The minimal type-I seesaw model provides a simple explanation of neutrino flavor oscillations and induces charged lepton flavor violation (cLFV). Despite extensive previous studies, semileptonic cLFV channels remain underexplored. Using updated form factors, decay constants, and oscillation data, we revisit $\textit{τ}$ and meson decay channels, performing a systematic comparison across the seesaw parameter space. Surprisingly, we find that decays such as $\textit{τ}$ → $\textit{ℓρ}$ can dominate over purely leptonic $\textit{τ}$-sector probes, including $\textit{τ}$ → 3$\textit{ℓ}$ and even $\textit{τ}$ → $\textit{ℓγ}$, in certain regions. In contrast, heavy-meson decays remain far below experimental sensitivity. Considering global constraints on the seesaw parameters, we derive branching ratios for the relevant cLFV processes and identify those within potential reach of next-generation experiments.

Denmark

Kevin Urquía-Calderón

Oleg Ruchayskiy

Revisiting lepton flavor violation: $\textit{τ}$ and meson decays

Heavy neutral leptons (HNLs) can explain the origin of neutrino masses and oscillations over a wide range of masses. Direct experimental probes of HNLs become unfeasible for masses significantly above the electroweak scale. Consequently, the strongest limits arise from the non-observation of charged lepton flavor-violating processes induced by HNLs at loop level. Counter-intuitively, these bounds tighten as the HNL mass increases, an effect that persists within the perturbative regime.

Inar Timiryasov

Tree-level unitarity constraints on heavy neutral leptons

Heavy neutral leptons (HNLs) are hypothetical particles proposed as a potential explanation for neutrino oscillations and the generation of the baryon asymmetry in the Universe. This paper focuses on HNLs with masses significantly above the electroweak scale. It is challenging to test for the presence of such particles directly. However, they leave behind effective interactions of Standard Model particles, leading in particular to charged lepton flavor violation (cLFV) processes. Non-observation of cLFV processes puts therefore constraints on the parameters of the HNLs. In this paper, we find the relations between the effective operators in the realistic case when neutrino masses are non-zero and the HNLs are non-degenerate. This allows us to strengthen the existing cLFV constraints on the tau-flavors from much stronger constraints on muon and electron flavors. We also link the baryon asymmetry of the Universe to the same higher-dimensional effective operators, providing complementary bounds on these parameters.

Ukraine

Volodymyr Gorkavenko

Oleksandr Khasai

Mariia Tsarenkova

Constraints on the parameters of the neutrino extension of the Standard Model

Dark photons, hypothetical feebly interacting massive vector bosons, appear in many extensions of the Standard Model. This study investigates their production and subsequent decay during supernova explosions. We demonstrate that the decay of dark photons, with masses ranging from 200 to 400 MeV, can lead to the emission of neutrinos with energies surpassing those emitted by supernovae. These neutrinos therefore serve as a distinct signal of new physics, allowing for the exploration of previously uncharted regions of the dark photon parameter space and complementing both accelerator-based searches and other astrophysical constraints. The signal is largely unaffected by the specifics of the supernova’s temperature and density radial profiles outside the SN core. Our results indicate that searching for high-energy neutrinos accompanying supernova explosions provides a novel approach to probe physics beyond the Standard Model, including dark photons, heavy neutral leptons, and other feebly interacting particles with masses in the hundreds of MeV range.

Netherlands

Vsevolod Syvolap

High-energy neutrino signals from supernova explosions: A new window into dark photon parameter space

The hot big bang model predicts the existence of a cosmic neutrino background. The number of particles and antiparticles in this primordial bath of neutrinos can be different—a memory of processes that took place at very early epochs. If neutrinos were massless, this asymmetry would not change once neutrinos froze out. However, in the case of massive particles, the asymmetry is not protected by conservation laws and can get erased via helicity-flipping scatterings off matter inhomogeneities. We evaluate this helicity-flipping rate and demonstrate that if relic lepton asymmetry ever existed, it would remained largely intact in the Earth’s neighborhood for realistic values of neutrino masses.

Switzerland

Robin Würsch

Lepton number survival in the cosmic neutrino background

Heavy neutral leptons (HNLs) are hypothetical particles able to explain neutrino oscillations and provide a mechanism for generating the baryon asymmetry of the Universe. Quantum corrections due to such particles give rise to flavor violating processes in the charged lepton sector. Based on the fact that these corrections grow with HNL masses, we improve existing constraints by orders of magnitude in mass and mixing angle. This allows us to probe part of the parameter space of leptogenesis with multi-TeV HNLs. We also show that one will be able to infer HNL parameters in a significant portion of the parameter space for TeV-PeV masses if charged lepton flavor violating signals are detected.

