07 August 2026

We investigate neutrino mass and thermal leptogenesis in the Babu-Nandi-Tavartkiladze (BNT) model featuring a scalar quadruplet ($Φ$) and a pair of vector-like fermion triplets ($Σ$). In this framework, neutrino masses are generated via an effective dimension-7 operator $LLHH(H^{\dagger}H)/Λ^3$ at the tree level and via the dimension-5 operator $LLHH/Λ$ at the one-loop level. It naturally accommodates sub-eV neutrino masses even if the new physics scale $Λ$ is $\mathcal{O}(\rm TeV)$, thus making the model a compelling target for experimental searches. We explore the viability of thermal leptogenesis in this model, which is distinct from the canonical seesaw-based leptogenesis due to the presence of vector-like fermions. We find that leptogenesis is viable for $M_Σ\gtrsim 10^{7}$ GeV for a hierarchical spectrum of fermion triplets. However, in the quasi-degenerate regime, resonant enhancement of the $CP$ asymmetry lowers this scale down to $\mathcal{O}({\rm TeV})$, reconciling successful leptogenesis with the originally motivated TeV-scale phenomenology and testability of the model at colliders.

We revisit a minimal scenario for spontaneous leptogenesis where the asymmetry generation is driven by a heavy axion-like field evolving via standard misalignment. We focus on the simplest framework in which lepton number violation originates from the dimension-five Weinberg operator and contributions from standard thermal leptogenesis become negligible. The asymmetry is computed by solving a set of transport equations, which incorporate fully flavor-dependent interaction rates for the Weinberg operator that are presented here for the first time. We also derive a simple algebraic master formula for the generated $B\!-\!L$ asymmetry, valid for arbitrary axion velocities and classically shift-symmetric couplings to the Standard Model, that reproduces the $B\!-\!L$-production dynamics of the full set of transport equations in controlled parameter regions. Along the way, we quantify the importance of low-scale neutrino masses and the two mass orderings on the final prediction for the generated asymmetry. Finally, we assess the cosmological viability of different axion couplings by accounting for entropy dilution from late-time axion decays and imposing constraints from baryonic isocurvature perturbations. We demonstrate that explaining the baryon asymmetry via standard Majoron misalignment is under severe tension due to dilution from late-time decays. In contrast, axions coupled to strong or weak sphalerons can successfully produce the observed asymmetry for decay constants near the energy scale of grand unification and axion oscillation temperatures $T_{\text{osc}} \gtrsim 10^{11\cdots12}$ GeV, provided a cosmological history featuring high-scale inflation and efficient reheating.

The Nelson-Barr mechanism solves the strong CP problem through spontaneous CP violation, but the resulting CP-conjugate vacua generically lead to stable domain walls. We propose that the Nelson-Barr scalar itself drives hilltop inflation. A CP-symmetric double-well potential for its imaginary component forms two CP-breaking valleys, with inflation proceeding along one of them. Since the lowest CP-invariant ridge is generically much higher than the energy available after inflation, transitions between the two branches are energetically inaccessible, and the domain walls cannot be regenerated after inflation. A small CP-preserving linear deformation raises the scalar spectral index of quartic hilltop inflation and yields CMB-compatible parameter regions. The Nelson-Barr sector necessarily provides a portal to visible-sector reheating. When coupled to right-handed neutrinos, it can also transmit the spontaneous CP phase to the lepton sector and, in an extension that allows a higher CP-breaking scale, even realize nonthermal leptogenesis. This establishes a unified cosmological realization of the Nelson--Barr mechanism in which the same CP-breaking dynamics solves the strong CP problem, can generate quark and lepton CP violation, drives inflation and reheating, and enables baryogenesis, while simultaneously eliminating the associated domain-wall problem.

