  We propose a model describing spin-half quantum particles in curved spacetime
in the framework of quantum field theory. Our model is based on embodying
Einstein's equivalence principle and general covariance in the definition of
quantum-particle states. With this model at hand, we compute several
observables which characterise spin-half quantum particles in a gravitational
field. In particular, we find that spin precesses in a normal Fermi frame, even
in the absence of torsion. The effect appears to be complementary to free-fall
non-universality we have recently reported about for spinless quantum
particles. Furthermore, we find that quantum-particle gravitational-potential
energy is insensitive to wave-packet spreading in the Earth's gravitational
field, that is responsible for the non-universality of free fall in quantum
theory. This theoretical result provides another channel for the experimental
study of our quantum-particle model by using gravitational spectrometers.
Finally, we also find that (elementary) fermions and antifermions are
indistinguishable in gravity.
