  We discuss spin-$\frac12$ Heisenberg antiferromagnet on the triangular
lattice using the recently proposed bond-operator technique (BOT). We use the
variant of the BOT which takes into account all spin degrees of freedom in the
magnetic unit cell containing three spins. Apart from conventional magnons
known from the spin-wave theory (SWT), there are novel high-energy collective
excitations in the BOT which are built from high-energy excitations of the
magnetic unit cell. We obtain also another novel high-energy quasiparticle
which has no counterpart not only in the SWT but also in the harmonic
approximation of the BOT. All observed elementary excitations produce visible
anomalies in dynamical spin correlators. We show that quantum fluctuations
considerably change properties of conventional magnons predicted by the SWT.
The effect of a small easy-plane anisotropy is discussed. The anomalous spin
dynamics with multiple peaks in the dynamical structure factor is explained
that was observed recently experimentally in $\rm Ba_3CoSb_2O_9$ and which the
SWT could not describe even qualitatively.

Strongly Correlated Electrons