14 September 2026

We study thermal leptogenesis in three predictive type-I seesaw models in which the neutrino Dirac mass matrix is equal to the mass matrix of up-type quarks, or down-type quarks, or charged leptons. In this framework, the seesaw relation permits a full reconstruction of the heavy right-handed neutrino mass matrix from low-energy neutrino parameters, which greatly reduces the parameter freedom. A systematic numerical scan based on density matrix Boltzmann equations is performed to examine whether the observed baryon asymmetry of the Universe can be obtained. Successful leptogenesis occurs for normal ordering of light neutrino masses with nonzero Majorana phases. In this case, viable solutions are found in model B, associated with down-type quarks, and model C, associated with charged leptons. Both point to a close-mass pair of heavy neutrinos satisfying $|M_i-M_j|/M_i<10^{-3}$, while remaining outside the conventional quasi-degenerate resonant regime. Four representative benchmark points are selected to show the evolution of the asymmetry and the impact of different treatments of spectator effects. Neutrinoless double beta decay is further studied for all parameter points that can generate an acceptable baryon asymmetry $\eta_B = (6.12 \pm 0.20)\times 10^{-10}$. The predicted effective Majorana mass for certain cases can be probed by next generation experiments with sub-10 meV sensitivity, such as LEGEND-1000, nEXO, JUNO 50 tons, and CUPID-1T. This framework therefore provides clear targets for future searches.

We revisit precision muon decay in the framework of the LEFT by considering how the LEFT perturbs the total rate as well as the standard and polarization-dependent Michel parameters. We derive the dependence of these observables on the dimension-five and -six LEFT operators, including lepton number violating operators, allowing for arbitrary neutrino flavor assignments. We consider fits of the LEFT to the muon decay data, in particular an approach to neutrino flavor general fits that does not respect the LEFT power counting and assuming new physics couples to all flavors equally and therefore allows for a consistent fit. We then consider how new physics imprints on the SMEFT and subsequently the LEFT in order to constrain specific scenarios of single-field extensions as well as two motivated two-field extensions. While we infer associated mass scales for these scenarios in the hundreds of GeV to multi-TeV range we also find that for the specific cases of lepton number violation, neutrino mass constraints from operator mixing are more restrictive than muon decay.

We interpret the recently reported 248~keV nuclear recoil event in the LUX-ZEPLIN (LZ) experiment via inelastic dark matter scattering within the minimal Scotogenic model. A sub-MeV mass splitting between neutral inert scalars suppresses low-energy scattering while permitting signals from the high-velocity halo tail. Crucially, co-annihilation with nearly degenerate right-handed fermions accommodates the thermal relic density for dark matter masses up to $\sim {\cal O} (1)$~TeV, extending the viable range significantly beyond the pure inert doublet model limit while evading direct detection bounds. However, to resolve the severe tension with IceCube neutrino limits on solar capture, we extend this minimal framework by introducing a hidden $U(1)_X$ gauge symmetry that naturally leads us to tiny $\lambda_5$ coupling at the one-loop level. This realizes an isospin-violating scenario that suppresses dark matter capture in the Sun while preserving the coherent scattering signal in the Xenon-based LZ detector. We numerically verify that this extended framework naturally generates neutrino masses and satisfies constraints from Big Bang Nucleosynthesis and indirect detection, providing a robust and testable solution to the LZ anomaly.

We perform a systematic study of an economical class of modular $S_3$ lepton-flavor models without enlarging the continuous gauge symmetry or introducing right-handed neutrinos or additional flavon fields. We provide a complete classification of all inequivalent realizations allowed by the $S_3$ singlet--doublet representation structure and the admissible modular-weight assignments. Their phenomenological viability is investigated for both normal ordering (NO) and inverted ordering (IO) using Bayesian model comparison and parameter-correlation analysis based on current neutrino-oscillation data. We identify 18 viable models for NO and 19 for IO, with 14 satisfying the experimental constraints in both orderings. The viable models successfully accommodate current neutrino-oscillation data while yielding nontrivial predictions for leptonic CP violation, Majorana phases, and observables probing the absolute neutrino-mass scale. Among these predictions, the absolute-mass observables provide the clearest separation between the two orderings. At the best-fit points, the predicted ranges of the effective electron-neutrino mass ($m_\beta$) are completely disjoint between NO and IO, whereas those of the sum of neutrino masses ($\sum_i m_i$) and the effective Majorana mass ($m_{\beta\beta}$) show only partial separation. The IO models generally predict a higher absolute neutrino-mass scale and are consequently more strongly constrained by cosmological observations and more accessible to neutrinoless double-beta-decay searches. Our results show that, despite its economical field content, the modular $S_3$ framework accommodates a diverse set of phenomenologically viable lepton-flavor realizations with experimentally testable predictions.

