Decaying vs Annihilating Dark Matter in Light of a Tentative …€¦ · Lars Bergstrom Department...

Decaying vs Annihilating Dark Matterin Light of a Tentative Gamma-Ray Line

Mathias Garny (DESY Hamburg, Germany)

TUM, 05.07.12

based on arXiv:1206.7056 with Wilfried Buchmuller

Mathias Garny (DESY) Decaying vs Ann. DM in Light of a Tentative γ-Ray Line

Outline

Tentative Gamma Ray Line from Fermi LAT at Eγ ∼ 130 GeV

Models

NeutralinoGravitino

Constraints

Continuum Gamma RaysAntiprotonsMorphology of the excess

The question about the actual existence and origin is under an active debate; in this

talk, consequences of the hypothesis that dark matter is responsible will be discussed


Standard Model of Cosmology


WIMP Dark Matter

mX (TeV)

X

DM

____

Feng 2010101 102 103

WIMP mass [GeV]

10-26

10-25

10-24

10-23

10-22

10-21

WIM

P c

ross

sect

ion [

cm3

/s]

Upper limits, Joint Likelihood of 10 dSphs

3 ·10−26

bb Channel

τ+ τ− Channel

µ+ µ− Channel

W+W− Channel

Fermi 1108.3546

101 102 103

10-44

10-43

10-42

10-41

10-40

10-39

10-38

mΧ @GeVD

WIM

P-nu

cleo

ncr

oss

sect

ion

ΣSI

@cm2 D

Limits: 90%

Contours: 90%, 3Σ

v0 = 220 kms, vesc = 550 kms

CDMS low-thrXenon

-100

KIMS

CRESST-II

DAMA

CRESST comm. run

CDMS

Kopp, Schwetz, Zupan 2011


Tentative Gamma Ray Line from Fermi LAT

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MPP-2012-73Prepared for submission to JCAP

A Tentative Gamma-Ray Linefrom Dark Matter Annihilationat the Fermi Large Area Telescope

Christoph Weniger

Max-Planck-Institut fur Physik, Fohringer Ring 6, 80805 Munchen, Germany

E-mail: [email protected]

Abstract. The observation of a gamma-ray line in the cosmic-ray fluxes would be a smoking-gun signature for dark matter annihilation or decay in the Universe. We present an improvedsearch for such signatures in the data of the Fermi Large Area Telescope (LAT), concentratingon energies between 20 and 300 GeV. Besides updating to 43 months of data, we usea new data-driven technique to select optimized target regions depending on the profileof the Galactic dark matter halo. In regions close to the Galactic center, we find a 4.6σindication for a gamma-ray line at Eγ ≈ 130 GeV. When taking into account the look-elsewhere effect the significance of the observed excess is 3.3σ. If interpreted in terms ofdark matter particles annihilating into a photon pair, the observations imply a dark mattermass of mχ = 129.8 ± 2.4+7

−13 GeV and a partial annihilation cross-section of 〈σv〉χχ→γγ =(1.27± 0.32+0.18

−0.28

)×10−27 cm3 s−1 when using the Einasto dark matter profile. The evidence

for the signal is based on about 50 photons; it will take a few years of additional data toclarify its existence.



-60

-30

-15

0

15

30

60

b[deg]

Reg1Cored Isothermal

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

-60

-30

-15

0

15

30

60

b[deg]

Reg2NFW

-60

-30

-15

0

15

30

60

b[deg]

Reg3Einasto

-60

-30

-15

0

15

30

60

b[deg]

Reg4Contr. α=1.15

-80 -60 -40 -20 0 20 40 60 80

ℓ [deg]

-60

-30

-15

0

15

30

60

b[deg]

Reg5Contr. α=1.3

Weniger 1204.2797

10-7

10-6

γ=2.57

Reg1

γ=2.28

10-6

γ=2.54

Reg2

γ=2.36

10-6

E2

Φ[G

eVcm

−2s−

1sr−1

]

