The Experiment
Predicted antideuteron flux as a function of kinetic energy per nucleon for several dark matter candidates. The expected GAPS sensitivity is shown in gray. Figure adapted from Aramaki et al., 2016.

The General AntiParticle Spectrometer (GAPS) aims to study dark matter through sensitive observations of cosmic-ray antiprotons, antideuterons, and antihelium. The science goals of the GAPS experiment are:

Many dark matter theories predict high antiproton and antideuteron fluxes below ~1 GeV, including some neutralinos, Kaluza-Klein neutrinos, and gravitinos. While ordinary matter cosmic rays are abundantly produced and accelerated at astrophysical sources (so called "primaries"), the astrophysical cosmic-ray antimatter background comes mainly from the spallation of primaries on the interstellar medium ("secondaries"). Spallation reactions result in the creation of very few low-energy particles due to kinematics, and energy-loss mechanisms also operate less efficiently on antideuterons (compared to antiprotons), resulting in a highly suppressed antideuteron flux at low energies. The antideuteron search exploits both the enhanced dark matter signal and background suppression in the low-energy antideuteron spectrum.

GAPS will fly over Antarctica using a long-duration balloon. The long observation time and low geomagnetic cutoff rigidity near the South Pole allow GAPS to achieve its high sensitivity extending toward low energies. The GAPS flight is also near the next solar-minimum, when suppression of fluxes due to solar modulation will be reduced.

Dark Matter
Illustration of the basic principle behind three of the four methods of dark matter.

The idea of dark matter was first put forward by Fritz Zwicky in 1937 to explain anomalous galaxy velocities in the Coma cluster. Since then, evidence for dark matter has come from a variety of observations of astrophysical phenomena, including galactic rotation curves, the colliding galaxies in the Bullet Cluster, gravitational lensing, and the cosmic microwave background. However, these observations alone are unable to discriminate between the myriad viable models for dark matter.

Searches for dark matter can be grouped into four classes: direct detection, indirect detection, colliders, and astrophysical probes. Direct detection experiments aim to detect the scattering of dark matter off of standard model particles in the detector. Indirect detection experiments seek to observe the standard model products of dark matter interactions (e.g. annihilations or decays). Collider experiments attempt to produce dark matter particles, then look for signatures of such production. Lastly, astrophysical probes refer to all astrophysical observables sensitive to non-gravitational interactions of dark matter. Such diversity in detection methods is necessary to explore the entire parameter space dark matter can inhabit; for example, the gravitino mentioned above cannot be seen by direct detection experiments but can be detected with cosmic-ray antideuterons.

Several results from indirect detection experiments (e.g. using positrons, antiprotons) have prompted dark matter explanations, but it has proven difficult to disentangle the dark matter and astrophysical contributions. An unambiguous dark matter detection would require an indirect detection experiment with better-understood or lower (relative to the expected dark matter signal) backgrounds. Low-energy antideuterons may provide such a channel, and so GAPS has been designed to specifically measure these particles. A precise measurement of the low-energy antiproton spectrum will also allow for cleaner separation of a dark matter signal from the conventional background.

An Exotic Atom Technique
Detection principle of the GAPS experiment. An antideuteron (black) enters the detector and slows down, depositing ever larger amounts of energy in the ToF and tracker, then stops in the Si(Li) target, forming an exotic atom. As the atom de-excites, X-rays (red) are emitted at characteristic energies. The atom then annihilates, producing pions (purple) and protons. Figure adapted from Aramaki et al., 2016.

To identify cosmic-ray antiparticles, GAPS uses a novel technique based on the physics of an "exotic atom" where an antiparticle replaces an electron. An antiparticle first slows down through dE/dx losses in the ToF system and tracker. The antiparticle then stops in the tracker, forming an exotic atom with near-unit probability. The exotic atom initially forms in a high excitation state, and will subsequently deexcite through both autoionizing transitions and radiative transitions. The radiative transitions release X-rays at energies uniquely determined by the components of the exotic atom. Shortly after the release of these X-rays, the antiparticle annihilates in the nucleus into a shower (or "star") of pions and/or protons, whose multiplicity scales with the antiparticle's mass.

Measurements of the atomic X-rays and pion/proton multiplicities, along with the different stopping ranges of incoming particles and dE/dx vs. velocity measurements, allow for rare antideuterons to be distinguished from background processes, including cosmic-ray antiprotons, protons, and incidental X-rays (e.g. from muons). Furthermore, eliminating the need for a conventional magnetic spectrometer presents numerous advantages, the most important being an increased active volume, translating into a large geometric acceptance.