1 dark matter indirect detection -...

Nicole Bell, The University of Melbourne Seminar, The Ohio State University, 13 May 2011

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Dark Matter Indirect Detection: Electroweak Bremsstrahlung

&

Other Topics

Nicole BellThe University of Melbourne, Australia

with

Ahmad Galea, Thomas Jacques, Kalliopi

Petraki

(Melbourne)James Dent, Lawrence Krauss (Arizona)

Tom Weiler

(Vanderbilt)


2

Outline

Introduction

Dark matter annihilation in the Sun

Late decaying dark matter & small scale structure

Enhanced annihilation via internal bremsstrahlung


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•Evidence for dark matter arises from a wide range of astrophysical observations (galaxy rotation curves, CMB, BBN, large scale structure, lensing, SN1a,…)

•All are sensitive to dark matter’s gravitational influence.

•As yet, very little information about the particle properties of dark matter.

Dark Matter


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Axions, Neutralinos, Gravitinos, Axinos, Sneutrinos, Kaluza-Klein particles, Heavy Fourth Generation Neutrinos, Mirror particles, superWIMPs, WIMPzillas, Sterile Neutrinos, Light Scalars, Q-Balls, Brane

World

Dark Matter, Primordial Black Holes, …. something we haven’t thought of ….

In other words …

we just don’t know …

Dark Matter Candidate Zoo


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Produce dark matter in a collider

Direct detection

Indirect detection

Ideal situation would be to obtain positive signals via all approaches and combine information/check for consistency.

Dark matter detection strategies


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(1) Assume dark matter initially in thermal equilib.:

(2) Universe cools and the non-relativistic DM is Boltzmann suppressed:

(3) “Freeze out”

--

relic density fixed:

vconstN

1.

TmEQ eNN /~

Final dark matter abundance proportional to inverse of the annihilation cross section.

Thermal Relic Dark Matter

ff


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“WIMP Miracle”

The thermal relic picture sets the “natural scale”

for

the dark matter annihilation cross section:

Correct relic density for:• electroweak strength couplings• GeV

–

TeV

masses

Realistic prospects of detecting annihilation signals!

Indirect detection also sensitive to decaying DM, in some scenarios

implies


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Annihilation in our Galaxy or in nearby galaxies Photons, antimatter, neutrinos

Annihilations in galaxies throughout the Universe cosmic diffuse fluxes

Annihilation in Sun/Earth Neutrinos only

Indirect DetectionIndirect DetectionSearch for fluxes of DM annihilation or decay products from regions where the DM density is high:


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Annihilation of dark matter

Baltz

et al., JCAP 2008


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Annihilation in the Sun


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Neutrinos from the Sun•

Dark matter accumulates in the centre of Sun (and Earth)

•

Neutrinos are the only annihilation products able to escape •

High energy neutrinos detected by IceCube, SuperK, etc.

•

Capture rate and (if in equilibrium) the annihilation rate controlled by WIMP-nucleon scattering cross section

•

Competitive limits for spin-dependent cross sections

D. Hooper


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IceCube, PRL 2010; Limits with 22 string detector.

IceCube

limits on Neutralinos


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Wilkstrom

and Edsjo, JCAP 2009

Full IceCube

Sensitivity -

Forecast


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Batell, Pospelov, Ritz and Shang, PRD 2010

Annihilation in the Sun –

secluded dark matter

Annihilation via metastable

mediators, V VV SM particles

The mediators propagate before decaying:

Can decay outside the Sun solar gamma rays


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Schuster, Toro, Weiner and Yavin, PRD 2010

Annihilation to long lived

force carriers which escape the Sun before decaying

Inelastically

interacting

dark matter forms halo of dark matter outside the Sun

gamma rays or charged particles from the Sun Test with Fermi data in near future


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Neutrino signal enhanced for secluded DM

High energy neutrinos produced at the solar core undergo

significant absorption in the Sun, for E > O(100) GeV.

If mediators propagate out of the dense core before

decaying neutrino signal greatly enhanced

Since the solar density falls exponentially with radius, this can have a large effect, even for short mediator lifetimes.

~100 GeV, x=optical depth in Sun


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Neutrino evolution in the Sun

Injection by mediator decays Absorption by CC and NC interactions

Regeneration of neutrinos at lower energy (from tau

decay, or from NC scattering) . Flavour

oscillations

Helpfully, the flavour

oscillations and the interactions approximately decouple.


