Speaker
Description
One of the defining features of the strong nuclear interaction is its near charge independence: the nuclear components of the proton-proton, neutron-proton, and neutron-neutron interactions are remarkably similar. This symmetry under proton-neutron exchange gives rise to the concept of isospin, in which the proton and neutron are treated as different projections of a single nucleon. Consequently, nuclei with the same mass number $A$ but different isospin projections, members of an isobaric multiplet are expected to exhibit closely related excitation spectra. Deviations from this symmetry arise from isospin non-conserving (INC) interactions and can be quantified through the triplet energy difference (TED), defined as the double difference in excitation energies across an isobaric triplet. In addition, the proton $E2$ matrix element is expected to linearly with isospin projection across a triplet, while in deformed even--even nuclei the excitation energy of the first $2^{+}$ state is inversely correlated with the $B(E2)$ transition strength [1-5].
A fusion-evaporation experiment at the Accelerator Laboratory of the University of Jyväskylä led to the first observation of the 2⁺ and tentatively the 4⁺ states in the N = Z − 2 nucleus ⁷⁸Zr and extended the T = 1 band in ⁷⁸Y [1]. These results were achieved using the JUROGAM 3 γ-ray spectrometer coupled to the MARA vacuum-mode mass separator, employing recoil–β correlation techniques.
In this presentation, we discuss the new experimental results for the $A = 78$ triplet, which represents the heaviest isobaric triplet for which complete excitation-energy information is available. The extracted TED values are inconsistent with contemporary shell-model and density functional theory calculations [6,7]. Furthermore, we highlight how the recent extrapolation of $B(E2)$ strengths across the triplet, under the assumption of isospin symmetry, implies a significantly smaller deformation in $^{78}$Zr. This apparent contradiction reveals a tension between the expected $B(E2)$ behaviour and the deformation systematics inferred from excitation energies.
References:
[1] G. L. Zimba, P. Ruotsalainen, D.G. Jenkins, W. Satula et al., Phys. Rev. Lett. 134 022502 (2025).
[2] K. Wimmer, P. Ruotsalainen, et al., Phys. Lett. B. 847 138249 (2023).
[3] G. L. Zimba, P. Ruotsalainen, G. De Gregorio et al., Phys. Rev. C. 110 024314 (2024).
[4] J. Henderson, D. G. Jenkins, J. Heery, C. Müller-Gatermann, P. Ruotsalainen, and G. L. Zimba, Phys. Rev. C 112, 014330(2025)
[5] K. Kaneko, Y. Sun, T. Mizusaki, Y. Sun, S. Tazaki, and G. de Angelis, Phys. Rev. Lett. 109, 092504 (2012). And reference therein.
[6] K. Kaneko, Y. Sun, T. Mizusaki, and S. Tazaki, Phys. Rev. C 89, 031302(R) (2014).
[7] W. Satuła, P. Bączyk, J. Dobaczewski, and M. Konieczka, Phys. Rev. C 94, 024306 (2016).
| Contribution category | Experiment |
|---|---|
| Presenter status | Postdoc |