It is well known that the differences between the lepton numbers can be gauged with the Standard Model matter content. Such extended gauge theories, dubbed as the gauged U(1) Lα−L β models, have been widely discussed so far as potential candidates for physics beyond the Standard Model. In this work, we study the minimal versions of these gauge theories, where three right-handed neutrinos as well as a single U(1) Lα−L β symmetry breaking Higgs field-an SU(2) L singlet or doublet-are introduced. In these minimal models, the neutrino mass terms are constrained by the gauge symmetry, which result in the two-zero texture or twozero minor structure of neutrino mass matrices. Such restrictive forms of neutrino mass matrices lead to non-trivial predictions for the neutrino oscillation parameters as well as the size of the mass eigenvalues. We find that due to this restriction the minimal gauged U(1) Lα−L β models are either incompatible with the observed values of the neutrino parameters or in strong tension with the Planck 2018 limit on the sum of the neutrino masses. Only the U(1) Lµ−Lτ model with an SU(2) L singlet U(1) Lµ−Lτ -breaking field barely evades the limit, which can be tested in the future neutrino experiments.

We study the structure of the neutrino-mass matrix in the minimal gauged U(1) L μ −L τ model, where three right-handed neutrinos are added to the Standard Model in order to obtain non-zero masses for the active neutrinos. Because of the U(1) L μ −L τ gauge symmetry, the structure of both Dirac and Majorana mass terms of neutrinos is tightly restricted. In particular, the inverse of the neutrinomass matrix has zeros in the (μ, μ) and (τ, τ ) components, namely, this model offers a symmetric realization of the socalled two-zero-minor structure in the neutrino-mass matrix. Due to these constraints, all the CP phases -the Dirac CP phase δ and the Majorana CP phases α 2 and α 3 -as well as the mass eigenvalues of the light neutrinos m i are uniquely determined as functions of the neutrino mixing angles θ 12 , θ 23 , and θ 13 , and the squared mass differences m 2 21 and m 2 31 . We find that this model predicts the Dirac CP phase δ to be δ 1.59π -1.70π (1.54π -1.78π ), the sum of the neutrino masses to be i m i 0.14-0.22 eV (0.12-0.40 eV), and the effective mass for the neutrinoless double-beta decay to be m ββ 0.024-0.055 eV (0.017-0.12 eV) at 1σ (2σ ) level, which are totally consistent with the current experimental limits. These predictions can soon be tested in future neutrino experiments. Implications for leptogenesis are also discussed.

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We study the minimal extensions of the Standard Model by a linear combination of U(1) L e −L μ , U(1) L μ −L τ and U(1) B−L gauge symmetries, where three right-handed neutrinos and one U(1)-breaking SU(2) L singlet or doublet scalar are introduced. Because of the dependence on the lepton flavor, the structures of both Dirac and Majorana mass matrices of neutrinos are restricted. In particular, the two-zero minor and texture structures in the mass matrix for the active neutrinos are interesting. Analyzing these structures, we obtain uniquely all the neutrino parameters, namely the Dirac CP phase δ, the Majorana CP phases α 2,3 and the mass eigenvalues of the light neutrinos m i as functions of the neutrino mixing angles θ 12 , θ 23 , and θ 13 , and the squared mass differences m 2 21 and m 2 31. In 7 minimal models which are consistent with the recent neutrino oscillation data, we also obtain the predictions for the sum of the neutrino masses i m i and the effective Majorana neutrino mass m ββ and compare them with the current experimental limits. In addition, we also discuss the implication of our results for leptogenesis.

We study capability of the ILC beam dump experiment to search for new physics, comparing the performance of the electron and positron beam dumps. The dark photon, axion-like particles, and light scalar bosons are considered as new physics scenarios, where all the important production mechanisms are included: electron-positron pair-annihilation, Primakoff process, and bremsstrahlung productions.We find that the ILC beam dump experiment has higher sensitivity than past beam dump experiments, with the positron beam dump having slightly better performance for new physics particles which are produced by the electron-positron pair-annihilation.

High energy collisions at the High-Luminosity Large Hadron Collider (LHC) produce a large number of particles along the beam collision axis, outside of the acceptance of existing LHC experiments. The proposed Forward Physics Facility (FPF), to be located several hundred meters from the ATLAS interaction point and shielded by concrete and rock, will host a suite of experiments to probe standard model (SM) processes and search for physics beyond the standard model (BSM). In this report, we review the status of the civil engineering plans and the experiments to explore the diverse physics signals that can be uniquely probed in the forward region. FPF experiments will be sensitive to a broad range of BSM physics through searches for new particle scattering or decay signatures and deviations from SM expectations in high statistics analyses with TeV neutrinos in this low-background environment. High statistics neutrino detection will also provide valuable data for fundamental topics in perturbative and non-perturbative QCD and in weak interactions. Experiments at the FPF will enable synergies between forward particle production at the LHC and astroparticle physics to be exploited. We report here on these physics topics, on infrastructure, detector, and simulation studies, and on future directions to realize the FPF’s physics potential.

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We study a simple Dirac fermion dark matter model in $$ \mathrm{U}{(1)}_{L_{\mu }-{L}_{\tau }} $$ U 1 L μ − L τ theory. The new light gauge boson X plays important roles in both dark matter physics and the explanation for the muon g− 2 anomaly. The observed dark matter relic density is realized by a large $$ \mathrm{U}{(1)}_{L_{\mu }-{L}_{\tau }} $$ U 1 L μ − L τ charge without introducing a resonance effect of the X boson. As a by-product of the model, characteristic neutrino signatures from sub-GeV dark matter ψ are predicted depending on the mass spectrum. We formulate the analysis of $$ \psi \overline{\psi}\to \nu \overline{\nu} $$ ψ ψ ¯ → ν ν ¯ , and of $$ \psi \overline{\psi}\to XX $$ ψ ψ ¯ → XX followed by $$ X\to \nu \overline{\nu} $$ X → ν ν ¯ in a model independent way. The energy spectrum of neutrinos in the former process is monochromatic while in the latter process is bowl-shape. We also evaluate sensitivity at Super-Kamiokande and future Hyper-Kamiokande detectors. The analysis is finally applied to the $$ \mathrm{U}{(1)}_{L_{\mu }-{L}_{\tau }} $$ U 1 L μ − L τ dark matter model.