astrophysical neutri- Current measurements have not yet reached the nos and identifyingtheir sources. The discussion sensitivity to detect cosmogenic neutrinos at the is organized by detection method. I do not in- expectedlevels,asshowninFig.6fromRef.[80]. cludeherethedenselyinstrumenteddetectorssuch LimitsshowninFig.6comefromseveraltypesof asMINOSandSuper-K,whichareaimedprimar- detectors,whicharesensitivetodifferentrangesof ilyatstudyofneutrinooscillationsand(inthecase energy and to different combinations of neutrino of Super-K) low energy neutrinos and proton de- flavors. The Auger limit [80] is for the ν chan- τ cay. nel for a period from January 2004 to December Becauseofoscillations,neutrinosfromastrophys- 2006thatcorrespondsto oneyear of operationof icalsourcesareexpectedtoconsistofcomparable thefulldetector. Thelimitisshownfortheenergy numbersofallthreeneutrinoflavorsafterpropaga- region that would generate 90% of the signal for anE−2differentialspectrum,whichoverlapswell tionfromdistantsources. Atproduction(whether ν in the atmosphere or in an astrophysical source) withtheexpectedspectrumofcosmogenicneutri- the production of ν is strongly suppressed rela- nos.AsimilarlimitfromHi-Res[81]isessentially τ tivetoν andν . Forthisreason,identificationof atthesamelevelastheAugerlimit.Theair-shower µ e τ-neutrinoswouldbeasignalofastrophysicalneu- trinos. The signatureof a tau neutrinointeraction