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Isospin symmetry holds beyond the proton drip line: First observation of excited states in 55Cu

Pigliapoco, S. ; Cortés, M.L. ; Recchia, F. ; Doornenbal, P. ; Lenzi, S.M. ; Bentley, M.A. ; Browne, F. ; Ferreira, L.S. ; Jungclaus, A. and Koiwai, T. , et al. (2026) In Physics Letters B 878.
Abstract
Isospin symmetry, combined with shell-model description, has long been successful in reproducing nuclear structure and predicting mirror energy differences. However, these descriptions have been primarily limited to mirror nuclei with well-bound ground states. As one approaches the proton drip line, coupling to the continuum becomes important. Despite this, little is known about medium-mass nuclei at the drip line. Here we report the measurement of the excitation energies of several unbound states in a nucleus beyond the proton drip line, 55Cu, performed via γ-ray spectroscopy at the Radioactive Isotope Beam Factory (RIKEN). Several low-lying yrast states are observed to decay predominantly by γ-ray emission. In contrast, the... (More)
Isospin symmetry, combined with shell-model description, has long been successful in reproducing nuclear structure and predicting mirror energy differences. However, these descriptions have been primarily limited to mirror nuclei with well-bound ground states. As one approaches the proton drip line, coupling to the continuum becomes important. Despite this, little is known about medium-mass nuclei at the drip line. Here we report the measurement of the excitation energies of several unbound states in a nucleus beyond the proton drip line, 55Cu, performed via γ-ray spectroscopy at the Radioactive Isotope Beam Factory (RIKEN). Several low-lying yrast states are observed to decay predominantly by γ-ray emission. In contrast, the 7/22− state – predicted to be strongly populated in the one-proton knockout reaction – is notably absent, suggesting that it decays primarily via prompt proton emission. We show that even for such an unbound system, shell-model calculations based on a harmonic oscillator potential reproduce the observed mirror energy differences remarkably well in all the observed states. Based on this result, we propose that a general pattern might emerge where, in medium-mass systems at the proton drip line, the stronger Coulomb barrier together with the centrifugal barrier effectively confines the wave functions, so that the shell model is able to describe low-lying states, enabling accurate predictions of mirror energy differences between isobaric analogue states. (Less)
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organization
publishing date
type
Contribution to journal
publication status
published
subject
keywords
Isospin symmetry, Proton-drip line, Mirror nuclei, In-beam -ray spectroscopy
in
Physics Letters B
volume
878
article number
140564
pages
7 pages
publisher
Elsevier
external identifiers
  • scopus:105039918756
ISSN
0370-2693
DOI
10.1016/j.physletb.2026.140564
project
Nuclear Structure at the Limits: Isotope-selective Spectroscopy
language
English
LU publication?
yes
id
15976296-78eb-4329-afa1-4778394e993d
date added to LUP
2026-05-29 15:43:15
date last changed
2026-08-12 04:00:42
@article{15976296-78eb-4329-afa1-4778394e993d,
  abstract     = {{Isospin symmetry, combined with shell-model description, has long been successful in reproducing nuclear structure and predicting mirror energy differences. However, these descriptions have been primarily limited to mirror nuclei with well-bound ground states. As one approaches the proton drip line, coupling to the continuum becomes important. Despite this, little is known about medium-mass nuclei at the drip line. Here we report the measurement of the excitation energies of several unbound states in a nucleus beyond the proton drip line, <sup>55</sup>Cu, performed via γ-ray spectroscopy at the Radioactive Isotope Beam Factory (RIKEN). Several low-lying yrast states are observed to decay predominantly by γ-ray emission. In contrast, the 7/2<sub>2</sub><sup>−</sup> state – predicted to be strongly populated in the one-proton knockout reaction – is notably absent, suggesting that it decays primarily via prompt proton emission. We show that even for such an unbound system, shell-model calculations based on a harmonic oscillator potential reproduce the observed mirror energy differences remarkably well in all the observed states. Based on this result, we propose that a general pattern might emerge where, in medium-mass systems at the proton drip line, the stronger Coulomb barrier together with the centrifugal barrier effectively confines the wave functions, so that the shell model is able to describe low-lying states, enabling accurate predictions of mirror energy differences between isobaric analogue states.}},
  author       = {{Pigliapoco, S. and Cortés, M.L. and Recchia, F. and Doornenbal, P. and Lenzi, S.M. and Bentley, M.A. and Browne, F. and Ferreira, L.S. and Jungclaus, A. and Koiwai, T. and Maglione, E. and Rudolph, D. and Taniuchi, R. and Tostevin, J.A. and Vaquero, V. and Wimmer, K. and Zago, L. and Aguilera, P. and Arici, T. and Benito, J. and Carollo, S. and Escudeiro, R. and Fernández, A. and Imai, N. and Ha, J. and Kitamura, N. and Longfellow, B. and Lozeva, R. and Mauss, B. and Napoli, D.R. and Niikura, M. and Pereira López, X. and Rezynkina, K. and Ruotsalainen, P. and Sakurai, H. and Uthayakumaar, S. and Wadsworth, R. and Yajzey, R.}},
  issn         = {{0370-2693}},
  keywords     = {{Isospin symmetry; Proton-drip line; Mirror nuclei; In-beam -ray spectroscopy}},
  language     = {{eng}},
  publisher    = {{Elsevier}},
  series       = {{Physics Letters B}},
  title        = {{Isospin symmetry holds beyond the proton drip line: First observation of excited states in <sup>55</sup>Cu}},
  url          = {{http://dx.doi.org/10.1016/j.physletb.2026.140564}},
  doi          = {{10.1016/j.physletb.2026.140564}},
  volume       = {{878}},
  year         = {{2026}},
}