Sensitivity of WIMP bounds on the velocity distribution in the limit of a massless mediator

Abstract: We discuss the sensitivity of the bounds on the spin-independent (SI) and spin-dependent (SD) WIMP-proton and WIMP-neutron interaction couplings $\alpha_{SI, SD}^{p,n}$ on the WIMP velocity distribution for a massless mediator in the propagator by combining direct detection and the neutrino signal from WIMP annihilation in the Sun (fixing the annihilation channel to $b\bar{b}$). We update the bounds in the Standard Halo Model (SHM) and using the halo-independent single-stream method. In the case of a massless mediator the SHM capture rate in the Sun diverges and is regularized by removing the contribution of WIMPs locked into orbits that extend beyond the Sun-Jupiter distance. We discuss the dependence of the SHM bounds on the Jupiter cut showing that it can be sizeable for $\alpha_{\rm SD}^p$ and a WIMP mass $m_\chi$ exceeding 1 TeV. Our updated SHM bounds show an improvement between about two and three orders of magnitude compared to the previous ones in the literature. Our halo-independent analysis shows that, with the exception of $\alpha_{\rm SD}^p$ at large $m_\chi$, the relaxation of the bounds compared to the SHM is of the same order of that for contact interactions, i.e. relatively moderate in the low and high WIMP mass regimes and as large as a few $\times\sim 10^2$ for $m_\chi\simeq$ 20 GeV. On the other hand, the exact determination of the relaxation of the bound becomes not reliable for $\alpha_{\rm SD}^p$ and $m_\chi\gtrsim$ 1 TeV due to the sensitivity of the SHM capture rate in the Sun to the details of the Maxwellian velocity distribution at low incoming WIMP speeds. The halo-independent bounds do not depend on the Jupiter cut needed to regularize the calculation of the capture rate.