Description of stellar weak-interaction rates within the relativistic energy density functional formalismHYBRID

Processes mediated by weak nuclear interaction are of significance not only in nuclear and particle physics but also in astrophysics. Beta decay played a historical role in discovering the parity-violating nature of the weak force and is also instrumental in verifying the unitarity of the CKM matrix. Furthermore, although seemingly marginal, weak-interaction processes can affect the most extreme objects in our Universe. For instance, electron capture on nuclei plays a prominent role in the dynamics of core-collapse supernovae. Beta decays determine the timescale of the r-process, responsible for creating more than half of elements heavier than iron. Describing a nucleus in extreme environments is a challenging task and requires an extension of the existing nuclear models. Namely, an extreme stellar environment implies nuclei at finite temperatures and high density surrounded by electrons moving at almost relativistic velocities. The temperatures required to alter the nuclear shell structure are on the order of billion kelvins, however, one finds temperatures above billions of kelvins in core-collapse supernovae and neutron star mergers. To describe the excitations in the initial nuclear state, we extend the relativistic Hartree-Bogoliubov (RHB) theory to account for finite-temperature effects (FT-RHB). On the other hand, the transitions to final nuclear states are treated within the relativistic finite-temperature quasiparticle random-phase approximation (FT-RQRPA) [1]. The excitation strength function obtained by the FT-RQRPA is more involved than its zero-temperature counterpart and requires a more careful extraction of the physical strength [2]. By employing the state-of-the-art relativistic energy density functionals, our model is readily applicable in describing the nuclear excitations across the nuclide chart. Initial applications of the model include electron capture and beta-decay rates [3,4], providing comparable results to other theoretical calculations while taking advantage of the speed and simplicity. Recently, the model was applied in calculating the electron capture rates around the N = 50 shell-closure and compared with the corresponding non-relativistic model, obtaining excellent agreement [5]. Such results give us confidence that theoretical models describing the stellar weak-interaction rates have converged to a point where their predictions will provide similar results when translated to the corresponding observables in the core-collapse supernovae simulations.

[1] A. Ravlić, Y. F. Niu, T. Nikšić, N. Paar, and P. Ring, Phys. Rev. C 104, 064302 (2021)
[2] E.M. Ney, A. Ravlić, J. Engel, N. Paar, arXiv:2209.10009 [nucl-th]
[3] A. Ravlić, E. Yüksel, Y. F. Niu, G. Colò, E. Khan, and N. Paar, Phys. Rev. C 102, 065804 (2020)
[4] A. Ravlić, E. Yüksel, Y. F. Niu, and N. Paar, Phys. Rev. C 104, 054318 (2021)
[5] S. Giraud, R. G. T. Zegers, B. A. Brown, J.-M. Gabler, J. Lesniak, J. Rebenstock, E. M. Ney, J. Engel, A. Ravlić, and N. Paar, Phys. Rev. C 105, 055801 (2022)