Long-Lived-Particle Signals of a Composite Hidden Sector through the Neutrino Portal

TL;DR

The paper investigates a strongly coupled hidden sector that is approximately conformal in the ultraviolet and confines below the weak scale, communicating with the SM through the neutrino portal to realize an inverse seesaw mechanism for neutrino masses. The lightest hidden-state is a dilaton, whose decays back to the SM are suppressed by angular momentum, producing long-lived particle signatures at colliders and beam-dump experiments. It derives current laboratory and astrophysical constraints, and analyzes the reach of future searches at FASER 2, SHiP, and Belle II, finding that a significant portion of the parameter space can be probed. The work also discusses cosmological viability (BBN/CMB) and potential connections to nanohertz gravitational-wave signals from early-universe phase transitions, highlighting the broad phenomenological implications of a conformal hidden sector coupled via the neutrino portal.

Abstract

We explore the signals of a scenario in which the composite states of a strongly coupled hidden sector couple to the Standard Model through the neutrino portal, giving rise to the neutrino masses. We consider a framework in which the hidden sector is conformal in the ultraviolet and the compositeness scale lies below the weak scale. If the lightest composite state in the hidden sector is a scalar, its decay rate back to the Standard Model is suppressed by angular momentum considerations and can naturally be small, giving rise to long-lived particle signals. We determine the current constraints on this class of models and explore the reach of future collider and beam dump searches. We find that FASER, SHiP, and Belle II can potentially probe a significant part of the unexplored parameter space.

Figures (13)

• Figure 2: Probability density function of $m_\mathcal{U}$ in the case of leptonic decays of pseudoscalar mesons, where we set $m_\ell=0, ~\Delta_N = 7/4, ~m_\sigma/m_N = 0.6$ and $\Lambda/m_\mathfrak{m} = 0.1$. As shown here, we divide the kinematically allowed range in $m_\mathcal{U}$ into intervals corresponding to final states of the form $n\, N + (n-1)\,\overline{N}+p\,\sigma$. Solid lines correspond to $n=1, 2, 3, \ldots$ and the dashed lines correspond to $p=1, 2, 3$.
• Figure 3: Diagrams contributing to decays of $\sigma$ at order $U^2_{N\ell}$ in the amplitude. As described in the text, decay modes corresponding to \ref{['fig: sigma_2_1']} and \ref{['fig: sigma_2_2']} are suppressed by factors of $m^2_f$ and $m^2_\ell$ respectively. Consequently, $\sigma$ primarily decays through one of the decay modes shown in \ref{['fig: sigma_4']} and \ref{['fig: sigma_2_3']}.
• Figure 4: Left: Relative decay widths of $\sigma$ to different parton-level four-body final states, as a function of $m_N$. Right: Contours of $c\tau_\sigma$ in meters as a function of $(m_N, |U_{N\ell}|^2)$. Here, we set $\Delta_N=7/4$, $\Delta_{2N}=17/4$ and $m_\sigma/m_N = 0.6$. The light and dark shaded regions correspond to $\mathcal{BR}_\mathrm{inv} > 50\%$ and $\mathcal{BR}_\mathrm{inv} > 99\%$ respectively.
• Figure 5: Distributions of the reconstructed HNL mass $m_{HNL}$ in our case (blue) and in the case of conventional HNLs (red), for events passing all the other kinematic cuts in the ATLAS search of Ref. ATLAS:2022atq. The left and right plots correspond to $m_N = 20~\mathrm{GeV}$ and $m_N = 40~\mathrm{GeV}$ respectively.
• Figure 6: (Left) Distributions in the $(p, \theta)$ plane for $B^+$ mesons produced in proton-proton collisions at $\sqrt{s}=14~\mathrm{TeV}$, where the events are generated using Pythia 8. (Right) Similar distributions corresponding to $\sigma$ produced in the decays of $B^+$ mesons, where $m_N = 2~\mathrm{GeV}$. The dashed vertical line corresponds to $\theta=\theta_\mathrm{max}$.