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Neutrino Signals from Dark Matter Decay
Michael Grefe
Deutsches Elektronen-SynchrotronDESY, Hamburg, Germany
Presentation Skills SeminarDESY Hamburg – 18 January 2011
Based on work in collaboration with Laura Covi, Alejandro Ibarra and David Tran: JCAP 1004 (2010) 017Michael Grefe (DESY Hamburg) Neutrino Signals from Dark Matter Decay Presentation Skills – 18 January 2011 1 / 13
Outline
◮ Motivation• Why are we studying cosmic ray signals in the context of dark matter searches?
◮ Neutrino Signals and Neutrino Detection• What neutrino signal from dark matter decay can be observed at Earth?
• How will these signals look in a neutrino detector?
◮ Neutrino Constraints on Decaying Dark Matter• How useful are neutrinos to detect or to constrain decaying dark matter?
◮ Conclusion
Michael Grefe (DESY Hamburg) Neutrino Signals from Dark Matter Decay Presentation Skills – 18 January 2011 2 / 13
Motivation
What do we know about dark matter?
◮ Dark matter is observed on various scales through its gravitational interaction
◮ Dark matter contributes significantly to the energy density of the universe
◮ Dark matter properties known from cosmological observations:
• No electromagnetic interactions (dark)
• Weak-scale (or smaller) interactions
• Non-baryonic
• Cold (maybe warm)
• Very long-lived (but not necessarily stable!)
Michael Grefe (DESY Hamburg) Neutrino Signals from Dark Matter Decay Presentation Skills – 18 January 2011 3 / 13
Motivation
What do we know about dark matter?
◮ Dark matter is observed on various scales through its gravitational interaction
◮ Dark matter contributes significantly to the energy density of the universe
◮ Dark matter properties known from cosmological observations:
• No electromagnetic interactions (dark)
• Weak-scale (or smaller) interactions
• Non-baryonic
• Cold (maybe warm)
• Very long-lived (but not necessarily stable!)
Particle dark matter could be a (super)WIMP with lifetime ≫ age of the Universe!
Michael Grefe (DESY Hamburg) Neutrino Signals from Dark Matter Decay Presentation Skills – 18 January 2011 3 / 13
Motivation
How can we unveil the nature of dark matter?
◮ There are three strategies for detecting dark matter based on non-gravitationalinteractions:
• Production of dark matter particles at colliders
• Direct detection of dark matter in the scattering off nuclei
• Indirect detection of dark matter in cosmic ray signatures
Michael Grefe (DESY Hamburg) Neutrino Signals from Dark Matter Decay Presentation Skills – 18 January 2011 4 / 13
Motivation
How can we unveil the nature of dark matter?
◮ There are three strategies for detecting dark matter based on non-gravitationalinteractions:
• Production of dark matter particles at colliders
• Direct detection of dark matter in the scattering off nuclei
• Indirect detection of dark matter in cosmic ray signatures
A combination of these search strategies will be necessary to reliably identify theparticle nature of the dark matter!
Michael Grefe (DESY Hamburg) Neutrino Signals from Dark Matter Decay Presentation Skills – 18 January 2011 4 / 13
Motivation
Why are we interested in neutrino signals?
◮ Recently, anomalies were observed in the cosmic ray positron and electron spectra
Energy (GeV)0.1 1 10 100
))
-(eφ
)+
+(eφ
) / (
+(eφ
Po
sitr
on
fra
ctio
n
0.01
0.02
0.1
0.2
0.3
0.4
Muller & Tang 1987
MASS 1989
TS93
HEAT94+95
CAPRICE94
AMS98
HEAT00
Clem & Evenson 2007
PAMELA
[PAMELA Coll. (2008)] [Fermi LAT Coll. (2010)]
◮ New sources of positrons and electrons are needed to explain the spectra:• Astrophysical sources (e.g. pulsars)
• Dark matter annihilations
• Dark matter decays (typically a lifetime 1026 s is needed)
◮ To obtain a consistent dark matter explanation, a multi-messenger approach thatincludes all cosmic ray channels will be necessary: This includes gamma rays,positrons, antiprotons, antideuterons, and neutrinos!
Michael Grefe (DESY Hamburg) Neutrino Signals from Dark Matter Decay Presentation Skills – 18 January 2011 5 / 13
Motivation
Why are we interested in neutrino signals?
