Dark matter gamma-ray line searches toward the Galactic Center halo - - PowerPoint PPT Presentation

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Dark matter gamma-ray line searches toward the Galactic Center halo - - PowerPoint PPT Presentation

Sep 10, 2023 246 likes •482 views

Dark matter gamma-ray line searches toward the Galactic Center halo with H.E.S.S. I Emmanuel Moulin, Lucia Rinchiuso, Aion Viana, Christopher van Eldik, Johannes Veh for the H.E.S.S. collaboration The H.E.S.S. experiment H.E.S.S. phase I

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Emmanuel Moulin, Lucia Rinchiuso, Aion Viana, Christopher van Eldik, Johannes Veh for the H.E.S.S. collaboration

Dark matter gamma-ray line searches toward the Galactic Center halo with H.E.S.S. I

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§ H.E.S.S. phase I

§ four 12m telescopes § FoV 5 deg § energy threshold 100 GeV § angular resolution < 0.1 deg § Operating since 2003

The H.E.S.S. experiment

§ H.E.S.S. phase II

§ four 12m telescopes § one 28m telescope (FoV 3.5 deg) § energy threshold O(30 GeV) § angular resolution from 0.4 deg to less than 0.1 deg

H.E.S.S. phase I H.E.S.S. phase II

Emmanuel Moulin . ICRC 2017, Busan, Korea

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Why VHE gamma-rays ?

§ Do not suffer from propagation effects at Galactic Scale : they point back to the source § Can reveal the abundance and distribution of DM § Characteristic features may be present in the spectrum at these energies

Emmanuel Moulin . ICRC 2017, Busan, Korea

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Why VHE gamma-rays ?

§ Do not suffer from propagation effects at Galactic scale : they point back to the source § Can reveal the abundance and distribution of DM § Characteristic features may be present in the spectrum at these energies § Identification of DM is possible

  • the gamma-ray distribution in the sky tells the DM density

distribution

  • the gamma-ray spectrum tells the reaction process and DM mass

Emmanuel Moulin . ICRC 2017, Busan, Korea

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Dark matter search with VHE gamma rays

Particle Physics :

  • Cross sections
  • Differential photon yield
  • DM particle mass

Astrophysics → DM distribution in the target

Emmanuel Moulin . ICRC 2017, Busan, Korea

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Dark matter search with VHE gamma rays

Particle Physics :

  • Cross sections
  • Differential photon yield
  • DM particle mass

Astrophysics → DM distribution in the target

Emmanuel Moulin . ICRC 2017, Busan, Korea

§ Weakly Interacting Massive Particles (WIMPs) – The weak interaction mass scale and ordinary gauge couplings give right relic DM density without fine-tuning. – Mass scale GeV-TeV, makes them Cold Dark Matter – Provides benchmark for indirect detection: thermally-produced WIMPs

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Spectral signatures in gamma rays

Continuum emission (“Secondary photons”) → from fragmentation of quarks/massive gauge bosons (via π0 decay) Virtual Internal Bremsstrahlung (VIB) → radiative correction to processes with charged final states → generically suppressed by O(α) Gamma-ray lines → from two-body annihilation into photons → forbidden at tree-level, generically suppressed by O(α2)

Emmanuel Moulin . ICRC 2017, Busan, Korea

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Energy spectrum from dark matter annihilation

cut-off of the signal at DM mass “power-law” background

The spectral information can be exploited for a better signal vs. background discrimination γ-line w/o energy resolution

not at tree level: suppressed but clear signature at DM mass !

1. Continuum: hadronization and/or decay of W/Z, quarks, leptons… 2. Line from prompt annihilation in two photons 3. Final state radiation 4. Virtual internal bremsstrahlung

Emmanuel Moulin . ICRC 2017, Busan, Korea

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Dark matter targets gamma-ray searches

§ DM density matters § Astrophysical background matters as well

Galactic Centre

  • Proximity (~8kpc)
  • Possibly high DM

concentration : DM profile : core? cusp?

