Searches for ultra-high energy cosmic
neutrinos with the IceCube neutrino detector
石原安野（Aya Ishihara)
aya@hepburn.s.chiba-u.ac.jp
Chiba University

Aya Ishihara 2012/12/5 Kyoto University

The High Energy Deep Universe Mystery
The fisrt star
million years

Big Bang

The first galaxy
800million years

Present

Gamma-ray burst
AGN – Massive BH•

巨大ブラックホールである活動銀河核（AGN)や宇宙で最も激しい爆発現象であるガンマ線爆発（GRB)といった極限爆発現象は遠方（若い）宇宙に分布• Was young Universe more active, if so why/how?

•

起源のわからない高エネルギー粒子が宇宙から地球に飛来している。どこから？
エネルギーはLHC加速器の１０００万倍以上に達する（１００EeV＝１０keV(x-ray) x 10,000,000,000,000,000!)•

解明に向けた障壁

地球で観測される最高エネルギー粒子は電荷を持っているため宇宙磁場に曲げられてきており来た方向が分からない。

•

高エネルギー深宇宙を直接光で観測することが出来ない。
o

ガンマ線は３Kの宇宙背景輻射と反応してしまい、エネルギーを失ってしまうo

見られる範囲は10万光年（天の川銀河の直径）ほどで、現在の宇宙の大きさのわずか約1/47万

ガンマ線＞１００TeV

宇宙背景輻射
e+
e－
e－

荷電粒子

Neutrinos in the Astronomical Objects
high energy cosmic-ray sources, e.g. AGN, GRB…

Cosmicaccelerator
photonCMB

star-lightB

p + g  D+  p + p0  p + gg
p + g  D+  n + p+  n + m + n
2012/11/28

Aya Ishihara

Neutrino

Cosmicray

4

Why Ultra-high Energy Neutrinos?
PeV and above•

Cosmic frontier - PeV
gamma-ray horizon

limited to a few tens ofkpc (our galaxy radius)•

Cosmogenic neutrino
production is a‘guaranteed’ n source•

Energies above
dominant atmospheric
neutrinos

2012/11/28

Aya Ishihara

5

The highest energy neutrinos
cosmogenic neutrinos induced by the interactions of cosmic-ray and CMB photons
Off-Source (<50Mpc) astrophysical neutrino production via
GZK (Greisen-Zatsepin-Kuzmin) mechanism
p >100EeV
n
n
m
m
p+
ne
The main energy range: En ~ 108-10 GeV
+
m
p0
e+
pg 2.7 K  p + + X  m + + n  e + + n ' sCarries important physics• Location of the cosmic-ray
sources• Cosmological evolution of the
Variouscosmic-ray sources
GZK n• Cosmic-ray spectra at sources
models • The highest energy of thecosmc-rays• Composition of the cosmicrays• Particle physics beyond theenergies accelerators canreach2012/11/30

Aya Ishihara

6どのような宇宙ニュートリノ検出器が必要か？
ニュートリノ流量のエネルギー分布

超新星1987

大気ニュートリノ
宇宙ニュートリノ

超新星爆発ニュートリノ
1987年2月23日にIMB実験とカミオカ
ンデ実験により初めて観測（小柴昌俊氏
が2002年にノーベル物理学賞）爆発的天体現象でニュートリノが出来ることがわ
かった

初の宇宙ニュートリノ発見以来、さまざまな実験によって超新星爆発ニュートリノエネルギーを超える,他の天体からの宇宙ニュートリノの検出研究が続けられているが、25年間観測されていない。
何故か？
９桁以上のエネルギー領域にわたって
背景事象となる大気ニュートリノの量が宇宙ニュートリノ量より何桁も多く、宇宙ニュートリノを区別して観測することが出来ない！

High energy neutrino telescopes in the worldSince 1976 DUMAND, Lake Baikal. NESTOR, ANTARES, and NEMO…

ICECUBE at
the SouthPoleandtime to see

We arehere!