The NEXT-100 detector is a high-pressure xenon time projection chamber utilising electroluminescence amplification for sub-1% FWHM energy resolution and topological discrimination, two key attributes required to achieve the NEXT programme's overarching goal of detecting neutrinoless double beta decay ($0 \nu \beta \beta$). The detector has completed its first physics run at the Laboratorio Subterr\'aneo de Canfranc (LSC), with xenon at a pressure of $\sim$4 bar. In this paper we report on the first validation of NEXT-100's topological performance and present the first topological analysis conducted at low pressure within the detector programme. A Monte Carlo study characterising the effects of pressure on track topology is presented, with qualitative agreement observed in data. We then demonstrate the topological discrimination capabilities for $0\nu \beta \beta$-like events from $^{208}$Tl decays using a cut-based method considered the 'baseline' in NEXT's topological programme, upon which all future analyses will improve. The analysis described applies a set of selection cuts before implementing a background discrimination algorithm that yields a reported signal efficiency for double-electron tracks and background acceptance for single-electron tracks of 75.6 $\pm$ 1.9 (stat.) $^{+3.4}_{-4.1}$ (syst.) % and 14.7 $\pm$ 0.4 (stat.) $^{+0.7}_{-0.8}$ (syst.) %. These results demonstrate the excellent topological discrimination capabilities of the NEXT-100 detector in line with NEXT-White, which achieved a signal efficiency and background acceptance of 71.6 $\pm$ 1.5 (stat.) $\pm$ 0.3 (syst.) % and 20.6 $\pm$ 0.4 (stat.) $\pm$ 0.3 (syst.) % respectively.

We investigate the prospects for constraining neutrinophilic axion-like particles via measurements of the tritium beta decay spectrum, as performed by the KATRIN experiment and its planned TRISTAN detector upgrade. We study in detail the resulting spectral modifications and the corresponding experimental sensitivity. The relevant complementary searches are also discussed for comparison and we derive the most up-to-date and robust constraints on keV-scale neutrinophilic axion-like particles, covering both lepton-number-conserving and lepton-number-violating interactions. Cosmological constraints are generally more stringent; however, this conclusion relies on the assumption that the particles remain unchanged from the early universe to the present day. We therefore construct a model in which the neutrinophilic axion-like particle, being a pseudo-Nambu-Goldstone boson of an extended scalar sector, emerges from a spontaneous symmetry breaking featuring a non-trivial thermal history. We show that the model naturally evades the conventional cosmological bounds, allowing tritium beta decay measurements to provide the leading constraints.

We study a predictive class of flavoured DFSZ axion models in which the Peccei--Quinn symmetry is fully horizontal and the domain wall number is unity. These Minimal F-DFSZ models simultaneously resolve the strong CP problem and avoid the post-inflationary domain wall problem, while enforcing a predictive set of texture-zero quark mass matrices. Remarkably, the entire quark Yukawa sector can be reconstructed in terms of the measured quark masses and Cabibbo--Kobayashi--Maskawa parameters. We derive analytic expressions for the resulting flavour-violating Higgs and axion couplings, thereby making explicit how the new-physics effects are determined by Standard Model observables. We then analyse the full flavour phenomenology of the framework and obtain bounds on the scale of new physics from precision flavour observables.

We introduce a projective-kernel framework for the study of replicable functions in modular function theory. The main point is that the same two-point kernel simultaneously encodes the differential projective geometry of a modular function and the Faber--Grunsky data governing its replicability. This makes it possible to translate between Schwarzian invariants, coefficient identities, and modular correspondences within a single structure. The kernel yields a reconstruction theorem showing that the ordinary Schwarzian determines the normalized Grunsky matrix and hence the underlying Laurent expansion. When Norton's replicability relations are imposed, the projective kernel produces strong arithmetic restrictions on the possible cusp data. In the degree-one case this leads to a precise classification: replicability and complete replicability are equivalent to solvability of the projective monodromy, while the icosahedral case is excluded by an explicit Grunsky obstruction. The same principle extends to the arithmetic Hecke triangle groups. The projective-kernel viewpoint also isolates the remaining global difficulty in Norton's Hauptmodul conjecture and provides a natural setting in which replicability, projective monodromy, and modular differential invariants can be studied together.

The short-baseline neutrino anomalies observed by LSND, MiniBooNE, and the gallium experiments can be explained by additional neutrino states beyond the three established by solar, atmospheric, and reactor oscillation measurements. The minimal 3+1 sterile neutrino model that parameterizes these anomalies is in significant tension with null results from other oscillation experiments and with cosmological constraints, motivating the exploration of non-minimal extensions in which the additional mass eigenstate is unstable. This thesis presents a search for an unstable sterile neutrino model, in which the heavy mass eigenstate $\nu_4$ decays to two invisible particles, using 10.67 years of high-energy atmospheric neutrino data from the IceCube Neutrino Observatory. The analysis finds no preference for sterile decay over the no-decay 3+1 hypothesis and excludes most of the parameter space preferred by global fits to short-baseline data at 90% confidence level, providing a strong constraint on this non-minimal explanation of the anomalies. The thesis additionally develops the foundation for a future sterile neutrino search at IceCube targeting the resonant disappearance of antineutrinos. Three contributions are presented: a comprehensive calculation of neutrino-nucleon and neutrino-nucleus deep-inelastic scattering cross sections, new machine-learning reconstruction techniques, and an improved event selection with twice the signal efficiency of the previous iteration. Together, these tools enable statistical separation of neutrinos and antineutrinos and establish the infrastructure for the next generation of sterile neutrino searches with IceCube.

