Searching for cosmic sources of neutrinos with ANTARES Aart Heijboer

1. Searching for cosmic sources of neutrinos with ANTARES Aart Heijboer NIKHEF/University of Amsterdam outline: • neutrino astronomy • cosmic rays, gamma's and neutrinos • ANTARES • status • reconstruction • point source searches

2. The birth of neutrino astronomy • Solar neutrinos • SN1987a (12+6 's) • neutrino oscillations • atmospheric • Solar

3. through-going muon in SK Going to TeV energies MeV neutrinos (SNO, SK): • produced in nuclear reactions • detected from Sun and Supernova • fluxes from other stars or extra-galactic SN (probably) too low TeV neutrinos: • produced in collisions of high energy hadrons (or decay of massive particles) • observational advantages • neutrino cross-section rises with E • small scattering angle (due to large boost)allows for pointing accuracy • energetic reaction products: can usesparse detector to monitor large volume cost effectively put PMTs furhter apart!

4. High energy multi-messenger astronomy Need stable particles Protons Photons Neutrinos Satellites Balloons Air shower arrays Water Cherenkov (air) shower detection GeV: satellites TeV: Air Cherenkov

5. Cosmic Rays The existence of (ultra) high energy cosmic rays is a major motivation for high energy gamma and neutrino astronomy SNR standard candidate for acceleration upto Z  1015 eV light elements disappear first? microquasar Highest energies: AGN, GRB? • Fermi acceleration probable • Sources still unknown, due to deflection my magnetic fields

6. Cosmic ray interactions, photons and neutrinos Photons and neutrinos are produced in ~equal amounts by CR interactions cosmic ray proton (or nucleus) neutrinos photons straight propagation sources transparent to neutrinos no absorption while propagating smoking gun for hadrons hard to detect straight propagation reprocessed to low E in (opaque) source absorbed on extra-galactic photon background alternative production mechanism: electrons > 100 GeV air-Cherenkov technique

7. The Sky in TeV Photons up to 70 TeV evidence for hadrons? H. Völk, TAUP 2003 many types of Galactic and extra-Galactic sources

8. Evidence for hadrons? RX J1713.7-3946 Recently: Cangaroo galactic center astro-ph/0403592 well fitted with protons Enomoto et al. (Cangaroo), Nature 2002 Reimer et al., 2002 0 inverse Compton detection neutrinos would provide direct proof of hadron component

9. Other signals in neutrino telescopes • GZK neutrinos UHECR+gcmbn+X • Dark matter Wimps can be gravitationally bound to heavy objects (Earth, Sun, Galactic center). Neutralinos can annihilate e.g. cc W+W-n+X • Decay of very massive, long lived particles topological defects, GUT particles • Magnetic Monopoles direct detection when traversing detector

10. Water/ice Cherenkov: Detection principle  43o  Cherenkov Photons  p  • Use the Earth as target and the sea as detector • reconstruct muon trajectory from arrival times of Cherenkov photons on PMTs 

11. need accurate muon reconstruction Water/ice Cherenkov: Backgrounds • backgrounds • atmospheric m: • reject by looking for up-going muons • beware of mis-reconstructed atm. muons (need factor 107 rejection) • atmospheric n: • largely irreducible, but • isotropic with steeply falling (soft) energy spectrum • rejection: • energy (look for hard, diffuse fluxes) • direction (look for point sources)..... • time (transients (GRB), correlate to other detectors) p  backgrounds: atmospheric  atmospheric  p  p 

12. Cherenkov telescopes around the world Mediterranean Sea: deep, clear water ANTARES MACRO NEMO NESTOR SNO Baikal SuperK BAIKAL Dumand DUMAND AMANDA running since 1996 at shallow depth. pioneering experiment in good, deep water funding stopped in 1995 taking data at South Pole next step: IceCube

13. Three projects in Mediterranean ANTARES NEMO NESTOR NEMO R&D for km3 scale detector in Italy NESTOR planning detector in Pylos, Greece ANTARES building 0.1 km2 scale detector KM3NeT Joint initiative for km3-scale detector somewhere in Med.

