New Gamma-Ray Contributions to Supersymmetric Dark Matter Annihilation

TL;DR

The paper tackles the problem of predicting gamma-ray signatures from neutralino dark matter annihilation by systematically computing electromagnetic radiative corrections, including internal bremsstrahlung (FSR and VIB), across MSSM and mSUGRA scenarios and constraining models to the observed relic density. It employs large-scale scans with DarkSUSY to assess how IB alters the high-energy gamma-ray spectrum, finding that IB can boost the yield by up to orders of magnitude in regions with near-degeneracies (e.g., stau coannihilation) and create a sharp spectral feature at $E_6gamma = m_6chi$, often outperforming line signals. The results indicate that these corrections are essential for accurate gamma-ray templates and could significantly impact analyses with GLAST (Fermi-LAT) data. The authors provide benchmark models and integrate the IB calculations into DarkSUSY to facilitate broader phenomenological use and future work on astrophysical backgrounds and non-perturbative effects.

Abstract

We compute the electromagnetic radiative corrections to all leading annihilation processes which may occur in the Galactic dark matter halo, for dark matter in the framework of supersymmetric extensions of the Standard Model (MSSM and mSUGRA), and present the results of scans over the parameter space that is consistent with present observational bounds on the dark matter density of the Universe. Although these processes have previously been considered in some special cases by various authors, our new general analysis shows novel interesting results with large corrections that may be of importance, e.g., for searches at the soon to be launched GLAST gamma-ray space telescope. In particular, it is pointed out that regions of parameter space where there is a near degeneracy between the dark matter neutralino and the tau sleptons, radiative corrections may boost the gamma-ray yield by up to three or four orders of magnitude, even for neutralino masses considerably below the TeV scale, and will enhance the very characteristic signature of dark matter annihilations, namely a sharp step at the mass of the dark matter particle. Since this is a particularly interesting region for more constrained mSUGRA models of supersymmetry, we use an extensive scan over this parameter space to verify the significance of our findings. We also re-visit the direct annihilation of neutralinos into photons and point out that, for a considerable part of the parameter space, internal bremsstrahlung is more important for indirect dark matter searches than line signals.

Figures (5)

• Figure 1: Types of diagrams that contribute to the first order QED corrections to WIMP annihilations into a pair of charged particle final states. The leading contributions to diagrams (a) and (b) are universal, referred to as final state radiation (FSR), with a spectral distribution which only depends slightly on the final state particle spin and has been calculated, e.g., in birkedal. Internal bremsstrahlung from virtual particles (or virtual internal bremsstrahlung, VIB) as in diagram (c), on the other hand, is strongly dependent on details of the short-distance physics such as helicity properties of the initial state and masses of intermediate particles.
• Figure 2: From top to bottom, the gamma-ray spectra for the benchmark models defined in Tab. \ref{['benchmark']} is shown. The contributions from IB and secondary photons is indicated separately (in these figures, the line signal is not included).
• Figure 3: Integrated internal bremsstrahlung flux from supersymmetric dark matter, above $0.6\,m_\chi$, as compared to the "standard" continuum flux produced by secondary photons (left) and the flux from both line signals (right). As for the following figures (\ref{['FSRcomp2']} and \ref{['FSRmSUGRA']}), two symbols at the same location always indicate the whole interval between the values corresponding to these symbols. Every model considered here features a relic density as determined by WMAP and satisfies all current experimental bounds.
• Figure 4: The observationally relevant quantity ${\cal S}\equiv N_\gamma\frac{\langle\sigma v\rangle}{10^{-29}\mathrm{cm}^3\mathrm{s}^{-1}}\left(\frac{m_\chi}{100\mathrm{GeV}}\right)^{-2}$ for IB (left panel) and the line signals (middle and right panel). See text for more details.
• Figure 5: As in the left panel of Fig. \ref{['FSRcomp1']}, but now for the individual contributions from various final states of neutralino annihilations in mSUGRA models. IB from light leptons covers a very similar region of the plotted parameter space as that from $\tau$ leptons. All (other) final states not shown here give always IB fluxes less than 10% of the flux from secondary photons.