Testing effective field theory with the most general neutron decay correlations

Built on the seminal works by Jackson-Treiman-Wyld and Ebel-Feldman, we derive the most general free neutron differential decay rate where all massive particles (neutron, proton, and electron) are polarized. This introduces 33 new correlations in addition to the 18 existing ones, which overconstrain the coupling constants in the low-energy effective field theory of charged weak interactions, and thus provides stringent tests of the validity of the theory framework itself. We classify the correlation coefficients in terms of their Standard Model limit and discrete symmetries, and study their expansion with respect to the new physics coupling strengths, supplemented by the experiment-independent $\mathcal{O}(\alpha )$ virtual electromagnetic radiative corrections.

Figure 1

The axion-induced one-particle-irreducible diagram.

Figure 2

The axion-induced breaking of the $a=\tilde{a}$ equality as functions of $m_{\mathbf{a}}$, $E_e$, and $c$.