Sprecher
Beschreibung
Relativistic heavy-ion collisions have emerged as a powerful tool to probe the shapes of the colliding nuclei [1]. Correlations among the final-state particles provide access to the spatial structure of the initial state, making an accurate description of the nuclear geometry essential. In this work, we study the isobars $^{76}$Ge and $^{76}$Se, which are also of particular interest for neutrinoless double-beta decay experiments. A precise understanding of their nuclear structure is crucial for reducing the uncertainties in the corresponding nuclear matrix elements [2]. Refs. [3] and [4] demonstrate the sensitivity of double-beta decay nuclear matrix elements to nuclear shape differences between the parent and daughter nuclei.
The nuclear shell model provides an accurate description of nuclear spectroscopy, including excitation energies and quadrupole-related observables.
These are the key ingredients for assessing nuclear deformation from a low-energy perspective. We characterize the intrinsic shapes of the nuclei using the quadrupole shape invariants $(Q^n)$ [5]. In particular, the expectation value of $\langle Q^2\rangle$ probes the magnitude of the quadrupole deformation $(\beta_2)$, while $\langle Q^3\rangle$ characterizes its triaxiality ($\gamma$).
We perform shell-model calculations to extract the deformation parameters $(\beta_2,\gamma)$ and charge radii of the $^{76}$Ge and $^{76}$Se isobars from the shape invariants. We then determine the corresponding Woods-Saxon parameters that reproduce these microscopic results [6]. The triaxial degree of freedom is of particular importance for these nuclei, as low-energy data suggests they are close to maximal triaxiality ($\gamma\approx30^\circ$) [7]. Using this nuclear-structure input, we calculate the initial-state eccentricities as a function of collision centrality, providing predictions for the expected behavior of relativistic heavy-ion collisions involving these isobars [8].
[1] STAR Collaboration, Nature 635 (2024)
[2] J. Engel and J. Menéndez Rep. Prog. Phys. 80, 046301 (2017)
[3] D. Castillo, D. Frycz, B. Beneavente, J. Men\'endez, Phys. Lett. B 875, 140306 (2026)
[4] Y. Li, X. Zhang, G. Giacalone, J. Yao, Phys. Rev. Lett. 135, 022301 (2025)
[5] A. Poves, F. Nowacki, Y. Alhassid, Phys. Rev. C 101, 054307 (2020)
[6] B. Bally, G. Giacalone, M. Bender Eur. Phys. J. A 59 (2023)
[7] A. D. Ayangeakaa et al., Phys. Rev. C 107, 044314 (2023)
[8] The details of the heavy-ion collisions simulations will be discussed in V. Magas talk