Initial State Parton Broadening and Energy Loss Probed in d+Au at RHIC

TL;DR

This work extends perturbative QCD calculations of hadron production in d+Au collisions by incorporating initial-state elastic energy loss in addition to transverse momentum broadening. Using a Glauber geometry and a k_T-broadening framework, the authors predict the centrality- and rapidity-dependent nuclear modification factor R_dAu(p_T), illustrating modest midrapidity enhancement and pronounced rapidity-dependent features tied to small-x shadowing and large-x EMC/energy-loss effects. They introduce a concrete energy-loss implementation via shifts in parton momentum fractions, controlled by an opacity parameter chi and transport coefficients, and compare shadowing scenarios to explore cold nuclear-matter constraints and their energy dependence. The results offer a means to distinguish initial-state vs final-state explanations for jet quenching in heavy-ion collisions and to constrain the gluon antishadowing/EMC region in nuclear PDFs.

Abstract

The impact parameter and rapidity dependence of the Cronin effect for massless pions in $d+Au$ reactions at $\sqrt{s}_{NN}=200$ GeV at RHIC is computed in the framework of pQCD multiple elastic scattering on a nuclear target. We introduce a formalism to incorporate initial state energy loss in perturbative calculations and take into account the elastic energy loss in addition to the transverse momentum broadening of partons.We argue that the centrality dependence of the Cronin effect can distinguish between different hadron production scenarios at RHIC. Its magnitude and rapidity dependence are shown to carry important experimental information about the properties of cold nuclear matter up to the moderate- and large-$x$ antishadowing/EMC regions.

Figures (4)

• Figure 1: The ratio of $A$-scaled $p+W/p+Be$ data on $\pi^+,\pi^-$ production at $\sqrt{s}=27.4, 38.8$ GeV from Cronin:zm. Calculations are for ${ \frac{1}{2}} (\pi^+ + \pi^-$): top/bottom panel include/do not include anti-shadowing/EMC effect as described in the text. The anticipated $\sqrt{s}=200$ GeV $p+W/p+Be$ ratio is shown to illustrate the energy dependence of the Cronin effect.
• Figure 2: The Cronin effect, represented by the nuclear modification ratio $R_{dAu}(p_T)$, for ${ \frac{1}{2}}(\pi^+ + \pi^-)$ is shown for the total invariant cross section and impact parameters $b=0,6$ fm.
• Figure 3: Illustration of the difference between the computed small $\sim 5-10\%$ Cronin enhancement at $p_T\geq 5$ GeV in central reactions and the argued Gallmeister:2002us factor of $30-50\%$ suppression. The estimated $\sqrt{R_{AA}(p_T)}$ for ${ \frac{1}{2}}(h^+ + h^-)$ and $\pi^0$ from preliminary STAR and PHENIX data Adcox:2001jp and the $N_{part}/2$ scaling for $b=0,6$ fm are shown.
• Figure 4: Rapidity dependence of the Cronin effect in $d+Au$ reactions at $\sqrt{s}_{NN}=200$ GeV with and without antishadowing/EMC effect. The result of switching off elastic energy loss is also shown via the upper bands for $y=\pm 3$.