Recent Results on Dark
Matter Searches withFermi
Simona Murgia, SLAC-KIPAC
on behalf of the Fermi-LAT Collaboration2009 Fermi Symposium
2-5 November 2009 - Washington DC

Fermi 1 year sky

Fermi 1 year sky

Fermi’s great capabilities give us a unique perspective in investigating
the existence of dark matter particles indirectly, primarily throughtheir annihilation or decay into photons and into electronsFermi1 yearfrom
sky DM annihilation
Simulated sky mapof γ-rays(Pieri et al, arXiv:0908.0195, based on Via Lactea II simulation)

Fermi’s great capabilities give us a unique perspective in investigating
the existence of dark matter particles indirectly, primarily throughtheir annihilation or decay into photons and into electrons

Dark Matter Distribution
The dark matter annihilation (or decay) signal strongly depends on the dark
matter distribution.
Cuspier profiles and clumpiness of the dark matter halo can provide large boost
factors
Bertone et al., arXiv:0811.3744

NFW profiler0 1 + (r0 /a0 )2
ρ(r) = ρ0
r 1 + (r/a0 )23

ρ0= 0.3 GeV/cm

a0= 20 kpc, r0 = 8.5 kpc✓ Via Lactea II predicts a cuspierprofile, ρ(r) r-1.2✓ Aquarius predicts a shallower

than r-1 innermost profile

WIMP Dark MatterSpectrum
Several theoretical models have been proposed that predict the existence ofWIMPs (Weakly Interacting Massive Particle) that are excellent DM candidates
In addition to photons, with Fermi we can also probe electron+positron final states

Continuum spectrum with
cutoff at MW
Annihilation (or decay) into γ

Spectral line at MW
Prompt annihilation into γγ, γZ, γH0...
(also prompt decay into photons)

"!

??

!

" , Z, ...

Search Strategies
Galactic center:
Satellites:
Low background and goodsource id, but low statistics

Good statistics but source
confusion/diffuse background

Milky Way halo:
Large statistics but
diffuse backgroundAll-sky map of gamma raysfrom DM annihilation
arXiv:0908.0195 (based on
Via Lactea II simulation)

And electrons!
Anisotropies

Spectral lines:

Extra-galactic:

No astrophysicaluncertainties, goodsource id, but lowstatistics

Large statistics,
but astrophysics,
galactic diffusebackgroundGalaxy clusters:
Low backgroundbut low statistics

Pre-launch sensitivities published in Baltz et al., 2008, JCAP 0807:013 [astro-ph/0806.2911]

Search Strategies
Galactic center:
Satellites:
Low background and goodsource id, but low statistics

Good statistics but source
confusion/diffuse background

Milky Way halo:
Large statistics but
diffuse background
See B. Winer’s poster

All-sky map of gamma raysfrom DM annihilation
arXiv:0908.0195 (based on
Via Lactea II simulation)

And electrons!
Anisotropies

Spectral lines:

Extra-galactic:

No astrophysicaluncertainties, goodsource id, but lowstatistics

Large statistics,
but astrophysics,
galactic diffuse
backgroundGalaxy clusters:
Low backgroundbut low statistics

Pre-launch sensitivities published in Baltz et al., 2008, JCAP 0807:013 [astro-ph/0806.2911]

Fermi Early Impact on DMSearches
With the measurement of the galactic diffuse emission at intermediate latitudes and theCR e+e- spectrum, the Fermi-LAT data have made significant impact in the dark matterinterpretation of potential signals from other experiments➡ DM contribution is not required, however cannot be ruled out
HaloElectrons

PRELIMINARY

See L. Latronico’s talkPhys. Rev. Lett. 102, 181101 (2009)

Fermi Galactic Center Source
Very crowded region of the sky!!
0FGL J1746.0-2900 is the closest source to
the GC in Fermi Bright Source List (>10 σdetection, first 3 months of data).
Marginal variability is not confirmed withlarger statistics➡ See J. Cohan-Tanugi’s talk for results onthe Fermi GC source with 11 months ofdata

Search for DM in the GC

Steep DM profiles

Expect large DM annihilation/decay signal from the GC!

Good understanding of the astrophysical background is crucial to extract apotential DM signal from this complicated region of the sky:source confusion: energetic sources near to or in the line of sight of the GC
diffuse emission modeling: uncertainties in the integration over the line ofsight in the direction of the GC, very difficult to model:➡ See S. Digel’s talk on diffuse emission from the inner galaxy and T. Porter’stalk on galactic diffuse emission

See V. Vitale’s poster

Search for DM in the GC
Preliminary analysis of a 7o x7o region
centered at the GC:‣binned likelihood analysis of 11 months ofdata, >400 MeV, front-converting events‣

Model: galactic diffuse (GALPROP) andisotropic emission. Point sources in theregion (from Fermi 1 year catalog, to bereleased)➡ Model generally reproduces data well within
uncertainties. The model somewhat underpredicts the data in the few GeV range(spatial residuals under investigation)
In a 3o region around the GC, the largest
residual in the same energy range is ~40%, a~2σ effect (sources not subtracted, see S.
Digel’s talk)black: data (stat error)
red: diffuse emission
other: sources in ROIPRELIMINARYPRELIMINARY

blue band:sys uncertainty on effective area

See V. Vitale’s poster

Search for DM in the GC➡ Any attempt to disentangle a potential dark matter signal from thegalactic center region requires a detailed understanding of theconventional astrophysics
More prosaic explanations must be ruled out before invoking acontribution from dark matter if an excess is found (e.g. modeling of
the diffuse emission, unresolved sources, ....)
Analysis in progress to updated constraints on annihilation crosssection

Search for Spectral Lines➡ Smoking gun signal of dark matter
Search for lines in the first 11 months of Fermi data in the 30-200 GeV energy range
Search region‣ |b|>10o and 30o around galactic center
Remove point sources (for |b|>10o). The data selection includes additional cuts toremove residual charged particle contamination.