14. The ANTARES collaboration

15. The ANTARES detector buoy 40 km electro-optical cable to the shore floors consisting of 3 PMTs, with electronics for digitisation, local clock 14.5 m floor spacing junction box bottom 100m not instrumented 60 m string spacing depth =2.4 km 12 strings x 25 floors x 3 = 900 PMTs

16. Site of Detector and shore station

17. Prototype Sector Line: in the lab Offset of ~110 ns between adjacent floors due to difference in fibre length, Check of internal clock-system calibration Timing resolution of 2.0/21.4 ns. dominated by TTS of the PMT Deliver simultaneous laser pulses to OMs Take data with full DAQ system Measure time differences between hits on different OMs

18. Movie of sector line connection

19. Prototype Sector Line: in the water period of highoptical background background relatedto sea currents screenshot of online monitoring

20. Muon Track Reconstruction Reconstruction of muon direction (and position) based on hit-times v=c straight line (multiple scattering very small) beware of backgroundphotons muon does not go 'through'the hits (non-linear problem) Energy reconstruction based on amount of light emitted

21. Find track parameters so that residuals are 'small' Muon Track Reconstruction v theoretical arrival time residual w.r.t actual hit time

22. Distribution of residuals • MC simulation • simulation of water propertiestaking in situ measurementsinto account. • PDF strongly peaked despite • light scattering • secondary electrons (pair production and bremsstrahlung) • optical background total E = TeV number of hits (a.u.) muon electrons & scattered residual (ns) Optical background due to decaying 40K and bioluminescence.

23. For each hit, the signal and background PDFs are weighed differently background hits (flat) signal hits (peaked with tail) log(P) time residual 0 Amplitude of hit distance track-PMT angle between PMT and photon .... but there is a problem

24. Finding the maximum of the likelihood function scan of -likelihood around true direction (position fixed to true) problem: PDF is 'flat' for small or very large residuals Fitting algorithms rely on derivatives of PDF. Scanning unfeasible in 5-dim parameter space Need good starting point for the fit (1o accurate) -log(L) + Constant azimuth angle (deg) zenith angle (deg)

25. linear prefit linear prefit PM position x PM position x hit time t hit time t Step 1: Linear prefit linear prefit PM position x hit time t roughly 10o accurate not good enough, but it's a start linear 2 fit yielding dx/dt, dy/dt, dz/dt and x0, y0, z0

26. buzzword: robust estimation (see e.g. numerical recipes) Step 2: M-estimator Trade-off needed between accurate PDF of residuals providing gradient to the true minimum Fitting technique that is resistant to 'outliers', but still is able to find the global minimum by minimising a 'modified 2': called M many events with few degree accuarcy ri2 M =  g(ri) r2 rises only linearly: outliers are not so important hit residual (ns)

27. Full reconstruction algorithm multiple stages with increasing accuracy linear prefit try a few different starting points hit selection fit with M-estimator hit selection final fit with full likelihood

28. up hor Absoption in Earth Detector Performance angular resolution • below 0.2o for high energies • dominated by physics below ~3 TEV Effective area cut on MC truth: known sources Aeff= Rdet / F

29. Point Source Searches 1555 events, 667 up-going AMANDA-II 197 days (simulation of) One year of atmospheric neutrinos seen by ANTARES Excess of events: Discovery? no excess: Upper limit

30. for both methods Conventional methods cone method grid method count events falling in square-like bins clusters are built by selecting all events in a cone  • only event counting • bin/cone size chosen to give constant background • optimal bin/cone sizes found for exclusion and discovery • significances can be calculatedanalytically from the data • results for both methods arelargely similar but......

31. Likelihood ratio method 2 clusters with 5 events Idea: use all available information • precise configuration of the events • energy of the events • detailed knowledge of angular resolution not so signal like signal like optimal observable (test statistic) = likelihood ratio  hypothesis that, in addition to the background, there is a point source of neutrinos. how to calculate P(data|s+b) ? hypothesis that there is only background

32.   ra Likelihood calculation probability of the data can be expressed as a sum over the events source position: ra, d flux: dFsig/dEn = fEn-g unknown parameters in Fsig probability of getting reconstructed muon energy neutrino energy unknown: integrate signal hypothesis spectrum angle between source position and reconstructed muon direction effective area for neutrinos point spread function