◮ Recently, anomalies were observed in the cosmic ray positron and electron spectra
Energy (GeV)0.1 1 10 100
))
-(eφ
)+
+(eφ
) / (
+(eφ
Po
sitr
on
fra
ctio
n
0.01
0.02
0.1
0.2
0.3
0.4
Muller & Tang 1987
MASS 1989
TS93
HEAT94+95
CAPRICE94
AMS98
HEAT00
Clem & Evenson 2007
PAMELA
[PAMELA Coll. (2008)] [Fermi LAT Coll. (2010)]
◮ New sources of positrons and electrons are needed to explain the spectra:• Astrophysical sources (e.g. pulsars)
• Dark matter annihilations
• Dark matter decays (typically a lifetime 1026 s is needed)
◮ To obtain a consistent dark matter explanation, a multi-messenger approach thatincludes all cosmic ray channels will be necessary: This includes gamma rays,positrons, antiprotons, antideuterons, and neutrinos!
Neutrinos are an important complementary channel for indirect dark matter detection!
Michael Grefe (DESY Hamburg) Neutrino Signals from Dark Matter Decay Presentation Skills – 18 January 2011 5 / 13
Motivation
What is the Difference of Dark Matter Annihilations and Decays?
◮ Different angular distribution of the gamma-ray/neutrino flux from the galactic halo:
Dark Matter Annihilation
dJhalo
dE=
〈σv〉DM
8π m2DM
dNdE
Z
l.o.s.
ρ2halo(~l) d~l
particle physics astrophysics
Dark Matter Decay
dJhalo
dE=
14π τDM mDM
dNdE
Z
l.o.s.
ρhalo(~l) d~l
particle physics astrophysics
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
0 30 60 90 120 150 180
σ =
S/√
B c
ompa
red
to fu
ll sk
y
cone half angle towards galactic center (°)
NFW decayNFW annihilation
Einasto decayEinasto annihilation
Isothermal decayIsothermal annihilation
Annihilation• Strong signal from peaked structures
• Enhancement of cross section needed
• Best statistical significance for small conearound galactic centre
Decay• Less sensitive to the halo model
• Best statistical significance for full-skyobservation
Michael Grefe (DESY Hamburg) Neutrino Signals from Dark Matter Decay Presentation Skills – 18 January 2011 6 / 13
Motivation
What is the Difference of Dark Matter Annihilations and Decays?
◮ Different angular distribution of the gamma-ray/neutrino flux from the galactic halo:
Dark Matter Annihilation
dJhalo
dE=
〈σv〉DM
8π m2DM
dNdE
Z
l.o.s.
ρ2halo(~l) d~l
particle physics astrophysics
Dark Matter Decay
dJhalo
dE=
14π τDM mDM
dNdE
Z
l.o.s.
ρhalo(~l) d~l
particle physics astrophysics
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
0 30 60 90 120 150 180
σ =
S/√
B c
ompa
red
to fu
ll sk
y
cone half angle towards galactic center (°)
NFW decayNFW annihilation
Einasto decayEinasto annihilation
Isothermal decayIsothermal annihilation
Annihilation• Strong signal from peaked structures
• Enhancement of cross section needed
• Best statistical significance for small conearound galactic centre
Decay• Less sensitive to the halo model
• Best statistical significance for full-skyobservation
Different search strategies required!