The strong CP problem and the origin of neutrino masses are among the clearest motivations for physics beyond the Standard Model. The Peccei-Quinn mechanism offers an elegant dynamical solution to the strong CP problem, predicting the QCD axion, while neutrino masses point to new degrees of freedom or interactions beyond the Standard Model. We explore the possibility that these two phenomena arise from a common origin by identifying the Peccei-Quinn symmetry-breaking scale with the scale responsible for neutrino mass generation. We perform a systematic and unified analysis of this connection in both DFSZ and KSVZ realizations of the Peccei-Quinn mechanism, considering the Type-I, Type-II, and Type-III seesaw mechanisms, the Zee radiative model, and the colored seesaw. For each scenario, we determine the axion couplings to photons and neutrinos and examine their phenomenological consequences. A robust prediction of all the frameworks considered is the existence of axion-neutrino couplings. We show that these interactions can significantly affect the axion decays. If the QCD axion makes up the observed dark matter, these couplings also lead to a large monochromatic neutrino flux from axion decays, opening a complementary path to test these theories in future neutrino experiments.

Neutrinoless double beta decay is the flagship laboratory probe of a Majorana contribution to the neutrino mass. However, besides the standard mass mechanism other higher- dimensional lepton number-violating interactions can enter, or even dominate, this process. The corresponding positron-emitting neutrinoless modes, such as electron capture or double-positron emission, have long been considered out of reach experimentally, due to their naturally smaller rates. Recently, innovative detector technologies as used in the proposed NuDoubt++ experiment are changing the game. In this work, we explore how a positron- and electron-mode detector can be complementary in the search for new physics. Assuming an observation of neutrinoless double beta decay, we predict the expected discovery half-life for the positron-modes. Using half-life ratios, especially between the double beta and electron capture modes, we demonstrate how underlying long-range interactions can be distinguished, in particular to identify a purely right-handed leptonic current.

The ATLAS search for pair-produced doubly charged Higgs bosons in multi-lepton final states using the full Run 2 dataset [Eur. Phys. J. C 83 (2023) 605] reports the strongest limits to date on the mass of doubly charged scalars, driven by an essentially background-free four-lepton channel. We show that the four-lepton signal efficiency implied by the auxiliary cutflow of that analysis exceeds a strict, mass-independent upper bound derived from the equal-branching-ratio assumption of the search, the leptonic $τ$ branching fractions, and the ATLAS lepton reconstruction efficiencies. We show that this excess cannot be explained by hadronic $τ$ or jet misidentification without invoking fake rates far above realistic values. We regenerate the signal independently and recompute the exclusion limit, using the corrected signal yields, the ATLAS background predictions and uncertainties, and the same $CL_s$ procedure implemented in pyhf. The resulting expected limit lies systematically above the ATLAS expected limit, by roughly a factor of two or more. This shifts the expected lower mass bound from $1065$ GeV to $\sim950$ GeV in the left-right symmetric type-II seesaw model, and from $880$ GeV to $\sim770$ GeV in the Zee--Babu model.

We investigate the sensitivity of the Pierre Auger Observatory to physics beyond the Standard Model arising from magnetic-moment-induced transitions between active and heavy sterile neutrinos. Such dipole portal interactions can enhance neutrino-nucleon cross sections above a kinematic threshold set by the sterile neutrino mass, leading to observable modifications of neutrino detection rates at ultrahigh energies (UHE). We estimate the impact of these interactions on both down-going and Earth-skimming neutrino detection channels, the contrasting responses of which enable discrimination between an enhanced neutrino flux and a modified interaction cross section. Using the non-observation of UHE neutrino candidates, we derive neutrino-flux-dependent constraints with 90% confidence-level on the transition magnetic moment for sterile neutrino masses in the range 1 TeV-100 TeV. Under the assumed flux scenarios, the resulting flavor-independent limits extend existing bounds into previously unexplored parameter space.

We study the phenomenology of the complex scoto-singlet model, an extension of the scotogenic framework that simultaneously accounts for radiative neutrino masses and fermionic dark matter. We perform a Markov Chain Monte Carlo scan of the parameter space, imposing theoretical consistency together with constraints from neutrino oscillation data, lepton-flavor violation, collider searches and the observed dark matter relic abundance. Viable dark matter solutions are found both near the Higgs resonance and in coannihilation regions, extending the allowed mass range with respect to the minimal scotogenic model. Direct- and indirect-detection signals are strongly suppressed, placing most of the viable parameter space beyond the reach of current experiments. At colliders, monojet searches offer the best prospects to probe our model in compressed-spectrum scenarios, while long-lived charged scalars may give rise to displaced or detector-stable signatures.

