Probing electroweak pair production of heavy neutral leptons with displaced vertices at the LHC

Abstract

We study the sensitivity of displaced vertex searches at the LHC to heavy neutral leptons (also known as sterile neutrinos) that are produced in pairs with an electroweak-size cross section. We work within the context of a supersymmetric model in which the sterile neutrino is produced along with Standard Model particles in higgsino decays. By making use of model-independent reconstruction efficiencies provided by the ATLAS collaboration in their search for displaced vertices with multiple jets, we obtain constraints on this model from $139$ fb$^{-1}$ of data collected by ATLAS during the LHC Run~2, and assess the discovery reach of Run~3 and of the high-luminosity LHC (HL-LHC). Depending on the higgsino mass parameter, sterile neutrino masses between $20~\mathrm{GeV}$ and $230~\mathrm{GeV}$ and active-sterile neutrino mixings in the range $4 \times 10^{-14} \lesssim V^2_N \lesssim 3 \times 10^{-10}$ can be excluded. At the HL-LHC, discovery-level significances could be reached for sterile neutrinos masses up to $295~\mathrm{GeV}$ and values of $V^2_N$ down to $3 \times 10^{-14}$. Finally, moving away from the supersymmetric scenario, we study to which extent these results can be generalized to a broader class of models in which the sterile neutrinos are produced in the decays of heavier particles that are themselves pair-produced with an electroweak-size cross section.

Figures (9)

• Figure 1: Production (left) and decay (right) of the sterile neutrino. In the left diagram, $V$, $V'$ and $V"$ are electroweak gauge bosons $W^\pm$ and $Z$, while $\tilde{\chi}_i$ and $\tilde{\chi}_j$ stand for higgsino-like electroweakinos $\tilde{\chi}^0_{1,2}$ and $\tilde{\chi}^\pm_1$. In the right diagrams, the intermediate gauge bosons can be on shell or off shell, depending on the sterile neutrino mass. The coefficients $V_{N \alpha}$ at the sterile neutrino--gauge boson vertices are the active-sterile mixing angles, with $\alpha = e, \mu, \tau$.
• Figure 2: $95\%$ C.L. exclusion regions in the $(m_N, \mu)$ parameter space of the model of Section \ref{['sec:model']} from the recasting of the ATLAS analysis ATLAS:2023oti, assuming maximal real mixing (left plot) and minimal mixing (right plot).
• Figure 3: $95\%$ C.L. exclusion regions in the $(m_N, V^2_N)$ parameter space of the model of Section \ref{['sec:model']} from the recasting of the ATLAS analysis ATLAS:2023oti, assuming $\mu = 500~\mathrm{GeV}$ (left plot) and $\mu = 800~\mathrm{GeV}$ (right plot). The bottom dark grey area is inconsistent with neutrino oscillation data, while the top light grey region corresponds to a fine-tuning larger than $\approx 5\%$ in the light neutrino mass matrix.
• Figure 4: Projected discovery regions in the $(m_N, \mu)$ parameter space of the model of Section \ref{['sec:model']} for LHC Run 3 with $\mathcal{L}=300~\mathrm{fb}^{-1}$, assuming maximal real mixing (left plot) and minimal mixing (right plot).
• Figure 5: Projected discovery regions in the $(m_N, V^2_N)$ parameter space of the model of Section \ref{['sec:model']} for LHC Run 3 with $\mathcal{L}=300~\mathrm{fb}^{-1}$, assuming $\mu = 500~\mathrm{GeV}$ (upper left plot), $800~\mathrm{GeV}$ (upper right plot) and $1.2~\mathrm{TeV}$ (lower plot).