  • High astrophysical

bck / source confusion Galaxy satellites of the Milky Way

  • Many of them within the 100 kpc from GC
  • High M/L
  • Low astrophysical background

Aquarius, Springel et al. Nature 2008

Substructures in the Galactic halo

  • Lower signal
  • Cleaner signal

(once found) Galactic halo

  • Large statistics
  • Galactic diffuse

background

Emmanuel Moulin . ICRC 2017, Busan, Korea

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Dark matter distribution in the Galactic Centre region (1)

We assume cusped DM density profiles, e.g. the Einasto, NFW, …

ρ$%&' r = ρ*exp −2 α r r*

2

− 1 parametrized with α = 0.17 r* = 21 𝑙𝑞𝑑 ρ* = 0.07 𝐻𝑓𝑊𝑑𝑛>? Einasto profile

as used in HESS GC 2011, 2013 papers

per bin of 0.02degx0.02deg

Emmanuel Moulin . ICRC 2017, Busan, Korea

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Dark matter distribution in the GC region (2)

We assume cusped DM density profiles, e.g. the Einasto, NFW, …

Einasto profile

per bin of 0.02degx0.02deg

Spatial information can be used to discriminate from the isotropic residual background

Emmanuel Moulin . ICRC 2017, Busan, Korea

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Dark matter distribution in the GC region (3)

We assume cusped DM density profiles, e.g. the Einasto, NFW, …

Einasto profile Example for one OFF region for a given pointing

ON region pointing position

§ Significant DM gradient between ON and OFF regions

per bin of 0.02degx0.02deg

Emmanuel Moulin . ICRC 2017, Busan, Korea

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Optimized background measurement

§ OFF regions are chosen symmetrically to ON regions with respect to the pointing position in the same observational field of view § Overlapping areas are discarded to maintain the dark matter gradient

→ same acceptance in ON and

OFF region (due to azimuthal symmetry) → strong dark matter gradient that improves the limits

Emmanuel Moulin . ICRC 2017, Busan, Korea

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The Galactic Centre region seen by H.E.S.S. I

§ Largest H.E.S.S. dataset from observations accumulated over 10 years § Most detailed view of GC @VHE : pointlike sources (PWN, SNR, Unid) and diffuse emission → GC region is a crowded environment at VHE § DM density in the central region of the Milky Way expected to be large

Emmanuel Moulin . H.E.S.S. Moriond EW 2015

Excess map, E>200 GeV

75 pc

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Region of interest (ROI) definition

§ 10-year observations with H.E.S.S. 1 toward the GC § Gamma-ray statistics : 254 h compared to 112 h (2011/2013 papers) § Novel analysis method : 2D likelihood analysis with spectral and spatial information of signal and background

Galactic Longitude Galactic Latitude G0.9 Sgr A*

Gamma-ray excess map

HESS J1745-303

Emmanuel Moulin . ICRC 2017, Busan, Korea

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§ Excluded region ±0.3° : dominated by astrophysical sources and diffuse emission § Whole region of Interest: 1° in radius § ROI divided in 7 sub-regions of 0.1°: spatial binning

Emmanuel Moulin . ICRC 2017, Busan, Korea

Region of interest (ROI) definition

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Gamma-ray line rate

§ Spectrum : Dirac delta function at 𝐧𝐄𝐍 § Energy resolution with

𝛕 𝐅 = 𝟐𝟏%

  • almost independent of the offset and the

azimuth angle

𝐞𝐎 𝐞𝐅′ = 𝟑𝛆 𝐅O − 𝐧𝐄𝐍 𝐒 𝐅, 𝐅O = 𝟐 𝟑𝛒

  • 𝛕

𝐟

> 𝐅>𝐅U 𝟑 𝟑𝛕𝟑

𝐎𝐣𝐤

𝛅 = 𝟐

𝟓𝛒 𝛕𝐰 𝟑𝐧𝐄𝐍

𝟑

[ 𝐞𝐎 𝐞𝐅O 𝐒 𝐅, 𝐅O 𝐔𝐩𝐜𝐭𝐁𝐟𝐠𝐠(𝐅)𝐞𝐅′

  • x 𝐊(∆𝛁)

sigma/E bias

Emmanuel Moulin . ICRC 2017, Busan, Korea

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2D-binned likelihood analysis

