Abstract
Adopting the Standard Halo Model (SHM) of an isotropic Maxwellian velocity distribution for dark matter (DM) particles in the Galaxy, the most stringent current constraints on their spin-dependent scattering cross-section with nucleons come from the IceCube neutrino observatory and the PICO-60 hbox {C}_3hbox {F}_8 superheated bubble chamber experiments. The former is sensitive to high energy neutrinos from the self-annihilation of DM particles captured in the Sun, while the latter looks for nuclear recoil events from DM scattering off nucleons. Although slower DM particles are more likely to be captured by the Sun, the faster ones are more likely to be detected by PICO. Recent N-body simulations suggest significant deviations from the SHM for the smooth halo component of the DM, while observations hint at a dominant fraction of the local DM being in substructures. We use the method of Ferrer et al. (JCAP 1509: 052, 2015) to exploit the complementarity between the two approaches and derive conservative constraints on DM-nucleon scattering. Our results constrain sigma _{mathrm{SD}} lesssim 3 times 10^{-39} mathrm {cm}^2 (6 times 10^{-38} mathrm {cm}^2) at gtrsim 90% C.L. for a DM particle of mass 1 TeV annihilating into tau ^+ tau ^- (bbar{b}) with a local density of rho _{mathrm{DM}} = 0.3~mathrm {GeV/cm}^3. The constraints scale inversely with rho _{mathrm{DM}} and are independent of the DM velocity distribution.
Highlights
Based on inferences from observations of gravitational effects, it has long been believed that a significant fraction of the Universe is made up of dark matter (DM)
For the “hard” channels ( W +W −and τ +τ −), which produce a relatively large number of neutrinos at energies just below the DM mass, the DM-velocity-independent constraints are in general worse only by a factor of 2–4 compared to the PICO Standard Halo Model (SHM) constraints
At a DM Mass of ∼250 GeV (∼700 GeV for bb), the constraints are significantly worse because the DM particle velocities just below the PICO threshold are still too high to be captured by scattering off protons in the Sun
Summary
Based on inferences from observations of gravitational effects, it has long been believed that a significant fraction of the Universe is made up of dark matter (DM) (see [2]). In the absence of a convincing detection, constraints have been derived on the interaction cross-sections of these hypothetical particles with Standard Model particles Such an inference requires knowledge both of the density of DM ρDM and of its velocity distribution function (VDF) f (v). In this work we use the method of [1], which is independent of the velocity distribution of the halo model to exploit this complementarity and derive conservative, upper limits on the spindependent DM-nucleon scattering cross-section by combining the results from [10,11]. Constraints from individual searches will be dependent on the stream velocity, by exploiting the complementarity of the IceCube and PICO searches, constraints independent of the stream velocity can be obtained This method improves on previous assessments of halo model uncertainties on indirect DM detection [13], by allowing the velocity distribution to be anisotropic. The resulting constraints are a factor of 2–4 worse than the PICO SHM constraints at low DM masses and up to an order of magnitude worse at high DM masses, depending upon the annihilation channel, but are independent of the halo model
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