We study the flavour structure of an $SU(15)_p$ confining chiral gauge theory in which the Standard Model (SM) quarks, leptons, and Higgs emerge as composite bound states. The couplings of two scalar fields in the conjugate antisymmetric ($\overline{\mathbf{105}}$) and conjugate symmetric ($\overline{\mathbf{120}}$) representations of $SU(15)_p$ provide two $SU(4)_F$ flavour-breaking spurions that generate both the SM Yukawa couplings and the flavour-changing processes. The up and down Yukawa matrices are tightly-correlated due to a "right-handed isospin" symmetry, which predicts a trivial CKM matrix in the absence of spontaneous symmetry breaking. The lepton Yukawas are correlated with the quarks due to a common source of flavour spurions. With a judicious Froggatt-Nielsen-like texture for the two flavour spurions we find that a benchmark fit with $\mathcal{O}(1)$ non-perturbative coefficients reproduces all six quark masses, three charged lepton masses, and the CKM matrix. The same spurions mediate charged lepton flavour violation, neutral meson mixing, rare kaon decays, and induce electric dipole moments. We compare the reach on the compositeness scale $Λ_{\rm pre}$ across these observables in the numerical benchmark and find that the electron EDM and $K^0$-$\bar{K}^0$ mixing provide the strongest sensitivity, reaching $\mathcal{O}(10^4)$ TeV, the lower end of the range probed by proton decay, while $μ\to eγ$, $D^0$-$\bar{D}^0$ mixing, and $μ$-$e$ conversion give complementary reach at $10^2$-$10^3$ TeV. The projected electron EDM sensitivity extends this to $\mathcal{O}(10^6)$ TeV, beyond the reach of planned proton decay searches.

We record a compact numerical regularity in charged-fermion mass ratios and fermion mixing parameters. The construction uses discrete generation labels G=1,2,3, fixed structural integers Nc=3 and Nw=2, sector exponent functions LA(G), and simple phase assignments. No continuous numerical optimization is performed: after one overall mass scale is chosen in each charged sector, the charged-fermion mass ratios and the leading mixing inputs follow directly from the stated formulae. The same exponent structure generates six charged-fermion mass ratios, six mixing sines, and the CKM phase as correlated numerical outputs. The full CKM and PMNS matrices are then obtained by standard unitary reconstruction. The construction is phenomenological and does not claim a first-principles derivation of the numerical constants.

Moving a detector through a beam with a spatially varying energy spectrum exposes the same target and apparatus to distinct incident spectra, enabling interaction spectroscopy. While movable neutrino detectors were put forth primarily to control systematic uncertainties, we show that detector motion enables probing the structure of fundamental interactions. We demonstrate this with the Intermediate Water Cherenkov Detector (IWCD), the movable detector of Hyper-Kamiokande in the J-PARC off-axis beam, considering neutrino non-standard interactions (NSI) as a benchmark. Exploiting ratios of neutral current to charged current event rates reduces common normalization uncertainties, while measurements at multiple off-axis positions can break degeneracies that persist for a single incident spectrum. Combining three off-axis positions and adopting a $5\%$ correlated normalization uncertainty benchmark, we project $95\%$ CL sensitivities to axial neutral current NSI of $-0.07 \lesssim \varepsilon^{uA}_{μμ} \lesssim 0.06$ and vector NSI of $-0.10 \lesssim \varepsilon^{uV}_{μμ} \lesssim 0.12$, as well as for charged current NSI of $-0.05 \lesssim \varepsilon^{udL}_{μμ} \lesssim 0.05$. Axial NSI, which do not contribute to the ordinary matter potential, are complementary to neutrino oscillation and high energy scattering measurements. More broadly, detector motion provides a new way to distinguish interactions with different energy dependence.

