Pseudorapidity Dependence of Particle Production and Elliptic Flow in Asymmetric Nuclear Collisions of $p+\mathrm{Al}$, $p+\mathrm{Au}$, $d+\mathrm{Au}$, and ${}^3\mathrm{He}+\mathrm{Au}$ at $\sqrt{s_{NN}}=200\mathrm{GeV}$

Asymmetric nuclear collisions of $p+\mathrm{Al}$, $p+\mathrm{Au}$, $d+\mathrm{Au}$, and ${}^3\mathrm{He}+\mathrm{Au}$ at $\sqrt{s_{NN}}=200\mathrm{GeV}$ provide an excellent laboratory for understanding particle production, as well as exploring interactions among these particles after their initial creation in the collision. We present measurements of charged hadron production $d{N}_{\text{ch}}/d\eta$ in all such collision systems over a broad pseudorapidity range and as a function of collision multiplicity. A simple wounded quark model is remarkably successful at describing the full data set. We also measure the elliptic flow $v_2$ over a similarly broad pseudorapidity range. These measurements provide key constraints on models of particle emission and their translation into flow.

Figure 1

Charged hadron $d{N}_{\text{ch}}/d\eta$ as a function of pseudorapidity in high-multiplicity 0%–5% central ${}^3\mathrm{He}+\mathrm{Au}$, $d+\mathrm{Au}$, $p+\mathrm{Au}$, and $p+\mathrm{Al}$ collisions at $\sqrt{s_{NN}}=200\mathrm{GeV}$. Also shown are results in inelastic $p+p$ collisions at $\sqrt{s_{NN}}=200\mathrm{GeV}$ as measured by the PHOBOS Collaboration [27]. Predictions from the wounded-quark [3] and hydrodynamical [4] models are shown. The calculations have an overall normalization factor ($S$) to best match the data. These factors are $S=0.88$, 0.93, 0.85, and 0.77 for the wounded-quark model for $p+\mathrm{Al}$, $p+\mathrm{Au}$, $d+\mathrm{Au}$, and ${}^3\mathrm{He}+\mathrm{Au}$, respectively; and $S=0.81$, 0.96, and 0.75 for the hydrodynamical model for $p+\mathrm{Au}$, $d+\mathrm{Au}$, and ${}^3\mathrm{He}+\mathrm{Au}$, respectively.

Figure 2

Charged hadron $d{N}_{\text{ch}}/d\eta$ as a function of pseudorapidity in various multiplicity classes of $p+\mathrm{Al}$, $p+\mathrm{Au}$, $d+\mathrm{Au}$, and ${}^3\mathrm{He}+\mathrm{Au}$ collisions at $\sqrt{s_{NN}}=200\mathrm{GeV}$. Predictions from the wounded-quark model [3] are shown.

Figure 3

Midrapidity charged hadron $d{N}_{\text{ch}}/d\eta$ per participating quark pair ($N_{qp}/2$) as a function of the number of participating quarks ($N_{qp}$). Results are shown for $p+\mathrm{Al}$, $p+\mathrm{Au}$, $d+\mathrm{Au}$, and ${}^3\mathrm{He}+\mathrm{Au}$ collisions in various multiplicity classes. Also shown are previously published results in $p+p$ collisions from PHENIX [15] and PHOBOS [27]. The line is the best fit to all the data to a constant level.

Figure 4

Elliptic flow $v_2$ as a function of pseudorapidity in high-multiplicity 0%–5% central $p+\mathrm{Al}$, $p+\mathrm{Au}$, $d+\mathrm{Au}$, and ${}^3\mathrm{He}+\mathrm{Au}$ collisions at $\sqrt{s_{NN}}=200\mathrm{GeV}$. Also shown are predictions from the hydrodynamical model [4]. Lastly, the measured $d{N}_{\text{ch}}/d\eta$ results are shown scaled to match the $v_2$ at forward rapidity for shape comparison with the elliptic flow coefficients.