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

Recent Results from Searchesfor Astrophysical Neutrinos with the IceCube Neutrino Telescope

Alexander KappesSeminarAPC, Paris, June 14, 2013

2Alexander Kappes | Seminar, APC, Paris | 14.06.2013 |

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


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

109 1012 1015 1018 1021

energy (eV)

10-27

10-21

10-15

10-9

10-3

103

Flu

x (G

eV-1 m

-2 s

-1 s

r-1)

LHC(beam energy)

No one knows how the Universe looks like in neutrinos

→ expect many surprises !


Detection of cosmic neutrinos

muon

νμν-nucleoninteraction

(νμ + N → μ + X)

Time & position of hits

μ trajectory → ν trajectory

Energy

Light intensity


Current (planned) neutrino telescope projects

Baikal (GVD)

(KM3NeT)

IceCube

ANTARES


~250 authors from 39 institutes in 10 countries


-1450 m

-2450 m

The IceCube Observatory


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)


Optical ice properties

‣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)

dust logger data

ice model

10

Alexander Kappes | Seminar, APC, Paris | 14.06.2013 |

Cascades: Energy reconstruction (EM showers)

plot

sho

ws

stat

istic

al e

rror

onl

y

IceCube Preliminary

11


Cascade: Directional reconstruction

time delayvs. direct

light“on

time”delayed

12


Cascades: Directional reconstruction

hit time [μs]

num

ber

of p

hoto

ns

IceCube Preliminary

best directionreversed direction

13


Cascades: Directional reconstruction

resolution for an individual example event

from re-simulation

IceCube Preliminary

14

conventional νe

conventional νμ


Background: Atmospheric spectrum

prompt

c,(b)

νe,μe,μ

cosmic ray (p)

conventional

νμ

μ

νμνe

e

π

prompt νμ, νe

‣Prompt componentstill unmeasured

‣Is there a diffuse flux from unresolved cosmic sources?

‣Disentangling cosmic and prompt fluxes challenging

Waxman&Bahcall upper boundastrophysical neutrinos

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

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)

17


The high-energy tail from previoussearches in muons and cascades

18


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

AMANDAANTARES

atmosphericneutrinos

Models:AGN: Mannheim (1995)AGN: Mücke et al (2003)AGN: Stecker et al (2005)GRB: Waxman et al (1997)

‣Observed spectrum slightlyharder than predicted

→ Limit > sensitivity

19


High-energy cascades in IC40

+prompt

~220 TeV

one event intest sample

20


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)

IceCube Preliminary

21


Cosmogenic neutrinos in IC79+IC86

Two very interesting events

‣Shown at Neutrino’12

‣Both downgoing

‣Expected background: 0.082→ 2.8σ excess

“Bert”

“Ernie”

Aug. 9, 2011~1.0 PeVδ ≈ ‑28º

Jan. 3, 2012~1.1 PeVδ ≈ ‑67º

arXiv:1304.5356, accepted by PRL

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)

22


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


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


Follow-up analysis on the two PeV events

μ Veto

μ

νμ

✓

✘

25


Background 1 - Atmospheric Muons

Time/µs

Q/pe

250

0 31 2

Q/pe

Time/µs0 31 2

Q/pe

Time/µs0 31 2

Q/pe

Time/µs0 31 2

250dQ/dt

Q/pe

Time/µs0 31 2

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

μVeto νμ

T250= time at which Q= 250 pe

T250= time at which Q= 250 pe

Requirement: Qveto-region (T<T250) < 3 p.e.

→ remaining events: 6±3.4 (preliminary)

✗ ✔

✔

26


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)

atmospheric neutrino spectra

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

28


Results from follow-up search

29


Results of contained vertex event search

Track events (×) can have much higher neutrino energies(also true on smaller scale for CC events except νe)

Combined 4.1σ (preliminary)

30


Event distribution in detector: Vertex position

Uniform in fiducial volume(atmospheric muons would pile up at detector boundary)

31


Event distribution in detector: Directions (x vs. z)

IceCube Preliminary

32


Event distribution in detector: Directions (y vs. z)

IceCube Preliminary

33


Some example events

declination: -0.4°deposited energy: 71TeV

declination: -13.2°deposited energy: 82TeV

declination: 40.3°deposited energy: 253TeVIc

eCub

e P

relim

inar

y

IceC

ube

Pre

limin

ary

IceC

ube

Pre

limin

ary

34


Event Reconstruction

IceCubePreliminary

IceCubePreliminary

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)

35


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% e

ne

rgy:

10

% R

MS

zenith:9° RMS

Outliers included in systematic errors

IceCube Preliminary

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

37


Zenith distribution

‣Excess compatible with isotropic flux

‣Events from northern hemisphere absorbed in Earth

‣Minor excess in southern hemisphere but not significant

38


Significance skymap

‣Test of 28 events vs. uniform distribution in right ascension

‣Likelihood analysis using full-sky reconstruction

39


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

muon bkg.estimatedfrom data

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)

41


Backup

42


‣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


Scattering length compared to dust logger data

44


Tilt of dust layers

45


Directional Resolution for Showers

IceCube Preliminary

IceCube Preliminary

plot shows statistical error only

46


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

μVeto Tagging Region

IceCube Preliminary

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

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