25 September 2026

The ordinary and dark matter in the Universe may share a common origin in the framework of a minimal extension of the Standard Model by three right-handed neutrinos. A pair of such heavy neutral leptons (HNLs) can give masses to neutrinos, generate the baryon asymmetry at the electroweak scale and produce a large lepton asymmetry at the QCD scale, which is then resonantly transformed into an abundance of the third state that acts as sterile neutrino dark matter. The minimality of this setup, known as Neutrino Minimal Standard Model ($ν$MSM), makes it highly predictive. Earlier attempts to pin down the properties of the HNLs based on the requirement to simultaneously explain the neutrino masses as well as the baryonic and dark matter abundances were hampered by an incomplete understanding of the quantum kinetic equations describing the HNLs throughout cosmic history. We perform the first parameter scan of the $ν$MSM in which this shortcoming has been overcome. We find that the $ν$MSM provides a common explanation for all aforementioned phenomena only within a well-defined parameter space that is limited in all directions, with HNL masses kinematically accessible by the SHiP experiment as well as searches at the LHC and future lepton colliders. Further predictions for the HNL decay branching ratios, CP-violation, and lepton number violation (LNV) make the model highly testable, bringing a discovery of the common origin of neutrino masses and all matter in the Universe within reach of existing and planned experiments.

We investigate how one-loop operator mixing reshapes the flavor sensitivity of lepton-number-violating dimension-7 SMEFT interactions. Using both bottom-up and top-down EFT approaches, we study the interplay of neutrinoless double beta decay ($0νββ$) and neutral current rare meson decays $K\toπνν$, and $B\to K^{(*)}νν$, which at tree level probe apparently disconnected quark- and lepton-flavor sectors. We show that loop mixing qualitatively changes this picture: flavor structures radiatively feed into one another, substantially extending the sensitivity to higher new-physics scales across operators and flavors. In the bottom-up EFT approach, this can make $0νββ$ highly sensitive even to operators involving heavier-generation quarks, while rare kaon and $B$ decays retain crucial complementary sensitivity to other flavor combinations. A top-down EFT approach example using a ultraviolet model involving scalar-leptoquark illustrates how the same effects can reorder the relative importance of different observables.

We propose a chain seesaw mechanism to generate not only the masses of neutral neutrinos but also the masses of charged fermions, by supplementing the standard model $SU(3)_c^{}\times SU(2)_L^{}\times U(1)_Y^{}$ gauge symmetries with an additional $U(1)_{Y'}^{}$ gauge symmetry. The chains for the down-type and up-type quarks contain different numbers of links, because the corresponding mediator vector-like fermions and new Higgs scalars carry different $U(1)_{Y'}^{}$ charges. The Peccei-Quinn global symmetry is automatically embedded in this $U(1)_{Y'}^{} $ gauge symmetry which is sufficient to guarantee the high quality of axion.

The scotogenic model generates sub-eV neutrino masses radiatively through small Yukawa couplings $\sim\mathcal{O}(10^{-3})$ that also set the decay length of the heavy neutral fermion $\Ntwo$; in the compressed regime a separate, much smaller Yukawa entry $\sim\mathcal{O}(10^{-6})$ additionally renders the charged scalar $\etapm$ long-lived. We study the compressed-spectrum regime of its two-inert-doublet variant ($\DMcomp = \mHpm - \mNtwo \simeq 200\MeV$), in which small Yukawa entries render both the charged scalar $\etapm$ and the heavy neutral fermion $\Ntwo$ long-lived and produce a nested cascade at the High-Luminosity LHC~(HL-LHC): $\etapm$ leaves a disappearing track~(DT), decays to an invisible $\Ntwo$, which travels macroscopically before forming a displaced dilepton vertex~(DV). The two slow flights combine into a 4D time-of-flight delay $\dt\sim200$--$800\ps$ resolvable by the CMS MIP Timing Detector, while the DT-to-DV displacement vector inherits the parent track direction, yielding a spatial back-pointing correlation with no SM analogue. These two handles are expected to suppress the background to $B\sim10^{-5}$--$10^{-3}$ events, and the discovery remains $5σ$-significant for any background up to $B\sim10$ events. At the benchmark ($\mNtwo=150\GeV$ and proper decay lengths $\ctauH=300\mm$, $\ctauN=189\mm$) the strategy yields $N_s\approx1212$ signal events at $3000\ifb$ and establishes sensitivity across $15\mm\lesssim\ctauH\lesssim300\mm$, $10\mm\lesssim\ctauN\lesssim1000\mm$, a correlated region left uncovered by any standalone DT or DV search. Because the $\Ntwo$ decay length is controlled by the same Yukawa couplings that generate sub-eV neutrino masses, mapping this nested cascade turns the HL-LHC into a direct collider probe of the interaction responsible for neutrino mass.