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Density of SunNucleon density

Note: scale height of approximately 0.1 solar radius

Define (dimensionless) optical depth:

Or, approximately,


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And, to a very good approximation:


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Bell and Petraki, JCAP 2011

Bottom to top:mediator lifetimes of

= 0.001 s, 0.1 s, 0.3 s,

1 s, and 10 s.

Note: solar radius ~ 2.2s

Enhanced HE neutrino

fluxes from Sun


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The ratio of μ±

track events observable at IceCube

in the presence of mediators of lifetime

over those in the absence of metastable

mediators, integrated over muon

energies E > 100GeV

Bell and Petraki, JCAP 2011

Enhancement of the high energy region of the nu flux

big enhancement of event rates, because detection

cross sections grow with energy


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Late decaying dark matter


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Structure & decaying dark matter Structure & decaying dark matter

Problems with CMD:CDM simulations correctly reproduce observations at

large

scales, but discrepancies seen at small scales

cuspy

central galactic density profiles (observations prefer cores)overprediction

of satellite galaxies

i.e. too much power on small scales.

Possible resolution:Warm dark matter inhibits growth of structure on small scalesalternatively: Decaying dark matter may alleviate the problem


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Dark matter decay to almost Dark matter decay to almost degenerate daughter degenerate daughter

• DM energy density approx. unchanged by decays

• Very little energy transferred to radiation

• The daughter DM particle receives a recoil velocity

•

The recoil velocities affect galactic structure if decays occurs at appropriate time. Lifetimes of about 1Gyr are interesting.


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Dark matter kicksDark matter kicks

Structure formation problems eased:Recoil velocities disrupt small halos (e.g. Abdelqader

& Melia)Heating causes central cores to expand and form cores (e.g. Sanchez-Salcedo)

Early decays (before recombination)Kaplinghat

(2005); Cembranos, Feng, Rajaraman

& Takayama

(2005)

Late decaysSanchez-Salcedo

(2003); Abdelqader

& Melia

(2008); Peter, Moody & Kamionkowski

(2010); Peter and Benson (2010)


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Adapted from Peter et al. (2010)

Dark matter decay parameter spaceDark matter decay parameter space


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Dark matter decayDark matter decay--

Constraints from observed fluxes of radiationConstraints from observed fluxes of radiation

If l is a standard model particle, constrain the lifetime using observational measurements of radiation backgrounds

Kuksel

& Kistler

(2007)

Bell, Galea

& Petraki

(2010)

Bell, Galea

& Petraki

(2010)


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Excluded Excluded lifetimeslifetimes


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Excluded lifetimesExcluded lifetimes

Bell, Galea, Petraki

(2010)


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Annihilation to Antimatter


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Fermi e++e-

excessPhys. Rev. Lett. 102, 181101 (2009)

PAMELA e+ excessNature 458, 607-609

PositronsAnnihilation to Antimatter


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Antiproton data consistent with theory expectation (for secondary production of antiprotons via cosmic ray propagation in the Galaxy).

AntiprotonsAnnihilation to Antimatter


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Positrons from astrophysics?Re-examine the expected positron flux from: pulsars supernova remnantsmodified cosmic ray propagation/acceleration

Positrons from Dark Matter? Challenging because: Must produce enough e+e-

without overproducing pbar

or

gamma ray, or radio fluxes Need big cross sections!

Boost via DM clumping/substructure or enhanced cross

sections”Sommerfeld”, non-thermal DM, … But annihilation to leptons is often suppressed…...

Resolution of positron anomalies?


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Pulsars?

Yuksel, Kistler

and Stanev, PRL 2009

Possible contribution from Geminga

pulsar to positron fraction:


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Parameterize the annihilation cross section as:

<v> = a + bv2 + …

a --

from s-wave (L=0) annihilationb --

both s-wave and p-wave (L=1) contributions

The Lth partial wave contribution is suppressed as v2L

In galactic halos, v~ 10-3c, so only the s-wave contribution will be significant.

However, in many models, s-wave annihilation to a fermion pair is helicity

suppressed by a factor of 2

DMf mm

Annihilation cross section


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Majorana

neutralinos

annihilate to a fermion

pair via:

• t-

and u-channel exchange of sfermions helicity

suppressed

• s-channel exchange of Zhelicity

suppressed

• s-channel exchange of higgs suppressed by yukawa

couplings

2 vevfm

Example: SUSY


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When is annihilation suppressed? For s-channel annihilation:

Suppressed: scalaraxial-vector

Non-suppressed: pseudo-scalar vector (not allowed for Majorana

DM)

tensor

(not allowed for Majorana

DM)

s-channel exchange of a pseudo-scalar is the sole non-suppressed mode for Majorana

DM.

