Multiple parton interactions and forward double pion production in pp and dA scattering

TL;DR

The paper investigates double-parton scattering as a significant contributor to forward two-pion production at RHIC, comparing leading-twist and double-scattering mechanisms in pp and dA collisions. It employs a simplified 2pGPD-inspired framework with an effective interaction area to estimate double-parton contributions, and demonstrates that these interactions become especially important at forward rapidities and are markedly enhanced in dA due to geometric and nuclear effects. The authors argue that double-scattering largely explains the pedestal observed in pion azimuthal correlations and profoundly affects the interpretation of backward-peak suppression via shadowing and energy loss, while also offering a new window into double quark distributions in nucleons. The work highlights forward two-pion measurements as a potential probe of multi-parton dynamics and correlations, with implications for understanding nuclear effects and initial-state interactions in high-energy collisions.

Abstract

We estimate the contributions by double-parton interactions to the cross sections for pp->pi^0 pi^0 X and dA->pi^0 pi^0 X at RHIC. We find that such contributions become important at large forward rapidities of the produced pions. This is in particular the case for dA scattering, where they strongly enhance the azimuthal-angular independent "pedestal" component of the cross section, providing a natural explanation of this feature of the RHIC dA data. We argue that the discussed processes open a window to studies of double quark distributions in nucleons. We also briefly address the roles of shadowing and energy loss in dA scattering, which we show to affect the double-inclusive pion cross section much more strongly than the single-inclusive one. We discuss the implications of our results for the interpretation of pion azimuthal correlations.

Figures (6)

• Figure 1: Distributions of the leading-power LO cross section for $pp\to \pi^0\pi^0X$ (see Eq. (\ref{['eq1']})) at $\sqrt{S}=200$ GeV in momentum fraction $x_a$, for $\eta_1=2$ (left) and $\eta_1=3$ (right). We have chosen $p_{T,1}=2.5$ GeV and integrated over $1.5~\mathrm{GeV}\leq p_{T,2}\leq p_{T,1}$ and various bins in $\eta_2$. Units are arbitrary. The inserts show the corresponding normalized distributions on a linear scale.
• Figure 2: Comparison of the leading-twist cross section for $pp\to \pi^0\pi^0X$ (solid, see Eq. (\ref{['eq1']})) and the double-interaction contribution estimated from Eq. (\ref{['eq4']}) (dashed), as functions of $p_{T,1}$ (left) and $\eta_1$ (right). The plots in the upper row are for $1.5< \eta_2 < 2$, the ones in the lower row for $2.5< \eta_2 < 4$. For all plots we have chosen $1.5~\mathrm{GeV}<p_{T,2}<p_{T,1}$. The inserts in each plot show the ratio $r_{pp}$ of the double-interaction contribution to the leading-twist one, see Eq. (\ref{['eq7']}).
• Figure 3: As Fig. \ref{['MIfig']}, but at $\sqrt{S}=500$ GeV and for $2~\mathrm{GeV}<p_{T,2}<p_{T,1}$ and $1.5< \eta_2 < 4$.
• Figure 4: Contributions to two-pion production in $dA$ collisions through the double-interaction mechanism.
• Figure 5: Ratio $r_{dA}$ (defined in Eq. (\ref{['rdef']})) of double-parton and leading-twist contributions in $dA\to\pi^0\pi^0X$. The plots in the upper row are for $1.5< \eta_2 < 2$, the ones in the lower row for $2.5< \eta_2 < 4$. For all plots we have chosen $1.5~\mathrm{GeV}<p_{T,2}<p_{T,1}$. The inserts show the ratios $r_a/r_c$ and $r_c/r_b$.