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