recent results from searches for astrophysical neutrinos with the icecube neutrino telescope
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Recent Results from Searches for Astrophysical Neutrinos with the IceCube Neutrino Telescope. Alexander Kappes Seminar APC, Paris, June 14, 2013. Outline. The IceCube neutrino observatory Event reconstruction in IceCube Previous results from diffuse searches in muons and cascades - PowerPoint PPT PresentationTRANSCRIPT
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
3Alexander Kappes | Seminar, APC, Paris | 14.06.2013 |
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 !
4Alexander Kappes | Seminar, APC, Paris | 14.06.2013 |
Detection of cosmic neutrinos
muon
νμν-nucleoninteraction
(νμ + N → μ + X)
Time & position of hits
μ trajectory → ν trajectory
Energy
Light intensity
5Alexander Kappes | Seminar, APC, Paris | 14.06.2013 |
Current (planned) neutrino telescope projects
Baikal (GVD)
(KM3NeT)
IceCube
ANTARES
6Alexander Kappes | Seminar, APC, Paris | 14.06.2013 |
~250 authors from 39 institutes in 10 countries
7Alexander Kappes | Seminar, APC, Paris | 14.06.2013 |
-1450 m
-2450 m
The IceCube Observatory
8Alexander Kappes | Seminar, APC, Paris | 14.06.2013 |
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)
9Alexander Kappes | Seminar, APC, Paris | 14.06.2013 |
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
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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
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Alexander Kappes | Seminar, APC, Paris | 14.06.2013 |
Cascade: Directional reconstruction
time delayvs. direct
light“on
time”delayed
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Alexander Kappes | Seminar, APC, Paris | 14.06.2013 |
Cascades: Directional reconstruction
hit time [μs]
num
ber
of p
hoto
ns
IceCube Preliminary
best directionreversed direction
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Alexander Kappes | Seminar, APC, Paris | 14.06.2013 |
Cascades: Directional reconstruction
resolution for an individual example event
from re-simulation
IceCube Preliminary
14
conventional νe
conventional νμ
Alexander Kappes | Seminar, APC, Paris | 14.06.2013 |
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
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Alexander Kappes | Seminar, APC, Paris | 14.06.2013 |
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
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Alexander Kappes | Seminar, APC, Paris | 14.06.2013 |
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)
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Alexander Kappes | Seminar, APC, Paris | 14.06.2013 |
The high-energy tail from previoussearches in muons and cascades
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Alexander Kappes | Seminar, APC, Paris | 14.06.2013 |
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
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Alexander Kappes | Seminar, APC, Paris | 14.06.2013 |
High-energy cascades in IC40
+prompt
~220 TeV
one event intest sample
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Alexander Kappes | Seminar, APC, Paris | 14.06.2013 |
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
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Alexander Kappes | Seminar, APC, Paris | 14.06.2013 |
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)
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Alexander Kappes | Seminar, APC, Paris | 14.06.2013 |
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
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Alexander Kappes | Seminar, APC, Paris | 14.06.2013 |
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
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Alexander Kappes | Seminar, APC, Paris | 14.06.2013 |
Follow-up analysis on the two PeV events
μ Veto
μ
νμ
✓
✘
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Alexander Kappes | Seminar, APC, Paris | 14.06.2013 |
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)
✗ ✔
✔
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Alexander Kappes | Seminar, APC, Paris | 14.06.2013 |
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
Alexander Kappes | Seminar, APC, Paris | 14.06.2013 |
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
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Alexander Kappes | Seminar, APC, Paris | 14.06.2013 |
Results from follow-up search
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Alexander Kappes | Seminar, APC, Paris | 14.06.2013 |
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)
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Alexander Kappes | Seminar, APC, Paris | 14.06.2013 |
Event distribution in detector: Vertex position
Uniform in fiducial volume(atmospheric muons would pile up at detector boundary)
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Alexander Kappes | Seminar, APC, Paris | 14.06.2013 |
Event distribution in detector: Directions (x vs. z)
IceCube Preliminary
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Alexander Kappes | Seminar, APC, Paris | 14.06.2013 |
Event distribution in detector: Directions (y vs. z)
IceCube Preliminary
33
Alexander Kappes | Seminar, APC, Paris | 14.06.2013 |
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
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Alexander Kappes | Seminar, APC, Paris | 14.06.2013 |
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)
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Alexander Kappes | Seminar, APC, Paris | 14.06.2013 |
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
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Alexander Kappes | Seminar, APC, Paris | 14.06.2013 |
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
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Alexander Kappes | Seminar, APC, Paris | 14.06.2013 |
Zenith distribution
‣Excess compatible with isotropic flux
‣Events from northern hemisphere absorbed in Earth
‣Minor excess in southern hemisphere but not significant
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Alexander Kappes | Seminar, APC, Paris | 14.06.2013 |
Significance skymap
‣Test of 28 events vs. uniform distribution in right ascension
‣Likelihood analysis using full-sky reconstruction
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Alexander Kappes | Seminar, APC, Paris | 14.06.2013 |
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
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Alexander Kappes | Seminar, APC, Paris | 14.06.2013 |
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)
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Backup
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Alexander Kappes | Seminar, APC, Paris | 14.06.2013 |
‣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
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Scattering length compared to dust logger data
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Alexander Kappes | Seminar, APC, Paris | 14.06.2013 |
Tilt of dust layers
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Alexander Kappes | Seminar, APC, Paris | 14.06.2013 |
Directional Resolution for Showers
IceCube Preliminary
IceCube Preliminary
plot shows statistical error only
46
Alexander Kappes | Seminar, APC, Paris | 14.06.2013 |
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