Recent Results from Searches for Astrophysical Neutrinos with the IceCube Neutrino Telescope

1. Recent Results from Searchesfor Astrophysical Neutrinos with the IceCube Neutrino Telescope • Alexander Kappes • Seminar • APC, Paris, June 14, 2013

2. Outline • The IceCube neutrino observatory • Event reconstruction in IceCube • Previous results from diffuse searches in muons and cascades • The two PeV events “Ernie” and “Bert” • Results from a follow-up search on the two PeV events Alexander Kappes | Seminar, APC, Paris | 14.06.2013 |

3. Cosmic-ray spectrum No one knows how the Universe looks like in neutrinos → expect many surprises ! 103 10-3 Flux (GeV-1 m-2 s-1 sr-1) 10-9 10-15 10-21 LHC (beam energy) 10-27 109 1012 1015 1018 1021 energy (eV) Mission of neutrino telescopes • Search for the sources of the cosmic rays (CRs) • What are the accelerators? • How do they work? • Search for diffuse neutrino fluxes • Is there a cosmogenic flux? (interaction of CRs with CMB) • Does the Galactic plane shine in neutrinos? • Are there extended structures? • What is the nature of Dark Matter? • Are there exotic particles in the Universe?(magnetic monopoles, Q-balls . . .) Alexander Kappes | Seminar, APC, Paris | 14.06.2013 |

4. Time & position of hits Light intensity Energy μ trajectory → ν trajectory Detection of cosmic neutrinos muon νμ ν-nucleon interaction (νμ + N → μ + X) Alexander Kappes | Seminar, APC, Paris | 14.06.2013 |

5. Current (planned) neutrino telescope projects Baikal (GVD) ANTARES (KM3NeT) IceCube Alexander Kappes | Seminar, APC, Paris | 14.06.2013 |

6. ~250 authors from 39 institutes in 10 countries Alexander Kappes | Seminar, APC, Paris | 14.06.2013 |

7. The IceCube Observatory -1450 m -2450 m Alexander Kappes | Seminar, APC, Paris | 14.06.2013 |

8. Neutrino signatures • Track-like: • Source: νμ CC interaction • Good angular resolution (< 1°) • Factor of 2 resolution in muon energy • Sensitive ≫ instrumented volume • Cascade-like: • Source: νe, νμ, ντ NC + νe CC interaction • Good energy resolution (≳ 10%) • Limited angular resolution ( ≳ 10° ) • Sensitive ≈ instrumented volume • Composites: • Source: ντ CC + νμ CC inside instrumented volume • Challenging to reconstruct muon (data) cascade (data) tau (simulation) Alexander Kappes | Seminar, APC, Paris | 14.06.2013 |

9. ice model Optical ice properties dust logger data • Photon propagation dominated by scatteringλeff.scat 5 − 100 mλabs 20 − 250 m • Ice below South Pole inhomogeneous (horizontal “dust layers”) • Calibration devices • Dust logger (8 holes across detector) • LED flashers (12 on each DOM) • In-ice calibration laser (2) • Uncertainty on ice properties ~10% IceCube, NIM A711 (2013) Alexander Kappes | Seminar, APC, Paris | 14.06.2013 |

10. Cascades: Energy reconstruction (EM showers) IceCube Preliminary plot shows statistical error only Alexander Kappes | Seminar, APC, Paris | 14.06.2013 |

11. time delay vs. direct light delayed “on time” Cascade: Directional reconstruction Alexander Kappes | Seminar, APC, Paris | 14.06.2013 |

12. best direction reversed direction number of photons hit time [μs] Cascades: Directional reconstruction IceCube Preliminary Alexander Kappes | Seminar, APC, Paris | 14.06.2013 |

13. Cascades: Directional reconstruction IceCube Preliminary resolution for an individual example event from re-simulation Alexander Kappes | Seminar, APC, Paris | 14.06.2013 |

14. c,(b) Waxman&Bahcall upper bound cosmic ray (p) astrophysical neutrinos prompt νμ, νe νe,μ e,μ prompt conventional νμ conventional νe π μ νμ e conventional νμ νe Background: Atmospheric spectrum • Prompt componentstill unmeasured • Is there a diffuse flux from unresolved cosmic sources? • Disentangling cosmic and prompt fluxes challenging Alexander Kappes | Seminar, APC, Paris | 14.06.2013 |

15. Signals and backgrounds • Signal (astrophysical) • Cascade dominated from full mixing(~80% per volume) • Expected to be high energy(typically E-2) • Mostly in southern sky due toEarth absorption • Background (atmospheric) • Track-like from atmospheric muonsand neutrinos • Soft spectrum(E-3.7 ‑ E-2.7) • Muons in southern and neutrinos in northern hemisphere Alexander Kappes | Seminar, APC, Paris | 14.06.2013 |

16. Observables of interest • Spectral slope: separate extraterrestrial fluxes from atmospheric,probe properties of source • Existence of a cutoff: maximum energy of source;(galactic ↔ extragalactic) • Flavor composition: discriminates against νμ dominated by background,probes physics of production process • Zenith distribution: comparison to backgrounds,probes source location (together with azimuth) Alexander Kappes | Seminar, APC, Paris | 14.06.2013 |

