Mon·Jan 13, 2025Monday·January 13, 2025

1. 01*

   Lifetime measurement of the muonic atoms of enriched Si isotopes

   R. Mizuno🇯🇵 · M. Niikura🇯🇵 · S. Akamatsu🇯🇵 · T. Fujiie🇯🇵 · K. Ishida🇯🇵 · T. Ito🇯🇵 · T. Kikuchi🇯🇵 · T. Matsuzaki🇯🇵 · F. Minato🇯🇵 · J. Murata🇯🇵 · T. Naito🇯🇵 · K. Shimomura🇯🇵 and 3 other authors

   Background: A muonic atom, composed of a negative muon and an atomic nucleus, undergoes two primary decay processes: muon-electron decay and muon nuclear capture. The branching ratio between these two processes can be determined from the measured lifetime of the muonic atom. While past researches have examined the general trend of muon capture rates across different nuclei, experimental and theoretical investigations into the isotope dependence of the lifetime remain limited. Purpose: The present study aims to measure the lifetimes of the muonic atom of isotopically enriched silicon isotopes. Methods: The experiment was conducted at the muon facility in the Material and Life Science Facility (MLF), J-PARC. A muon beam was stopped in various target materials, including isotopically enriched ${}^{28,29,30}$Si. The lifetimes of the muonic atoms were measured by detecting decay electrons using a $\mu$SR spectrometer. The decay spectra were analyzed by fitting with multi-exponential functions to account for contributions from the target nucleus and surrounding materials. Results: For the first time, the lifetimes of the muonic atom of isotopically enriched ${}^{28,29,30}$Si were measured. Additionally, seven other targets were studied to validate the experimental method and analysis procedure. The results were compared with theoretical models such as Primakoff, Goulard-Primakoff, and the recently-developed microscopic and evaporation model (MEM). Conclusions: The Primakoff and Goulard-Primakoff formulas, while reproducing certain aspects of the isotope dependence, require further refinement. The comparison with MEM calculations constrains the axial vector ($g_A$) and induced pseudoscalar ($g_P$) coupling constants. The present experiment establishes a method for measuring the lifetimes of the muonic atom and will contribute to future systematic investigations.

   nucl-exPRC(2025)·4 citations
2. 02*

   Bound-State Beta Decay of ${}^{205}{\mathrm{Tl}}^{81+}$ Ions and the LOREX Project

   R. S. Sidhu🇩🇪 · G. Leckenby🇨🇦 · R. J. Chen🇩🇪 · R. Mancino🇩🇪 · Yu. A. Litvinov · G. Martínez-Pinedo🇩🇪 · G. Amthauer🇦🇹 · M. Bai🇩🇪 · K. Blaum🇩🇪 · B. Boev🇲🇰 · F. Bosch🇩🇪 · C. Brandau🇩🇪 and 46 other authors

   Stable ${}^{205}$Tl ions have the lowest known energy threshold for capturing electron neutrinos (${\nu }_e$) of $E_{{\nu }_e}\ge 50.6$\,keV. The Lorandite Experiment (LOREX), proposed in the 1980s, aims at obtaining the longtime averaged solar neutrino flux by utilizing natural deposits of Tl-bearing lorandite ores. To determine the ${\nu }_e$ capture cross section, it is required to know the strength of the weak transition connecting the ground state of ${}^{205}$Tl and the 2.3 keV first excited state in ${}^{205}$Pb. The only way to experimentally address this transition is to measure the bound-state beta decay (${\beta }_b$) of fully ionized ${}^{205}\mathrm{T}{\mathrm{l}}^{81+}$ ions. After three decades of meticulous preparation, the half-life of the ${\beta }_b$ decay of ${}^{205}\mathrm{T}{\mathrm{l}}^{81+}$ has been measured to be $291_{-27}^{+33}$ days using the Experimental Storage Ring (ESR) at GSI, Darmstadt. The longer measured half-life compared to theoretical estimates reduces the expected signal-to-noise ratio in the LOREX, thus challenging its feasibility.

   nucl-exastro-ph.SRphysics.ins-detPRL(2024)·11 citations

* Reconstructed cohort: no mailing for this day survives in the archive. Papers are grouped by their submission times and arXiv's announcement cut-off, assuming announcement without delay; positions follow identifier order. Validated at ~91% exact-day agreement against the archived era.