  Electron spin resonance (ESR) is usually interpreted as a single-particle
phenomenon protected from the effect of many-body correlations. We show that
this is not the case in a two-dimensional Fermi liquid (FL) with spin-orbit
coupling (SOC). Depending on whether the magnetic field is below or above some
critical value, ESR in such a system probes up to three collective chiral-spin
modes, augmented by the presence of the field, or the Larmor mode, augmented
both by SOC and FL renormalizations. We argue that ESR can be used as a probe
not only for SOC but also for many-body physics.