γ=2.52

Reg3

γ=2.42

10-7

10-6

γ=2.52

Reg4

γ=2.45

20 50 100 200

E [GeV]

10-5

10-4

γ=2.64

Reg5

γ=2.6



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Draft version June 15, 2012Preprint typeset using LATEX style emulateapj v. 03/07/07

STRONG EVIDENCE FOR GAMMA-RAY LINE EMISSION FROM THE INNER GALAXYMeng Su1,3, Douglas P. Finkbeiner1,2

Draft version June 15, 2012

ABSTRACT

Using 3.7 years of Fermi-LAT data, we examine the diffuse 80 − 200 GeV emission in the inner

Galaxy and find a resolved gamma-ray feature at ∼ 110−140 GeV. We model the spatial distributionof this emission with a ∼ 3 FWHM Gaussian, finding a best fit position 1.5 West of the GalacticCenter. Even better fits are obtained for off-center Einasto and power-law profiles, which are preferred

over the null (no line) hypothesis by 6.5σ (5.0σ/5.4σ after trials factor correction for one/two line case)assuming an NFW density profile centered at (ℓ, b) = (−1.5, 0) with a power index α = 1.2 . The

energy spectrum of this structure is consistent with a single spectral line (at energy 127.0± 2.0 GeVwith χ2 = 4.48 for 4 d.o.f.). A pair of lines at 110.8±4.4 GeV and 128.8±2.7 GeV provides a marginally

better fit (with χ2 = 1.25 for 2 d.o.f.). The total luminosity of the structure is (3.2±0.6)×1035 erg/s,or (1.7± 0.4)× 1036 photons/sec. The energies in the two-line case are compatible with a 127.3± 2.7

GeV WIMP annihilating through γγ and γZ (with χ2 = 1.67 for 3 d.o.f.). We describe a possiblechange to the Fermi scan strategy that would accumulate S/N on spectral lines in the Galactic center

4 times as fast as the current survey strategy.

Subject headings: gamma rays — diffuse emission — milky way — dark matter

1. INTRODUCTION

Although various cosmological and astrophysical ob-

servations provide compelling evidence for dark matter(DM), which constitutes ∼ 80% of the matter in the Uni-

verse, we still know little about its intriguing nature (e.g.Bertone et al. 2005; Hooper & Profumo 2007). Among aforest of dark matter models, stable Weakly Interacting

Massive Particles (WIMP) have been predicted in manyextensions of the Standard Model of particle physics (e.g.

Bergstrom 2000). The WIMP with the virtues of weakscale masses and couplings is an excellent dark matter

particle candidate which can annihilate into high en-ergy gamma-rays (e.g. Bergstrom et al. 1998). The in-

ner Galaxy provides one of the most promising regionson the sky to search for WIMP annihilation produced

gamma rays (Ackermann et al. 2011; Abramowski et al.2011; Gondolo & Silk 1999). The expected relativelyhigher annihilation rate due to higher dark matter par-

ticle density provides a potential window to identify anynon-gravitational dark matter signatures.

The “smoking-gun” signal of annihilating dark mat-ter would be a monochromatic gamma-ray line (or lines)

in a region of high dark matter density, either in lo-cal dwarf galaxies or in the Galactic center. This line

could be produced by dark matter decays or annihila-tions into two photons, or two-body final states involv-

1 Institute for Theory and Computation, Harvard-SmithsonianCenter for Astrophysics, 60 Garden Street, MS-51, Cambridge, MA02138 USA

2 Physics Department, Harvard University, Cambridge, MA02138 USA

3 [email protected]

ing one photon plus a Higgs boson, Z boson, or otherchargeless non-SM particle. In most models, dark mat-ter does not annihilate directly to photons, but in models

where it annihilates to charged lepton pairs, there maybe loop interactions that produce photons in two-body

final states, yielding spectral lines. These lines can pro-vide a signature of dark matter even in a complicated

astrophysical environment, because no known astrophys-ical process produces gamma-ray lines at E ≫ 1 GeV.

