GAPS: Antarctic Balloon Mission to Search for Dark Matter Signatures in Cosmic-ray Antinuclei

The General Antiparticle Spectrometer (GAPS) is an Antarctic balloon mission to search for dark matter by measuring low-energy cosmic-ray antinuclei using a novel detection technique. GAPS is the first experiment optimized to detect cosmic-ray antideuterons below 0.25 GeV/n. Because antideuteron production in this energy range is kinematically suppressed in standard astrophysical processes but expected from a wide range of well-motivated dark matter models, detection of antideuterons by GAPS would be a smoking-gun signature of new physics. Meanwhile, GAPS will also provide a precision cosmic-ray antiproton spectrum in a previously unprobed low-energy range, as well as leading sensitivity to low-energy antihelium-3. The GAPS measurement is possible thanks to a novel particle identification method based on exotic atom formation, de-excitation, and decay. Exotic atoms provide a unique event topology for negatively-charged antinuclei and enable a large sensitive area for detecting rare events within the constraints of a long-duration balloon. The instrument contains two sensitive detector systems. A time-of-flight system, composed of 160 scintillator panels with <400 ps timing resolution, provides the system trigger as well as the GAPS energy scale. Completely enclosed within the time-of-flight, the 2.5 m3 tracker volume is instrumented with 1000 10-cm-diameter silicon sensors which serve as the target, X-ray spectrometer, and particle tracker. An oscillating heat pump thermal system cools the tracker without the need for a bulky cryostat. Calibration of the integrated payload is underway, with the first of several Antarctic flights expected in 2024. This contribution will briefly review the dark matter motivation for a cosmic-ray antinucleus detector. It will then introduce the GAPS instrument and its exotic-atom detection capability before finally reporting the calibration on the sensitive detector systems and the performance of the integrated payload.