Michael Grefe (DESY Hamburg) Neutrino Signals from Dark Matter Decay Presentation Skills – 18 January 2011 6 / 13
Motivation
Models of Decaying Dark Matter
◮ A non-exhaustive list of models predicting decaying dark matter includes:
• Gravitino dark matter with R-parity violationDecay rate suppressed by Planck scale and small R-parity violation.[Takayama, Yamaguchi (2000)], [Buchmüller, Covi, Hamaguchi, Ibarra, Yanagida (2007)], [Chen, Mohapatra, Nussinov, Zhang (2009)]
• Sterile neutrinosDecay rate suppressed by small Majorana mass of the sterile neutrino[Asaka, Blanchet, Shaposhnikov (2005)]
• Right-handed Dirac sneutrinosDecay rate suppressed by small neutrino Yukawa couplings[Pospelov, Trott (2008)]
• Bound state of strongly interacting particlesDecay via GUT-scale or Planck-suppressed higher-dimensional operators.[Hamaguchi, Nakamura, Shirai, Yanagida (2008)], [Nardi, Sannino, Strumia (2008)]
• Hidden sector fermionsDecay via GUT-scale suppressed dimension-6 operators.[Hamaguchi, Shirai, Yanagida (2008)], [Arvanitaki, Dimopoulos, Dubovsky, Graham, Harnik, Rajendran (2008)]
• Hidden sector gauge bosons and gauginosDecay rate suppressed by tiny kinetic mixing between U(1)hid and U(1)Y .[Chen, Takahashi, Yanagida (2008)], [Ibarra, Ringwald, Tran, Weniger (2009)]
Michael Grefe (DESY Hamburg) Neutrino Signals from Dark Matter Decay Presentation Skills – 18 January 2011 7 / 13
Neutrino Signals and Neutrino Detection
Neutrino Flux and Atmospheric Background
◮ Decay channels of a scalar dark matter particle:• DM → νν: two-body decay with monoenergetic line at E = mDM/2
• DM → ℓ+ℓ−: soft spectrum from lepton decay (no neutrinos for e+e−)
• DM → Z 0Z 0/W +W−: low-energy tail from gauge boson fragmentation
10-8
10-7
10-6
10-5
10-4
10-3
10-2
10-1
100 101 102 103 104 105
Eν2 ×
dJ/
dEν
(GeV
cm
-2 s
-1 s
r-1)
Eν (GeV)
νe
νµ
ντ
atmospheric neutrinos
mDM = 1 TeV , τDM = 1026 s
DM → ννDM → µµ/ττ
DM → ZZ/WWSuper-K νµ
Amanda-II νµFrejus νeFrejus νµ
Amanda-II νµIceCube-22 νµ
• Triangular tail from extragalactic dark matter decays
• Neutrino oscillations distribute the flux equally into all neutrino flavours
• Atmospheric neutrinos are dominant background for TeV scale decaying dark matter
Michael Grefe (DESY Hamburg) Neutrino Signals from Dark Matter Decay Presentation Skills – 18 January 2011 8 / 13
Neutrino Signals and Neutrino Detection
Neutrino Flux and Atmospheric Background
◮ Decay channels of a fermionic dark matter particle:• DM → Z 0ν: narrow line near E = mDM/2 and tail from Z 0 fragmentation
• DM → ℓ+ℓ−ν: hard prompt neutrino spectrum and soft spectrum from lepton decay
• DM → W±ℓ∓: soft spectrum from W± fragmentation and lepton decay
10-8
10-7
10-6
10-5
10-4
10-3
10-2
10-1
100 101 102 103 104 105
Eν2 ×
dJ/
dEν
(GeV
cm
-2 s
-1 s
r-1)
Eν (GeV)
νe
νµ
ντ
atmospheric neutrinos
mDM = 1 TeV , τDM = 1026 s
DM → ZνDM → eeν/µµν/ττνDM → We/Wµ/Wτ
Super-K νµAmanda-II νµ
Frejus νeFrejus νµ
Amanda-II νµIceCube-22 νµ
• Triangular tail from extragalactic dark matter decays
• Neutrino oscillations distribute the flux equally into all neutrino flavours
• Atmospheric neutrinos are dominant background for TeV scale decaying dark matter
Michael Grefe (DESY Hamburg) Neutrino Signals from Dark Matter Decay Presentation Skills – 18 January 2011 9 / 13
Neutrino Signals and Neutrino Detection
Neutrino Signals IUpward Through-going Muons
◮ Muon tracks from CC DIS of muon neutrinos off nuclei outside the detector
Advantages• Muon track reconstruction is well-understood at neutrino telescopes
Disadvantages• Neutrino–nucleon DIS and propagation energy losses shift muon spectrum to lower energies
• Bad energy resolution (0.3 in log10 E) smears out cutoff energy
100
101
102
103
104
105
106
1 10 100 1000 10000
φ (k
m-2
yr-1
)
Eµ (GeV)
through-going muons
mDM = 1 TeV , τDM = 1026 s
DM → ννDM → µµ (ττ)
DM → ZZ (WW)atmospheric
0
1
2
3
4
5
1 10 100 1000 10000σ
= S
/√B
Eµ (GeV)
through-going muons
mDM = 1 TeV
τDM = 1026 s
DM → ννDM → µµ (ττ)DM → ZZ (WW)
Michael Grefe (DESY Hamburg) Neutrino Signals from Dark Matter Decay Presentation Skills – 18 January 2011 10 / 13
