Opening

• Jouni Suhonen (University of Jyväskylä)
• Sabin Stoica (CIFRA)

NEPTUN project: opportunities, managerial challenges, results and future prospects

• Sabin Stoica (CIFRA)

The “NEutrino Properties Through Use of Nuclei” (NEPTUN) project addresses some of the most fundamental open questions in modern physics by developing advanced theoretical methods and computational tools to study neutrinos produced in beta decay and neutrinoless double-beta decay. By combining the internationally recognised expertise of the theory groups from Jyväskylä, Bucharest, and Michigan, together with close collaborations with leading experimental teams, the project aims to provide reliable theoretical support for the nuclear-, particle-, and neutrino-physics communities. Such a project would not come without its opportunities, managerial challenges, results and future prospects, which I will present in my talk.

TBA

• Doru Delion

TBA

Effective axial-vector strength within the proton-neutron deformed quasiparticle random-phase approximation

• Alexandru DUMITRESCU (IFIN-HH)

We use the available experimental Gamow-Teller beta and electron-capture decay rates between 0 and 1+ ground states in neighboring even-even and odd-odd nuclei, combined with two-neutrions double beta half-lives, to analyze the influence of the nuclear environment on the weak axial-vector strength. For this purpose, the proton-neutron deformed quasiparticle random-phase approximation with a schematic dipole residual interaction is employed. The Hamiltonian contains particle-hole and particle-particle channels with mass-dependent strengths. In deriving the equations of motion we use a self-consistent procedure in terms of a single-particle basis with projected angular momentum provided by the diagonalization of a spherical mean field plus the quadrupole-quadrupole interaction.

Extending and accelerating the NEPTUN goals in the era of AI

• Mihai Horoi (CIFRA, CMU)

After reviewing some recent results obtained under the present NEPTUN goals I will show how modern AI agents can be efficiently used to accelerate discovery and therefore extend the reach of the NEPTUN project goals.

Physics of NEPTUN and beyond

• Jouni Suhonen (University of Jyväskylä)

The physics of the NEPTUN project has included topics within the range of reactor antineutrinos, beta spectral shapes, the phenomenology of the axial-vector coupling, low-Q electron-capture and beta-minus decays for neutrino-mass and antineutrino-mass studies. Going beyond NEPTUN will inspire new projects and extension of the studies towards new developments in the double beta decay, ordinary muon capture,background investigations of rare-events experiments,giant and Pygmy resonances, precision oncology, radio theranostics, etc.

Improving Shell Model Calculations Using Statistical Approaches

• Andrei Neacsu (CIFRA)

Shell model calculations rely on effective Hamiltonians to convey observables. While the results from different Hamiltonians are generally consistent, residual uncertainties leave predictions open to question. To address this, we apply advanced statistical techniques to assign meaningful error bars to both known observables and those yet to be measured. Two unmeasured observables are of particular interest: the neutrinoless double-beta decay rate and the absolute neutrino mass. Both have motivated extensive experimental and theoretical efforts. Observing this decay would establish that neutrinos are Majorana particles (i.e., their own antiparticles), confirm that neutrino mass is nonzero, and require exploring beyond-Standard-Model mechanisms for generating that mass. Our recent work tackles this problem in steps. First, we compute observables relevant to this decay for both the parent and daughter nuclear structures. We then compare our results against experimental data and use them to predict neutrinoless nuclear matrix elements. Starting from several well-established effective Hamiltonians, we introduce small perturbations to their matrix elements while preserving the magicity of the core. Hundreds of perturbed Hamiltonians generate statistical distributions for the observables and predictions, yielding standard deviations and quantified theoretical uncertainties. Finally, we examine the correlations between the calculated observables.

Dynamical features of deformed nuclei

• Andreea Budaca (CIFRA)

The dynamics associated with two types of deformed nuclear systems is studied based on a geometrical perspective. One such problem refers to a semiclassical treatment of a set of particles coupled to a triaxial core, which can be put into a chiral configuration. The proposed model explores the possibility of the harmonic chiral vibration. The quadrupole-octupole dynamics associated with alternate parity bands is the other topic discussed. This is achived by a collective model which treats consistently the parity splitting mechanism and reproduces successfully the corresponding experimental observables.

Dipole Polarizability in Neutron-Rich Ni and Sn Isotopes

• Cosmin-Marian Cipu-Draghici (CIFRA, University of Bucharest)

Our study investigates the response of neutron-rich nickel and tin isotopes under a dipole excitation, with emphasis on the properties of the Pygmy Dipole Resonance (PDR), the Giant Dipole Resonance (GDR) and the role of the symmetry energy. Using a framework based on Random Phase Approximation with Separable Interactions (RPA-SI), applying linear response theory and the Migdal model, we study the evolution of dipole polarizability across the two chains isotopes, comparing them with experimental data, and we analyze the pygmy mode`s contribution to the dipole polarizability.