AMANDA/IceCube

The first test bed
in 1991/1992
2012/11/28

Aya Ishihara

8

The IceCube Collaboration
http://icecube.wisc.edu

36 institutions, ~270 members
CanadaUniversity of Alberta

US
Bartol Research Institute, Delaware
Pennsylvania State University
University of California - Berkeley
University of California - Irvine
Clark-Atlanta University
University of Maryland
University of Wisconsin - Madison
University of Wisconsin - River Falls
Lawrence Berkeley National Lab.
University of Kansas
Southern University, Baton Rouge
University of Alaska, Anchorage
University of Alabama, Tuscaloosa
Georgia Tech
Ohio State University

SwedenGermany

Universität Mainz
Uppsala Universitet
Stockholms Universitet DESY-Zeuthen
Universität Dortmund
Universität Wuppertal
UK
Humboldt-Universität zu Berlin
Oxford University
MPI Heidelberg
Japan
RWTH Aachen
Chiba
Universität Bonn
University
Ruhr-Universität Bochum

Belgium
Université Libre de Bruxelles
Vrije Universiteit Brussel
Universiteit Gent
Université de Mons-Hainaut

Switzerland
EPFL, Lausanne

Barbados

New Zealand

University of West Indies

2012/11/28

The first
results fromthe full
IceCube

ANTARCTICA
Amundsen-Scott Station

University of Canterbury

9

Aya Ishihara

The Largest Neutrino Detector in the world:

The IceCube Detector
1km

1.5km

Array of 78 sparseand 8 dense strings

5160
opticalsensors

South Pole Dome (old station)

Amundsen-Scott
South Pole station

1km
2012/11/28

Aya Ishihara

10

検出原理

暗く、しかし光をよく通す巨大なマテリアルνll,W, Z νlhadronic
showerm, t or cascades
チェレンコフ光

光電子増倍管の格子n11from MC simulation

Ultra-high Energy Signal Events
20PeV muon

Track

300PeV nu induced cascade

CascadeNot flavor sensitive except some special cases, however, we distinguish muon/tau
tracks induced by nu mu, nutau CC and cascades induced by nu e CC and NC by 3
flavors of neutrinos
12Comparison between IceCube’s 3D events• 丸一つ一つが光電子増倍管
and SK 2D events••super-KAMIOKANDE
検出器の大きさ40m x 40mx40m>15,000倍

丸の大きさが観測された光の強さ
色が光が届くタイミングICECUBE
検出器の大きさ
1000m x 1000m x 1000m

2Dから3Dへ
事象の大きさ6m x 6m

ICECUBE
事象の大きさ800m x 300m x 300m

A big challenge: IceCube検出器の建設
2006年に建設開始2010年末に建設終了 IC86 = full IceCube (2011~)

IC79 (2009-2010)

IC59 (2008-2009)

IC40 (2007-2008)

IC22 (2006-2007)14

Event rates
Strings

Data(year)

Livetime

AMANDAII(19) 2000-2006 3.8 years(Hz)HE ν rate(per day)100~5 / daytrigger rate

IC40

2008-09375 days

1100~40/ day

IC59

2009-10350 days

1900~70/ day

IC79

2010-11320 days

2250~100/dayIC86-I2011- 2012

~ year2700

processing

IC86-II

current

2700

running

IC86 achieving ~ 99% uptime
2012/11/28

Aya Ishihara

15

Neutrino Example

With 40 strings, 2009 May

2012/11/28

Aya Ishihara

16

IceCube Events
With 40 strings, 2008 Dec

With 79 strings, 2010 June

Energy threshold ~10 GeV>108 muons/day>100 neutrinos/day

2012/11/30

With 22 strings
2007

Aya Ishihara
130TeV

Cascade-like17

Digital Optical Module
•
•
•
•
•
•

PMT: 10 inch Hamamatsu
Waveforms, times digitized in each DOM
Power consumption: 3 W
Digitize at 300 MHz for 400 ns with custom chip
40 MHz for 6.4 μs with fast ADC• Dynamic range 500 photoelectron/15ns
Flasherboard with 12 LEDs
Local HV

Clock stability: 10-10 ≈ 0.1 nsec / sec
Synchronized to GPS time every ≈10 sec
Time calibration resolution = 2 nsec

25 cm PMT
33 cm Benthosphere

18

Waveform examples
from spe to 10000 pe

single pe level

Digitized
Waveform

7V

O(10000)peSC

4

2012/11/30

10

NPE = Integrated charge/PMT gain

Aya Ishihara

19

Time resolution:~1ns for bright pulses• Time difference

betweenneighboring DOMs
fired with (bright)
flasher pulses: 1 ns.