The sensitivity of the Deep Underground Neutrino Experiment (DUNE) to neutrino oscillation is evaluated using a Bayesian Markov Chain Monte Carlo (MCMC) approach. This analysis uses the same underlying sensitivity inputs as previous DUNE studies [Eur. Phys. J. C 80, 978 (2020)], and therefore does not present updated DUNE sensitivities, but instead explores the additional inferences accessible using a Bayesian approach. We present four-dimensional posterior probability distributions of the oscillation parameters, highlighting the breadth of correlation in the parameter space of interest, especially between $\sin^2 θ_{23}$ and $\sin^2 θ_{13}$. We exploit the flexibility of the Bayesian framework to incorporate parameter constraints post hoc and assess the impact of applying a reactor short-baseline $θ_{13}$ constraint. A significant increase in the sensitivity to the $θ_{23}$ octant is found when including the constraint. Posterior distributions of derived quantities can be easily constructed from MCMC results. This work presents the first study of DUNE's sensitivity to the Jarlskog invariant, $J$, a quantity that provides a parametrisation-independent measure of charge-parity violation in the leptonic sector.

Neutrino quantum kinetics is computationally intractable in the neutrino-dense arenas of core-collapse supernovae and neutron star mergers. Flavor-wave (or flavomon) transport is an emerging approach to this problem in which small-scale flavor inhomogeneities are treated as quasiparticles with properties determined by the local mean background. We present the first numerical calculations of flavor-wave transport under slow driving. Our results show some quantitative successes but also underscore the theoretical challenges that need to be overcome. Two issues are particularly concerning. (1) Nonlinear wave-wave coupling may be important. (2) Degeneracies and small energy gaps are responsible for significant nonadiabatic transitions. Future work will need to address these points if flavor-wave transport is to be viable in settings of astrophysical interest. Other topics examined include flavor-wave parallel transport and flavor-space symmetry breaking.

We report on the search performed for a diffuse astrophysical neutrino flux from the full sky and from a specific region of the Galactic plane, the Galactic Ridge. The study uses the dataset collected with the first KM3NeT/ARCA configurations with 6, 8, 19, and 21 active detection units, corresponding to a total livetime of 640 days. For the all-sky analysis, the fitted single-flavour astrophysical neutrino flux parameters, under the single power-law assumption, are $φ_0^{1f} = 3.0^{+2.1}_{-2.0} \times 10^{-18}$ GeV$^{-1}$ cm$^{-2}$ s$^{-1}$ sr$^{-1}$ with spectral index $γ= 3.00^{+0.30}_{-0.35}$ at 68% credible level. The Galactic Ridge fit does not yield constraints for the flux parameters with the current sensitivity. For both analyses upper limits are derived and compared with state-of-the-art measurements. While the analysis of these datasets has not yielded statistically significant results, the developed methods provide a solid basis for future measurements with the KM3NeT detector.

Muon decays in a muon collider ring would produce TeV neutrino and antineutrino beams of electron and muon flavor. We study this flux in the forward $μ^+$ and $μ^-$ beam directions at a 10 TeV muon collider, introducing MINT, a dedicated Monte Carlo simulation to model neutrino fluxes including the muon beam dynamics. We find that a benchmark detector at 5 km from the interaction point would see about $\mathcal{O}(10^{9})$ neutrino interactions per year in a $\sim3$ tonne fiducial volume with a beam spot size of $\mathcal{O}(1)$ meter. We calculate the number of secondary muons and neutrinos generated by neutrino interactions in the rock upstream of the forward detectors and find that about two secondary high-energy and highly polarized muons from the rock would cross each detector per bunch crossing. Neutrino productions of charmed mesons and taus in the rock generate a small $ν_τ+\barν_τ$ secondary flux, with $\mathcal{O}(0.2)$ events per year in the detectors, likely too small to be observed. Wrong-sign neutrinos from similar processes, such as $ν_e+\barν_μ$ in the $μ^-$ beam, are more numerous but still of $\mathcal{O}(10^{-9})$ of the number of TeV neutrino interactions. Finally, we outline how the large forward neutrino exposure can be used to search for beyond-the-Standard-Model particles produced in neutrino interactions, with examples of heavy neutral leptons coupled to electron and muon flavors through mixing or electromagnetic dipole operators.

