Hidden Physics at the Neutrino Frontier: Tridents, Dark Forces, and Hidden Particles

The unexplained origin of neutrino masses suggests that these neutral and weakly interacting particles might provide a portal to physics beyond the Standard Model. In view of the growing prospects in experimental neutrino physics, we explore new theoretical models and experimental searches that can shed light on the existence of low-scale particles with very small couplings to ordinary matter. Our efforts highlight a vast landscape of models where neutrino physics offers our best chance of discovering such hidden sectors. Along the way, we revisit the Standard Model physics of neutrino trident production with a modern calculation and explore its phenomenology at neutrino facilities. As shown here, this type of rare neutrino scattering process can probe unexplored anomaly-free extensions of the Standard Model with a complementary, and often more powerful, search strategy to to the well-known searches in neutrino-electron scattering. As to new models, we propose a novel neutrino mass model resembling the inverse seesaw, where neutrino mixing stands as the most prominent portal to dark sectors and dark matter. In our dark neutrino model, neutrino masses are generated radiatively, with the vector, scalar, and neutrino phenomenology displaying an unique interplay. Later, we devise new methods to search for these dark neutrinos using neutrino-electron scattering data, aiming to discriminate among new physics explanations of the MiniBooNE anomalous results. Finally, we discuss light and heavy conventional sterile neutrinos in the context of $\nu$STORM, an entry-level neutrino factory for precision neutrino physics

Pion decay constraints on exotic 17 MeV vector bosons

We derive constraints on the couplings of light vector particles to all first-generation Standard Model fermions using leptonic decays of the charged pion, $\pi^+\to e^+ \nu_e X_\mu$. In models where the net charge to which $X_\mu$ couples to is not conserved, no lepton helicity flip is required for the decay to happen, enhancing the decay rate by factors of ${O}(m_\pi^4/m_e^2m_X^2)$. A past search at the SINDRUM-I spectrometer severely constrains this possibility. In the context of the hypothesized $17$ MeV particle proposed to explain anomalous $^8$Be, $^4$He, and $^{12}$C nuclear transitions claimed by the ATOMKI experiment, this limit rules out vector-boson explanations and poses strong limits on axial-vector ones.Comment: 10 pages, 5 figure

Decaying sterile neutrinos at short baselines

Long-standing anomalous experimental results from short-baseline neutrino experiments have persisted for decades. These results, when interpreted with one or more light sterile neutrinos, are inconsistent with numerous null results experimentally. However, if the sterile neutrino decays en route to the detector, this can mimic $\nu_\mu \to \nu_e$ oscillation signals while avoiding many of these external constraints. We revisit this solution to the MiniBooNE and LSND puzzles in view of new data from the MicroBooNE experiment at Fermilab. Using MicroBooNE's liquid-argon time-projection chamber search for an excess of $\nu_e$ in the Booster beam, we derive new limits in two models' parameter spaces of interest: where the sterile neutrino decays (I) via mixing with the active neutrinos, or (II) via higher-dimensional operators. We also provide an updated, comprehensive fit to the MiniBooNE neutrino- and antineutrino-beam data, including appearance ($\nu_e$) and disappearance ($\nu_\mu$) channels. Despite alleviating the tension with muon neutrino disappearance experiments, we find that the latest MicroBooNE analysis rules out the decaying sterile neutrino solution in a large portion of the parameter space at more than $99\%$ CL.Comment: 16 pages, 12 figures, 1 tabl

New physics in multi-electron muon decays

We study the exotic muon decays with five charged tracks in the final state. First, we investigate the Standard Model rate for $\mu^+ \to 3e^+\,2e^-\,2\nu$ ($B = 4.0\times 10^{-10})$ and find that the Mu3e experiment should have tens to hundreds of signal events per $10^{15}$ $\mu^+$ decays, depending on the signal selection strategy. We then turn to a neutrinoless $\mu^+ \to 3e^+\,2e^-$ decay that may arise in new-physics models with lepton-flavor-violating effective operators involving a dark Higgs $h_d$. Following its production in $\mu^+ \to e^+ h_d$ decays, the dark Higgs can undergo a decay cascade to two $e^+e^-$ pairs through two dark photons, $h_d \to \gamma_d \gamma_d \to 2(e^+e^-)$. We show that a $\mu^+ \to 3e^+\,2e^-$ search at the Mu3e experiment, with potential sensitivity to the branching ratio at the $O(10^{-12})$ level or below, can explore new regions of parameter space and new physics scales as high as $\Lambda \sim 10^{15}$ GeV.Comment: 20 pages, 7 figure

Semi-Visible Dark Photon Phenomenology at the GeV Scale

In rich dark sector models, dark photons heavier than tens of MeV can behave as semi-visible particles: their decays contain both visible and invisible final states. We present models containing multiple dark fermions which allow for such decays and inscribe them in the context of inelastic dark matter and heavy neutral leptons scenarios. Our models represent a generalization of the traditional inelastic dark matter model by means of a charge conjugation symmetry. We revisit constraints on dark photons from $e^+e^-$ colliders and fixed target experiments, including the effect of analysis vetoes on semi-visible decays, $A^\prime \to \psi_i (\psi_j \to \psi_k \ell^+\ell^-)$. We find that in some cases, the BaBar and NA64 experiments no longer exclude large kinetic mixing, $\varepsilon \sim 10^{-2}$, and, specifically, the related explanation of the discrepancy in the muon $(g-2)$. This reopens an interesting window in parameter space for dark photons with exciting discovery prospects. We point out that a modified missing-energy search at NA64 can target short-lived $A^\prime$ decays and directly probe the newly-open parameter space.Comment: 41 pages, 22 figures, version published in PR

Resonant Neutrino Flavor Conversion in the Atmosphere

Neutrinos produced in the atmosphere traverse a column density of air before being detected at neutrino observatories like IceCube or KM3NeT. In this work, we extend the neutrino flavor evolution in the {nuSQuIDS} code accounting for the varying height of neutrino production and the variable air density in the atmosphere. These effects can lead to sizeable spectral distortions in standard neutrino oscillations and are crucial to accurately describe some new physics scenarios. As an example, we study a model of quasi-sterile neutrinos that induce resonant flavor conversions at neutrino energies of ${O}(300)\text{ MeV}$ in matter densities of $1 \text{ g/cm}^3$. In atmospheric air densities, the same resonance is then realized at neutrino energies of ${O}(300- 700)$~GeV. We find that the new resonance can deplete the $\nu_\mu + \overline{\nu}_\mu$ flux at the IceCube Neutrino Observatory by as much as $10\%$ in the direction of the horizon.Comment: 12 pages,9 figure