  The first study of the angular distribution of $\mu^+ \mu^-$ pairs produced
in the forward rapidity region via the Drell-Yan reaction $pp \rightarrow
\gamma^{*}/Z +X \rightarrow l^+ l^- + X$ is presented, using data collected
with the LHCb detector at a centre-of-mass energy of 13TeV, corresponding to an
integrated luminosity of 5.1 $\rm{fb}^{-1}$. The coefficients of the five
leading terms in the angular distribution are determined as a function of the
dimuon transverse momentum and rapidity. The results are compared to various
theoretical predictions of the $Z$-boson production mechanism and can also be
used to probe transverse-momentum-dependent parton distributions within the
proton.
