Illuminating dark matter and primordial black holes with interstellar antiprotons

Interstellar antiproton fluxes can arise from dark matter annihilating or d ecaying into quarks or gluons that subsequently fragment into antiprotons. Evaporation of primordial black holes can also produce a significant antipr oton cosmic-ray flux. Since the background of secondary antiprotons from sp allation has an interstellar energy spectrum that peaks at similar to 2 GeV and falls rapidly for energies below this, low-energy measurements of cosm ic antiprotons are useful in the search for exotic antiproton sources. Howe ver, measurement of the flux near the Earth is challenged by significant un certainties arising from the effects of the solar wind. We suggest evading this problem and more effectively probing dark matter signals by placing an antiproton spectrometer aboard an interstellar probe currently under discu ssion. We address the experimental challenges of a light, low-power-consumi ng detector, and present an initial design of such an instrument. This expe rimental effort could significantly increase our ability to detect, and hav e confidence in, a signal from exotic, nonstandard antiproton sources. Furt hermore, solar modulation effects in the heliosphere could be better quanti fied and understood by comparing results to inverse modulated data derived from existing balloon and space-based detectors near the Earth.