However, these loop processes would be suppressed by1-4 orders of magnitude compared to the total annihi-

lation or decay rate (e.g. Bergstrom & Ullio 1997). Theunprecedented sensitivity and spectral resolution of theLarge Area Telescope (LAT; Gehrels & Michelson 1999;

Atwood et al. 2009) aboard the Fermi Gamma-ray SpaceTelescope make it possible to search for dark matter an-

nihilation lines over the whole sky up to a few hundredGeV.

However, other signals can mimic a weak line. Featuressuch as a spectral edge or a broken power law can be mis-

taken for a line in noisy data, especially when smoothedby the instrumental response. Thus careful separation ofpossibly diffuse dark matter emission from other diffuse

components is crucial.For example, the Fermi bubbles extend ∼10 kpc above

and below the Galactic center (GC) (Su et al. 2010).Their emission has a flat energy spectrum in E2dN/dE

from hundreds of MeV to ∼100 GeV implying the abil-ity of the Fermi bubbles to accelerate cosmic ray (CR)

electrons up to ∼TeV, if the gamma rays are producedby inverse Compton (IC) scattering of the Cosmic Mi-

crowave Background (CMB) photons (Su et al. 2010).


Outline


Models

NeutralinoGravitino

Constraints



Neutralino

PHYSICAL REVIEW D VOLUME 37, NUMBER 12 15 JUNE 1988

Observable monochromatic photons from cosmic photino annihilation

Lars BergstromDepartment of Theoretical Physics, University ofStockholm, Vanadisvagen 9, S-11346 Stockholm, Sweden

Hkkan SnellmanDepartment of Theoretical Physics, Royal Institute of Technology, S 100-44 Stockholm, Sweden

(Received 3 November 1987)

A recent suggestion that the observations of monochromatic photons from annihilation of pho-tinos into quarkonium plus a photon could be a signature of dark-matter photinos is extended to in-clude all S- and P-wave bound quarkonium states for a general interaction Lagrangian and an esti-mate of the cross sections is given. We also propose the process A, A, ~yy as a potentially richsource of monochromatic photons and estimate its strength.

I. INTRODUCTION

The dark-matter problem in the Universe has recentlyattracted attention among astrophysicists and particlephysicists. ' Much work has been devoted to the task offinding experimental signatures for the various candidatesfor dark matter —axions, photinos, Higgsinos, heavyneutrinos, etc. In the case of photinos (A, ) it has recentlybeen suggested by Srednicki, Theisen, and Silk that theannihilation process kA, ~ Vy might give a detectablephoton signal (V is the vector-meson bound state ofheavy quarks, e.g. , cc). This analysis, which was basedon a pointlike coupling between the gg pair and the vec-tor meson, has been criticized by Rudaz who found alower branching ratio for A.A,~ Vy using a more realisticbound-state description of the vector meson. For mz &4GeV, photons from kA, ~J/gy should still be detectableusing one of the high-resolution detectors that have beenproposed. High-energy resolution enables taking advan-tage of the near monochromaticity of the lines which isdue to the low (galactic) relative velocities of the annihi-lating A, pair (u„~/c=10 ).

In this paper we extend the analysis of Rudaz to in-clude the processes A, A.~( +'LJ)+y for all bou~d QQP- and S-wave states and for a more general interactionLagrangian. In addition, we propose the processA,A, ~yy as a potentially rich source of monochromaticphotons and make an estimate of its strength.

man rules a& effectively gets multiplied by a factor of 2whereas U& vanishes. We will write the various ampli-tudes as if A, is a Dirac fermion, but will take the last re-mark into account when presenting cross sections forphotinos. For supersymmetric particles such as photinos,Higgsinos, etc. , the effective couplings in (1 have beenexpressed in terms of gauge couplings and scalar super-partner masses by Ellis et al.

