  We have studied three realistic benchmark geometries for a new far detector
GAZELLE to search for long-lived particles at the \superkekb accelerator in
Tsukuba, Japan. The new detector would be housed in the same building as Belle
II and observe the same $e^+e^-$ collisions. To assess the discovery reach of
GAZELLE, we have investigated three new physics models that predict long-lived
particles: heavy neutral leptons produced in tau lepton decays, axion-like
particles produced in $B$ meson decays, and new scalars produced in association
with a dark photon, as motivated by inelastic dark matter. We do not find
significant gains in the new physics discovery reach of GAZELLE compared to the
Belle II projections for the same final states. The main reasons are the
practical limitations on the angular acceptance and size of GAZELLE,
effectively making it at most comparable to Belle II, even though backgrounds
in the far detector could be reduced to low rates. A far detector for
long-lived particles would be well motivated in the case of a discovery by
Belle II, since decays inside GAZELLE would facilitate studies of the decay
products. Depending on the placement of GAZELLE, searches for light long-lived
particles produced in the forward direction or signals of a confining hidden
force could also benefit from such a far detector. Our general findings could
help guide the design of far detectors at future electron-positron colliders
such as the ILC, FCC-ee or CEPC.
