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1Scan for outdated or missing drivers - takes under a minute2Repair Windows errors before they cause bigger problems3Fix the driver behind crashes, sound loss and screen glitchesDark-matter annihilation and decay can both produce gamma rays, but they predict different sky patterns and particle-physics parameters. Annihilation involves two dark-matter particles and weights the signal by density squared; decay involves one particle and weights it by density. In ideal two-photon line channels, the photon energy is the dark-matter mass for annihilation and half that mass for decay.
How annihilation and decay differ
| Feature | Annihilation | Decay |
|---|---|---|
| Particles involved in each event | Two dark-matter particles | One dark-matter particle |
| Local rate’s density dependence | Density squared, ρ² | Density, ρ |
| Astrophysical line-of-sight factor | J-factor: an integral of density squared along the line of sight and over the relevant angular region | D-factor: the corresponding integral of density |
| Particle parameter setting normalization | Velocity-averaged annihilation cross-section, ⟨σv⟩ | Decay rate, Γ = 1/τ, or lifetime, τ |
| Ideal two-photon line energy | Eγ = mDM | Eγ = mDM/2 |
These are distinct parts of the prediction: particle physics determines such factors as the interaction rate and photon spectrum, while the target’s dark-matter distribution determines the J- or D-factor. The predicted flux also depends on the final state and its photon yield. A larger astrophysical factor alone does not guarantee a detectable signal.
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Why the sky pattern is different
At a given location, annihilation requires two particles to meet, so its event rate rises with the square of the local density. Decay is a one-particle process, so its rate rises linearly with density. Integrating those rates along each viewing direction produces the J-factor for annihilation and the D-factor for decay. This makes annihilation predictions more strongly weighted toward dense parts of a halo, including its center.
That distinction describes the underlying weighting, not a universal image shape. Whether a signal looks extended or concentrated depends on the target’s halo profile, its distance and angular extent, the instrument’s angular resolution, and the analysis. Annihilation predictions are especially sensitive to assumptions about dense central regions. For dwarf galaxies, estimates of the density distribution and halo extent are important; changing the adopted extent can affect the inferred astrophysical factor and resulting particle constraints. The cited dwarf-galaxy analysis discusses these modeling considerations.
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What a gamma-ray line can—and cannot—tell you
A line’s energy reflects the energy budget of a particular particle-physics event. In the ideal case of annihilation into two photons, the line energy is Eγ = mDM. In the two-photon decay example, each photon has energy Eγ = mDM/2. Those relations apply to those specific final states; not every annihilation or decay produces a monochromatic line.
Other final states can produce unstable particles that decay, or particles that hadronize, creating a spread of gamma-ray energies rather than a single line. Consequently, a line energy by itself is not a general test that identifies annihilation or decay. The expected spectrum must be considered together with the spatial distribution and the particle model.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.How searches interpret a possible signal
A gamma-ray search compares a predicted spectrum and sky distribution with instrument response, backgrounds, and the target’s properties. For annihilation, the calculation includes the J-factor alongside the cross-section, branching fractions into possible channels, and the photon spectrum. A cluster analysis, for example, sets out these dependencies and notes that bright Galactic diffuse emission can weaken results and introduce uncertainty through foreground templates. Its analysis illustrates why both the astrophysical target and foreground modeling matter.
Searches test annihilation and decay as possible sources of gamma rays; the cited search and review literature does not establish a confirmed dark-matter gamma-ray detection. A 2015 review of indirect searches describes the broader approach of looking for gamma rays and cosmic rays generated by dark-matter annihilation or decay.
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A historical Fermi-LAT example
A Fermi Large Area Telescope collaboration paper published in 2012 reported gamma-ray line-flux upper limits over 7–200 GeV and diffuse gamma-ray-background limits over 4.8–264 GeV, using two years of LAT data. These are energy ranges covered by that historical analysis—not a dark-matter mass measurement, a detection, or a statement of current global limits. The paper presents those results and their analysis context.
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