青山 一天

The upcoming GRAMS (Gamma-Ray and AntiMatter Survey) experiment aims to provide unprecedented sensitivity to a poorly explored region of the cosmic gamma-ray spectrum from 0.1-100 MeV, often referred to as the “MeV gap”. Utilizing Liquid Argon Time Projection Chamber (LArTPC) technology to detect these MeV gamma rays, GRAMS has the potential to uncover crucial details behind a variety of processes in multi-messenger astrophysics. Various theories on particle interactions beyond the standard model predict that dark matter annihilations may contribute to the cosmic gamma spectrum via monochromatic gamma emissions (spectral lines), the annihilation of decay products, and the radiation of electromagnetically charged final states (FSR). MeV gamma rays may also be emitted from primordial black holes (PBHs) that are currently gaining interest as candidates for dark matter. By looking for the Hawking radiation from such objects, GRAMS can likely probe for ultra-light PBHs, which theoretically may comprise the majority of dark matter seen in the Universe. Here, we will describe how the analyses of the targeted gamma-ray regime will enable GRAMS to uniquely and complementarily place constraints on low-mass dark matter models.

It has been demonstrated that the liquid argon scintillation detector is an excellent particle detector for use in various physics experiments, particularly those seeking evidence of WIMPs dark matter. The detector observes primary scintillation signal (S1) and/or secondary electroluminescence signal (S2). It is crucial that the properties of the detector are understood comprehensively so that such projects reduce systematic uncertainty and improve sensitivity. This work covers the recent measurements, taken using the liquid argon test facility at Waseda University, of the liquid argon scintillation and ionization responses for nuclear and electronic recoils. Additionally, a basic study of the gaseous argon luminescence signal, which involves the S2 signal, is presented herein. The luminescence process of the S2 signal, based on this measurement, is then discussed along with a suggested new mechanism called neutral Bremsstrahlung.

Liquid argon is known as an excellent target material for WIMP dark matter direct search experiment. Relatively small atomic mass (A=40) gives high nuclear recoil energy for WIMP-Ar nuclear scattering, thus it potentially has high sensitivity for low mass WIMP (<10 GeV/c2) . There are two crucial R&D topics for the liquid argon detector to explore such low mass WIMP signal, namely, deep understanding the liquid argon scintillation and ionization process for low energy deposition (<10 keV), and increasing the signal light yield by improving the argon scintillation light collection efficiency. In this paper, we report recent results from Waseda university liquid argon test facility.