Forward $J/\psi$ production at high energy: Centrality dependence and mean transverse momentum

Forward rapidity $J/\psi$ meson production in proton-nucleus collisions can be an important constraint of descriptions of the small-$x$ nuclear wave function. In an earlier work we studied this process using a dipole cross section satisfying the Balitsky-Kovchegov equation, fit to HERA inclusive data and consistently extrapolated to the nuclear case using a standard Woods-Saxon distribution. In this paper we present further calculations of these cross sections, studying the mean transverse momentum of the meson and the dependence on collision centrality. We also extend the calculation to backward rapidities using nuclear parton distribution functions. We show that the parametrization is overall rather consistent with the available experimental data. However, there is a tendency towards a too strong centrality dependence. This can be traced back to the rather small transverse area occupied by small-$x$ gluons in the nucleon that is seen in the HERA data, compared to the total inelastic nucleon-nucleon cross section.

Figure 1

Nuclear modification factor $R_{\mathrm{pPb}}$ at negative rapidity as a function of $Y$ (left) and $P_{\perp }$ (right) at $\sqrt{s_{NN}}=5\mathrm{TeV}$. Data are from Refs. [8, 9, 10].

Figure 2

Forward to backward ratio in proton-lead collisions as a function of $Y$ (left) and $P_{\perp }$ (right) at $\sqrt{s_{NN}}=5\mathrm{TeV}$. Data are from Refs. [8, 9].

Figure 3

Nuclear modification factor $Q_{\mathrm{pPb}}$ as a function of $P_{\perp }$ at $\sqrt{s_{NN}}=5\mathrm{TeV}$ in different centrality bins compared with ALICE data [12].

Figure 4

Left: saturation scale of the lead nucleus in the ${\mathrm{MV}}^e$ parametrization as a function of the impact parameter compared with the saturation scale of the proton at $x={10}^{-4}$. Right: number of binary collisions in proton-lead collisions as a function of the impact parameter in the optical Glauber model.

Figure 5

$Q_{\mathrm{pPb}}$ as a function of $P_{\perp }$ at $\sqrt{s_{NN}}=5\mathrm{TeV}$ in the 60%–80% centrality class, both when using a fixed impact parameter and when integrating explicitly over $b$ using the $N_{\text{coll}}$ distribution obtained in two different models. Data are from Ref. [12].

Figure 6

Nuclear modification factor $Q_{\mathrm{pPb}}$ as a function of $N_{\text{coll}}$ at $\sqrt{s_{NN}}=5\mathrm{TeV}$ compared with ALICE data [12], both when using a fixed impact parameter and when integrating explicitly over $b$.

Figure 7

Mean transverse momentum as a function of $Y$ in proton-proton collisions at $\sqrt{s}=7\mathrm{TeV}$ (left) and in proton-lead collisions at $\sqrt{s_{NN}}=5\mathrm{TeV}$ (right). Data are from Ref. [2].

Figure 8

Average transverse momentum squared $⟨P_{\perp }^2{⟩}_{\mathrm{pPb}}$ (left) and nuclear transverse momentum broadening $⟨P_{\perp }^2{⟩}_{\mathrm{pPb}}-⟨P_{\perp }^2{⟩}_{\mathrm{pp}}$ (right) as a function of $N_{\text{coll}}$ at $\sqrt{s_{NN}}=5\mathrm{TeV}$ compared with ALICE data [12]. For the latter both the result using a fixed impact parameter and that when integrating explicitly over $b$ are shown.

Figure 9

Ratio of the proton-proton cross section at $\sqrt{s}=13$ and 8 TeV as a function of $Y$ (left) and $P_{\perp }$ (right) compared with LHCb data [6].

Figure 10

Nuclear modification factor $R_{\mathrm{pPb}}$ at $\sqrt{s_{NN}}=5$ and 8 TeV as a function of $Y$ (left) and $P_{\perp }$ (right). Data are from Refs. [8, 9, 10].