In models where the scalars associated with the left-and right-handed fermions, respectively, are nearly de-generate in mass the coupling Uf is small whereas it maybe important for large mass differences [generallyuf /af (m t —m tt )/(m r +m it )]

With the effective Lagrangian (1) governing the cou-pling specifically between kA, and a heavy-quark pair, wemust prescribe the bound-state model before we can cal-culate the process AA, ~( +'LJ)+y depicted in Fig. 1.We adopt the point of view of Rudaz that an adequatedescription of the process is provided by a nonrelativistic

II. AMPLITUDES FOR RADIATIVE QUARKONIUMPRODUCTION

We write the effective interaction Lagrangian for theinteraction between a A. pair and a fermion (f ) pair as

ff l„[fy" ( uI +y'a1 )fjj

where l„=)(,y„(u~+a&y5)A,. When f is a quark field, asum over colors is implied. There is an important dis-tinction between the case when A, is a Majorana fermion(such as a photino) and in the straightforward case whenA, is a Dirac fermion (such as a heavy neutrino). In theMajorana case, when passing from Eq. (1 to the Feyn-

FIG. 1. Diagrams contributing to the process A,A, ~Py,where P is a +'LJ quarkonium bound state.

37 3737 1988 The American Physical Society


Neutralino

χχ→ γγ Eγ = mDM

χχ→ γZ Eγ = mDM

(1− M2

Z

4m2DM

)

Bergstrom, Ullio 97

χ1

0

χ1

0

f

fff

∼

γ

γ1 a

χ1

0

χ1

0

f

f

f∼

f∼

γ

γ

1 b

χ1

0

χ1

0

Z0

f

f

f

γ

γ

1 c

χ1

0

χ1

0

H3

0f

f

f

γ

γ

1 d

χ1

0

χ1

0

χ+

χ+

χ+

H+

γ

γ2 a

χ1

0

χ1

0

χ+

χ+

H+

H+

γ

γ

2 b

χ1

0

χ1

0

Z0

χ+

χ+

χ+

γ

γ

2 c

χ1

0

χ1

0

H3

0χ

+

χ+

χ+

γ

γ

2 d

χ1

0

χ1

0

W+

W+

W+χ

+

γ

γ3 a

χ1

0

χ1

0

χ+

χ+

W+

W+

γ

γ

3 b

χ1

0

χ1

0

χ+

χ+

W+ χ

+

γ

γ3 c

χ1

0

χ1

0

χ+

χ+

χ+

G+

γ

γ4 a

χ1

0

χ1

0

χ+

χ+

G+

G+

γ

γ

4 b


Neutralino

Example: higgsino (H) and wino-like (W) LSP

µ M2 mχ10

mχ20

mχ1±

σvγγ(σvγZ ) σvWW (σvZZ )

H 139 1000 135.89 144.44 139.20 1.0(3.4) · 10−28 2.1(1.4) · 10−25

W 400 143 139.79 408.08 139.94 2.0(10.9) · 10−27 3.4(0.0) · 10−24

BRγ =σvγγ + 0.5σvγZ

σv∼ 0.08%(0.2%)


Gravitino

Volume 174, number 1 PHYSICS LETTERS B 26 June 1986

B A R Y O G E N E S I S W I T H O U T GRAND UNIFICATION

M F U K U G I T A

Research lnstttute for Fundamental Ph)'sl~ s, Kyoto Umc'ervtO', Kvoto 606, Japan

and

T Y A N A G I D A

lnstuute of Phvstcs, College of General Education, Tohoku Um~'ersu),, Sendat 980, Japan and Deutsches Elektronen-Synehrotron DESY, D-2000 Hamburg, Fed Rep German)'

Received 8 March 1986

A mechanism is pointed out to generate cosmological baryon number excess without resorting to grand unified theories The lepton number excess originating from Majorana mass terms may transform into the baryon number excess through the unsuppressed baryon number wolatlon of electroweak processes at high temperatures