Core-collapse supernova serve as a powerful laboratory for testing physics beyond the Standard Model (BSM), particularly regarding new, light states interacting feebly with SM particles. In this work, we investigate for the first time the production of dark matter (DM) via the neutrino-neutrino scattering processes inside a core-collapse supernova, which contributes to the excessive cooling. By incorporating state-of-the-art supernova simulation data , we derive stringent and robust limits on sub-GeV dark matter with effective couplings to neutrinos. We find that the existing and projected constraints from indirect detection are quite weak. Our supernova cooling bounds on DM-neutrino reference cross section can improve indirect detection limits more than ten orders of magnitude for DM masses below $\mathcal{O}(100)$ MeV, and it can also provide strong complementarity with other cosmological constraints. Our results highlight the exceptional sensitivity of core-collapse supernova to feebly interacting particles and motivate future supernova neutrino observations as a powerful probe of light dark sectors.

Explosions from stellar collapse are important in nearly all aspects of astronomy: formation of compact remnants (including the potential seeds of supermassive black holes), the production of compact binaries the produce X-ray binaries and radio pulsars, the origin of many of the heavy elements (a critical aspect of galactic chemical evolution), and as standard candles to probe the early universe (including the nature of star formation). Because these explosions occur in extreme conditions, if we can understand them, they can be used to probe fundamental physics: matter at extreme densities (exceeding nuclear densities, quark formation), neutrino physics (including neutrino oscillations) and, in black-hole forming systems, the nature of relativity. Gravitational waves and neutrinos provide the most direct probe of stellar collapse, but these diagnostics require nearby events. Electromagnetic probes, from shock breakout to observations of supernova remnants, can be much more common and provide complementary diagnostics of the explosive engines behind stellar collapse. In this review, we review these electromagnetic diagnostics, the physics behind them and the theory and modeling work we require to take advantage of these probes.

We investigate superluminal Lorentz invariance violation (LIV) in the neutrino sector using cosmogenic neutrino fluxes generated with ultrahigh-energy cosmic-ray propagation models. Standard fluxes are calculated with \texttt{CRPropa 3.2} and subsequently modified using a prescription based on LIV-induced neutrino splitting. Superluminal LIV suppresses the flux at the highest energies while producing an enhancement at PeV - EeV energies. We use the KM3-230213A event as a benchmark to evaluate the sensitivity of current observations to these spectral modifications. Although the available statistics do not allow a formal constraint, the predicted fluxes are particularly sensitive to coefficients in the range $10^{-24} \mathrm{eV}^{-1} \lesssim \delta_{\nu,1} \lesssim 10^{-22}\,\mathrm{eV}^{-1}$, with the results strongly depending on the assumed cosmic-ray source properties. Intermediate coefficients can enhance the expected event rate within the reconstructed energy range of KM3-230213A, whereas larger coefficients may overproduce neutrinos in energy intervals constrained by IceCube and the Pierre Auger Observatory. These results identify a region of observational sensitivity to LIV and provide testable predictions for future neutrino telescopes.

We have developed techniques for a competitive sub-TeV time-integrated neutrino search and applied it to 11.1 years of IceCube-DeepCore data. The DeepCore subarray lowers the sensitivity of IceCube down to sub-TeV energies and is especially interesting for objects with soft spectra. Three studies were performed: a search for neutrino emission from AGN exhibiting high intrinsic X-ray flux, including NGC 1068, as identified by SWIFT/BAT; a search for neutrino emission from Galactic objects identified by Fermi-LAT as exhibiting a spectral shape consistent with neutral pion decay; and an all-sky search for neutrino point sources. Objects for this study were selected given their prospects for sub-TeV neutrino emission. No evidence for sub-TeV neutrino emission is found in any of the searches performed. Finally, for each catalog of objects, we use a statistical combination of the p-values via a binomial test to search for aggregated neutrino emission from a subset of the objects. Neither of the binomial tests yields significant results. For NGC 1068, assuming a power law spectrum with index 3.4, the 90% confidence level upper limit on per-flavor neutrino emission in the 30--400 GeV range is $\Phi_{\nu+\bar{\nu}}|_{\mathrm{1 TeV}} < 9.5 \times 10^{-11}$ TeV$^{-1}$ cm$^{-2}$ s$^{-1}$, a factor of two higher than the extrapolation of IceCube's measurement at higher energies. We additionally provide neutrino flux upper limits for a variety of spectra.