  We demonstrate that rotational superradiance can be efficient in millisecond
pulsars. Measurements from the two fastest known pulsars PSR J1748-2446ad and
PSR B1937+21 can place bounds on bosons with masses below 10^{-11} eV. The
bounds are maximally good at masses corresponding to the rotation rate of the
star, where scalar interactions that mediate forces ~ 10^6 times weaker than
gravity are ruled out, exceeding existing fifth force constraints by 3 orders
of magnitude. For certain neutron star equations of state, these measurements
would also constrain the QCD axion with masses between 5 10^{-13} and 3
10^{-12} eV. Despite the ability of most neutron star equations of state to
support frequencies as high as ~ 1500 Hz, the observed absence of pulsars above
~ 700 Hz could be due to the existence of a new particle of mass ~ 10^{-11} eV
with a Yukawa coupling to nucleons.

High Energy Physics - Phenomenology;High Energy Astrophysical Phenomena;General Relativity and Quantum Cosmology;High Energy Physics - Theory