Long-lived sterile neutrinos from axionlike particles at the Super Tau-Charm Facility

TL;DR

This paper explores the potential of the Super Tau-Charm Facility to detect long-lived heavy neutral leptons produced via axionlike particles, using simulations to estimate sensitivity improvements over existing bounds.

Contribution

It introduces a novel search strategy for HNLs at STCF via ALP decays, providing sensitivity estimates and comparing with beam-dump experiment constraints.

Findings

STCF can probe $|V_{eN}|^2$ values 10-100 times lower than current bounds.

Displaced-vertex searches at STCF significantly enhance detection prospects.

Beam-dump experiments offer complementary constraints on HNLs.

Abstract

We study the search prospect of long-lived heavy neutral leptons (HNLs) pair produced in decays of axionlike particles (ALPs) at the proposed Super Tau-Charm Facility (STCF), focusing on the center-of-mass energy of $\sqrt{s}=3.773$ GeV. The ALPs are assumed to originate from $D^\pm$-meson decays in association with a charged pion. We perform both truth-level and detector-level Monte Carlo simulations and obtain the expected sensitivity reach to the mixing parameter between the HNL and the electron neutrino, $|V_{eN}|^2$, with a displaced-vertex search at STCF. We find that STCF can probe values of $|V_{eN}|^2$ about one-to-two orders of magnitude beyond the existing bounds. We also perform an approximate reinterpretation of a search for HNLs at the CHARM experiment, which is subject to model-dependent assumptions on the production and kinematic distributions, and find that beam-dump experiments may provide strong complementary constraints.