Gamma-ray triangles: A possible signature of asymmetric dark matter in indirect searches

Alejandro Ibarra, Sergio Lopez-Gehler, Emiliano Molinaro, and Miguel Pato

Phys. Rev. D 94, 103003 – Published 10 November 2016

Abstract

We introduce a new type of gamma-ray spectral feature, which we denominate gamma-ray triangle. This spectral feature arises in scenarios where dark matter self-annihilates via a chiral interaction into two Dirac fermions, which subsequently decay in flight into another fermion and a photon. The resulting photon spectrum resembles a sharp triangle and can be readily searched for in the gamma-ray sky. Using data from the Fermi-LAT and H.E.S.S. instruments, we find no evidence for such a spectral feature and, therefore, set strong upper bounds on the corresponding annihilation cross section. A concrete realization of a scenario yielding gamma-ray triangles consists of an asymmetric dark matter model where the dark matter particle carries lepton number. We show explicitly that this class of models can lead to intense gamma-ray spectral features, potentially at the reach of upcoming gamma-ray telescopes, opening a new window to explore asymmetric dark matter through indirect searches.

Received 2 May 2016

DOI:https://doi.org/10.1103/PhysRevD.94.103003

Physics Subject Headings (PhySH)

Gamma ray astronomy Particle dark matter

Gravitation, Cosmology & AstrophysicsParticles & Fields

Authors & Affiliations

Alejandro Ibarra¹, Sergio Lopez-Gehler^1,2, Emiliano Molinaro³, and Miguel Pato⁴

¹Physik-Department T30d, Technische Universität München, James-Franck-Straße, D-85748 Garching, Germany
²Excellence Cluster Universe, Technische Universität München, Boltzmannstraße 2, D-85748 Garching, Germany
³CP3-Origins, University of Southern Denmark, Campusvej 55, DK-5230 Odense M, Denmark
⁴The Oskar Klein Centre for Cosmoparticle Physics, Department of Physics, Stockholm University, AlbaNova, SE-106 91 Stockholm, Sweden

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Vol. 94, Iss. 10 — 15 November 2016

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Images

Figure 1
The triangular photon spectrum of a cascade annihilation $χ かい χ かい \to ψ ぷさい ψ ぷさい \to 2 ν にゅー 2 γ がんま$ with an intermediate fermionic state $ψ ぷさい$ . In the left panel, sample spectra are plotted for different values of $α あるふぁ$ , while fixing the mass splitting ${δ でるた}_{ψ ぷさい χ かい} = 0.25$ . In the right panel, we show the effect of varying the mass splitting ${δ でるた}_{ψ ぷさい χ かい}$ ; for the extreme values of ${δ でるた}_{ψ ぷさい χ かい}$ we adopt the physical values of $α あるふぁ$ , i.e. $α あるふぁ = 1$ for ${δ でるた}_{ψ ぷさい χ かい} = 0.99$ and $α あるふぁ = 0$ for ${δ でるた}_{ψ ぷさい χ かい} = 0.001$ (see text for a detailed discussion).
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Figure 2
The signal expected from triangular spectral features against current Fermi-LAT data. The signal plotted is produced by a dark matter model with $(σ しぐま v)_{0}^{2 γ がんま} = 3 \times 10^{- 26} {cm}^{3} / s$ , $m_{χ かい} = GeV$ , ${δ でるた}_{ψ ぷさい χ かい} = 0.25$ and $α あるふぁ = 0$ , $\pm 0.8$ , taking into account the energy resolution of Fermi-LAT. Note that these features correspond to the spectra shown in the left panel of Fig. 1. The Fermi-LAT data correspond to a circular region of radius 16° around the Galactic center (R16; cf. Appendix pp1). The cross section value $3 \times 10^{- 26} {cm}^{3} / s$ used here is solely for illustrative purposes and has no physical relevance, since we are not considering thermal relics but asymmetric dark matter models.
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Figure 3
The one-sided 95% confidence level upper limits on the annihilation cross section $(σ しぐま v)_{0}^{2 γ がんま}$ for triangular features of fixed width and different slopes (left) and different widths (right). The benchmark configurations shown here mimic the ones in the left and right panels of Fig. 1. The black and red lines correspond to the bounds obtained using Fermi-LAT R16 and H.E.S.S. data sets, respectively (cf. Appendix pp1 for further details on data treatment).
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Figure 4
The branching ratios of $N_{D}$ (semi-)leptonic decays into SM particles. In the upper panel, $P^{0, +}$ ( $V^{0, +}$ ) denote the pseudoscalar (vector) mesons which are kinematically accessible (see e.g. [47]) and it is assumed that $N_{D}$ couples only to the third lepton family. In the lower panel, the branching ratio of $N_{D}$ radiative decay is shown for a coupling to the first, second and third lepton families.
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Figure 5
The one-sided 95% confidence level upper limits on the coupling $f$ for the three benchmarks of Fig. 3 (right). The limits are shown for a generic mass splitting ${δ でるた}_{S χ かい}$ (left axis) and for $m_{S} = 5 m_{χ かい}$ (right axis), where in the latter case $f$ becomes nonperturbative for $m_{χ かい} ≳ 1.3 TeV$ .
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Figure 6
Correlation between the spin polarization $α あるふぁ$ of the decaying fermion $N_{D}$ and the relative width of the photon spectrum. We impose gamma-ray constraints on our simplified asymmetric dark matter model for the three benchmarks indicated in the plot: $({δ でるた}_{N χ かい}, α あるふぁ) = (0.25, 0.8)$ (square), $({δ でるた}_{N χ かい}, α あるふぁ) = (0.99, \approx 1)$ (circle) and $({δ でるた}_{N χ かい}, α あるふぁ) = (0.001, 0.06 \approx 0)$ (triangle). The photon spectra corresponding to these configurations are displayed in Fig. 1 (right). Notice that for the third benchmark we assume a vanishing $α あるふぁ$ for simplicity; this approximation has little impact on our results.
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