  Particle energization in shear flows is invoked to explain non-thermal
emission from the boundaries of relativistic astrophysical jets. Yet, the
physics of particle injection, i.e., the mechanism that allows thermal
particles to participate in shear-driven acceleration, remains unknown. With
particle-in-cell simulations, we study the development of Kelvin-Helmholtz (KH)
instabilities seeded by the velocity shear between a relativistic
magnetically-dominated electron-positron jet and a weakly magnetized
electron-ion ambient plasma. We show that, in their nonlinear stages, KH
vortices generate kinetic-scale reconnection layers, which efficiently energize
the jet particles, thus providing a first-principles mechanism for particle
injection into shear-driven acceleration. Our work lends support to
spine-sheath models of jet emission - with a fast core/spine surrounded by a
slower sheath - and can explain the origin of radio-emitting electrons at the
boundaries of relativistic jets.
