Constraining Supersymmetric Dark Matter With Synchrotron Measurements

Abstract

The annihilations of neutralino dark matter (or other dark matter candidate) generate, among other Standard Model states, electrons and positrons. These particles emit synchrotron photons as a result of their interaction with the Galactic Magnetic Field. In this letter, we use the measurements of the WMAP satellite to constrain the intensity of this synchrotron emission and, in turn, the annihilation cross section of the lightest neutralino. We find this constraint to be more stringent than that provided by any other current indirect detection channel. In particular, the neutralino annihilation cross section must be less than ~ 3 x 10^-26 cm^3/s (1 x 10^25 cm^3/s) for 100 GeV (500 GeV) neutralinos distributed with an NFW halo profile. For the conservative case of an entirely flat dark matter distribution within the inner 8 kiloparsecs of the Milky Way, the constraint is approximately a factor of 30 less stringent. Even in this conservative case, synchrotron measurements strongly constrain, for example, the possibility of wino or higgsino neutralino dark matter produced non-thermally in the early universe.

Figures (3)

• Figure 1: The specific intensity (in kilo-Janskys per steradian) observed by WMAP in its 22 and 33 GHz bands, as a function of the angle from the Galactic Center. In each frame, the dashed line denotes the flux of synchrotron emission from the annihilation products of a 200 GeV neutralino annihilating to $W^+ W^-$ with an annihilation cross section of $\sigma v = 5 \times 10^{-26}$ cm$^3$/s and distributed with an NFW halo profile. We have used $U_B/(U_B+U_{\rm rad}) = 0.26$ and the diffusion parameters described in the text.
• Figure 2: Top: The upper limit on the neutralino annihilation cross section from the synchrotron constraint as a function of mass, for the case of an NFW halo profile (dashed) and a flat (homogeneous) distribution of dark matter within the solar circle (dot-dashed). These limits were arrived at considering neutralinos which annihilate largely to $W^+ W^-$ (as is the case for wino or higgsino-like neutralinos), $U_B/(U_B+U_{\rm rad}) = 0.26$ and the diffusion parameters described in the text. Shown for comparison are the annihilation cross sections for a pure-wino (red solid) and a pure-higgsino (green solid). Bottom: The upper limit found with an NFW profile, and for several dominant annihilation modes, $b\bar{b}$ (dotted), $ZZ$ (blue dashed), $W^+W^-$ (black dashed) and $\tau^+ \tau^-$ (solid).
• Figure 3: A comparison of the limits placed on the dark matter's annihilation cross section from several astrophysical channels. The black dashed and dot-dashed lines represent the synchrotron constraints (see Fig. \ref{['limit']}) for the case of a NFW halo profile and the conservative case of a flat dark matter distribution, respectively. The dotted blue line represents the constraint which can be arrived at from measurements of the cosmic positron spectrum by the HEAT experiment positron. The red dashed line is the limit from the EGRET gamma-ray satellite for the case of an NFW halo profile dingus. The upper dot-dashed curve is the conservative limit from the diffuse neutrino flux, assuming dark matter annihilates only to neutrinos beacom. With the exception of the neutrino constraint, each of these limits were arrived at considering neutralinos which annihilate largely to $W^+ W^-$ (as is the case for wino or higgsino-like neutralinos).