The current view ascribes the origin of cosmological baryon excess to the macroscopic baryon number vaolataon process in the early stage of the Universe [1,2]. The grand unified theory (GUT) of particle ln- teractaons is regarded as the standard candidate to account for thas baryon number vaolatlon: The theory can gave the correct order of magnitude for baryon to entropy ratio. I f the Unaverse undergoes the anflation epoch after the baryogenesis, however, generated baryon numbers are diluted by a huge factor. The re- heatang after the anflataon is unlakely to rinse the temperature above the GUT energy scale. A more arri- taring problem as that no evidences are gaven so far experimentally for the baryon number vaolation, which might cast some doubt on the GUT idea.

Some time ago 't Hooft suggested that the instan- ton-like effect violates baryon number in the Welnberg- Salam theory through the anomaly term, although the effect is suppressed by a large factor [3]. It has been pointed out, however, that this effect is not suppressed and can be efficient at hagh temperatures above the Welnberg-Salam energy scale [4]. This baryon number wolatlng process conserves B - L, but it erases rapidly the baryon asymmetry which would have been generated at the early Umverse with B - L

0370-2693/86/$ 03.50 © Elsevier Science Pubhshers B V (North-Holland Physacs Publashlng Division)

conserving baryon number violation processes as m the standard SU(5) GUT. (Baryon numbers would re- main, if the baryon productaon takes place at low temperatures T<~ O(100 GeV), e.g., after reheating [5,6] .) The process atself can not produce the baryon asymmetry, sance at is unhkely to suppose a partxculal mechanism leading to departures from equfllbraum [4].

In this letter, we point out that this electroweak baryon number violation process, if it is supplement- ed by a lepton number generation at an earher epoch, can generate the cosmological baryon asymmetry without resorting to the GUT scenarao: The lepton number excess m the earher stage can efficiently be transformed into the baryon number excess. It is rather easy to find an agent leadang t~y the lepton number generataon. A candidate is the decay process revolving Majorana mass terms.

Let us present a specific model which gaves lepton number generation. We assume the presence of a right- handed Majorana neutrano N~ (l = 1 - n) an addatlon to the conventional leptons. We take the lagrangaan to be

45


Gravitino

Consistent cosmology with leptogenesis (TR ∼ 109 GeV), gravitino darkmatter (Ω3/2h

2 = 0.11) and BBN (τNLSP . tBBN)

Ω3/2h2 ' 0.27

(TR

109 GeV

)(10 GeV

m3/2

)( mg

1 TeV

)2

Buchmuller, Bolz, Brandenburg hep-ph/0012052

Gravitino meta-stable due to bilinear R-parity violation

τ3/2(ψ → γν) ' 1027s

(ζ

10−7

)−2(M1

100 GeV

)2 ( m3/2

10 GeV

)−3

Buchmuller, Covi, Hamaguchi, Ibarra, Yanagida 07; Bobrovski, Buchmuller, Hajer 1007.5007

Lifetimes of this order can be probed by indirect detectionBuchmuller, Ibarra, Shindou, Takayama, Tran 09; Ibarra, Tran, Grefe; . . .

Two-body decay channels ψ → γν,Zν,W `, hν

L =i√2M

(χγνγµ(Dµφ)ψν + c.c.)︸︷︷︸ψ → Zν,W `, hν

− 1

4MλγνσµρψνFµρ︸︷︷︸ψ → γν


Gravitino

BR

[%]

m3/2 [GeV]

Gravitino Dark Matter

Wino NLSP

Higgsino NLSP

BR(γν)

BR(hν)

BR(Zν)

BR(Wℓ)

0.001

0.01

0.1

1

10

100

260100 1000

Branching ratios of two-body gravitino decays for two representative examplesWino NLSP: M2 = 1.1 m3/2, M1 = µ = 10 m3/2

Higgsino NLSP: µ = 1.1 m3/2, M1 = 10 m3/2, M2 = 1.9 M1.


