Gamma-Ray and AntiMatter Survey(GRAMS) experiment

TL;DR

GRAMS targets the MeV-gap in gamma-ray astronomy by deploying a large-scale liquid argon time projection chamber (LArTPC) to simultaneously measure MeV gamma rays and low-energy antinuclei as indirect dark matter signatures. The instrument leverages a segmented LArTPC plus a time-of-flight system to reconstruct Compton events for gamma directions and to capture antiparticles as exotic atoms whose annihilation products reveal the particle type; GEANT4 simulations with QGSP_BERT project significant sensitivity gains over prior missions, including MeV gamma-ray improvements by over an order of magnitude and competitive antinuclei sensitivities such as $\sim 10^{-6}$ for antideuterons and $1.47 \times 10^{-7}$ for antihelium-3 on balloons, extending to $1.55 \times 10^{-9}$ / $3.10 \times 10^{-10}$ for 2-year/10-year space missions. The program advances through a staged path with pGRAMS and a 2026 science flight in mind, supported by NASA APRA and international partners, and builds toward a long-term satellite capability following prior missions like GAPS and COSI. If realized, GRAMS would offer a cost-effective route to a dual-measurement platform for MeV gamma-ray astronomy and indirect dark matter detection with transformative improvements in sensitivity. The integration of TOF, Compton reconstruction, and exotic-atom signatures enables cross-checks with other experiments and provides a unique probe of nucleosynthesis, particle acceleration, and dark matter interactions in the MeV regime.

Abstract

The Gamma-Ray and AntiMatter Survey (GRAMS) is a next-generation experiment using a Liquid Argon Time Projection Chamber (LArTPC) detector to measure MeV gamma rays and antiparticles. MeV gamma-ray observations are important for understanding multi-messenger and time-domain astronomy, enabling exploration of the universe's most potent events, such as supernovae and neutron star mergers. Despite the significance of MeV gamma-rays, GRAMS could also explore the so-called 'MeV gap' region to improve MeV gamma-ray measurement sensitivity that was restricted by the challenge of accurately reconstructing Compton events. Aside from gamma-ray detection, the GRAMS proposed method also serves as an antiparticle spectrometer, targeting the low-energy range of cosmic antinuclei measurements. This work will provide updates on the current status and progress towards the prototype balloon flight with a small-scale LArTPC (pGRAMS) scheduled for early 2026, as well as the recent progress on antihelium-3 sensitivity calculation.

Figures (4)

• Figure 1: GRAMS detection concept. The LArTPC is segmented into “cells” to minimize coincident background events. The right figure shows charged-particle and gamma-ray interactions inside the detector. For the charged-particle case, including antinuclei, the TOF system will measure the velocity and energy deposition. Then, they will slow down as they deposit energy, through ionization, in the LArTPC. The antiparticle will be captured by an argon nucleus, forming an exotic atom. The exotic atom in the excited state will de-excite, emitting Auger electrons and X-rays aramaki2013measurement. The antiparticle will eventually be captured by the nucleus and produce annihilation products, including charged pions and protons. The number of pions and protons produced will be related to the number of antinucleons, providing additional information to identify the incoming antiparticle aramaki2020dualZENG2025103152. For an incoming gamma ray, multi-hit Compton scatterings will be measured and used to reconstruct the Compton ring to determine the source direction. GRAMS has also developed an algorithm to reconstruct the Compton event inside our detector Takashima:2022YONEDA2023102765.
• Figure 2: The left figure shows the GRAMS MeV gamma-ray sensitivity projection for a 35 day long-duration balloon flight and a 1-year satellite mission. Both cases would provide more than an order of magnitude improvement over previous missions, including COMPTEL aramaki2020dual. The middle plot shows the GRAMS antideuteron sensitivity in comparison with other projects aramaki2020dual. The right plot shows the GRAMS antihelium-3 sensitivity in grey lines compared with other experiments. Theoretical primary Dark Matter model predictions are shown in colored lines and areas, while background models are shown in dashed linesZENG2025103152.
• Figure 3: GRAMS timeline
• Figure 4: (a) Year 2023 eGRAMS payload. The highlighted red box shows the LArTPC carrier chamber. (b) Year 2025 antiproton beam test at J-PARC. The orange arrow shows the $700MeV/n$ antiproton beam direction. Red xyz arrows show LArTPC chamber orientation. (c) Detector that stays inside the beam test chamber. (d) Year 2023 eGRAMS launch site at Taiki Aerospace Research Field in Hokkaido, Japan. (e) pGRAMS payload design, in preparation for a flight in 2026 at Tucson, Arizona.