17. The high-energy tail from previoussearches in muons and cascades Alexander Kappes | Seminar, APC, Paris | 14.06.2013 |

18. Observed spectrum slightlyharder than predicted → Limit > sensitivity Models: AGN: Mannheim (1995) AGN: Mücke et al (2003) AGN: Stecker et al (2005) GRB: Waxman et al (1997) atmospheric neutrinos AMANDA ANTARES W&B bound Atmospheric muon neutrinos in IC59 • Non-significant excess (1.8σ) in high-energy tail found IC59 atm. ν cosmic ν (E-2) Alexander Kappes | Seminar, APC, Paris | 14.06.2013 |

19. +prompt ~220 TeV High-energy cascades in IC40 one event in test sample Alexander Kappes | Seminar, APC, Paris | 14.06.2013 |

20. IceCube Preliminary Cosmogenic neutrinos in IC79+IC86 • Aim: Simple search to look for extremely high-energy (109 GeV) neutrinos from proton interaction with CMB • Upgoing muons • always neutrinos • background: atmospheric neutrinos • high energy threshold (1 PeV) • Downgoing muons • atmospheric muon background • very high energy threshold (100 PeV) Alexander Kappes | Seminar, APC, Paris | 14.06.2013 |

21. arXiv:1304.5356, accepted by PRL Cosmogenic neutrinos in IC79+IC86 Aug. 9, 2011 ~1.0 PeV δ ≈ ‑28º “Bert” • Two very interesting events • Shown at Neutrino’12 • Both downgoing • Expected background: 0.082→ 2.8σ excess • What we had learned • At least two PeV neutrinos intwo-year dataset • Events are downgoing • Don’t seem to be cosmogenic • More than expected fromatmospheric background • Compatible with IC59 upper limit • Spectrum doesn’t seem to extend to much higher energies(unbroken E-2 would have produced 8‑9 more events above 1 PeV) Jan. 3, 2012 ~1.1 PeV δ ≈ ‑67º “Ernie” Alexander Kappes | Seminar, APC, Paris | 14.06.2013 |

22. Ernie Bert Things we wanted to learn • Isolated events or tail of spectrum? • Spectral slope/cutoff • Flavor composition (ratio tracks/cascades) • Where do they come from? • Astrophysical or air-shower physics (e.g. charm)? → Needed more statistics to answer all of these Alexander Kappes | Seminar, APC, Paris | 14.06.2013 |

23. Neutrino identification • How to identify neutrinos? • Upgoing (through-going) muon tracks • filter out atmospheric muons with bulk of Earth • unknown vertex → hard to measure energy • Excess over background (all directions) • works only for extremely bright/high-energy neutrinos • Contained vertex • filter out atmospheric muons using outer detector layer for anti-coincidence • neutrino vertex observed → good energy estimation Alexander Kappes | Seminar, APC, Paris | 14.06.2013 |

24. μ νμ Veto μ Follow-up analysis on the two PeV events ✘ ✓ Alexander Kappes | Seminar, APC, Paris | 14.06.2013 |

25. Q/pe Q/pe Q/pe μ νμ Q/pe (cumulative) ✗ Q/pe ✔ Q/pe Veto dQ/dt 250 250 2 2 2 3 3 0 3 1 0 0 1 1 2 2 3 3 Time/µs 0 0 1 1 Time/µs Time/µs Time/µs ✔ Requirement: Qveto-region (T<T250) < 3 p.e. → remaining events: 6±3.4 (preliminary) T250= time at which Q= 250 pe T250= time at which Q= 250 pe Background 1 - Atmospheric Muons Through-going muon Contained cascade Total detector Total detector dQ/dt Time/µs Veto region Veto region – barely contained cascade Veto region – well contained cascade Alexander Kappes | Seminar, APC, Paris | 14.06.2013 |

26. atmospheric neutrino spectra Background 2 ‑ Atmospheric neutrinos • Typically separated by energy • Very low at PeV energies (order of 0.1 events/year) • Large uncertainties in spectrum at high energies(normalization of prompt component) • 4.6+3.3‑1.2 events expected in two years (662 days)(preliminary) • Southern sky: atmospheric neutrinos vetoed by accompanying muons from same air shower !(effective above ~100 TeV) • Prompt baseline model: Enberg et al., PRD 78 (2008)(updated with cosmic-ray “Knee” model) Alexander Kappes | Seminar, APC, Paris | 14.06.2013 |

27. Effective area / volumes • Effective Area • Differences at low energies dueto constant charge threshold Q • Peak at 6.3 PeV due to Glashowresonance (only νe) • Effective volume • Fully efficient above 100 TeV for CC electron neutrinos • About 400 Mton effective target mass Effective area Effective volume Alexander Kappes | Seminar, APC, Paris | 14.06.2013 |

