The observation of neutrino masses strongly motivates $U(1)_{B-L}$ extensions of the Standard Model, in which heavy neutral leptons acquire Majorana masses through spontaneous $U(1)_{B-L}$ symmetry breaking and generate light neutrino masses via the seesaw mechanism. In this framework, the singlet scalar responsible for symmetry breaking mixes with the SM Higgs boson, allowing it to be produced in rare meson decays. We investigate a scenario in which this light scalar promptly decays into a pair of long-lived heavy neutrinos that subsequently decay into visible charged leptons and hadrons through light-heavy neutrino mixing inside the proposed Forward Physics Facility (FPF) at the FCC-hh and the SHiP beam-dump experiment. Taking into account realistic detector geometries, decay probabilities, and visible branching fractions, we estimate the projected sensitivities to the scalar-Higgs mixing angle as a function of the scalar mass and to the light-heavy neutrino mixing as a function of the heavy neutrino mass. We find that FPF and SHiP can significantly extend the discovery reach for both light scalars and long-lived heavy neutrinos beyond existing experimental limits, providing powerful and complementary probes of neutrino-mass generation and hidden-sector physics.
We study leptogenesis via out-of-equilibrium decays of right-handed neutrinos in a parity symmetric extension of the Standard Model with gauge group $SU(3)_c \times SU(2)_L \times SU(2)_R \times U(1)_X$, spontaneously broken to the Standard Model gauge group at a scale $v_R$. We focus on the minimal Higgs realization, in which the strong CP problem is resolved without invoking additional symmetries. In this framework, lepton number is violated through Yukawa interactions, while neutrino masses arise only at higher loop order, a feature that permits the CP-violating quantum corrections to right-handed neutrino decays to be parametrically large. For thermal production of right-handed neutrinos, we find that successful leptogenesis requires $v_R \gtrsim 6\times 10^{12}$ GeV. Non-thermal production, however, relaxes this bound dramatically, requiring only $v_R \gtrsim 10$ TeV, a scale accessible to collider searches for new particles and searches for rare processes.
Heavy Neutral Lepton (HNL)-mediated t-channel processes provide a unique opportunity to probe mass scales beyond the kinematic reach of direct production at high-energy colliders. We revisit these processes using the vector boson scattering channel $WW\to\ell\ell$ at the LHC as a case study, highlighting the essential role of the light neutrinos in restoring the proper high-energy unitary behavior of the scattering amplitude. Their inclusion, overlooked in some previous studies, leads to destructive interference that strongly suppresses lepton number violating signatures, demonstrating that a consistent treatment of the full seesaw spectrum qualitatively alters the phenomenology of t-channel HNL searches. This motivates the exploration of lepton number conserving but lepton flavor violating final states instead. We present a detailed analysis of the $pp\to e\mu jj$ channel and show that it provides a promising probe of TeV-scale HNLs in low-scale seesaw scenarios with sizable active-sterile mixing, extending the LHC sensitivity beyond existing direct searches.
C. García-García, Manuel González-López, Xabier Marcano et al.· 0 citations
Large-volume neutrino telescopes offer a unique opportunity to search for decaying dark matter through events containing a pair of energetic, highly non-collimated muon tracks emerging from a common vertex. Such events would have negligible Standard Model backgrounds and would constitute a striking signature of new physics. Conventional dark matter annihilation or decay, however, is too strongly constrained to produce an observable rate of such events. We therefore consider scenarios in which an excited dark matter state is extremely long-lived in vacuum but decays much more rapidly in the presence of ordinary matter. We present two realizations of this mechanism. In the first, a long-range scalar field sourced by ordinary matter modifies the dark-sector mass spectrum, kinematically opening the decay $\chi_2 \rightarrow \chi_1 Z'$ near the Earth while leaving it forbidden in vacuum. In the second, the scalar background induces kinetic mixing between a heavy $Z'$ and the photon, greatly enhancing the three-body decay $\chi_2\to\chi_1\mu^+\mu^-$ in matter-rich environments. We calculate the resulting distributions of muon energies and opening angles and show that viable regions of parameter space can yield observable event rates in IceCube, KM3NeT, and other large-volume neutrino telescopes while remaining consistent with existing constraints. We also briefly consider the sensitivity of IceCube to multi-muon events produced by the decays of cosmologically long-lived charged particles with masses $\gtrsim 1$ TeV.
H. Davoudiasl, Dan Hooper, Samyak Jain· 1 citation
Neutrino self-interactions mediated by a light scalar offer a compelling resolution to cosmological tensions and may naturally arise in neutrino mass generation mechanisms. When the scalar also couples to a light dark-sector fermion, supernova neutrinos can resonantly annihilate with the cosmic neutrino background (C$\nu$B) into invisible dark radiation, depleting the flux en route to Earth. We use this depletion to constrain the neutrino-scalar coupling from the SN1987A data. Within a Bayesian framework we analyze the data for two supernova neutrino emission models, a parameterized model and a 2D hydrodynamic simulation, and for two coupling types, a mass-independent one and a mass-proportional one. Our results show that new-physics limits from SN1987A cannot be quoted independently of the heavy-flavor emission, the flavor conversion, or the coupling structure. Finally, we forecast that a future high-statistics burst recorded by Hyper-Kamiokande would restore a meaningful upper bound even when flavor conversion is included.
Christina Gao, Ke Hu, Kun-Feng Lyu et al.· 0 citations
We investigate the dark matter (DM) phenomenology and subsequent collider signatures of a dark leptoquark (LQ) model containing dark vector-like quarks (VLQs) and a scalar singlet. The dark LQs and VLQs participate in the radiative generation of Majorana neutrino masses at one loop. Since the coloured LQs and VLQs cannot serve as viable DM candidates, a $\mathbb{Z}_2$-odd singlet scalar is introduced as the DM candidate. The $\mathbb{Z}_2$-odd nature of the LQs forbids their conventional decays into a quark and a lepton, allowing them to evade the standard LHC constraints that apply to visible LQ signatures. This framework therefore offers the distinctive possibility of sub-TeV dark LQs coexisting with TeV-scale dark VLQs. These sub-TeV dark LQs help in achieving the observed relic abundance of the singlet through co-annihilation. We analyse the resulting DM phenomenology and assess the prospects for probing this scenario at a future muon collider.
Priyotosh Bandyopadhyay, Kirtiman Ghosh, Anirban Karan et al.· 0 citations
The observation of neutrino oscillations and masses motivates extensions of the Standard Model containing right-handed neutrinos. If additional new physics exists at scales beyond the direct reach of present experiments, its effects can be systematically described within the neutrino Standard Model Effective Field Theory ($\nu$SMEFT). In this framework, heavy neutral leptons provide a promising target for collider searches. In this work, we reinterpret a CMS search for heavy Majorana neutrinos in the same-sign dimuon plus two jets final state to constrain the parameter space of the dimension-six $\nu$SMEFT. Using a detailed recast of the experimental analysis, we present results for both a pure-agnostic benchmark and a benchmark incorporating neutrinoless double-beta-decay bounds. For heavy-neutrino masses in the range $200~\mathrm{GeV} \le m_N \le 15~\mathrm{TeV}$, the observed upper limits on the effective coupling strength range from $5.8\times10^{-7}\,\mathrm{GeV}^{-2}$ to $2.3\times10^{-6}\,\mathrm{GeV}^{-2}$. These results constitute the first experimental exclusion limits on the agnostic $\nu$SMEFT parameter space obtained from a dedicated recast of an LHC heavy-neutrino search.
L. Duarte, Agustı́n Guillenea· 0 citations
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