Restricting CP violation in the low-scale type-I seesaw with three right-handed neutrinos to the Dirac phase δ alone yields specific testable subregions of heavy-neutrino flavor mixings and permits low-scale leptogenesis to generate the observed baryon asymmetry even for O(10^{-5}) deviations from C
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The present and future status of heavy neutral leptons
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abstract
The existence of non-zero neutrino masses points to the likely existence of multiple SM neutral fermions. When such states are heavy enough that they cannot be produced in oscillations, they are referred to as Heavy Neutral Leptons (HNLs). In this white paper we discuss the present experimental status of HNLs including colliders, beta decay, accelerators, as well as astrophysical and cosmological impacts. We discuss the importance of continuing to search for HNLs, and its potential impact on our understanding on key fundamental questions, and additionally we outline the future prospects for next-generation future experiments or upcoming accelerator run scenarios.
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Thermal Resonant Leptogenesis produces the observed baryon asymmetry via a dominant thermal channel from Higgs decays and lepton-doublet coherences, without requiring quasi-degenerate sterile neutrinos.
Bubble collisions in a seesaw model produce right-handed neutrinos that source novel gravitational waves detectable by LISA, ET, and LVK while allowing the lightest RHN to explain dark matter or enable leptogenesis.
Correlated HNL discovery at SHiP and flavor ratio shifts in astrophysical neutrinos at telescopes would establish neutrinos as Majorana fermions.
DELIGHT-SHIELD uses a dedicated shield followed by tracking to suppress hadronic and electromagnetic backgrounds by up to seven orders of magnitude, reaching branching ratio sensitivity of O(10^{-9}) for h to phi phi in a dark scalar model.
A proposed Chinese charm factory could detect heavy neutral leptons from axionlike-particle decays with a displaced-vertex search, reaching electron-neutrino mixing one to two orders of magnitude below current bounds.
Tera-Z factories can produce millions of heavy neutral leptons and billions of axion-like particles under optimistic assumptions, turning them into exotics factories for discovery and detailed study.
Solar data constrains the non-unitary parameter (1-α11) to less than 0.046 at 99% credible interval, with strong correlation to the solar mass splitting.
GLoBES simulations show that DUNE can probe quasi-Dirac sterile-neutrino mixing down to θ_sa ≈ 0.05 with uniform coverage, while current NOvA+T2K data constrains θ_sa ≳ 0.08.
In a U(1) gauged seesaw model, Z'Z' fusion processes at muon colliders enable HNL pair production via Z'Z'→H→NN and Z'Z'→NN, yielding LNV signals not suppressed by Higgs mixing.
Dynamical inverse seesaw predicts low-frequency stochastic GW signals from a first-order phase transition, with complementarity to heavy neutral lepton searches at small active-sterile mixing.
The paper identifies promising parameter regions for observing same-sign tetralepton events from charged Higgs pair and single production decaying to muons and heavy neutral leptons at μTRISTAN.
citing papers explorer
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Dirac-Phase CP-Violation in the Low-Scale Type-I Seesaw with Three Right-Handed Neutrinos
Restricting CP violation in the low-scale type-I seesaw with three right-handed neutrinos to the Dirac phase δ alone yields specific testable subregions of heavy-neutrino flavor mixings and permits low-scale leptogenesis to generate the observed baryon asymmetry even for O(10^{-5}) deviations from C
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Dominant Thermal Resonant Mechanism for Low-Scale Leptogenesis
Thermal Resonant Leptogenesis produces the observed baryon asymmetry via a dominant thermal channel from Higgs decays and lepton-doublet coherences, without requiring quasi-degenerate sterile neutrinos.
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Cosmic Collider Gravitational Waves sourced by Right-handed Neutrino production from Bubbles: Testing Seesaw, Leptogenesis and Dark Matter
Bubble collisions in a seesaw model produce right-handed neutrinos that source novel gravitational waves detectable by LISA, ET, and LVK while allowing the lightest RHN to explain dark matter or enable leptogenesis.
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Are neutrinos Majorana? Fixed-target and high-energy astrophysical searches decide
Correlated HNL discovery at SHiP and flavor ratio shifts in astrophysical neutrinos at telescopes would establish neutrinos as Majorana fermions.
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A new approach to long-lived particle detection at hadron colliders: the $\textsf{DELIGHT-SHIELD}$ concept
DELIGHT-SHIELD uses a dedicated shield followed by tracking to suppress hadronic and electromagnetic backgrounds by up to seven orders of magnitude, reaching branching ratio sensitivity of O(10^{-9}) for h to phi phi in a dark scalar model.
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Long-lived sterile neutrinos from axionlike particles at the Super Tau-Charm Facility
A proposed Chinese charm factory could detect heavy neutral leptons from axionlike-particle decays with a displaced-vertex search, reaching electron-neutrino mixing one to two orders of magnitude below current bounds.
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New Particles at the Z-Pole: Tera-Z factories as discovery and precision machines
Tera-Z factories can produce millions of heavy neutral leptons and billions of axion-like particles under optimistic assumptions, turning them into exotics factories for discovery and detailed study.
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A non-unitary solar constraint for long-baseline neutrino experiments
Solar data constrains the non-unitary parameter (1-α11) to less than 0.046 at 99% credible interval, with strong correlation to the solar mass splitting.
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Probing Quasi-Dirac Neutrino Oscillations at Long Baseline Experiments
GLoBES simulations show that DUNE can probe quasi-Dirac sterile-neutrino mixing down to θ_sa ≈ 0.05 with uniform coverage, while current NOvA+T2K data constrains θ_sa ≳ 0.08.
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New Avenues of Heavy Neutral Lepton at Muon Collider
In a U(1) gauged seesaw model, Z'Z' fusion processes at muon colliders enable HNL pair production via Z'Z'→H→NN and Z'Z'→NN, yielding LNV signals not suppressed by Higgs mixing.
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Probing Dynamical Inverse Seesaw with Low-frequency Gravitational Waves
Dynamical inverse seesaw predicts low-frequency stochastic GW signals from a first-order phase transition, with complementarity to heavy neutral lepton searches at small active-sterile mixing.
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Same-Sign Tetralepton Signature at $\mu$TRISTAN
The paper identifies promising parameter regions for observing same-sign tetralepton events from charged Higgs pair and single production decaying to muons and heavy neutral leptons at μTRISTAN.
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