Long-Lived HNLs via ALP Portal at the LHC

TL;DR

The paper investigates HL-LHC prospects for long-lived heavy neutral leptons produced through an axion-like particle portal or via dimension-7 N_R SMEFT operators coupling HNLs to gluons. By combining ALP-gluon interactions and ALPHNL couplings with EFT descriptions, the authors compute production cross sections and ALP decays, and perform detector simulations for ATLAS and several far-detector concepts (MATHUSLA-40, ANUBIS-C) using the Displaced Decay Counter. Sensitivity studies in the |V_{eN}|^2–m_N plane show that HL-LHC can probe mixing values down to ~10^{-24} in favorable ALP-mass scenarios, while ATLAS can reach Λ up to ~300 TeV and far detectors can probe tens of TeV scales, with ANUBIS-C typically offering the strongest reach for small mixings. These results highlight the HL-LHC’s potential to explore HNL parameter space far beyond current limits and demonstrate the synergy between on-shell ALP production and displaced-vertex searches across multiple detector concepts.

Abstract

Heavy neutral leptons (HNLs) and axion-like particles (ALPs) are both considered well-motivated candidates for beyond the standard model (BSM) physics. ALPs with couplings to gluons will be abundantly produced at the LHC. Therefore, HNLs produced via the ALP portal may provide unprecedented sensitivities to HNL parameters. Here, we study the prospects for the high-luminosity LHC to search for long-lived HNLs. We consider future far detectors as well as ATLAS in our simulations. In the limit where the ALP mass is large, HNLs are effectively produced by a dimension-8 operator connecting HNL pairs to gluons. For completeness, we therefore also calculate future LHC sensitivities for HNLs produced via $N_R$SMEFT operators with gluons.

Figures (9)

• Figure 1: Contours of branching ratios Br$(a\to NN)$ for $c_{NNa}=1$ as a function of Log$_{10}(c_{G{\tilde{G}}a})$ and $x=m_N/m_a$. All other Wilson coefficients are assumed to be zero in this plot.
• Figure 2: Cross section $\sigma(pp \to a^{(*)} \to NN)$ in fb for $\sqrt{s}=14$ TeV as a function of the ALP mass, $m_a$, for two different choices of $m_N$ and $c_{G{\tilde{G}}a}$. There are three different kinematic regimes: (i) $m_a \le 2 m_N$; (ii) $2 m_N \le m_a \hbox{$\;<$$\sim\;$} 1$ TeV and (iii) $m_a \hbox{$\;>$$\sim\;$}$ TeV. In regime (ii) the ALP can be on-shell and the cross section is enhanced due to the small width of the ALP. In regime (iii) the ALP is too heavy to be produced on-shell and the cross section reduces to the contribution of an effective operator coupling HNLs to gluons, see text.
• Figure 3: Cross section $\sigma(pp \to NN)$ in fb for $\sqrt{s}=14$ TeV as a function of the HNL mass, $m_N$, for three different choices of the operator scale, $\Lambda$. The Wilson coefficient of the $d=7$ effective operator has been chosen as $c_{GN}=1$ in this example.
• Figure 4: Sensitivity estimates for the high-luminosity LHC for HNL parameters for two different ALP masses, $m_a$ and two different combinations of Wilson coefficients at a fixed value of $\Lambda$. For discussions, see text.
• Figure 5: Sensitivity estimates for the high-luminosity LHC for HNL parameters for two different cases: To the left, a calculation putting $m_a=5$ TeV; to the right, using the $d=7$ operator with $\Lambda = 10$ TeV.