Gravitino

Branching ratio for ψ → γν for Wino NLSP

BRmaxγ ' 3πα

2√

2GFm23/2

' 3% for m3/2 ' 260 GeV


Outline


Models

NeutralinoGravitino

Constraints



Continuum Gamma Rays

Line + continuum

dJ

dE= α

(δ(E − Eγ) +

dNEG

dE+

1− BRγNγ BRγ

dNγcontdE

)+ β

(E

Eγ

)−γ

Extragalactic contribution e.g. Bertone, Buchmuller, Ibarra

dNEG

dE=

ΩDMρc0√ΩM(H0/c)Jψ

E 1/2

E3/2γ

(1 +

ΩΛ

ΩM

(E

Eγ

)3)−1/2

Θ(Eγ − E)

Continuum gamma spectrum (from PYTHIA)

dNγcontdE

≡ 1∑f 6=γ BRf

∑f 6=γ

BRfdNγfdE

Independent of DM distribution and CR propagation



ψ → γν,Zν,W `, hν

10−8

10−7

10−6

10−5

20 300100

E2Φ

[GeV/cm

2/s/sr]

E [GeV]

Decaying Dark Matter (SOURCE)

mDM/2 = 128.1GeV

Fermi LAT Reg 3

γ-ray line only

EG contribution

BR(ψ → γν) = 0.1

BR(ψ → γν) = 0.01

BR(ψ → γν) = 0.0025



χχ→ γγ,WW

10−8

10−7

10−6

10−5

20 300100

E2Φ

[GeV

/cm

2/s/sr]

E [GeV]

Annihilating Dark Matter (SOURCE)

Fermi LAT Reg 3

γ-ray line only

σvγγ/σv = 0.1

σvγγ/σv = 0.01

σvγγ/σv = 0.0025

mDM/2 = 127.9GeV



10−7

10−6

10−5

10−4

20 300100

E2Φ

[GeV

/cm

2/s/sr]

E [GeV]

Higgsino Dark Matter

Einasto, Ωχ/Ωdm = 1

mχ = 135.9GeV

Fermi LAT Reg 4 SOURCE

lines only, BF=5.6 (4.1σ)

lines+continuum, BF=5.6

lines+continuum, BF=1



10−7

10−6

10−5

10−4

20 300100

E2Φ

[GeV

/cm

2/s/sr]

E [GeV]

Wino Dark Matter

Einasto, BF=1

mχ = 139.8GeV

Fermi LAT Reg 4 SOURCE

lines only, Ωχ/Ωdm = 0.45 (4.1σ)

lines+continuum, Ωχ/Ωdm = 0.45

lines+continuum, Ωχ/Ωdm = 1


Continuum Gamma Rays + Antiprotons

ψ → γν,Zν,W `, hν

1025 1026 1027 1028 1029

10- 3

10- 2

10-1

100

lifetime Τ @ secD

BR

HΨ®

ΓΝLDecaying DM - SOURCE

MA

Xexcluded by

PAMELA pbar p ME

D

MIN

Fermi L

ATΓ - lin

e

Gravitino DM

Ψ ® ΓΝ ZΝ hΝ Wl

Einasto



χχ→ γγ,WW

ææ

àà

10- 28 10- 27 10- 26 10- 25 10- 24 10- 23

10- 3

10- 2

10-1

100

total cross section Σv @ cm3 secD

ΣvH

ΧΧ®

ΓΓLΣ

vAnnihilating DM - SOURCE

MA

X

ME

D

MIN

PA

ME

LApb

arp

excl

uded

Fermi LAT

Γ - line

H Higgsino DML

H Wino DMLΧΧ ® ΓΓ WW

Einasto



χχ→ γγ, γZ ,WW ,ZZ

ææ

àà

10- 28 10- 27 10- 26 10- 25 10- 24 10- 23

10- 3

10- 2

10-1

100

total cross section Σv @ cm3 secD

HΣv Γ

Γ+

0.5

Σv Γ

ZLΣ

vAnnihilating DM - SOURCE

MA

X

ME

D

MIN

PA

ME

LApb

arp

excl

uded

Fermi LAT

Γ - line

Higgsino DM

Wino DM

ΧΧ ® ΓΓ ΓZ WW ZZ

Einasto


Morphology

arX

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Draft version June 15, 2012Preprint typeset using LATEX style emulateapj v. 03/07/07

