$t$+$t$ cluster states in $^{6}$He

TL;DR

This work investigates fermionic $t+t$ clustering in $^6$He by combining a fragmentation-excitation experiment with a microscopic Generator Coordinate Method (GCM) analysis that includes $t+t$ and $\\alpha+n+n$ configurations. Experimentally, two resonances above the $t+t$ threshold are observed at $E_x = 13.9 \,\pm\ 0.3$ MeV and $15.0 \,\pm\ 0.3$ MeV via invariant-mass reconstruction of coincident $t+t$, with background estimated by event-mixing and energy-resolution-driven non-uniform binning. The theoretically driven GCM framework reproduces the low-lying spectrum and, upon coupling to $\\alpha+n+n$, lowers the ground-state energies to align with data; reduced-width amplitudes (RWA) reveal strong $t+t$ clustering in the identified resonances. The study highlights the rich and channel-sensitive nature of $t+t$ resonances in $^6$He and stresses the need for spin-parity determinations in future high-statistics experiments to unambiguously map the fermionic clustering landscape in neutron-rich nuclei.

Abstract

The study of $t$+$t$ cluster states in $^{6}$He provides valuable insights into exotic nuclear structures and the behavior of fermionic cluster systems. This study shows rich cluster resonant state structures above the threshold, identified by experimental reconstruction and theoretical calculations. The excitation energy spectrum above the $t$+$t$ threshold in $^{6}$He is measured via the fragmentation excitation process during the breakup reaction of $^{9}$Li on a $^{208}$Pb target at an incident energy of 32.7 MeV/nucleon. The resonant states are reconstructed from the final state coincident particles $t$+$t$ using the invariant mass method, while the non-resonant background is estimated using the event mixing method. Two states of energy level peaks at $13.9\pm0.3$ and $15.0\pm0.3$ MeV are observed. Microscopic cluster model calculations exploring the $t+t$ resonant states in $^6\mathrm{He}$ yield theoretical energy spectra which are then compared with the current experimental results. The calculated reduced width amplitudes (RWA) of the $t+t$ channels further confirm the clustering structure of the identified $t+t$ resonant states.

Figures (5)

• Figure 1: (Color online) Schematic view of the detector setup WMa1.
• Figure 2: (color online) The $E_{x}$ spectrum, reconstructed from the $t$+$t$ coincident fragments, is fitted with Breit-Wigner-shaped resonance functions (red line) together with the term of nonresonant contribution (pink dashed line marked as EM), taking the resolution (dE) and acceptance efficiency into account. The curve of the acceptance efficiency is scaled by a factor of 50 for better visibility alongside the event counts.
• Figure 3: (color online) The energy levels of $^{6}$He, calculated using the GCM, are provided for the 0$^{+}$, 2$^{+}$, 1$^{-}$, 2$^{-}$, and 3$^{-}$ states. The energy spectrum obtained from the $t+t$ cluster model is displayed in the left columns. In the middle columns, we present the results from coupling the $t+t$ configuration with the shell-model-like $\alpha+n+n$ configurations and states exhibiting significant $t+t$ structure are highlighted in red. In the right columns, the levels depicted in black represent data from experiments, including this work and previous studies. The dashed lines are the candidates for further confirmation.
• Figure 4: The calculated RWA of $t+t$ configuration in the ground state, the first excited state, and positive-parity resonant states with prominent $t+t$ structure.
• Figure 5: The calculated RWA of $t+t$ configuration in negative-parity resonant states with prominent $t+t$ structure.