  We propose a simple mechanism that may explain the observed
particle-antiparticle asymmetry in the Universe. In the
Einstein-Cartan-Sciama-Kibble theory of gravity, the intrinsic spin of matter
generates spacetime torsion. Classical Dirac fields in the presence of torsion
obey the nonlinear Hehl-Datta equation which is asymmetric under a
charge-conjugation transformation. Accordingly, at extremely high densities
that existed in the very early Universe, fermions have higher effective masses
than antifermions. As a result, a meson composed of a light quark and a heavy
antiquark has a lower effective mass than its antiparticle. Neutral-meson
oscillations in thermal equilibrium therefore favor the production of light
quarks and heavy antiquarks, which may be related to baryogenesis.
