Session DA: Neutrons: From Particles to Stars

Measuring the Neutron Lifetime

Neutron beta decay is the simplest example of nuclear beta decay and is crucial in our understanding of weak processes. The neutron lifetime, when combined with other neutron decay parameters, provides a test of the unitarity of the CKM matrix in the Standard Model. The value of the neutron lifetime is also an important input in Big Bang Nucleosynthesis models, as well as playing a role in other areas including solar physics and the detection of reactor antineutrinos. Two main methods have been utilized to measure the neutron lifetime: the “bottle” method and the “beam” method. In the bottle method, ultracold neutrons are confined in a material and/or magnetic trap. After varying lengths of storage time, the number of neutrons remaining in the trap are counted. In the beam method a cold neutron beam is passed through a fiducial volume. The absolute neutron beam flux is measured, as well as the absolute number of decay particles (protons or electrons) resulting from neutron decay inside the fiducial volume. While the most precise neutron lifetime experiments have reached uncertainties of less than 1 s, there remains significant scatter in the results, including a large discrepancy between the bottle average value and the beam average value for the neutron lifetime. An overview of the measurement methods and recent or ongoing experiments will be given, with an emphasis on current efforts to improve and check the results of the beam method.   

Advances in the Ab Initio Description of Neutron-Rich Nuclei

Today, computationally efficient many-body methods can be used to perform first-principles calculations for nuclei as heavy as the tin isotopes. This progress has made it possible to confront modern two- and three-nucleon interactions from Chiral Effective Field Theory (EFT) with a wealth of experimental data, in particular for neutron-rich nuclei, and provide important guidance in their ongoing refinement. Significant challenges remain when it comes to the treatment of collective correlations in doubly open-shell nuclei (e.g., due to intrinsic deformation) or the coupling to the continuum\footnote{S. R. Stroberg, H. Hergert, S. K. Bogner and J. D. Holt, Ann. Rev. Nucl. Part. Sci. 69, 307 (2019)}. These issues have sparked new lines of research about combining complementary techniques, e.g., particle-hole expansions with symmetry breaking and restoration. The In-Medium Similarity Renormalization Group (IMSRG) offers a particular useful framework for such efforts$^{2,}$\footnote{H. Hergert, S. K. Bogner, T. D. Morris, A. Schwenk and K. Tsukiyama, Phys. Rept. 621, 165 (2016)}$^{,}$\footnote{H. Hergert, Phys. Scripta 92, 023002 (2017)}. I will give a brief overview of the state of the art of \emph{Ab initio} nuclear many-body theory, and discuss applications of “hybrid” IMSRG approaches$^{2,}$\footnote{E. Gebrerufael, K. Vobig, H. Hergert and R. Roth, Phys. Rev. Lett. 118, 152503 (2017) }$^{,}$\footnote{J. M. Yao, B. Bally, J. Engel, T. R. Rodriguez and H. Hergert, Phys. Rev. Lett. 124, 232501 (2020)} to the first-principles description of selected medium-mass open-shell nuclei, including candidates for fundamental symmetry tests$^{6}$.   

Recent astrophysical observations of gravitational waves and some implications

The Laser Interferometer Gravitational-wave Observatory (LIGO) detectors in Livingston, Louisiana, and Hanford, Washington, USA, and the Virgo detector in Italy, after decades of development and recent upgrades, have completed three observational runs in recent years. We have observed waves emitted during the inspiral and coalescence of pairs of black holes and neutron stars and have exchanged low-latency alerts with other astrophysical messengers. LIGO's detector technology and recent runs will be summarized, together with select observational results and some implications.