28. Results from follow-up search Alexander Kappes | Seminar, APC, Paris | 14.06.2013 |

29. Results of contained vertex event search Combined 4.1σ (preliminary) Track events (×) can have much higher neutrino energies(also true on smaller scale for CC events except νe) Alexander Kappes | Seminar, APC, Paris | 14.06.2013 |

30. Event distribution in detector: Vertex position • Uniform in fiducial volume(atmospheric muons would pile up at detector boundary) Alexander Kappes | Seminar, APC, Paris | 14.06.2013 |

31. Event distribution in detector: Directions (x vs. z) IceCube Preliminary Alexander Kappes | Seminar, APC, Paris | 14.06.2013 |

32. Event distribution in detector: Directions (y vs. z) IceCube Preliminary Alexander Kappes | Seminar, APC, Paris | 14.06.2013 |

33. Some example events IceCube Preliminary IceCube Preliminary IceCube Preliminary declination: -13.2° deposited energy: 82TeV declination: -0.4° deposited energy: 71TeV declination: 40.3° deposited energy: 253TeV Alexander Kappes | Seminar, APC, Paris | 14.06.2013 |

34. Tracks • Good angular resolution (<1º) • Inherently worse resolution on energy due to leaving muon • Cascades • Larger uncertainties on angle (about 10°-15°) • Good resolution on deposited energy(might not be total energy for NC and ντ) IceCube Preliminary IceCube Preliminary Event Reconstruction Alexander Kappes | Seminar, APC, Paris | 14.06.2013 |

35. IceCube Preliminary zenith: 9° RMS energy: 10% RMS Outliers included in systematic errors Systematic studies and cross-checks • Systematics were checked using an extensive per-event re-simulation(analysis repeated with ice model and energy scale varied within uncertainties) • Second fit method based on continuous re-simulation of events • can include ice systematics in fit !(anisotropy in scattering angle, tilted dust layers) • very slow • Comparison to standard method: all results compatible to within 10% Alexander Kappes | Seminar, APC, Paris | 14.06.2013 |

36. Distribution of deposited energy • Harder than expected fromatmospheric background • Merges well into expected backgrounds at low energies • Potential cutoff at 1.6+1.5‑0.4 PeV Alexander Kappes | Seminar, APC, Paris | 14.06.2013 |

37. Zenith distribution • Excess compatible with isotropic flux • Events from northern hemisphere absorbed in Earth • Minor excess in southern hemisphere but not significant Alexander Kappes | Seminar, APC, Paris | 14.06.2013 |

38. Significance skymap • Test of 28 events vs. uniform distribution in right ascension • Likelihood analysis using full-sky reconstruction Alexander Kappes | Seminar, APC, Paris | 14.06.2013 |

39. muon bkg. estimated from data What have we found? • Events seem to be neutrinos • Energy spectrum very hard, but with cutoff • Flavor distribution consistent with ( 1 : 1 : 1 ) • Angular distribution disfavorsatmospheric explanation(air showers missing) • Compatible with isotropic flux • No evidence for clustering Alexander Kappes | Seminar, APC, Paris | 14.06.2013 |

40. Conclusions • Two 1 PeV neutrinos observed at threshold of search for cosmogenic neutrinos (significance 2.8σ) • Follow-up analysis reveals 26 more events at lower energies(preliminary significance 3.3σ) • Increasing evidence for high-energy component beyond atmospheric spectrum • inconsistent with standard atmospheric backgrounds at 4.1σ (preliminary) • neutrinos from charm decay an uncertainty factor but unlikely to explain PeV events • Less clear what it actually is . . . • compatible with isotropic astrophysical flux with PeV cutoff • no clustering • Publication in preparation • More data coming soon (one more year of data already waiting on disk) Alexander Kappes | Seminar, APC, Paris | 14.06.2013 |

41. Backup Alexander Kappes | Seminar, APC, Paris | 14.06.2013 |

42. Pointing accuracy • No neutrino reference point-source to validate absolute pointing • Use lack of atmospheric muons from Moon direction (point-sink) • Moon diameter 0.5° • Angular muon resolution < 1° • Observed in IC59 with > 12σ • Pointing accuracy < 0.2° IceCube 59 strings Alexander Kappes | Seminar, APC, Paris | 14.06.2013 |

43. Scattering length compared to dust logger data Alexander Kappes | Seminar, APC, Paris | 14.06.2013 |

44. Tilt of dust layers Alexander Kappes | Seminar, APC, Paris | 14.06.2013 |

45. Directional Resolution for Showers IceCube Preliminary plot shows statistical error only Alexander Kappes | Seminar, APC, Paris | 14.06.2013 | IceCube Preliminary

46. Estimating Muon Background From Data Veto Tagging Region • Add one layer of DOMs on the outside to tag known background events • Then use these events to evaluate the veto efficiency • Avoids systematics from simulation assumptions/models! • Can be validated at charges below our cut (6000 p.e.) where background dominates μ Alexander Kappes | Seminar, APC, Paris | 14.06.2013 | IceCube Preliminary