A quantum field-theoretical description of neutrino oscillations in matter in adiabatic approximation is developed. Estimates of the oscillation coherence lengths have been made for several solar neutrino production and detection processes. It has been shown that, within the framework of the developed approach, the results of solar neutrino detection experiments can be correctly described in terms of neutrino mass eigenstates without taking oscillations into account.

In string model building, flavor symmetries can arise from the geometric features of the compactification space. In the case of heterotic orbifold models, modular and traditional flavor symmetries can be related to both geometric and algebraic properties of the orbifold space group. Our goal is to study the origin of moduli-independent discrete flavor symmetries in symmetric heterotic orbifolds with non-Abelian point groups. We develop various methods based on the Abelianization and the geometric automorphisms of the orbifold space group to determine these symmetries and their associated charges for matter fields. Notably, our methods were applied to all 331 non-Abelian affine geometries, including roto-translations, suitable for delivering N=1 supersymmetric heterotic compactifications in four dimensions. This provides additional motivation to pursue the derivation of new types of phenomenologically viable models from string theory.

Dark matter that is weakly mixed with particles in a thermal plasma can be produced through both, oscillatory and collisional freeze-in. While plasma effects have long been known to suppress the former, their role in collisional freeze-in remains less clear quantitatively, leading to conflicting results in sterile neutrino dark matter production via collisional freeze-in. Using first-principles nonequilibrium Kadanoff-Baym equations in the flavour-covariant formulation, we establish the general result that plasma effects also suppress collisional freeze-in: the vacuum mixing angle in the collision rate is replaced by its plasma-corrected counterpart. Applying this result, we conclude that sterile neutrinos cannot be dark matter in the keV-MeV mass range when produced from the Standard Model plasma.

The LUX-ZEPLIN (LZ) experiment has reported an event consistent with a $248$ keV nuclear recoil. Explaining the absence of lower-energy recoil events typically calls for some form of upscattering that kinematically forbids such events. In this work, we present a framework in which the LZ observation has a neutrino origin, with atmospheric neutrinos providing the dominant flux in the required energy range. A scenario in which atmospheric neutrinos upscatter to a heavier neutral state would also produce a large number of neutral-current events in neutrino experiments. In particular, scattering on lighter nuclear targets results in much larger nuclear recoil energies compared to xenon, yet no such excess has been observed. We show that this constraint from neutrino experiments can be evaded if atmospheric neutrinos within a narrow energy window first produce a nearly monoenergetic state $N_1$, followed by the upscattering of $N_1$ to its heavier partner $N_2$ in LZ. In such a scenario, scattering on xenon becomes kinematically allowed for $N_2$ masses around $250$ MeV, while scattering on oxygen, carbon, and other targets used in large-scale neutrino experiments remains kinematically forbidden. We show that this two-step process, $ν\to N_1 \to N_2$, can be realized through a parametric resonance induced by a dark matter background that efficiently produces $N_1$, followed by $N_1 \to N_2$ upscattering mediated by a vector boson in a model with gauged $U(1)_B$. The latter interaction can be sufficiently stronger than the weak interaction, which is necessary to lift the neutrino floor and yield $\mathscr{O}(1)$ event at LZ.

Chiral anomalies in baryon and lepton number currents in the GUT-inspired $SO(5) \times U(1) \times SU(3)$ gauge-Higgs unification model in the Randall-Sundrum (RS) warped space are examined. Total anomalies including contributions of all Kaluza-Klein (KK) excited modes of fermions running along internal triangular loops are expressed in terms of the values of wave functions of gauge-boson KK towers at the ultraviolet (UV) and infrared (IR) branes in the RS space. The covariant divergence of 5D baryon number current picks up an anomaly term proportional to ${\rm Tr} \,F_{μν} \tilde F^{μν} |_{SU(2)_L} - {\rm Tr} \, F_{μν} \tilde F^{μν} |_{SU(2)_R}$ at the UV and IR branes where $SU(2)_L \times SU(2)_R$ is a subgroup of $SO(5)$.