Experimental investigation of ground-state properties of $^7$H with transfer reactions

TL;DR

This study provides experimental evidence for the existence and structure of the extremely neutron-rich nucleus $^7$H, revealing it as a resonance with a halo structure, which advances understanding of nuclear forces in neutron-rich matter.

Contribution

First experimental measurement of $^7$H's ground state properties, confirming its resonance nature and halo structure, and providing insights into neutron-rich nuclear systems.

Findings

$^7$H exists as a resonance at ~0.7 MeV above $^3$H+4n

$^7$H has a narrow width of ~0.2 MeV and $1/2^+$ spin-parity

$^7$H likely has a $^3$H core with an extended four-neutron halo

Abstract

The properties of nuclei with extreme neutron-to-proton ratios, far from those naturally occurring on Earth, are key to understand nuclear forces and how nucleons hold together to form nuclei. $^7$H, with six neutrons and a single proton, is the nuclear system with the most unbalanced neutron-to-proton ratio known so far. However, its sheer existence and properties are still a challenge for experimental efforts and theoretical models. Here we report experimental evidences on the formation of $^7$H as a resonance, detected with independent observables, and the first measurement of the structure of its ground state. The resonance is found at $\sim$0.7 MeV above the $^3$H+4n mass, with a narrow width of $\sim$0.2 MeV and a $1/2^+$ spin and parity. These data are consistent with a $^7$H as a $^3$H core surrounded by an extended four-neutron halo, with a unique four-neutron decay and a relatively long half-life thanks to neutron pairing; a prime example of new phenomena occurring in what would be the most pure-neutron nuclear matter we can access in the laboratory.