33. all events preselection: clustering (large cone size) background like signal like Likelihood ratio method fit source position and spectrum, maximising P(data| ra,  , sig) likelihood P(data|ra,  , sig) takes into account: energy of the events point spread function  Cluster with the highest likelihood corresponds to best source candidate ra Likelihood ratio is observable discriminating between signal+bg and bg-only hypotheses

34. signal event background event true source position fitted source position Likelihood ratio method Three rare examples of 'best candidate' clusters: Discovery: finding a cluster that is so signal-like that the probability for the background to produce it is very small (i.e. 2.7´10-3/ 5.7´10-7, for 3/5 )

35. Likelihood ratio method optimised grid method =-80o LR method can discover a source at 5 CL, which is only a 3 excess in grid method Alternatively: grid method needs ~40% stronger flux to reach same CL

36. E-2 spectrum Likelihood ratio method As an extra, we get a ML estimate of the source position 6 events seen from source single-evt resolution/Ns For pinpointing a source, ANTARES has a resolution < 0.1o !

37. 2001 AMANDA-II: 197 days in 2000 discoverable at 5  CL 90% CL average exclusion limit 2009 Point source sensitivity of ANTARES detect in specialised analysis

38. Summary • Motivation for high energy neutrino astronomy • Cosmic ray origin? • Do TeV gamma rays show evidence of hadrons? • Neutrino telescopes in Mediterranean • ANTARES 0.1 km2 is under construction • Joint effort with NESTOR and NEMO for KM3NeT • Reconstruction • Take into account optical background • Reaches 0.2o accuracy • Point source searches • Profit from accurate reconstruction • Likelihood ratio method to improve discovery potential (3) • 2007 (hopefully): observe southern sky • improve existing limits by factor 10 in 1 year • complement AMANDA, which studies northern hemisphere • Future km3-scale detector in the Mediterranean

39. In ice: (Wiebush, vlvnt) but no background error estimates from the fit describes the actual error to within ~10%. fit = 1.09

40. GLUE 2002 Anita 2004 AUGER nt AABN 2007 EUSO Auger Salsa 2012 km3 Neutrino telescopes up to very high E RICE AGASA Water/ice Cherenkov • best for E<107 GeV • good angular resolution • beat the background • identify the source Amanda, Baikal Air shower neutrinos produce deep, horizontal showers Radio detection UHE showers emit coherent radiation inradio frequencies. very cost effective C. Spiering water/ice Cherenkov (radio) detection of -induced showers

41. well reconstructed events are selected by cutting on the value of the likelihood Event selection 10 1.0 log10 (reconstruction error /deg) 0.1 0.01 -log(L)/ndof Also rejects background from atmospheric muons which are mis-reconstructed as upward-going.

42. junction box deployment ANTARES 0.1 km2 data taking sector line recovered 2000 2001 2002 2003 2004 2005 2006 2007+ cable to shore deployed sector line connected first line deployment line 12 deployment SEPT 2002 2002: September 2002: deployment of sector line 2002: Building of the 'sector line' detector prototype line consistingof 5 floors (15 PMTs),final electronics. connected via finaljunciton box and cable October 2001: Deployment of 40 km cable to detector site March 2003 Connecting sector line to junction box ANTARES History and future sector line built and deployed

43. Prototype Sector Line: in the water median of countrate in 15 min. intervals baseline (kHz) fraction of time the rate exceeds 1.2 x baseline burst fraction periods of high activity, partially correlated with sea current impact on detector performance under study

44. Events above E (year-1) atmospheric neutrino background • atm 's have diffuse, steeply falling spectrum • rejection: • energy (look for hard, diffuse fluxes) • direction (look for point sources)..... • time (transients (GRB), correlate to other detectors) uncertainty on contribution from charmed meson decay (Costa, Astroparticle phys 16,2001) astrophysical diffuse flux shows up above atm. neutrino background

45. oscillations: e ≈ 1:1:1 for astrophysical source NC and CC e and t produce showers. Still focus on muons: good direction reconstruction increase effective volume

46. Atmospheric muon background rejection full simulation of 8 hours of atmospheric showers Contribution of mis- reconstructed atm. muons must be estimated from extrapolation :-( log(L)/Ndof Select events with >5.3: ~1 atmospheric muon/day 10.0 atmospheric neutrinos /day 74% efficient for E-2 signal events with an error < 1o