Speaker
Description
We explore the discovery potential for long-lived particles at
the 250-GeV ILC. The goal is to investigate possible gains of a dedicated far
detector over the main detector ILD. For concreteness, we perform our study for
sub-GeV axion-like particles $a$ produced via $e^+e^- \to a \gamma$ or $e^+e^-
\to Z \gamma \to (a\gamma)\gamma$ and decaying into pairs of charged leptons.
In the ideal case of zero background and perfect detection efficiency, we find that
far detectors placed in the planned underground cavities or a large cuboid on the
ground can enhance the sensitivity to long-lived pseudo-scalars at best
moderately. On the other hand, the ILD itself is a perfect environment to search for
long-lived particles, due to its excellent angular coverage and radial thickness. For
long-lived particles produced with cross sections of a few picobarns, the ILD could
probe lifetimes up to 300 ns or proper decay lengths up to 100 m in
$250\,$fb$^{-1}$ of data. For axion-like particles produced through weak
interactions, the ILC can reach an even higher sensitivity than searches for
displaced vertices in meson decays at \belletwo. Our findings apply similarly to
other proposed electron-positron experiments with a high angular coverage, such
as the FCC-ee and CEPC.