Nonperturbative-transverse-momentum broadening in dihadron angular correlations in $\sqrt{s_{{}_{NN}}}=200\mathrm{GeV}$ proton-nucleus collisions

The PHENIX collaboration has measured high-$p_T$ dihadron correlations in $p+p, p+\mathrm{Al}$, and $p+\mathrm{Au}$ collisions at $\sqrt{s_{{}_{NN}}}=200$ GeV. The correlations arise from inter- and intrajet correlations and thus have sensitivity to nonperturbative effects in both the initial and final states. The distributions of $p_{\mathrm{out}}$, the transverse-momentum component of the associated hadron perpendicular to the trigger hadron, are sensitive to initial- and final-state transverse momenta. These distributions are measured multidifferentially as a function of $x_E$, the longitudinal momentum fraction of the associated hadron with respect to the trigger hadron. The near-side $p_{\mathrm{out}}$ widths, sensitive to fragmentation transverse momentum, show no significant broadening between $p+\mathrm{Au}, p+\mathrm{Al}$, and $p+p$. The away-side nonperturbative $p_{\mathrm{out}}$ widths are found to be broadened in $p+\mathrm{Au}$ when compared to $p+p$; however, there is no significant broadening in $p+\mathrm{Al}$ compared to $p+p$ collisions. The data also suggest that the away-side $p_{\mathrm{out}}$ broadening is a function of $N_{\mathrm{coll}}$, the number of binary nucleon-nucleon collisions, in the interaction. The potential implications of these results with regard to initial- and final-state transverse-momentum broadening and energy loss of partons in a nucleus, among other nuclear effects, are discussed.

Figure 1

A schematic diagram showing a dihadron correlation in the transverse plane. Vector quantities are shown in bold. The red vectors are the two partons, which are acoplanar because of initial-state partonic $k_T$, while the two black vectors are the measured trigger and associated hadron, slightly displaced from the partons because of final-state transverse momentum $j_T$ from fragmentation. The quantities $p_{\mathrm{out}}$ and $x_E$ are shown as blue and green vectors, respectively.

Figure 2

The away-side $p_{\mathrm{out}}$ per-trigger yields are shown in both $p+\mathrm{Au}$ and $p+p$ collisions for several bins of $x_E$. Gaussian fits, shown as dotted lines for $p+\mathrm{Au}$ and solid lines for $p+p$, are shown to the small $p_{\mathrm{out}}$ distributions, highlighting the nonperturbative to perturbative transition.

Figure 3

The near-side $p_{\mathrm{out}}$ per-trigger yields are shown in both $p+\mathrm{Au}$ and $p+p$ collisions for several bins of $x_E$. Gaussian fits, shown as dotted lines for $p+\mathrm{Au}$ and solid lines for $p+p$, are performed at small $p_{\mathrm{out}}$ where nonperturbative behavior is dominant in the $\mathrm{\Delta }\varphi \approx 0$ region.

Figure 4

The Gaussian width differences are shown for the near side (a) and away side (b) between $p+\mathrm{Al}$ and $p+p$ collisions and for the near side (c) and away side (d) between $p+\mathrm{Au}$ and $p+p$ collisions as a function of $x_E$.

Figure 5

The Gaussian width differences between $p+\mathrm{A}$ and $p+p$ are shown in two $x_E$ bins as a function of $N_{\mathrm{coll}}$. Linear fits are shown for each $x_E$ bin, which exhibit a positive dependence with $N_{\mathrm{coll}}$.

Figure 6

The correlation between $x_E$ and $z$ is shown as determined in a pythia simulation for ${\pi }^0$-hadron correlations in the same kinematic regime as measured in the data. The correlation may provide insight into the origins of the inclusive hadron enhancement at moderate $p_T$ in $p+\mathrm{A}$ collisions.