Dihadron momentum imbalance and correlations in d+Au collisions

TL;DR

This work develops a perturbative QCD framework to quantify cold nuclear matter effects on back‑to‑back dijet and dihadron production in p+A/d+A collisions by computing the nuclear‑induced broadening Δ⟨q_⊥^2⟩ from initial‑ and final‑state double scattering using twist‑4 correlations, and extends it to dihadron production with fragmentation. The imbalance is expressed as a ratio of twist‑4 matrix elements T_{b/A}^{(I,F)} to LO cross sections with hard parts H^I, H^F, H^U, and then combined with dynamical shadowing and cold nuclear matter energy loss to describe RHIC measurements by PHENIX and STAR across mid‑ and forward rapidities, showing good agreement. The results connect the observed away‑side broadening and dihadron suppression to a consistent cold nuclear matter baseline, constrained by DIS data, and provide a framework for interpreting dihadron observables in p+A/d+A collisions. This has implications for baselining hot‑nuclear effects in heavy‑ion collisions and for understanding parton propagation in nuclei.

Abstract

We calculate in perturbative QCD the transverse momentum imbalance of dijet and dihadron production in high energy p+A (d+A) collisions. We evaluate the effect of both initial- and final-state multiple scattering, which determines the strength of this transverse momentum imbalance. Combining this new result with the suppression of the cross section in d+Au collisions, which arises from cold nuclear matter energy loss and coherent power corrections, we are able to describe the dihadron correlations measured by both PHENIX and STAR collaborations at RHIC, including mid-mid, mid-forward, and forward-forward rapidity hadron pairs.

Figures (6)

• Figure 1: Left: Feynman diagram for the partonic $qq'\to qq'$ scattering channel. Right: initial-state double scattering Feynman diagram that contributes to the dijet transverse momentum imbalance increase.
• Figure 2: Final-state double scattering Feynman diagrams that contribute to the dijet transverse momentum imbalance increase, for the partonic channel $qq'\to qq'$.
• Figure 3: Sample diagrams that contribute to the dijet transverse momentum imbalance for the partonic channels $qg\to qg$ (left two) and $gg\to gg$ (right two), where (a) and (c) are for initial-state double scattering, (b) and (d) are for final-state double scattering.
• Figure 4: The width $\sigma_F$ of the away-side correlation is plotted as a function of the associated hadron transverse momentum $p_{\perp,\rm assoc}$. Both hadrons are at mid-rapidity $|y_{1,2}|<0.35$. The upper panel is for $3<p_{\perp,\rm trig}<5$ GeV and the lower panel is for $5<p_{\perp,\rm trig}<10$ GeV. Solid curves represent the width calculated from $\langle q_\perp^2\rangle_{dA}=\langle q_\perp^2\rangle_{pp}+\Delta\langle q_\perp^2\rangle$ and include multiple scattering effect. Dashed are calculated from $\langle q_\perp^2\rangle_{pp}$. Data is from PHENIX Adler:2005ad.
• Figure 5: Nuclear modification factor $R_{dA}^{(2)}$ for back-to-back dihadron production in d+A collisions. Solid curves contain both dynamical shadowing and energy loss effect, whereas dashed curves contain only dynamical shadowing. Both top and bottom panels have an associated particle in the forward-rapidity $3.0<y_2<3.8$ and $1.0<p_{\perp,\rm assoc}<1.5$ GeV region. From left to right there are four groups corresponding to a mean binary collision number $\langle N_{\rm coll}\rangle$=3.2, 6.6, 10.2, and 15.1, respectively, even though they have been offset from these actual values for visual clarity. Top panel: trigger particle at mid-rapidity $|y_1|<0.35$. Within each centrality selection, from left to right, the transverse momentum of the trigger particle is in the [1, 2], [2, 4], [2.5, 3], [3, 4], and [4, 7] GeV interval. Bottom panel: trigger particle at forward rapidity $3.0<y_1<3.8$. Within each centrality selection, from left to right, the transverse momentum of the trigger particle is in the [1.1, 1.6], [1.6, 2.0], and [2.0, 5.0] GeV interval. Data is from PHENIX Adare:2011sc.