Ordinary Muon Capture on 136Ba: A Comparative Nuclear Structure Study

• Gabriel Filipciuc (CIFRA)

I present a study of ordinary muon capture on 136Ba, the daughter nucleus of 136Xe double-beta decay, using two independent nuclear structure approaches: the interacting shell model (Jokiniemi code) and a proton–neutron QRPA framework (Ydrefors code). Both calculations employ an exact Dirac muon wave function and cover all relevant multipole contributions to the total capture rate. Within the shell-model approach, the effects of chiral two-body meson-exchange currents on the capture rates are assessed. Differences between the two approaches are discussed in the context of their implications for neutrinoless double-beta decay matrix elements.

Spectral Observables and Short Range Contributions in Double Beta Decay

• Jenni Kotila

Double beta decay provides a sensitive probe of neutrino properties and physics beyond the Standard Model. Its interpretation relies on nuclear matrix elements and, increasingly, on detailed spectral observables. In this talk, some recent developments in the description of these aspects are discussed, with emphasis on how improved nuclear‑theory inputs and precision measurements can be combined to enhance the physics reach of current and future experiments.

Advances in Theoretical Studies of Electron Capture

• Vasile-Alin Sevestrean (IFIN-HH, FF-UB, CIFRA)

We present a comprehensive theoretical investigation of electron-capture processes and capture ratios across a broad range of atomic numbers, employing relativistic Dirac–Hartree–Fock–Slater calculations combined with improved treatments of electron correlations, overlap and exchange effects, and shake-up/shake-off processes. An energy-balance approach based on atomic masses is implemented to achieve a more accurate description of neutrino energies, transition probabilities, and capture ratios, particularly for low-energy decays. The framework also enables a detailed assessment of uncertainties and the study of higher-shell dominance in electron capture. Energy distributions for the decays of 95Tc, 97Tc, and 113Sn are analysed as potential candidates for neutrino-mass-determination experiments and compared with the experimentally relevant 163Ho case. Overall, the developed approach provides a consistent and precise description of electron-capture decay, with applications in neutrino physics, nuclear structure, detector calibration, and nuclear medicine.

SPADES and Related Activities concerning 2nu/0nu Double Beta Decay

• Stefan Ghinescu (CIFRA)

My activities in the NEPTUN project were in part focused on elaborating the SPADES code, capable of producing phase space factors and spectra for all two-neutrino double beta decay channels and the light neutrino exchange model of their neutrinonless conterparts. The codebase is mature, able to compute PSFs and spectra to the highest precision attainable numerically, includes the most relevant corrections and effects (e.g. screening, radiative, exchange effects). Other activities included helping in the quantification of uncertainties for studies related to electron capture. I have also spent some time performing a couple of studies with Aagrah Agnihotri and Jouni Suhonen. These studies were related to the reactor antineutrino anomalies and involved computing antineutrino spectra and their associated uncertainties accounting for the uncertainty in the q-value, half life and branching ratio of the 94-98Y nuclei. The result here is a set of code that can compute such spectra by modifying only configuration files, which are well documented and intuitive to use. These codes combine several tools created by the jyväskylä group. My direct contributions were related to the statistical treatment of uncertainties. Finally, with the help of A. Neacsu, I have learned how to use KSHELL and NuSHELLX and am now able to extract quantities relevant for the computation of double beta decay (nuclear excitation energies and one body transition densities).

Charged-current neutrino--nucleus scattering off Xe-129 and Xe-131 including the meson-exchange contributions

• Joona Kasurinen

Tanks of liquid xenon are used in many present and upcoming dark matter direct detection experiments. As shielding these detectors from neutrinos is impossible, it is necessary to know the scattering cross sections of neutrinos with the detector material. The lack of measured cross sections necessitates theoretical methods. Here we provide the charged-current scattering cross sections for the target nuclei Xe-129 and Xe-131. Such calculations have already been done before, but the effect of the meson-exchange two-body currents on the cross sections has thus far never been studied. We here include these for the first time and compare our results to a one-body-currents-only model where the quenching of the weak axial coupling constant was dealt with phenomenologically.

Study of Nuclear Weak Processes for Physics Beyond the Standard Model

• Agnitory Aagrah

The study of nuclear weak processes processes play a key role in multiple avenues of searches for physics beyond the standard model. The search for the rare neutrinoless double β-decay(0νββ) and exotic dark matter in nuclear laboratory-scale experiments is among such searches that aim to answer foundational questions in physics. In the searches for exotic dark matter, unknown, rare forbidden electron-capture decay can appear as an irreducible internal background. Therefore, giving theoretical estimates for branching ratios of such unknown decays is of utmost importance in experimental confirmation of the detection of exotic dark matter. On the 0νββ decay front, understanding the phenomenology of effective axial vector coupling (g_A^eff) in nuclear weak processes in general, and β-decays in particular, is key for determining the sensitivity of underground experiments designed to detect this rare decay. In addition to this, investigating the connection between 0νββ decay amplitudes and Ordinary Muon Capture (OMC) rates can be a window to interesting new physics. The talk aims to walk through these novel facets of physics that lie at the forefront of nuclear weak-interaction physics.

TBA

• Elina Kauppinen

TBA

Collaborative discussions

• Jouni Suhonen (University of Jyväskylä)
• Sabin Stoica (CIFRA)