Neutrino Signals and Neutrino Detection
Neutrino Signals IIImprovements using Showers
◮ Hadronic and electromagnetic showers from CC DIS of electron and tau neutrinos and NCinteractions of all neutrino flavours inside the detector
Disadvantages• TeV-scale shower reconstruction is not yet well understood
Advantages• 3× larger signal and 3× lower background compared to other channels• Better energy resolution (0.18 in log10 E) helps to distinguish spectral features• Potentially best channel for dark matter searches
100
101
102
103
104
105
106
1 10 100 1000 10000
dN/d
Vdt
(km
-3 y
r-1)
Eshower (GeV)
shower events
mDM = 1 TeV , τDM = 1026 s
DM → ννDM → µµ (ττ)
DM → ZZ (WW)atmospheric
0
10
20
30
40
50
1 10 100 1000 10000σ
= S
/√B
Eshower (GeV)
shower events
mDM = 1 TeV
τDM = 1026 s
DM → ννDM → µµ (ττ)DM → ZZ (WW)
Michael Grefe (DESY Hamburg) Neutrino Signals from Dark Matter Decay Presentation Skills – 18 January 2011 11 / 13
Neutrino Constraints on Decaying Dark Matter
Limits on the Dark Matter Parameter Space
Super-Kamiokande• Limit on the integrated flux of upward
through-going muons
• PAMELA and Fermi LAT preferred regionsare not constrained
IceCube• Observation of the integrated flux of upward
through-going muons will soon test thePAMELA and Fermi LAT preferred regions
• Use of spectral information and newdetection channels like showers will allowto greatly improve the sensitivity
1023
1024
1025
1026
101 102 103 104
τ DM
(s)
mDM (GeV)
Super-Kamiokande exclusion region
OO O
PAMELA / FermiDM → ννDM → ZνDM → eeνDM → µµν (ττν )DM → µµ (ττ)DM → ZZ (WW)DM → WeDM → Wµ (Wτ)
1023
1024
1025
1026
1027
1028
101 102 103 104
τ DM
(s)
mDM (GeV)
exclusion from through-going muons
after 1 year at IceCube + DeepCore
OO O
PAMELA / Fermi
DM → ννDM → ZνDM → eeνDM → µµν (ττν )DM → µµ (ττ)DM → ZZ (WW)DM → WeDM → Wµ (Wτ)
PAMELA and Fermi LAT preferred regions taken from [Ibarra, Tran, Weniger (2009)]
Michael Grefe (DESY Hamburg) Neutrino Signals from Dark Matter Decay Presentation Skills – 18 January 2011 12 / 13
Conclusion
Conclusion
• The determination of the nature of particle dark matter via indirect detectionsrequires a multi-messenger approach – including neutrinos
• Results from Super-Kamiokande do not constrain the dark matter parameterrange fitting the PAMELA and Fermi LAT observations
• Present and future neutrino experiments like IceCube have the capability to detectdark matter signals, in particular at large masses
Use of new detection channels like showers and use of spectral information willallow to greatly improve the sensitivity of these experiments
• After detection, directional observation with gamma rays and neutrinos will allowto distinguish between annihilating and decaying dark matter
• Then, the neutrino channel will give important additional information about thedark matter decay modes and hence about the nature of dark matter
Michael Grefe (DESY Hamburg) Neutrino Signals from Dark Matter Decay Presentation Skills – 18 January 2011 13 / 13
Conclusion
Conclusion
• The determination of the nature of particle dark matter via indirect detectionsrequires a multi-messenger approach – including neutrinos
• Results from Super-Kamiokande do not constrain the dark matter parameterrange fitting the PAMELA and Fermi LAT observations
• Present and future neutrino experiments like IceCube have the capability to detectdark matter signals, in particular at large masses
Use of new detection channels like showers and use of spectral information willallow to greatly improve the sensitivity of these experiments
• After detection, directional observation with gamma rays and neutrinos will allowto distinguish between annihilating and decaying dark matter
• Then, the neutrino channel will give important additional information about thedark matter decay modes and hence about the nature of dark matter
Thanks for your attention!
Michael Grefe (DESY Hamburg) Neutrino Signals from Dark Matter Decay Presentation Skills – 18 January 2011 13 / 13