2012/11/30

Aya Ishihara

20

UHE neutrino analysis 2010-2012
Total 75 GBytes/day

from the South Pole to the North

Name in Filter

Actual BW
used(MB/day)

MuonFilter_11
SlopFilterTime_11
EHEFilter_11
SlopFilterTrig_11
DeepCoreFilter_11
CascadeFilter_11

18400
480
3500
2850
9040
8750

Rate ofselected
events
(Hz)
30.25
0.45
2.33
0.81
26.86
27.12

IceTopSTA3_InIceSMT_11 2100

3.17

IceTopSTA3_11
SDST_LowUp_11
SDST_VEF_11
GCLEStarting_11

2460
600
160
2070

6.40
31.36
7.96
6.62

SDST_GCMinBias_11

7900

270.16

SDST_GCHE_11

3040

104.38

SDST_GCNWStarting_11

4700

190.94FilterMinBias_11

860

2012/11/30

PhysicsMinBiasTrigger_11 290

2.69

Aya Ishihara

1.28

NPE > 1000

21

Data samples
Effective livetime of 670.1days
2010-2011 - 79 strings config.
May/31/2010-May/12/2011
Effective livetime 319.9days

2011-2012 – 86 strings config
May/13/2011-May14/2012
Effective livetime 350.1 days

IceCube Top View

9 strings (2006)
22 strings (2007)
40 strings (2008)
59 strings (2009)79 strings (2010)
86 strings (2011)

IceCube Top View

new strings

IceCube has been in a stable operation for more than 5 years
2012/11/30

Aya Ishihara

22

BackgroundAtmospheric muonsandAtmospheric neutrinos

2013/2/28

Aya Ishihara

23

Major background is cosmic-ray muons(muon bundles) in rates
O(100-1000) muon multiplicity

2012/11/30

Aya Ishihara

24

Signal and Background events
Signal Track

Muon bundles

MC

data
Burn sample
NPE~ 1x105
μ bundle with ~ 3PeV

data

25

Atmospheric muon distribution
Relatively easy to cut
them away

2012/11/30

Aya Ishihara

26

Basic strategies for search for GZK neutrinos

GZK signal

Atmospheric m
background(CORSIKA)
2012/11/30

Background energy smaller thansignalBackground is vertical downwardgoing while signal comes near horizon

Aya Ishihara

27

Atmospheric neutrinos in PeV••••

Conventional atmospheric neutrinos from decays of pion and kaons
Prompt atmospheric neutrinos form decays of heavy flavor short lived mesons(charm, bottom)
Prompt harder than conventional still steeper than astronomical spectra
Transition around 3 x 105 GeV depending on the models• No clear evidence of prompt
atmospheric n observed so far
Prompt Slope= 2.7

Physics of heavy flavor particle production

ourdefault
relatively
high but
notexcluded
Conventional Slope = 3.7
2012/11/30

Aya Ishihara

28

Atmospheric neutrinos in a wide energy range

Astrophysical Neutrinos

8
2012/11/30

Aya Ishihara

929pair-creation

The Energy estimation

e+e-

photo-nuclear
m and t tracks lose their energy byradiative processes

m

g

p

e+e-

dE
E
dx

bremsstrahlung
Energy of incoming particle ∝ Energy-losses in detector ∝ number of photoelectrons (NPE)channel # > 300

2012/11/30

Aya Ishihara

30

contained cascade

Energy NPE relations
Systematics

Cascademuon

2/28/2013

cascadeAya Ishihara

NPE and #of channels serve as a containment cut

31

Absolute Calibration of DOMs
QE×CE Absolute calibration
Reference PMT
Absolutely calibrated
X-stage
DOM