§ Likelihood function binned in energy (bin j) and space (RoI, bin i) where expected NB obtained from dL/dNB = 0 § Test statistics: likelihood-ratio test § No significant excess if found in any of the ROIs :

𝓜 = h 𝓜𝐣𝐤

𝐣,𝐤

Observed signal (ON region) Observed background (OFF region) Line signal (expected in ON) Line signal (expected in OFF) Expected background 𝐎𝐏𝐎 𝐎𝐏𝐆𝐆 𝐎𝐓 𝐎′𝐓 𝐎𝐂

𝐔𝐓 = −𝟑 𝐦𝐩𝐡 𝓜𝐗 𝓜𝐗𝐏 = −𝟑 𝐦𝐩𝐡 𝓜 𝐎𝐏𝐎, 𝐎𝐏𝐆𝐆 𝐎𝐓, 𝐎𝐓

O , 𝐎𝐂

𝓜 𝐎𝐏𝐎, 𝐎𝐏𝐆𝐆 𝐎𝐂 TS = 2.71 for a 95% CL limit

𝓜𝐣𝐤 𝐎𝐏𝐎, 𝐎𝐏𝐆𝐆 𝐎𝐓, 𝐎𝐂 = 𝐎𝐓 + 𝐎𝐂 𝐎𝐏𝐎 𝐎𝐏𝐎! 𝐟> 𝐎𝐓r𝐎𝐂 𝐎𝐓

O + 𝐎𝐂 𝐎𝐏𝐆𝐆

𝐎𝐏𝐆𝐆! 𝐟> 𝐎𝐂r𝐎𝐓U

Emmanuel Moulin . ICRC 2017, Busan, Korea

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95% C.L. expected limits on the annihilation flux

§ Improvement of a factor about 8 observed @ 1 TeV

  • n the mean expected

limits § Improvement from the analysis: exclusion regions, 2D binned likelihood analysis approach, improved raw data danalysis, higher statistics

Emmanuel Moulin . ICRC 2017, Busan, Korea

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95% C.L. expected limits on the annililation cross section σv

§ best limit 2x10-28cm3s-1 @1TeV § mass range extended down to 300 GeV and up to 70 TeV § lower energy threshold thanks to the improved raw data analysis: best limit shifted down to lower masses

Emmanuel Moulin . ICRC 2017, Busan, Korea

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Comparison with previous result and other experiments

§ best limit 2x10-28cm3s-1 @1TeV § lower energy threshold thanks to the improved raw data analysis: best limit shifted down to lower masses § Fermi-LAT limits surpassed

  • f a factor about 6 @300

GeV

Emmanuel Moulin . ICRC 2017, Busan, Korea

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Summary

§ Full H.E.S.S.-I GC dataset analysis for dark matter line searches in the inner GC halo in the DM mass range 300 GeV – 70 TeV § Improvement at 1 TeV with respect to 2013 results is about 8 on the mean expected sensitivity

  • higher photon statistics
  • higher sensitivity of the improved raw data analysis
  • 2D likelihood analyis method

§ Next steps :

  • Observed limits for publication
  • Follow-up study with H.E.S.S.-II data: more statistics and extended

pointings from inner Galaxy survey started in 2015

  • Tests of different TeV DM models can be performed

Emmanuel Moulin . ICRC 2017, Busan, Korea

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Constraints on DM annihilation towards the GC for the continuum with 10 years of H.E.S.S. data (3)

For the Einasto profile, strongest limits so far in the TeV mass range:

  • in the WW channel: 6×10−26 cm3 s−1 at 1.5 TeV
  • in the ττ channel: 2×10−26 cm3 s−1 at 800 GeV

H.E.S.S. Collaboration, Phys. Rev. Lett. 117, 111301 (2016)

Emmanuel Moulin . ICRC 2017, Busan, Korea