STRONG EVIDENCE FOR GAMMA-RAY LINE EMISSION FROM THE INNER GALAXYMeng Su1,3, Douglas P. Finkbeiner1,2

Draft version June 15, 2012

ABSTRACT

Using 3.7 years of Fermi-LAT data, we examine the diffuse 80 − 200 GeV emission in the inner

Galaxy and find a resolved gamma-ray feature at ∼ 110−140 GeV. We model the spatial distributionof this emission with a ∼ 3 FWHM Gaussian, finding a best fit position 1.5 West of the GalacticCenter. Even better fits are obtained for off-center Einasto and power-law profiles, which are preferred

over the null (no line) hypothesis by 6.5σ (5.0σ/5.4σ after trials factor correction for one/two line case)assuming an NFW density profile centered at (ℓ, b) = (−1.5, 0) with a power index α = 1.2 . The

energy spectrum of this structure is consistent with a single spectral line (at energy 127.0± 2.0 GeVwith χ2 = 4.48 for 4 d.o.f.). A pair of lines at 110.8±4.4 GeV and 128.8±2.7 GeV provides a marginally

better fit (with χ2 = 1.25 for 2 d.o.f.). The total luminosity of the structure is (3.2±0.6)×1035 erg/s,or (1.7± 0.4)× 1036 photons/sec. The energies in the two-line case are compatible with a 127.3± 2.7

GeV WIMP annihilating through γγ and γZ (with χ2 = 1.67 for 3 d.o.f.). We describe a possiblechange to the Fermi scan strategy that would accumulate S/N on spectral lines in the Galactic center

4 times as fast as the current survey strategy.

Subject headings: gamma rays — diffuse emission — milky way — dark matter

1. INTRODUCTION

Although various cosmological and astrophysical ob-

servations provide compelling evidence for dark matter(DM), which constitutes ∼ 80% of the matter in the Uni-

verse, we still know little about its intriguing nature (e.g.Bertone et al. 2005; Hooper & Profumo 2007). Among aforest of dark matter models, stable Weakly Interacting

Massive Particles (WIMP) have been predicted in manyextensions of the Standard Model of particle physics (e.g.

Bergstrom 2000). The WIMP with the virtues of weakscale masses and couplings is an excellent dark matter

particle candidate which can annihilate into high en-ergy gamma-rays (e.g. Bergstrom et al. 1998). The in-

ner Galaxy provides one of the most promising regionson the sky to search for WIMP annihilation produced

gamma rays (Ackermann et al. 2011; Abramowski et al.2011; Gondolo & Silk 1999). The expected relativelyhigher annihilation rate due to higher dark matter par-

ticle density provides a potential window to identify anynon-gravitational dark matter signatures.

The “smoking-gun” signal of annihilating dark mat-ter would be a monochromatic gamma-ray line (or lines)

in a region of high dark matter density, either in lo-cal dwarf galaxies or in the Galactic center. This line

could be produced by dark matter decays or annihila-tions into two photons, or two-body final states involv-

1 Institute for Theory and Computation, Harvard-SmithsonianCenter for Astrophysics, 60 Garden Street, MS-51, Cambridge, MA02138 USA

2 Physics Department, Harvard University, Cambridge, MA02138 USA

3 [email protected]

ing one photon plus a Higgs boson, Z boson, or otherchargeless non-SM particle. In most models, dark mat-ter does not annihilate directly to photons, but in models

where it annihilates to charged lepton pairs, there maybe loop interactions that produce photons in two-body

final states, yielding spectral lines. These lines can pro-vide a signature of dark matter even in a complicated

astrophysical environment, because no known astrophys-ical process produces gamma-ray lines at E ≫ 1 GeV.