LED

NDfilter Slit
1mm

data

simulation

Reflectivity : 14.5%±0.73 Aya Ishihara – TMU July 3,
32
Nuclear Instruments and Methods A, 618 (2010)
Transmission : 50.7%±2.54
2011In-situ Absolute Calibration
Calibrated light source: Standard Candle (w. UC Berkeley) in-situ calibrated N2 pulsed laser
 light wavelength 337 nm
 at 100% intensity generates 4x1012 photons per pulse emitted at 41o output adjustable between 0.5% ~ 100%

33

The analysis flow in 2011-2012
zenith angle
reco.

Recalculation of NPE

coincident mcleaning

2012/11/30

log NPE > 3.5 ->Analysis Level sample

Aya Ishihara

34

Coincident m track cleaning
The “burn-sample” data
Example #1

After

BeforeExample #2

Before2012/11/30

After

Aya Ishihara

35

Analysis Level NPE Distributions
NPE distributions comparisons with burn sample

2/28/2013

Aya Ishihara

36

Analysis Level ZA Distributions
cos zenith angle distributions comparisons with burn sample

2/28/2013

Aya Ishihara

37

Analysis Level NPE vs ZA
IC86

Data

Background sum

SignalIC79

Data

Background sum

SignalBackground contributions
Background sum

atm mu+2012/11/30

coincident atm mu

conventional atm nu+

Aya Ishihara

39

Background event rates
BG
CR coinccontribution muons,

Totalbackground(IC79＋
IC86)

0.0004, 1%

Atmospheric m 0.0414
conventional
atm neutrinos,
0.022, 34%
CR single
muons,
0.0414, 65%

2012/11/30

Aya Ishihara

Atmospheric
0.0220conventional n

Coincidence m

0.0004

Total

0.064

prompt n0.13 3+/0.0007

Total withprompt0.197240

Signal Event Rates

Models

IceCube2010-2012per 615.9days

IceCube2008-2009
Phys. Rev D83 092003(2011)
333daysAll
contributions(B)Contributi
on> 100 PeVGZK (Yoshida m=4)*

0.57

2.0

1.9

GZK (Ahlers max) **

0.89

3.0

2.9

GZK (Ahlers best fit) **

0.43

1.5

1.4

GZK (Kotera, dip FRII) ***

4.2

2.7

GZK (Kotera, dip SFR1)***

0.9

0.6

2/28/2013

Aya Ishihara

41

Systematic Errors on Signal and BG(ice model, absolute sensitivity)(ice model, absolute sensitivity)

2/28/2013

Aya Ishihara

42Effective Area
Two years average

3 Flavors (1:1:1)sum• A factor of 2 increase from IC40
• NPE threshold difference changes the response below PeV• Larger for cascades than for track below 10PeV
2/28/2013

Aya Ishihara

43

After unblind - Observationof2
eventsRun118545-Event63733662
Run119316-Event36556705
NPE 9.628x104
GMT time: 2012/1/3 9:34:01

NPE 6.9928x104
GMT time: 2012/8/8 12:23:18

2 events / 615.9 days background (atm. m +conventional atm. n) expectation 0.060 events

Preliminary

p-value 1.8x10-3 (2.9 excess beyond conventional atmospheric neutrinos)(2.2 excess beyond bg with default prompt atmospheric neutrinos)
2012/11/30

Aya Ishihara

44

The highest Charged String Positions

Well
contained
events2012/11/30

Aya Ishihara

45

Two events observed in log NPE ~ 4.8-5.0
2011-2012

Final results are combined

2010-20112012/11/30

Aya Ishihara

46

Reconstruction of the two cascade
eventsDirection and Energy

2012/11/30

Aya Ishihara

47

Consistent with cascade events in detector
CC/NC interactions in the detector
Top view

MC

Side view

No indicationthat they are instrumental
artifactsthat they are cosmic-ray
muon induced
we can use dedicatedcascade hypothesis to thereconstructions of these special
events48

