Gamma Rays from Heavy Neutralino Dark Matter

TL;DR

The paper identifies a new radiative mechanism—internal bremsstrahlung in $\chi\chi \to W^+W^-\gamma$—that generates a pronounced high-energy peak in the gamma-ray spectrum of TeV-scale, higgsino-like neutralino dark matter near $E_\gamma \sim m_\chi$, potentially surpassing both the continuous spectrum from $W$ fragmentation and the $\gamma\gamma$/$Z\gamma$ lines after detector smearing. It provides an analytic expression for the photon yield ${dN_\gamma^W}/{dx}$ with $x=E_\gamma/m_\chi$ and discusses the roles of transverse versus longitudinal $W$ polarizations through the mass-shift parameter $\delta=(m_{\chi^\pm_1}-m_\chi)/m_W$ in the limit $v\to 0$. The work shows that, especially for small $\delta$, the high-energy peak remains robust against velocity effects and detector resolution, and can enhance line signatures by factor ~2 under realistic resolutions, improving prospects for gamma-ray indirect detection of heavy neutralinos. However, absolute flux predictions remain halo-dependent, underscoring the importance of spectral shape as a discriminant and its relevance for relic-density calculations in public SUSY tools.

Abstract

We consider the gamma-ray spectrum from neutralino dark matter annihilations and show that internal bremsstrahlung of W pair final states gives a previously neglected source of photons at energies near the mass of the neutralino. For masses larger than about 1 TeV, and for present day detector resolutions, this results in a characteristic signal that may dominate not only over the continuous spectrum from W fragmentation, but also over the γ-γand γ-Z line signals which are known to give large rates for heavy neutralinos. Observational prospects thus seem promising.

Figures (4)

• Figure 1: Contributions to $\chi\chi \rightarrow W^+ W^- \gamma$ for a pure higgsino-like neutralino (crossing fermion lines are not shown).
• Figure 2: The photon multiplicity for the radiative processes $\chi\chi\rightarrow W^+W^-\gamma$. The dots represent the MSSM model of Table \ref{['tab_susy']}, as computed with the FormCalc package formcalc for a relative neutralino velocity of $10^{-3}$. The thick solid line shows the full analytical result for the pure higgsino limit of the same model but with zero relative neutralino velocity. The thin solid line is the corresponding approximation as given in Eq.(\ref{['mult']}). Also shown, as dashed and dotted lines, are two pure higgsino models with a lightest neutralino (chargino) mass of 10 TeV (10 TeV) and 1.5 TeV (2.5 TeV), respectively.
• Figure 3: The total differential photon distribution from $\chi\chi$ annihilations (solid line) for the MSSM model of Table \ref{['tab_susy']}. Also shown separately is the contribution from radiative processes $\chi\chi\rightarrow W^+W^-\gamma$ (dashed), and the $W$ fragmentation together with the $\chi\chi\rightarrow\gamma\gamma, \, Z\gamma$ lines (dotted).
• Figure 4: The same spectra as in Fig. \ref{['fig_allspectra']}, as seen by a detector with an energy resolution of 15 percent.