In large-scale liquid scintillator detectors such as the Jiangmen Underground Neutrino Observatory (JUNO), high-intensity events like cosmic muons induce photomultiplier tube (PMT) afterpulses that can interfere with the analysis of delayed physics signals. To systematically evaluate this instrumental background, we present a dedicated measurement of long-delayed afterpulses in two types of JUNO 20-inch PMTs: a dynode-based PMT and a microchannel-plate (MCP) PMT. The afterpulse time profiles were first characterized within a direct 1.8~ms waveform window and were further extended to 20~ms using a sliding-window readout strategy. Distinct long-delayed components are observed, revealing a strong dependence on the PMT multiplication structure. The dynode PMT exhibits a broad afterpulse component peaking at approximately 260~$μ$s, whereas the MCP-PMT shows a pronounced peak around 90~$μ$s, an additional component around 550~$μ$s, and a much smaller, broadly distributed millisecond-scale component. For the microsecond-scale components, the afterpulse yield per primary photoelectron is at the $10^{-3}$ level in the selected delayed windows and increases approximately linearly with the primary light intensity. The accumulated delayed activity can therefore become non-negligible following high-intensity events. These quantitative findings provide critical inputs for PMT response characterization and for the accurate modeling of delayed correlated backgrounds in high-precision neutrino experiments.

Measuring distances in the Universe is one of the hardest problems in physics and astronomy. Almost every distance probe relies on photons, whose propagation across cosmic distances introduces extinction, absorption, scattering, and radiative-transfer effects. Neutrinos suffer none of these and propagate unattenuated through dust, intergalactic medium, and dense source environments alike. We introduce a new distance-ladder method for measuring the Hubble constant $H_0$ using high-energy astrophysical neutrinos from point sources as standardizable candles, and report its first observational realization. Using 12 X-ray-selected Seyfert galaxies for which IceCube reports significant per-source neutrino excesses in its 14-year public point-source release, we exploit the disk-corona correlation $L_ν= κ\, L_X^β$ between neutrino and X-ray luminosities to construct a neutrino distance ladder anchored by non-redshift distances to NGC 1068 (Cepheid + TRGB). We find $H_0 = 49^{+40}_{-30}\,\mathrm{km\,s^{-1}\,Mpc^{-1}}$ and $β= 0.67^{+0.16}_{-0.25}$ (68% credible intervals), with the corona slope disfavoring the calorimetric limit $β= 1$ at ${\sim}2σ$. The result is consistent with existing $H_0$ determinations from Planck and SH0ES within 1$σ$. While the uncertainty on $H_0$ is large, the measurement is free of electromagnetic propagation systematics and demonstrates the viability of neutrinos as a novel cosmographical probe.

Red-giant evolution is sensitive to non-standard channels of energy loss in stellar cores and therefore provides a probe of new physics. Such losses increase the luminosity of the tip of the red-giant branch (TRGB), the brightest point reached by a red giant in the color-magnitude diagram. Previous analyses of TRGB luminosities in globular clusters placed stringent bounds on the axion-electron coupling g_ae, but were limited by observational and theoretical uncertainties. To improve these constraints, we use a new homogeneous set of parameters for 27 globular clusters, derived from our fitting of the Stetson ground-based, Hubble Space Telescope, and Gaia data by isochrones from the Dartmouth Stellar Evolution Database and a Bag of Stellar Tracks and Isochrones together with state-of-the-art Modules for Experiments in Stellar Astrophysics models. For each cluster, we identify the most luminous non-variable red giant, infer its luminosity from the best-fit isochrones, and apply Monte-Carlo based corrections for discrete sampling of the red-giant branch and for the exclusion of variable stars. We model the TRGB luminosity as a function of g_ae, as well as its uncertainties, for each cluster individually, and combine the results in a likelihood analysis. We find no indication of anomalous energy losses and obtain g_ae<3.8*10^(-14) at 95% confidence level. The median TRGB bolometric absolute magnitude is -3.53+-0.05 mag. We briefly discuss the implications for the neutrino magnetic dipole moment, millicharged particles, and other new physics.