Search

Compilation of papers presented by the GAPS Collaboration at the 38th International Cosmic Ray Conference (ICRC), held July 26 through August 3, 2023 in Nagoya, Japan.

Many low-threshold experiments observe sharply rising event rates of yet unknown origins below a few hundred eV, and larger than expected from known backgrounds. Due to the significant impact of this excess on the dark matter or neutrino sensitivity of these experiments, a collective effort has been started to share the knowledge about the individual observations. For this, the EXCESS Workshop was initiated. In its first iteration in June 2021, ten rare event search collaborations contributed to this initiative via talks and discussions. The contributing collaborations were CONNIE, CRESST, DAMIC, EDELWEISS, MINER, NEWS-G, NUCLEUS, RICOCHET, SENSEI and SuperCDMS. They presented data about their observed energy spectra and known backgrounds together with details about the respective measurements. In this paper, we summarize the presented information and give a comprehensive overview of the similarities and differences between the distinct measurements. The provided data is furthermore publicly available on the workshop's data repository together with a plotting tool for visualization., Comment: 44 pages, 20 figures; Editors: A. Fuss, M. Kaznacheeva, F. Reindl, F. Wagner; updated copyright statements and funding information

Fully-depleted thick silicon Skipper-charge-coupled devices (Skipper-CCDs) are an important technology to probe neutrino and light-dark-matter interactions due to their sub-electron read-out noise. However, the successful search for rare neutrino or dark-matter events requires the signal and all backgrounds to be fully characterized. In particular, a measurement of the electron-hole pair creation energy below 150 eV and the Fano factor are necessary for characterizing the dark matter and neutrino signals. Moreover, photons from background radiation may Compton scatter in the silicon bulk, producing events that can mimic a dark matter or neutrino signal. We present a measurement of the Compton spectrum using a Skipper-CCD and a $^{241}$Am source. With these data, we estimate the electron-hole pair-creation energy to be $\left(3.71 \pm 0.08\right)$ eV at 130 K in the energy range between 99.3 eV and 150 eV. By measuring the widths of the steps at 99.3 eV and 150 eV in the Compton spectrum, we introduce a novel technique to measure the Fano factor, setting an upper limit of 0.31 at 90% C.L. These results prove the potential of Skipper-CCDs to characterize the Compton spectrum and to measure precisely the Fano factor and electron-hole pair creation energy below 150 eV.

The lithium-drifted silicon (Si(Li)) detector developed for the General Antiparticle Spectrometer (GAPS) experiment features a thick (~2.2 mm) sensitive layer, large (10 cm) diameter, and excellent energy resolution (~4 keV for 20-100 keV X-rays) at a relatively high operating temperature (approximately -40C). Mass production of GAPS Si(Li) detectors has been performed to construct a large-volume silicon tracker for GAPS. We achieved the first success of the mass production of large-area Si(Li) detectors with a high (~90%) yield rate. Valuable datasets related to detector fabrication, such as detector performance and manufacturing parameters, were recorded and collected during the mass production. This study analyzes the datasets using statistical methods with the aim of comprehensively examining the mass production and to gain valuable insight into the fabrication method. Sufficient uniformities of the performance parameters (leakage current and capacitance) between detectors and strips are found, demonstrating high-quality and stable mass production. We also search for correlations between detector performance and manufacturing parameters by using data-mining techniques. Conventional multivariate analysis (multiple regression analysis) and machine-learning techniques (regression tree analysis) are complementarily used, and it is found that the Li-drift process makes a significant contribution to the performance parameters of the finished detectors. Detailed investigation of the drift process is performed using environmental data, and physical interpretations are presented. Our results provide valuable insight into the fabrication methods for this kind of large-area Si(Li) detector, and encourages future projects that require large-volume silicon trackers., Comment: 41 pages, 18 figures, accepted for publication in Nuclear Instruments and Methods in Physics Research Section A

