Exclusion of canonical WIMPs by the joint analysis of Milky Way dwarfs with Fermi

Abstract

Dwarf spheroidal galaxies are known to be excellent targets for the detection of annihilating dark matter. We present new limits on the annihilation cross section of Weakly Interacting Massive Particles (WIMPs) based on the joint analysis of seven Milky Way dwarfs using a frequentist Neyman construction and Pass 7 data from the Fermi Gamma-ray Space Telescope. We exclude generic WIMP candidates annihilating into b-bbar with mass less than 40 GeV that reproduce the observed relic abundance. To within 95% systematic errors on the dark matter distribution within the dwarfs, the mass lower limit can be as low as 19 GeV or as high as 240 GeV. For annihilation into tau+tau- these limits become 19 GeV, 13 GeV, and 80 GeV respectively.

Figures (4)

• Figure 1: Illustration of the Neyman confidence belt construction used to generate upper limits on $\Phi_{\mathrm{PP}}$. Each axis represents the number of events that could be observed from a given dwarf (here, Dwarf A has a larger $J$ value than Dwarf B does). The shaded area, bordered by the solid line, represents the confidence belt for a particular value of $\Phi_{\mathrm{PP}}$. The dashed lines are the borders of the confidence belts for different values of $\Phi_{\mathrm{PP}}$, with $\Phi_{\mathrm{PP}}$ increasing from left to right. The borders are chosen to be normal to a vector of "sensitivities", which weights each dwarf according to the relative strength of its dark matter signal. Once a measurement is made (shown by the star) the confidence interval for $\Phi_{\mathrm{PP}}$ contains all values of $\Phi_{\mathrm{PP}}$ whose confidence belt contains the measured point. The dotted line shows the border for an alternative construction of the confidence belts which gives equal weight to each dwarf.
• Figure 2: Derived 95% upper limit on $\langle \sigma_A v \rangle$ as a function of mass for dark matter annihilation into $b{\bar{b}}$ and $\tau^+\tau^-$. The shaded area reflects the 95-percentile of the systematic uncertainty in the dark matter distribution of the dwarfs. The canonical annihilation cross section for a thermal WIMP making up the total observed dark matter abundance is shown by the dashed line. The inset figure shows detail for lower masses.
• Figure 3: This figure illustrates the ingredients and data required to derive upper limits on the dark matter annihilation cross section. Each plot corresponds to a different dwarf galaxy. Sampling the counts in 0.5$^\circ$ regions surrounding each dwarf results in an empirical background probability mass function (PMF) shown in red. The blue curves are Poisson distributions having the same mean as the empirical background PMFs. The vertical line represents the number of counts observed in the ROI centered on the dwarf's location. The dashed curve is the convolution of the background PMF with the Poisson distribution representing the contribution from dark matter annihilation when $\Phi_{\mathrm{PP}} = 5.0 \times10^{-30}\,{\mathrm{cm}}^3\, {\mathrm{s}}^{-1}\, \mathrm{GeV}^{-2}$ (the 95% upper limit on $\Phi_{\mathrm{PP}}$). This convolution is the probability distribution of the sum of signal and background. The label $w$ is the weight given to each dwarf in our construction of Neyman confidence belts. It is given by the ratio of the strength of the expected dark matter signal to the mean expected background.
• Figure 4: Derived limits on $\langle \sigma_A v \rangle$ as a function of mass for dark matter annihilation into $W^+W^-$, a heavy quark final state ($b{\bar{b}}$), and heavy lepton final states $\tau^+\tau^-$ and $\mu^+ \mu^-$. Limits on other heavy quark final states are similar to the $b{\bar{b}}$ channel. Annihilation spectra are derived using 2004JCAP...07..008Gdarksusy. All limits are 95% upper limits based on the central $J$ values for the dwarfs.