The measurement of the $^{99}$Tc $β$-decay spectrum and its implications for the effective value of weak axial coupling
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| Natura: | Preprint |
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2025
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| author | Song, J. W. Ramalho, M. Lee, M. K. Kim, G. B. Kim, I. Kim, H. L. Lee, Y. C. Woo, K. R. Kotila, J. Kostensalo, J. Suhonen, J. Kim, H. J. |
| author_facet | Song, J. W. Ramalho, M. Lee, M. K. Kim, G. B. Kim, I. Kim, H. L. Lee, Y. C. Woo, K. R. Kotila, J. Kostensalo, J. Suhonen, J. Kim, H. J. |
| contents | Measurements of $β$-spectral shapes is an important way to examine the effective value of the weak axial coupling $g_{\rm A}$. These stu\ dies focus specifically on forbidden non-unique $β^-$ transitions, as only in these cases is the spectral shape directly sensitive to th\ e ratio $g_{\rm A}/g_{\rm V}$. Here, the value of the weak vector coupling constant, $g_{\rm V}$, is fixed at 1.0 according to the Conserve\ d Vector Current (CVC) hypothesis. In previous studies for the fourth-forbidden non-unique $β^-$ decays of $^{113}$Cd [J.~Kostensalo \textit{et al.}, Phys. Lett. B 822, 136652 (2021)] and $^{115}$In [A.~F. Leder \textit{et al.}, Phys. Rev. Lett. 129, 232502 \ (2022) and L. Pagnanini \textit{et al.}, Phys. Rev. Lett. 133, 122501 (2024)] a quenched value was determined for the ratio $g_{\rm A}/g_{\rm V}$ using $g_{\rm V}=1.0$. A notable exception is the recent measurement and analysis of the second-forbidden non-unique $\ β$-decay transition in $^{99}$Tc, performed by M. Paulsen \textit{et al.}, Phys. Rev. C 110, 05503(2024). Where an enhanced ratio $g_{\\ rm A}/g_{\rm V}=1.526(92)$ was suggested. To resolve this apparently contradictory situation with the effective value of $g_{\rm A}$, we hav\ e performed calculations based on the nuclear shell model (NSM) Hamiltonians glekpn, jj45pnb, and the MQPM approach with a careful considera\ tion of the small relativistic vector nuclear matrix element (sNME). The theoretical spectra were compared to the $^{99}$Tc $β$-decay sp\ ectrum by using the 4$π$ gold absorber with a Metallic Magnetic Calorimeter (MMC). In all cases, we found that the data matches well with \ reduced $g_{\rm A}$/$g_{\rm V}$ values of 1.0--1.2. Our result contradicts the previously reported measurement for $^{99}$Tc and instead sup\ ports a quenched axial coupling as reported for other isotopes. |
| format | Preprint |
| id |
arxiv_https___arxiv_org_abs_2506_17544 |
| institution | arXiv |
| publishDate | 2025 |
| record_format | arxiv |
| spellingShingle | The measurement of the $^{99}$Tc $β$-decay spectrum and its implications for the effective value of weak axial coupling Song, J. W. Ramalho, M. Lee, M. K. Kim, G. B. Kim, I. Kim, H. L. Lee, Y. C. Woo, K. R. Kotila, J. Kostensalo, J. Suhonen, J. Kim, H. J. Nuclear Experiment Measurements of $β$-spectral shapes is an important way to examine the effective value of the weak axial coupling $g_{\rm A}$. These stu\ dies focus specifically on forbidden non-unique $β^-$ transitions, as only in these cases is the spectral shape directly sensitive to th\ e ratio $g_{\rm A}/g_{\rm V}$. Here, the value of the weak vector coupling constant, $g_{\rm V}$, is fixed at 1.0 according to the Conserve\ d Vector Current (CVC) hypothesis. In previous studies for the fourth-forbidden non-unique $β^-$ decays of $^{113}$Cd [J.~Kostensalo \textit{et al.}, Phys. Lett. B 822, 136652 (2021)] and $^{115}$In [A.~F. Leder \textit{et al.}, Phys. Rev. Lett. 129, 232502 \ (2022) and L. Pagnanini \textit{et al.}, Phys. Rev. Lett. 133, 122501 (2024)] a quenched value was determined for the ratio $g_{\rm A}/g_{\rm V}$ using $g_{\rm V}=1.0$. A notable exception is the recent measurement and analysis of the second-forbidden non-unique $\ β$-decay transition in $^{99}$Tc, performed by M. Paulsen \textit{et al.}, Phys. Rev. C 110, 05503(2024). Where an enhanced ratio $g_{\\ rm A}/g_{\rm V}=1.526(92)$ was suggested. To resolve this apparently contradictory situation with the effective value of $g_{\rm A}$, we hav\ e performed calculations based on the nuclear shell model (NSM) Hamiltonians glekpn, jj45pnb, and the MQPM approach with a careful considera\ tion of the small relativistic vector nuclear matrix element (sNME). The theoretical spectra were compared to the $^{99}$Tc $β$-decay sp\ ectrum by using the 4$π$ gold absorber with a Metallic Magnetic Calorimeter (MMC). In all cases, we found that the data matches well with \ reduced $g_{\rm A}$/$g_{\rm V}$ values of 1.0--1.2. Our result contradicts the previously reported measurement for $^{99}$Tc and instead sup\ ports a quenched axial coupling as reported for other isotopes. |
| title | The measurement of the $^{99}$Tc $β$-decay spectrum and its implications for the effective value of weak axial coupling |
| topic | Nuclear Experiment |
| url | https://arxiv.org/abs/2506.17544 |