Recent Results from Searches for Astrophysical Neutrinos with the IceCube Neutrino Telescope Alexander Kappes Seminar APC, Paris, June 14, 2013

2 Alexander Kappes | Seminar, APC, Paris | | 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

3 Alexander Kappes | Seminar, APC, Paris | | 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...) Cosmic-ray spectrum energy (eV) Flux (GeV -1 m -2 s -1 sr -1 ) LHC (beam energy) No one knows how the Universe looks like in neutrinos → expect many surprises !

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

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

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

7 Alexander Kappes | Seminar, APC, Paris | | m m The IceCube Observatory

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

9 Alexander Kappes | Seminar, APC, Paris | | Optical ice properties ‣ Photon propagation dominated by scattering λ eff.scat 5 − 100 m λ abs 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) dust logger data ice model

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

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

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

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

14 conventional ν e conventional ν μ Alexander Kappes | Seminar, APC, Paris | | Background: Atmospheric spectrum prompt c,(b) ν e,μ e,μ cosmic ray (p) conventional νμνμ μ νμνμ νeνe e π prompt ν μ, ν e ‣ Prompt component still unmeasured ‣ Is there a diffuse flux from unresolved cosmic sources? ‣ Disentangling cosmic and prompt fluxes challenging Waxman&Bahcall upper bound astrophysical neutrinos

15 Alexander Kappes | Seminar, APC, Paris | | 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 to Earth absorption Background (atmospheric) ‣ Track-like from atmospheric muons and neutrinos ‣ Soft spectrum (E -3.7 ‑ E -2.7 ) ‣ Muons in southern and neutrinos in northern hemisphere

16 Alexander Kappes | Seminar, APC, Paris | | 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 ν μ dominated by background, probes physics of production process ‣ Zenith distribution: comparison to backgrounds, probes source location (together with azimuth)

17 Alexander Kappes | Seminar, APC, Paris | | The high-energy tail from previous searches in muons and cascades

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

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

20 Alexander Kappes | Seminar, APC, Paris | | Cosmogenic neutrinos in IC79+IC86 Aim: Simple search to look for extremely high-energy (10 9 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) IceCube Preliminary

21 Alexander Kappes | Seminar, APC, Paris | | Cosmogenic neutrinos in IC79+IC86 Two very interesting events ‣ Shown at Neutrino’12 ‣ Both downgoing ‣ Expected background: → 2.8σ excess “Bert” “Ernie” Aug. 9, 2011 ~1.0 PeV δ ≈ ‑ 28º Jan. 3, 2012 ~1.1 PeV δ ≈ ‑ 67º arXiv: , accepted by PRL What we had learned ‣ At least two PeV neutrinos in two-year dataset ‣ Events are downgoing ‣ Don’t seem to be cosmogenic ‣ More than expected from atmospheric 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)

22 Alexander Kappes | Seminar, APC, Paris | | 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) ? Ernie Bert → Needed more statistics to answer all of these

23 Alexander Kappes | Seminar, APC, Paris | | 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

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

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

26 Alexander Kappes | Seminar, APC, Paris | | 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) ‣ ‑ 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) atmospheric neutrino spectra

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

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

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

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

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

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

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

34 Alexander Kappes | Seminar, APC, Paris | | Event Reconstruction IceCube Preliminary IceCube Preliminary 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 ν τ )

35 Alexander Kappes | Seminar, APC, Paris | | 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% energy: 10% RMS zenith: 9° RMS Outliers included in systematic errors IceCube Preliminary

36 Alexander Kappes | Seminar, APC, Paris | | Distribution of deposited energy ‣ Harder than expected from atmospheric background ‣ Merges well into expected backgrounds at low energies ‣ Potential cutoff at ‑ 0.4 PeV

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

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

39 Alexander Kappes | Seminar, APC, Paris | | 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 disfavors atmospheric explanation (air showers missing) ‣ Compatible with isotropic flux ‣ No evidence for clustering muon bkg. estimated from data

40 Alexander Kappes | Seminar, APC, Paris | | 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)

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

42 Alexander Kappes | Seminar, APC, Paris | | ‣ 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° Pointing accuracy IceCube 59 strings

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

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

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

46 Alexander Kappes | Seminar, APC, Paris | | Estimating Muon Background From Data 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 Veto Tagging Region IceCube Preliminary