  Development of new techniques to search for particles beyond the standard
model is crucial for understanding the ultraviolet completion of particle
physics. Several hypothetical particles are predicted to mediate exotic
spin-dependent interactions between particles of the standard model that may be
accessible to laboratory experiments. However, laboratory searches are mostly
conducted for static spin-dependent interactions, with only a few experiments
so far addressing spin- and velocity-dependent interactions. Here, we
demonstrate a search for exotic spin- and velocity-dependent interactions with
a spin-based amplifier. Our technique makes use of hyperpolarized nuclear spins
as a pre-amplifier to enhance the effect of pseudo-magnetic field produced by
exotic interactions by an amplification factor of > 100. Using such a
spin-based amplifier, we establish constraints on the spin- and
velocity-dependent interactions between polarized and unpolarized nucleons in
the force range of 0.03-100 m. Our limits represent at least two orders of
magnitude improvement compared to previous experiments. The established
technique can be further extended to investigate other exotic spin-dependent
interactions.
