A dark matter (DM) search experiment was flown on the IMAX balloon payload to search for a possible minor component of the dark matter in the Galactic halo: ionizing massive particles (IMPs) (mx≳104 GeV/c2) that cannot penetrate the atmosphere due to their low‐velocities and high energy‐loss. The DM search experiment consisted of a delayed coincidence between four large plastic scintillation detectors arranged in a vertical stack. In order to search for ultra‐slow particles which do not slow down in the IMAX telescope, the experiment contained TDCs which measured the time‐delay Ti,i+1∈(0.3, 14.0) μs between hits in successive counters to ∼2% precision. We present IMP flux limits for non‐slowing IMPs and also for IMPs which slow down significantly within the IMAX telescope. This experiment effectively closes much of a previously unconstrained ‘‘window’’ in the mass/cross‐section joint parameter spaces for massive particles as the dominant halo DM.

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