The General AntiParticle Spectrometer (GAPS) is an Antarctic balloon-borne detector designed to measure low-energy cosmic antinuclei (< 0.25 GeV/n), with a specific focus on antideuterons, as a distinctive signal from dark matter annihilation or decay in the Galactic halo. The instrument consists of a tracker, made up of ten planes of lithium-drifted Silicon Si(Li) detectors, surrounded by a plastic scintillator Time-of-Flight system. GAPS uses a novel particle identification method based on exotic atom capture and decay with the emission of pions, protons, and atomic X-rays from a common annihilation vertex. An important ingredient for the antinuclei identification is the reconstruction of the "annihilation star" topology. A custom antinucleus annihilation reconstruction algorithm, called the "star-finding" algorithm, was developed to reconstruct the annihilation star fully, determining the annihilation vertex position and reconstructing the tracks of the primary and secondary charged particles. The reconstruction algorithm and its performances were studied on simulated data obtained with the Geant4-based GAPS simulation software, which fully reproduced the detector geometry. This custom algorithm was found to have better performance in the vertex resolution and reconstruction efficiency compared with a standard Hough-3D algorithm.

The General Antiparticle Spectrometer (GAPS) will carry out a sensitive dark matter search by measuring low-energy ($\mathrm{E} < 0.25 \mathrm{GeV/nucleon}$) cosmic ray antinuclei. The primary targets are low-energy antideuterons produced in the annihilation or decay of dark matter. At these energies antideuterons from secondary/tertiary interactions are expected to have very low fluxes, significantly below those predicted by well-motivated, beyond the standard model theories. GAPS will also conduct low-energy antiproton and antihelium searches. Combined, these observations will provide a powerful search for dark matter and provide the best observations to date on primordial black hole evaporation on Galactic length scales. The GAPS instrument detects antinuclei using the novel exotic atom technique. It consists of a central tracker with a surrounding time-of-flight (TOF) system. The tracker is a one cubic meter volume containing 10 cm-diameter lithium-drifted silicon (Si(Li)) detectors. The TOF is a plastic scintillator system that will both trigger the Si(Li) tracker and enable better reconstruction of particle tracks. After coming to rest in the tracker, antinuclei will form an excited exotic atom. This will then de-excite via characteristic X-ray transitions before producing a pion/proton star when the antiparticle annihilates with the nucleus. This unique event topology will give GAPS the nearly background-free detection capability required for a rare-event search. Here we present the scientific motivation for the GAPS experiment, its design and its current status as it prepares for flight in the austral summer of 2021-22., Comment: 8 pages, 7 figures, Proc. 36th International Cosmic Ray Conference (ICRC 2019), Madison, USA

This study presents a fabrication process for lithium-drifted silicon (Si(Li)) detectors that, compared to previous methods, allows for mass production at a higher yield, while providing a large sensitive area and low leakage currents at relatively high temperatures. This design, developed for the unique requirements of the General Antiparticle Spectrometer (GAPS) experiment, has an overall diameter of 10 cm, with ~9 cm of active area segmented into 8 readout strips, and an overall thickness of 2.5 mm, with $\gtrsim$2.2 mm ($\gtrsim$90%) sensitive thickness. An energy resolution $\lesssim$4 keV full-width at half-maximum (FWHM) for 20-100 keV X-rays is required at the operating temperature ~-40C, which is far above the liquid nitrogen temperatures conventionally used to achieve fine energy resolution. High-yield production is also required for GAPS, which consists of $\gtrsim$1000 detectors. Our specially-developed Si crystal and custom methods of Li evaporation, diffusion and drifting allow for a thick, large-area and uniform sensitive layer. We find that retaining a thin undrifted layer on the p-side of the detector drastically reduces the leakage current, which is a dominant component of the energy resolution at these temperatures. A guard-ring structure and optimal etching of the detector surface are also confirmed to suppress the leakage current. We report on the mass production of these detectors that is ongoing now, and demonstrate it is capable of delivering a high yield of ~90%., Comment: 28 pages, 11 figures, submitted in 6/13/2019; re-submitted in 8/23/2019

