Measurement of dijet transverse momentum imbalance and azimuthal acoplanarity in $p$+$p$ collisions at $\sqrt{s}=200$ GeV with the sPHENIX detector

Abstract

This Letter reports on measurements of dijet transverse momentum ($p_{\mathrm{T}}$) imbalance and azimuthal acoplanarity in proton-proton collisions at $\sqrt{s}=200$~GeV, using data recorded by the sPHENIX detector at the Relativistic Heavy Ion Collider corresponding to an integrated luminosity of $41$~pb${}^{-1}$. Jets are reconstructed using the anti-$k_t$ algorithm with radius parameters $R=0.3$ to $0.8$ from electromagnetic and hadronic calorimeter energy deposits. The jet $p_{\mathrm{T}}$ resolution is determined directly in data using two independent methods. The dijet $p_{\mathrm{T}}$ imbalance is characterized by the ratio $x_{\mathrm{J}}=p_{\mathrm{T},2}/p_{\mathrm{T},1}$ where $p_{\mathrm{T},1(2)}$ is the highest (second-highest) jet $p_{\mathrm{T}}$ in the event. The dijet azimuthal acoplanarity $\hat{I}”\ddot{I}•=|\ddot{I}{•}_1-\ddot{I}{•}_2|$ is also reported. Results are reported for different $p_{\mathrm{T},1}$ selections and jet radius parameters, normalized per dijet pair, and compared to the results of \textsc{Pythia} and \textsc{Herwig} Monte Carlo event generators. These measurements provide a stringent quantitative test of the modeling of QCD parton shower and hadronization dynamics, place important constraints on event-generator descriptions at RHIC energies, and establish a comprehensive proton-proton baseline for forthcoming measurements of jet modification in heavy ion collisions.

19 pages total, 8 figures, 1 tables. All figures and tables can be found at https://www.sphenix.bnl.gov/PublicResults/sPH-JET-2026-01