Updated sensitivities to heavy neutral leptons at the LHC far detectors and SHiP

Abstract

In recent years, a number of experiments dedicated to searches for long-lived particles (LLPs) have been proposed, approved, or have entered operation. While the sensitivities of these experiments to various LLP scenarios have been extensively studied, key aspects--such as detector geometries, background estimates, and projected operational durations--for several facilities, including MATHUSLA, ANUBIS, and SHiP, have undergone significant updates. In this work, we implement the latest experimental designs in the Displaced Decay Counter tool for calculating detector acceptances and signal-event yields, and re-evaluate their sensitivity reach to one of the most widely studied LLP scenarios, namely minimal heavy neutral leptons.

Figures (1)

• Figure 1: Updated sensitivity reach of the LHC far detectors and the SHiP beam-dump experiment to minimal heavy neutral leptons that mix with the electron neutrino only, shown in the $(m_N, |V_{eN}|^2)$ plane. The dark gray area represents the current experimental bounds obtained at PIENUPIENU:2017wbj, KENUBryman:2019bjg, CHARMCHARM:1985nku, NA62NA62:2020mcv, T2KT2K:2019jwa, BEBCBarouki:2022bkt, and DELPHIDELPHI:1996qcc. The light gray band corresponds to the parameter region targeted by the type-I seesaw mechanism for active-neutrino masses between 0.05 eV and 0.12 eV. The sensitivity in the low-mass region ($m_N \lesssim m_D$) is dominated by HNLs produced from charm-meson decays, whereas for heavier masses ($m_D \lesssim m_N \lesssim m_B$) the contribution from bottom-meson decays dominates; only $D$- and $B$-meson channels are included in this analysis.