  Belle II is an experiment operating at the intensity frontier. Over the next
decades, it will record the decay of billions of bottom mesons, charm hadrons,
and tau leptons produced in 10 GeV electron-positron collisions at the
SuperKEKB high-luminosity collider at KEK. These data, collected in
low-background and kinematically known conditions, will allow us to measure
hundreds of parameters that test the standard model (SM) and probe for the
existence of new particles, at mass scales orders of magnitudes higher than
those studied at the energy frontier. We project our sensitivities for
measurements that are of primary relevance and where Belle II will be unique or
world leading for data corresponding to 1 to 50 ab$^{-1}$. Belle II will
uniquely probe non-SM contributions in sensitive $b \to q\bar q s$ decays and
charmless $b \to q\bar q d(u)$ decays, semileptonic $b \to s \nu \bar\nu$ and
$s \tau^+ \tau^-$ decays, fully leptonic $b \to \ell \nu$ decays, and select $c
\to u$ processes. Belle II will lead exploration of non-SM physics in $b \to c
\tau \nu$ and $b \to s \gamma$ decays and will most precisely determine the
quark-mixing parameters $|V_{ub}|$ and $|V_{cb}|$. Belle II will measure many
parameters in $\tau$ physics to precisions that will be world leading for the
foreseeable future, including the electric and magnetic dipole moments,
branching fractions for charged-lepton-flavor-violating decays, and quantities
that test lepton-flavor universality. Belle II will perform unique searches for
dark-sector particles with masses in the MeV-GeV range. We will also pursue a
broad spectroscopy program for conventional and multiquark $c \bar c$ and $b
\bar b$ states and provide essential inputs to sharpen the interpretation of
muon magnetic-anomaly results. Our exploration of uncharted regions of non-SM
parameter space with high precision will reveal non-SM particles or set
stringent constraints on their existence, guiding future endeavors.