However, these loop processes would be suppressed by1-4 orders of magnitude compared to the total annihi-

lation or decay rate (e.g. Bergstrom & Ullio 1997). Theunprecedented sensitivity and spectral resolution of theLarge Area Telescope (LAT; Gehrels & Michelson 1999;

Atwood et al. 2009) aboard the Fermi Gamma-ray SpaceTelescope make it possible to search for dark matter an-

nihilation lines over the whole sky up to a few hundredGeV.

However, other signals can mimic a weak line. Featuressuch as a spectral edge or a broken power law can be mis-

taken for a line in noisy data, especially when smoothedby the instrumental response. Thus careful separation ofpossibly diffuse dark matter emission from other diffuse

components is crucial.For example, the Fermi bubbles extend ∼10 kpc above

and below the Galactic center (GC) (Su et al. 2010).Their emission has a flat energy spectrum in E2dN/dE

from hundreds of MeV to ∼100 GeV implying the abil-ity of the Fermi bubbles to accelerate cosmic ray (CR)

electrons up to ∼TeV, if the gamma rays are producedby inverse Compton (IC) scattering of the Cosmic Mi-

crowave Background (CMB) photons (Su et al. 2010).


Morphology

Shape depends on line-of-sight integral over DM distribution (squared)

dJγdEdΩ

=1

4πδ(E − Eγ)

1

τγνmDM

∫l.o.s.

ds ρdm(r) decay

2σvγγm2

DM

∫l.o.s.

ds1

2ρdm(r)2 annihilation

NFW or Einasto profile

ρdm(r) ∝ 1

(r/rs)α(1 + r/rs)3−α , exp

(− 2

αE(r/rs)

αE

)with αE = 0.17, α ≥ 1 and scale radius rs = 20kpc


Morphology

0

2

4

6

8

10

12

14

-10 -5 0 5 10

dJ/d

E(arb.rescaled

units)

Galactic longitude ℓ

Spatial distribution (124.7 − 133.4GeV)

Fermi LAT excess

Dec. NFW α = 1.3

Dec. NFW α = 1.7

Ann. NFW α = 1.3

Ann. NFW α = 1.7


Morphology

0

2

4

6

8

10

12

14

-10 -5 0 5 10

dJ/d

E(arb.rescaled

units)

Galactic latitude b

Spatial distribution (124.7 − 133.4GeV)

Fermi LAT excess

Dec. NFW α = 1.3

Dec. NFW α = 1.7

Ann. NFW α = 1.3

Ann. NFW α = 1.7


Morphology

1028

1029

1030

Reg 2 Reg 3 Reg 4 Halo Limit

τ γν[sec]

search region

Decay lifetimes (SOURCE)

NFW

Einasto

NFW α = 1.3

NFW α = 1.7, ρ0 = 0.28


Conclusion

If the Fermi excess is real and due to DM...

Continuum gamma rays severely constrain annihilating DM

BRγ & 0.5%

Independent of DM distribution and CR propagation

Neutralino (higgsino/wino) ruled out as explanation of excess

Gravitino with wino NLSP compatible

Decaying DM would require enhanced DM density in theGalactic center region to fit the morphology of the excess


Gravitino

∆W = µiH`i ,∆L = BiHu˜i + m2

id˜†i Hd

ζi =ε′ivd + ε′′i vu

v

ε′i = −B ′i B + m2′

id

(m2

li −m2u

)(m2

li −m2u) (m2

li −m2d)− B2

ε′′i =B ′i(m2

li −m2d

)+ Bm2′

id

(m2li −m2

u) (m2li −m2

d)− B2

Bobrovski, Buchmuller, Hajer 1007.5007


Decaying vs Annihilating Dark Matter in Light of a Tentative …€¦ · Lars Bergstrom Department...

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