We have developed large-area lithium-drifted silicon (Si(Li)) detectors to meet the unique requirements of the General Antiparticle Spectrometer (GAPS) experiment. GAPS is an Antarctic balloon-borne mission scheduled for the first flight in late 2020. The GAPS experiment aims to survey low-energy cosmic-ray antinuclei, particularly antideuterons, which are recognized as essentially background-free signals from dark matter annihilation or decay. The GAPS Si(Li) detector design is a thickness of 2.5 mm, diameter of 10 cm and 8 readout strips. The energy resolution of <4 keV (FWHM) for 20 to 100 keV X-rays at temperature of -35 to -45 C, far above the liquid nitrogen temperatures frequently used to achieve fine energy resolution, is required. We developed a high-quality Si crystal and Li-evaporation, diffusion and drift methods to form a uniform Li-drifted layer. Guard ring structure and optimal etching of the surface are confirmed to suppress the leakage current, which is a main source of noise. We found a thin un-drifted layer retained on the p-side effectively suppresses the leakage current. By these developments, we succeeded in developing the GAPS Si(Li) detector. As the ultimate GAPS instrument will require >1000 10-cm diameter Si(Li) detectors to achieve high sensitivity to rare antideuteron events, high-yield production is also a key factor for the success of the GAPS mission., Comment: submitted to IEEE NSS/MIC 2018 Proceedings

The General AntiParticle Spectrometer (GAPS) is a balloon-borne instrument designed to detect cosmic-ray antimatter using the novel exotic atom technique, obviating the strong magnetic fields required by experiments like AMS, PAMELA, or BESS. It will be sensitive to primary antideuterons with kinetic energies of $\approx0.05-0.2$ GeV/nucleon, providing some overlap with the previously mentioned experiments at the highest energies. For $3\times35$ day balloon flights, and standard classes of primary antideuteron propagation models, GAPS will be sensitive to $m_{\mathrm{DM}}\approx10-100$ GeV c$^{-2}$ WIMPs with a dark-matter flux to astrophysical flux ratio approaching 100. This clean primary channel is a key feature of GAPS and is crucial for a rare event search. Additionally, the antiproton spectrum will be extended with high statistics measurements to cover the $0.07 \leq E \leq 0.25 $ GeV domain. For $E>0.2$ GeV GAPS data will be complementary to existing experiments, while $E<0.2$ GeV explores a new regime. The first flight is scheduled for late 2020 in Antarctica. These proceedings will describe the astrophysical processes and backgrounds relevant to the dark matter search, a brief discussion of detector operation, and construction progress made to date., Comment: Talk presented CIPANP2018. 13 pages, LaTeX, 7 figures. Higher resolution figs at http://gaps1.astro.ucla.edu/gaps/papers2018/cipanp18

The GAPS experiment is designed to carry out a sensitive dark matter search by measuring low-energy cosmic ray antideuterons and antiprotons. GAPS will provide a new avenue to access a wide range of dark matter models and masses that is complementary to direct detection techniques, collider experiments and other indirect detection techniques. Well-motivated theories beyond the Standard Model contain viable dark matter candidates which could lead to a detectable signal of antideuterons resulting from the annihilation or decay of dark matter particles. The dark matter contribution to the antideuteron flux is believed to be especially large at low energies (E < 1 GeV), where the predicted flux from conventional astrophysical sources (i.e. from secondary interactions of cosmic rays) is very low. The GAPS low-energy antiproton search will provide stringent constraints on less than 10 GeV dark matter, will provide the best limits on primordial black hole evaporation on Galactic length scales, and will explore new discovery space in cosmic ray physics. Unlike other antimatter search experiments such as BESS and AMS that use magnetic spectrometers, GAPS detects antideuterons and antiprotons using an exotic atom technique. This technique, and its unique event topology, will give GAPS a nearly background-free detection capability that is critical in a rare-event search. GAPS is designed to carry out its science program using long-duration balloon flights in Antarctica. A prototype instrument was successfully flown from Taiki, Japan in 2012. GAPS has now been approved by NASA to proceed towards the full science instrument, with the possibility of a first long-duration balloon flight in late 2020. Here we motivate low-energy cosmic ray antimatter searches and discuss the current status of the GAPS experiment and the design of the payload., Comment: 8 pags, 3 figures, Proc. 35th International Cosmic Ray Conference (ICRC 2017), Busan, Korea