Measurement of transverse single-spin asymmetries for dijet production in polarized proton-proton collisions at $\mathbf{\sqrt{s}}$ = 200 $\mathbf{\mathrm{GeV}}$

Abstract

We report a new measurement of transverse single-spin asymmetries for dijet production in collisions of polarized protons at $\sqrt{s}$ = 200 \mathrm{GeV}$. Correlations between the proton spin and the transverse momenta of its partons, each perpendicular to the proton momentum direction, are probed at high $Q^2$ $\approx $160 $\mathrm{GeV}^2$. Evidence for nonzero Sivers effects is measured for the first time in dijets from proton-proton collisions, but only when the jets are sorted by their net charge, which enhances the otherwise canceling opposite-sign $u$- or $d$-quark contributions to separate data samples. The resulting asymmetries are compared to recent theoretical calculations. Separately, the associated Sivers observable $\kt$, the average parton transverse momentum, is extracted using a simple kinematics approach which further enables a determination of the individual partonic contributions to the observed asymmetries.

Figures (5)

• Figure 1: Efficiency for associating beam and jet correctly in the embedding sample vs. $\eta^{total}$, the summed dijet $\eta$.
• Figure 2: The a) distribution of $Q$ and b) respective parton fraction from the embedded simulation. The tagging divides the data sample into 4 bins (separated by the dashed lines).
• Figure 3: The a) $\Delta\langle\zeta\rangle$ values and b) converted $\Delta\langle k_T \rangle$ plotted as a function of $\eta^{total}$. Rightmost points represent the average over the $\eta^{total}$ bins. Individual $0^+$ and $0^-$ points are suppressed in the lower panel to better view the $\Delta\langle k_T \rangle$ signal and systematic errors (dominated by fitting range contributions). Plotted points are offset in $\eta^{total}$ and outsize values omitted for clarity.
• Figure 4: Comparisons of $A_N$ for 200 $\mathrm{GeV}$$pp$ Sivers dijet asymmetries with theoretical predictions from Kang et al. 2021JHEP...02..066K and Liu et al. PhysRevD.102.114012. Measured $\Delta\langle\zeta\rangle$ asymmetries have been converted to $A_N$ via a numerical factor as described in the text (a 10% per point RMS conversion error is not included).
• Figure 5: The $\langle k_T \rangle$ for individual partons, inverted using parton fractions from simulation and tagged $\langle k_T \rangle$, plotted as a function of $\eta^{total}$, with rightmost points the $\eta^{total}$ average. Plotted points are offset in $\eta^{total}$ for clarity, and systematic uncertainties in $\eta^{total}$ are set nonzero to improve visibility.