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In this letter, measurements of the shared momentum fraction ($z_{\rm{g}}$)and the groomed jet radius ($R_{\rm{g}}$), as defined in the SoftDrop algorihm,are reported in \pp collisions at $\sqrt{s} = 200$ GeV collected by the STARexperiment. These substructure observables are differentially measured for jetsof varying resolution parameters from $R = 0.2 - 0.6$ in the transversemomentum range $15 < p_{\rm{T, jet}} < 60$ GeV$/c$. These studies show that, inthe $p_{\rm{T, jet}}$ range accessible at $\sqrt{s} = 200$ GeV and withincreasing jet resolution parameter and jet transverse momentum, the$z_{\rm{g}}$ distribution asymptotically converges to the DGLAP splittingkernel for a quark radiating a gluon. The groomed jet radius measurementsreflect a momentum-dependent narrowing of the jet structure for jets of a givenresolution parameter, i.e., the larger the $p_{\rm{T, jet}}$, the narrower thefirst splitting. For the first time, these fully corrected measurements arecompared to Monte Carlo generators with leading order QCD matrix elements andleading log in the parton shower, and to state-of-the-art theoreticalcalculations at next-to-leading-log accuracy. We observe that PYTHIA 6 withparameters tuned to reproduce RHIC measurements is able to quantitativelydescribe data, whereas PYTHIA 8 and HERWIG 7, tuned to reproduce LHC data, areunable to provide a simultaneous description of both $z_{\rm{g}}$ and$R_{\rm{g}}$, resulting in opportunities for fine parameter tuning of thesemodels for \pp collisions at RHIC energies. We also find that the theoreticalcalculations without non-perturbative corrections are able to qualitativelydescribe the trend in data for jets of large resolution parameters at high$p_{\rm{T, jet}}$, but fail at small jet resolution parameters and low jettransverse momenta.

Measurement of Groomed Jet Substructure Observables in \pp Collisions at $\sqrt{s} = 200$ GeV with STAR