Description
The vector and axial-vector charges of the nucleon ($g_V$ and $g_A$) play a pivotal role in the internal structure of the nucleon and the weak force processes it undergoes—such as superallowed nuclear beta decay and free neutron decay. Current experimental measurements of the neutron lifetime exhibit a tension from beam-based and bottle-based experiments. Beam-based experiments measure the neutron lifetime by measuring the resultant proton from a decay, whereas bottle-based experiments measure the lifetime by counting neutrons inside a trap—thereby measuring all possible decay processes. Under the standard model, these experiments should yield the same lifetime; however, they exhibit a $4\sigma$ tension of magnitude $\sim8$ seconds. This tension is concerning, as the neutron lifetime is relevant in both particle physics and cosmology as a test of the standard model and a key ingredient of nucleosynthesis models that describe the origin of elements in our universe. All of this demonstrates the need for first-principles calculations of $g_V$ and $g_A$ to help determine the source of this tension.
We present a first-principles lattice QCD determination of $g_V$ and $g_A$, investigating the effects of QCD isospin-breaking. Working over a suite of simulations where we change quark masses—by changing $m_d-m_u$ while holding $m_d+m_u$ constant—we determine the dependence of $g_V$ and $g_A$ on the nucleon mass splitting. The vector charge exhibits the quadratic dependence predicted by the Ademollo-Gatto theorem, while the axial-vector charge exhibits a linear correction near the per cent level. We note that a $\sim 0.5\%$ correction in the axial-vector charge corresponds to a $\sim8$ second change to the free neutron lifetime, meaning the isospin-breaking effects we recover may be a significant correction for future first-principles calculations of the neutron lifetime.
| I am the presenting author | Yes |
|---|