2012/11/30

Aya Ishihara

EHE-Jan-2012 Recorded pulses

63-26

Calibrated ATWD waveform above and below thehighest charged DOM (S63-29)

17m
63-27

S63-D29 and above
125m
63-28

63-29
S63-D29 and below

63-30

63-31

2012/11/30

Aya Ishihara

63-32

49

Event from Aug-2011
Calibrated ATWD waveform above and below the highest charged DOM (S53-23)“Bert”

S53-D23 and above

S53-D23 and below

2012/11/30

Aya Ishihara

50• Maximizing the Poisson likelihood
based on the recorded waveforms• Charges and timing information

Estimated Energy Deposit• Jan 2012 event 1.3 PeV• Aug 2011 event 1.14 PeV
Error of +- ~35% including absolute, ice, stat errors

Preliminary
2012/11/30
2/28/2013

Aya
Ishihara
Aya
Ishihara

51

51

log LLH distributions on directional
Downward-goingplane
PeV showers
zenith 11deg

zenith 67deg

2012/11/30

Aya Ishihara

52

PDF of the deposited energy
The “top-down” approach : Inject MC electrons with the event-relevant
phase space and reconstruct them by the same method
Preliminary

Jan 2012 event• Jan 2012 event 1.3 PeV• Aug 2011 event 1.14 PeV

Stat error~2%+
MC with the different ice models~15% (*)+
absolute sensitivity~10%+
Different reconstruction methods~10% (*)
 systematic error ~ 20-25%
but publically currently 35% is usedsince (*) not concluded
2012/11/30

Aya Ishihara

53

Deposited Energy  n Energy(At the IceCube depth = in-ice energy)ne  e + X (CC reaction) energy deposit = neutrino energy
nx  nx + X (NC reaction) energy deposit = a partial neutrino energy

Jan 2012 event

Aug 2011 event

Both events: ~1 PeV < En < ~4 PeV

CC
NC

2012/11/30

Aya Ishihara

54

In-ice n Energy  n Energy at the Earth surface
The in-earth n propagation effects
What is Ensurface that could induce the PeV event ?

when the primary n spectrum
(En) ~ En-2

when the primary n spectrum
(En) ~ a la GZK : harder spectrum

Log10(n energy at the Earth surface [GeV])

Log10(n energy at the Earth surface [GeV])

Sharp fall-off of ne and nm at 107 GeV
Note: Systematic errors NOT included

higher energy population should have converted
to the charge leptons (e or m) before reaching to
the IceCube instrumentation volume

EeV (=109 GeV) tail of nt

The regeneration: ntn

2012/11/30

Aya Ishihara

55

Earth-surface En probability All flavor sum (ne : nm : nt = 1:1:1)
Jan 2012 event
Aug 2011 event
Note: Systematic errors included“Bert”

Assuming E-2

Assuming GZK“Ernie”

Even assuming the hard GZK-type spectrum and the in-earth propagation effects,
probability above 100PeV is
2012/11/30

Aya Ishihara

56

ESS
YoshidaAhlers Max
Ahlers Best
Kotera FRII

Model energy
distributions

2/28/2013

Aya Ishihara

57

Two model tests
1. KS tests + full range event counting

c2 = -2 ln (pE) – 2 ln (Poisson(N=2,m))
The rate term
The energy term: p-value to the expected energy distribution
predicted by each of the cosmic n models
Use the Kolmogorov-Smirnov statistics

p-value = dlogE 
Bert(logE Bert) dlogE Ernie  Erine(logE Ernie) PKS(logE Bert, logE Ernie)

Bert

2. Counting events above 100 PeV
Most of the EHE n models predicts n with energies >100PeV
o Probability of two events being >100PeV is smallo2

N 100(1   )%   PnNn,(100   )%N 100(1   )%1
mn + m BG

2012/11/30

n 0

Pn probability of n events above 100 PeVAya Ishihara

p-value

58

The KS test
“Standard” GZK

The predicted En cumulative
probability“Low Energy Enhanced” GZK

Data

p-values O(1%)

