Mirror Symmetry Breaking Disclosed in the Decay of Three-Proton Emitter 20Al

TL;DR

This study reports the first observation of the unbound nucleus 20Al, revealing its three-proton decay energy and indicating possible mirror symmetry violation, supported by experimental data and theoretical calculations.

Contribution

First detection and characterization of 20Al's 3p decay, showing mirror symmetry breaking through combined experimental and theoretical analysis.

Findings

20Al is unbound with respect to 3p emission

Decay energy of 20Al's ground state is 1.93 MeV

Evidence of mirror symmetry violation in 3p emitters

Abstract

The previously-unknown nucleus 20Al has been observed for the first time by detecting its in-flight decays. Tracking trajectories of all decay products with silicon micro-strip detectors allowed for a conclusion that 20Al is unbound with respect to three-proton (3p) emission. The 3p-decay energy of 20Al ground state has been determined to be 1.93(+0.11,-0.09) MeV through a detailed study of angular correlations of its decay products, 17Ne+p+p+p. This value is much smaller in comparison with the predictions inferred from the isospin symmetry by using the known energy of its mirror nucleus 20N, which indicates a possible mirror symmetry violation in the structure of 3p emitters. Such an isospin symmetry breaking is supported by the calculations of the continuum embedded theoretical frameworks, describing the observed 20Al ground state as an 1p s-wave state with a spin-parity of 1-, which contradicts to the spin-parity (2-) of the 20N ground state. The 20Al ground state decays by sequential 1p-2p emission via intermediate ground state of 19Mg, which is the first observed case of daughter two-proton radioactivity following 1p decay of the parent state.