Neutrinos from off-shell final states and the indirect detection of dark matter

Abstract

We revisit the annihilation of dark matter to neutrinos in the Sun near the ${W}^{+}{W}^{\ensuremath{-}}$ and $t\overline{t}$ kinematic thresholds. We investigate the potential importance of annihilation to $W{W}^{*}$ in a minimal dark matter model in which a Majorana singlet is mixed with a vectorlike electroweak doublet, but many results generalize to other models of weakly interacting dark matter. We reevaluate the indirect detection constraints on this model and find that, once all annihilation channels are properly taken into account, the most stringent constraints on spin-dependent scattering for dark matter mass $60\text{ }\text{ }\mathrm{GeV}\ensuremath{\lesssim}{m}_{\ensuremath{\chi}}\ensuremath{\lesssim}{m}_{t}$ are derived from the results of the Super-Kamiokande experiment. Moreover, we establish the model-independent statement that Majorana dark matter whose thermal relic abundance and neutrino signals are both controlled by annihilation via an $s$-channel $Z$ boson is excluded for $70\text{ }\text{ }\mathrm{GeV}\ensuremath{\lesssim}{m}_{\ensuremath{\chi}}\ensuremath{\lesssim}{m}_{W}$. In some models, annihilation to $t{t}^{*}$ can affect indirect detection, notably by competing with annihilation to gauge boson final states and thereby weakening neutrino signals. However, in the minimal model, this final state is largely negligible, only allowing dark matter with mass a few GeV below the top quark mass to evade exclusion.