2012/11/30

Aya Ishihara

59

Results from KS tests + full range event counting
Combined with IC40

Neutrino
Model

KS TestPE

Expected
Event Rate

PoissonSignificanceFinalp-values

6.0x10-2

2.8

5.5x10-1

1.5x10-1

GZK
Yoshida/Teshima

m=4, Zmax=4

Marginally excluded

GZK
AhlersFermi Best

6.0x10-2

GZK
KoteraFR-II

3.4x10-2

GZK
KoteraGRB

4.4x10-2

2012/11/30

2.1

7.3x10-1

1.8x10-1
Marginally excluded

5.9

3.8x10-2

1.0x10-2
Excluded by 95% C.L.

1.1

4.2x10-1

9.2x10-2
Excluded by 90% C.L.

Aya Ishihara

60

Event rates(>100 PeV) and p-valuesnTopdown
GUT

GZK
Y&T

GZK
Sigl

GZK
Ahler

GZK
Ahler

GZK
KoteraGZK
Koteram=4,zmax=4

m=5, zmax=3

Fermi Best

Fermi Max

FR-II

SFR/GRB

2.6

4.0

2.0

4.1

3.8

0.6

5.0

Model
Rejection
Factor

0.98

0.65

1.27

0.64

0.69

3.6

0.53

p-value

9.6x10-2

2.4x10-2

1.6x10-1

2.3x10-2

3.1x10-2

6.7x10-1

<10-2

Model

Rate
>100PeV

Excluded
Mildly Excluded
Consistent
2012/11/30

Aya Ishihara

61

Differential Upper limits (Systematics included )90% C.L. upperlimit

F-C Upper fluctuation•••

A factor of ~4 improved
from the previous IceCube
results
The world’s best
sensitivity!
Will constrain theneutrino fluxes down to
mid-strong cosmologicalevolution models

Still at least a factorof 2 improvement tocome in 2 years
2012/11/30

Aya Ishihara

62

A short summary on the present constraints• None of the GZK cosmogenic n scenarios appears

consistent with the present observation.
Energies of the two events are too low• A model predicting n spectrum extending
beyond ~100 PeV would not account the observation.
IceCube have a greater exposure at EeV

Additional note: no sensitivity if UHECRs are heavy nuclei dominated2012/11/30

Aya Ishihara

63

Then what are they?
Reducing the energy threshold
Improvements in 100TeV region from the starting event analysis

Aya Ishihara

64

Starting event analysis• Bright (>6000pe)oNathan Whitehorn and Claudio Kopper (UW)

was > 63000pe, this reduces energy threshold an order• Outer veto cut to reject backgroundo

Events started in detector

Aya Ishihara

65Summary• Searched for neutrinos with PeV and greater energies in nearly full 2•

years of the IceCube data
IceCube is the largest neutrino detector and rejection of the atmospheric
neutrinos was achieved by setting energy threshold• Two candidate events observed
PeV to 10PeV energy cascade-channel neutrino events (CC/NC interactions
within the detector)
o The highest energy neutrino events observed ever!
o• Very likely beyond the conventional atmospheric neutrinos• Hints for the PeV events origin from different energy-region coming!?
26 additional starting events foundo p-values to be calculated – initial expectation ~3sigma beyond prompt taking
zenith angle information into accounto No events near PeV energies… The PeV 2 events population is a bit away from
E-2 flux expected from lower energy regiono• New information coming next years from IceCube would be very
interesting

2012/11/30

Aya Ishihara66

2/28/2013

Aya Ishihara

67

2/28/2013

Aya Ishihara

682/28/2013

Aya Ishihara69

2/28/2013

Aya Ishihara

70

Zenith angle resolutions••

resolution (RMS) for background is ~1.2 deg
resolution (RMS) for signal is ~10 deg due to its stochastic natureooresolution improves with increasing Nch
No strong dependence with NPE

resolution for Corsika cr muons

resolution for signal mu, tau tracks
qreco – qMC truth [deg]qreco – qMC truth [deg]tracksm

2012/11/30

in-ice m

Aya Ishihara

in-ice t

71