What about t-

and u-channel annihilation?

Fierz

transform to s-channel form. Non-suppressed only if a pseudo-scalar term present


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Cao, Ma, Shaughnessy, PLB 2009.Dark matter = gauge-singlet Majorana

fermion

=

Example: leptophillic

model

SUSY analogue

Annihilation of bino

dark matter to fermions via exchange of sfermions


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in CM frame, same helicity

But, opposite chirality!

In massless

limit, helicity

= chirality

not possible!

For small but non-zero masshighly suppressed!

Majoranaopposite

spins

Opposite Spins

Helicity

suppression of s-wave


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Emission of a photon can lift this suppression: Bergstrom, PLB 225, 372, 1989;Flores, Olive, Rudaz, PLB 232, 377, 1989;Bringmann, Bergstrom, Edsjo, 2008; Barger, Gao, Keung, Marfatia, 2009,…

Lifting the suppression (photons)

The photon carries away a unit of angular momentum no longer helicity

suppressed.


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Lifting the suppression (photons)

Final state radiation (FSR) “Virtual internal bremsstrahlung”

(VIB) Effect most pronounced for near-degenerate

and

(i.e. the co-annihilation region)

Bringmann, Bergstrom, Edsjo, 2008


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Bergstrom, Bringmann, Edsjo, PRD 2008

Bringmann, Bergstrom, Edsjo, JHEP 2008

e+e-

signals in SUSY models

Gamma rays Positrons


43Lifting the suppression: electroweak (W,Z) bremsstrahlung

Radiating a W or Z boson can also lift the suppression Both VIB and FSR are important similar to

brem, except for W/Z mass effects.

distinct phenomenology: W and Z bosons decay to charged leptons, neutrinos, gammas, and hadrons hadron

production even for “leptophillic”

models

Bell, Dent, Jacques & Weiler, 2010.

Bell, Dent, Galea, Jacques, Krauss & Weiler, 2011

Ciafaloni, Cirelli, Comelli, De Simone, Riotto

& Urbano, 2011


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W bremdiagrams


45W brem

rate larger than photon brem

rate, except near threshold

Bell, Dent, Galea, Jacques, Krauss and Weiler, arXiv:1104.3823


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W, Z, and

brem

In the limit where the W/Z mass is negligible, we have:

Adding all the bremsstrahlung

processes:


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Ratio of eW and e+e-

cross sections

• Annihilation to e+e-, e+e-Z & eW dominates over e+e-• Enhancement by up to three orders of magnitude!

(v~10-3c, Galactic halo)



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Ratio of eW and e+e-

cross sections

• Insensitive to the dark matter mass, except near threshold



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Total photon energy spectrumred = totalorange = primary photons from

brem

green = secondary photons from W/Z decay


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Total electron energy spectrumred = totalgreen = primary electrons from

brem

orange = primary electrons from W/Z bremblue = secondary electrons from W/Z decay


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Annihilation spectra:

/ W/ Z brem

p

γ

ν

e

Bell, Dent, Jacques & Weiler, 2011


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Maximum allowed brem

cross sections

Bremsstrahlung

can’t make significant contribution to e+ flux, without overproducing pbar

ee eee

/ppp

Bell, Dent, Jacques & Weiler, 2011


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Kachelriess, Serpico

and Solberg PRD 2009.

Models with no helicity suppression

EW-brem

still occurs, but is subdominant

W/Z decays ensures there is at least a minimal yield of hadrons, photons, charged leptons and neutrinos.

neutrinos-only

(+ W/Z brem)

electrons-only e+e-

(+ W/Z brem)


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Electroweak bremsstrahlung, even if a subdominate process, ensures that some minimal hadron production occurs, even for “leptophillic” models.

Obvious exception:If you are below threshold to make W and Z bosons.

--

This includes the light force carrier models (Arkani- Hamed

et al, Weiner et al, etc).


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Electroweak bremsstrahlung Annihilation to fermions is often helicity

suppressed

Internal bremstrahlung

of , W & Z can lift suppressions, enhance

annihilation yields, and result in interesting multi-messenger signals

Allows indirect detection of processes that would otherwise be too

suppressed to give observable signals. Antiproton limits are important.

Conclusions

Decay Dark matter decay to an almost degenerate daughter might ease small scale structure problems …. but parameter space somewhat constrained

Annihilation in the SunAnnihilation via metastable

mediators can greatly enhance the high

energy neutrino flux produced by dark matter annihilation in the

Sun

1 dark matter indirect detection -...

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