The study of two-neutrino double-beta decay ($2\nu\beta\beta$), an allowed process within the Standard Model in which two neutrons convert into two protons with the emission of two electrons and two antineutrinos, provides a unique window into nuclear structure and plays a crucial role in constraining theoretical descriptions of double-beta decay [1]. In particular, it offers valuable benchmarks for the nuclear matrix elements (NMEs) that also govern neutrinoless double-beta decay ($0\nu\beta\beta$), a hypothetical process beyond the Standard Model of particle physics (BSM) in which no antineutrinos are emitted. The observation of $0\nu\beta\beta$ would have important implications for particle physics, shedding light on open questions such as the origin of neutrino masses and the matter–antimatter asymmetry of the Universe [2].

In this talk, we present new predictions for $2\nu\beta\beta$ decay half-lives to the first excited $0_2^+$ states in nuclei of experimental interest, including $^{76}$Ge, $^{82}$Se, $^{130}$Te, and $^{136}$Xe. A major source of theoretical uncertainty in these calculations arises from the NMEs, on which the decay half-lives depend quadratically. We compute the NMEs within the nuclear shell-model framework, including contributions up to next-to-leading order in chiral effective field theory [3] and incorporating important corrections from the lepton energy expansion [4]. Finally, we discuss how uncertainties in the NMEs are connected to key features of nuclear structure, such as nuclear deformation [5]. We show that larger deformation differences between the initial and final states yield to smaller NMEs. 

[1] J. Engel, J. Menéndez, Rep. Prog. Phys. 80 (2017) 046301. 

[2] M. Agostini, G. Benato, J. A. Detwiler, J. Menéndez, F. Vissani, Rev. Mod. Phys. 95 (2023) 025002. 

[3] S. el Morabit, R. Bouabid, V. Cirigliano, J. de Vries, L. Gráf, E. Mereghetti, J. High Ener. Phys. 06 (2025).

[4] F. Simkovic, R. Dvornický, D. Stefánik, A. Faessler, Phys. Rev. C 97 (2018) 034315. 

[5] T. R. Rodríguez, G. Martínez-Pinedo, Phys. Rev. Lett. 105 (2010) 252503.

We compute the Form Factors of both neutral and charged pion using a non-perturbative running of the strong coupling constant αs obtained using a double-dilaton Holographic QCD model. These form factors remain poorly understood in the intermediate-energy region, which marks the transition between low- and high-energy physics. In particular, experimental data for the neutral pion Form Factor exhibits a deviation from the expected asymptotic behavior, and the charged pion form factor remains comparatively less explored. To address these issues, we employ the pion distribution amplitude formalism to investigate the Form Factor behavior in this intermediate regime. Our results suggests that non-perturbative physics of the strong interaction is relevant even at energy scales traditionally considered perturbative, implying that the perturbative regime could occur at higher energies than previously thought. Finally, our approach allows us to study isospin-breaking effects through the quadratic pion mass difference.

We utilize the universality of pion--pion (ππ) final-state interactions at small invariant masses to understand their enhanced localized CP violation in B±→K±π+π−, using a dispersive approach. From a fit to the integrated CP-asymmetry data, we successfully predict the Dalitz-plot kinematic distribution of the asymmetry in the low-energy ππ region, including the large localized CP violation recently observed by LHCb. An essential role is played by the contributions of isospin 2. This formalism, whose parameters have a physical meaning, can be adapted straightforwardly to other systems with CP violation enhanced by final-state interactions.

TBC

TBC