  Cavity optomechanical systems can be used for sensitive detection of
mechanical motion and to control mechanical resonators, down to the quantum
level. The strength with which optical and mechanical degrees of freedom
interact is defined by the photon-phonon coupling rate $g_0$, which is
especially large in nanoscale systems. Here, we demonstrate an optomechanical
system based on a sliced photonic crystal nanobeam, that combines subwavelength
optical confinement with a low-mass mechanical mode. Analyzing the transduction
of motion and effects of radiation pressure we find a coupling rate
$g_0$/2{\pi} = 11.5 MHz, exceeding previously reported values by an order of
magnitude. Using this interaction we detect the resonator's motion with a noise
imprecision below that at the standard quantum limit, even though the system
has optical and mechanical quality factors smaller than $10^3$. The broad
bandwidth is useful for application in miniature sensors, and for
measurement-based control of the resonator's motional state.