PRELIMINARY

See Y. Edmonds’ poster

Search for Spectral Lines➡ No line detection, 95% CL flux upper limits are placed
For each energy (WIMP mass) the flux ULs are combined with the integral over theline of sight of the DM density2 (or density) to extract UL (LL) on the annihilation
cross section <σv> (or lifetime for decaying DM particles)

PRELIMINARYPRELIMINARY

See Y. Edmonds’ poster

Search for Spectral Lines✓ Limits on <σv> are too weak (by O(1) or more) to constrain a typical thermal WIMP✓ Some models predict large annihilation cross sections into lines:
Wino LSP (Kane 2009): γZ line has <σv> ~1.4x10-26 cm3s-1
a factor of 2-5 depending on the halo profile

PRELIMINARY

already disfavored byPRELIMINARY

Search for DM Subhalos
Via Lactea II (Diemand et al. 2008)

Search for DM Subhalos➡ DM substructures: very low background targets for DM searches
Never before observed DM substructures (DM satellites):‣‣

Would significantly shine only in radiation produced by DM annihilation/decay.

‣All sky search for promising candidates with the LAT

Some of these satellites could be within a few kpc from the Sun (N-bodysimulations). Their extension could be resolved by the LAT

Optically observed dwarf spheroidal galaxies (dSph): largest clumps predicted by Nbody simulation. 25 have been discovered so far, many more are predicted.

‣‣‣

Most are expected to be free from other astrophysical gamma ray sources andhave low content in dust/gas, very few stars (Segue 1 might have 65 stars
associated with it, Geha&Simon 2009 )Given the distance and the LAT PSF, they are expected to appear as point sources
Select most promising candidates for observations

Search for DM Satellites
Search criteria:‣‣‣‣‣

More than 10o from the galactic plane
No appreciable counterpart at other wavelengths
Emission constant in time (1 week interval)
Spatially extended: ~1o average radial extension for nearby, detectable clumps
Spectrum determined by DM (both b-bbar and FSR spectra are tested vs a (soft)
power law hypothesis)

Blind analysis: finalize selection method with 3 months of data and apply to 10months
Search for sources (>5σ significance) passing these criteria in the 100 MeV to 300
GeV energy range.
Background: point sources+diffuse galactic and isotropic emission

See E. Bloom’s poster

Search for DM Satellites
4 sources above 5σ survive all criteria but the spectral requirement:
their spectra do not favor the DM hypothesis.

➡ No DM satellite candidates are found in 10 months of data
✓ Consistent with result of sensitivity study based on Via Lactea II predictions
for the DM distribution for a 100 GeV WIMP annihilating into b-bbar,<σv>=3x10-26 cm3 s-1 (paper in preparation)✓ Work is ongoing to evaluate the sensitivity for other models.

Search for DM in dSph
Select most promising dSph based on proximity, stellar kinematic data: less that 180
kpc from the Sun, more than 30o from the galactic plane
14 dSph have been selected for this analysis. More promising targets could be
discovered by current and upcoming experiments (SDSS, DES, PanSTARRS, ...)
Very large M/L ratio: 10 to ~> 1000 (M/L ~ 10 for Milky Way galaxy)
Distance: ~30 to 160 kpc• ultra-faint dwarfs• classical dwarfs

Ursa Major II
Segue 2
Willman 1
Coma Berenices
Bootes II
Bootes I
Ursa Minor
Sculptor
Draco
Sextans
Ursa Major IHercules
FornaxLeo IVSee C. Farnier’s talkand P. Scott’s posterSearch for DM in dSph

➡ No detection by Fermi with 11 months of data. 95% flux upper limits areplaced for several possible annihilation final states.
Flux upper limits are combined with the DM density inferred by the stellar data(*)
for a subset of 8 dSph (based on quality of stellar data) to extract constraints on<σv> vs WIMP mass for specific DM models

PRELIMINARYPRELIMINARY(*) stellar data from the Keck observatory (by Martinez, Bullock, Kaplinghat)

See C. Farnier’s talkand P. Scott’s poster

Search for DM in dSph
✓ Exclusion regions already cutting into interesting parameter space forsome WIMP models

PRELIMINARYPRELIMINARY

See C. Farnier’s talkand P. Scott’s poster

Search for DM in dSph
✓ Exclusion regions already cutting into interesting parameter space forsome WIMP models

Stronger constraints can be derived if IC of electrons and positrons from DM
annihilation off of the CMB is included, however diffusion in dwarfs is not known
use bracketing values of diffusion coefficients from cosmic rays in the Milky WayPRELIMINARYPRELIMINARY

See C. Farnier’s talkand P. Scott’s poster

Search for DM in dSph
✓ Exclusion regions already cutting into interesting parameter space forsome WIMP models

Stronger constraints can be derived if IC of electrons and positrons from DM
annihilation off of the CMB is included, however diffusion in dwarfs is not known
use bracketing values of diffusion coefficients from cosmic rays in the Milky WayPRELIMINARYPRELIMINARY

Conclusions/Outlook
No discovery....
.... however promising constraints on the nature of DM have been placed
In addition to increased statistics, better understanding of the astrophysical andinstrumental background will improve our ability to reliably extract a potential
signal of new physics or set stronger constraints
Further improvements are anticipated for analysis that benefits from multiwavelength observations (for example galactic center, dwarf spheroidal galaxies
and DM satellites)