  Excitons, Coulomb bound electron-hole pairs, are composite bosons and their
interactions in traditional semiconductors lead to condensation and light
amplification. The much stronger Coulomb interaction in transition metal
dichalcogenides such as WSe$_2$ monolayers combined with the presence of the
valley degree of freedom is expected to provide new opportunities for
controlling excitonic effects. But so far the bosonic character of exciton
scattering processes remains largely unexplored in these two-dimensional (2D)
materials. Here we show that scattering between B-excitons and A-excitons
preferably happens within the same valley in momentum space. This leads to
power dependent, negative polarization of the hot B-exciton emission. We use a
selective upconversion technique for efficient generation of B-excitons in the
presence of resonantly excited A-excitons at lower energy, we also observe the
excited A-excitons state $2s$. Detuning of the continuous wave, low power laser
excitation outside the A-exciton resonance (with a full width at half maximum
of 4 meV) results in vanishing upconversion signal.
