  We present the first inclusive measurements of the invariant and SoftDrop jet
mass in proton-proton collisions at $\sqrt{s}=200$ GeV at STAR. The
measurements are fully corrected for detector effects, and reported
differentially in both the jet transverse momentum and jet radius parameter. We
compare the measurements to established leading-order Monte Carlo event
generators and find that STAR-tuned PYTHIA-6 reproduces the data, while LHC
tunes of PYTHIA-8 and HERWIG-7 do not agree with the data, providing further
constraints on parameter tuning. Finally, we observe that SoftDrop grooming,
for which the contribution of wide-angle non-perturbative radiation is
suppressed, shifts the jet mass distributions into closer agreement with the
partonic jet mass as determined by both PYTHIA-8 and a
next-to-leading-logarithmic accuracy perturbative QCD calculation. These
measurements complement recent LHC measurements in a different kinematic
region, as well as establish a baseline for future jet mass measurements in
heavy-ion collisions at RHIC.
