\ Experiments in matter wave interferometry and optomechanics are increasing the spatial extent of wavefunctions of massive quantum systems; this gives rise to new sources of decoherence that must be characterized. Here\ we calculate the position space decoherence of a quantum particle due to interaction\ with a fluctuating classical background gas for several different force laws. We begin\ with the calculation of this effect for the Newton potential. To our knowledge, this\ calculation has not been done before. We then solve the same problem in the case\ of a Yukawa interaction, which interpolates between our long-range force result and\ the well-studied formula for collisional decoherence from a contact interaction. Unlike the contact interaction case, where the decoherence rate becomes independent\ of distance for large quantum particle separations, we observe that a long-range\ interaction leads to quadratic scaling of the decoherence rate with distance even at\ large separations. This work is relevant to the generation of massive superposition\ in optomechanical and atom beam experiments, and to conclude we comment on\ the use of this decoherence signal for gravitational detection of dark matter.\

}, url = {http://meetings.aps.org/Meeting/DAMOP20/Session/S08.5}, author = {Jonathan Kunjummen and Daniel Carney and J. M. Taylor} }