Heavy neutral leptons — Advancing into the PeV domain

We investigate how hypothetical particles—sterile neutrinos—can be produced in the interior of exploding supernovae via the resonant conversion of <math><msub><mover><mi>ν</mi><mo>¯</mo></mover><mi>μ</mi></msub></math> and <math><msub><mover><mi>ν</mi><mo>¯</mo></mover><mi>τ</mi></msub></math>. The novelty of our treatment lies in the proper account of the resulting lepton number diffusion. We compute the yield of sterile neutrinos and find that even after taking into account backreaction, sterile neutrinos can carry out a sizeable fraction of the total energy of the explosion comparable to that of active neutrinos. The production is, however, sensitive to the temperature in the inner supernovae regions, making robust predictions of challenging. In order to understand whether this production affects supernova evolution and can therefore be constrained, detailed simulations including the effects of sterile neutrinos are needed.

Alexey Boyarsky

Resonance production of keV sterile neutrinos in core-collapse supernovae and lepton number diffusion

Heavy neutral leptons (HNLs) appear in many extensions of the Standard Model of particle physics. In this study, we investigate to which extent the NA62 experiment at CERN could improve the existing bounds on the HNL mixing angle <math><mrow><mo>|</mo><msub><mrow><mi>U</mi></mrow><mrow><mi>e</mi></mrow></msub><msup><mrow><mo>|</mo></mrow><mrow><mn>2</mn></mrow></msup></mrow></math> by performing a missing mass search in <math><mrow><msup><mrow><mi>K</mi></mrow><mrow><mo>+</mo></mrow></msup><mo>→</mo><msup><mrow><mi>π</mi></mrow><mrow><mn>0</mn></mrow></msup><msup><mrow><mi>e</mi></mrow><mrow><mo>+</mo></mrow></msup><mi>N</mi></mrow></math> decays in flight. We show that the limit <math><mrow><mo>|</mo><msub><mrow><mi>U</mi></mrow><mrow><mi>e</mi></mrow></msub><msup><mrow><mo>|</mo></mrow><mrow><mn>2</mn></mrow></msup><mo>≃</mo><mn>2</mn><mo>×</mo><msup><mrow><mn>10</mn></mrow><mrow><mo>−</mo><mn>6</mn></mrow></msup></mrow></math> can be reached with the currently available data in the mass range 125–144 MeV, which is currently not well covered by production searches. Future data, together with a dedicated trigger and/or improvements in rejection of out-of-acceptance photons, can improve this limit by another order of magnitude.

Jean-Loup Tastet

United Kingdom

Evgueni Goudzovski

Projected NA62 sensitivity to heavy neutral lepton production in <math><mrow><msup><mrow><mi>K</mi></mrow><mrow><mo>+</mo></mrow></msup><mo>→</mo><msup><mrow><mi>π</mi></mrow><mrow><mn>0</mn></mrow></msup><msup><mrow><mi>e</mi></mrow><mrow><mo>+</mo></mrow></msup><mi>N</mi></mrow></math> decays

The extension of the Standard Model with two gauge-singlet Majorana fermions can simultaneously explain two beyond-the-Standard-model phenomena: neutrino masses and oscillations, as well as the origin of the matter-antimatter asymmetry in the Universe. The parameters of such a model are constrained by the neutrino oscillation data, direct accelerator searches, big bang nucleosynthesis, and requirement of successful baryogenesis. We show that their combination still leaves an allowed region in the parameter space below the kaon mass. This region can be probed by the further searches of NA62, DUNE, or SHiP experiments.

CERN

Kyrylo Bondarenko

Juraj Klaric

Oleksii Mikulenko

An allowed window for heavy neutral leptons below the kaon mass

The most general renormalizable interaction between the Higgs sector and a new gauge-singlet scalar is governed by two interaction terms: cubic and quartic. The quartic term is only loosely constrained by invisible Higgs decays and given current experimental limits about 10% of all Higgs bosons at the LHC can be converted to new scalars with masses up to 2. By including this production channel, one significantly extends the reach of the LHC-based Intensity Frontier experiments. We analyze the sensitivity of the FASER experiment to this model and discuss modest changes in the FASER 2 design that would allow exploring an order-of-magnitude wider part of the Higgs portal’s parameter space.