Rapidity and transverse-momentum dependence of the inclusive J/$\mathbfψ$ nuclear modification factor in p-Pb collisions at $\mathbf{\sqrt{\textit{s}_{NN}}}=5.02$ TeV

TL;DR

This paper analyzes the rapidity and transverse-momentum dependence of inclusive J/ψ production in p–Pb collisions at √sNN = 5.02 TeV to study cold nuclear matter effects. J/ψ yields are measured via e+e− decays at mid-rapidity and μ+μ− decays at forward/backward rapidities, enabling differential cross sections d²σJ/ψ/dy dpT and mean pT across three ycms regions. The nuclear modification factor RpPb is derived by interpolating the pp reference, revealing suppression at forward and mid-rapidity that decreases with pT, while backward rapidity shows no significant suppression. Comparisons to CNM models (shadowing, coherent energy loss, CGC) show fair agreement overall but tensions at forward rapidity and low pT, with implications for interpreting Pb–Pb results and possibly signaling charm-quark recombination at low pT in hot nuclear matter.

Abstract

We have studied the transverse-momentum ($p_{\rm T}$) dependence of the inclusive J/$ψ$ production in p-Pb collisions at $\sqrt{s_{\rm NN}} = 5.02$ TeV, in three center-of-mass rapidity ($y_{\rm cms}$) regions, down to zero $p_{\rm T}$. Results in the forward and backward rapidity ranges ($2.03 < y_{\rm cms} < 3.53$ and $-4.46 <y_{\rm cms}< -2.96$) are obtained by studying the J/$ψ$ decay to $μ^+μ^-$, while the mid-rapidity region ($-1.37 < y_{\rm cms} < 0.43$) is investigated by measuring the ${\rm e}^+{\rm e}^-$ decay channel. The $p_{\rm T}$ dependence of the J/$ψ$ production cross section and nuclear modification factor are presented for each of the rapidity intervals, as well as the J/$ψ$ mean $p_{\rm T}$ values. Forward and mid-rapidity results show a suppression of the J/$ψ$ yield, with respect to pp collisions, which decreases with increasing $p_{\rm T}$. At backward rapidity no significant J/$ψ$ suppression is observed. Theoretical models including a combination of cold nuclear matter effects such as shadowing and partonic energy loss, are in fair agreement with the data, except at forward rapidity and low transverse momentum. The implications of the p-Pb results for the evaluation of cold nuclear matter effects on J/$ψ$ production in Pb-Pb collisions are also discussed.

Figures (7)

• Figure 1: Charged particle specific energy loss (${\rm d} E/{\rm d}x$) as a function of momentum, as measured in the TPC in p--Pb collisions. The black lines are the corresponding Bethe-Bloch parametrizations for the various particle species.
• Figure 2: Opposite-sign dielectron invariant mass spectra (blue symbols) for various $p_{\rm T}$ intervals, compared to the background (black curve) estimated through mixed events. The background is scaled to match the data in the mass ranges $2.0<m_{{\rm e}^+{\rm e}^-}<2.5$ GeV/$c^2$ and $3.2<m_{{\rm e}^+{\rm e}^-}<3.7$ GeV/$c^2$.
• Figure 3: The opposite-sign dimuon invariant mass spectra for the various $p_{{\mathrm T}}$ bins, relative to the p--Pb data sample (blue symbols). The fits shown in this Figure (blue curves) were performed by using the sum of extended Crystal Ball functions for the J/$\psi$ and $\psi$(2S) signals, and a variable width Gaussian for the background. The signal and background components are shown separately as red curves.
• Figure 4: $p_{{\mathrm T}}$-differential inclusive J/$\psi$ cross sections for the various rapidity regions under study. The vertical error bars correspond to the statistical uncertainties, while open boxes represent the uncorrelated systematic uncertainties and the shaded boxes the quadratic sum of the fully and partially correlated ones. The numerical values can be read in Table \ref{['tab:2']}. The horizontal bars correspond to the widths of the $p_{\rm T}$ bins.
• Figure 5: The J/$\psi$ nuclear modification factor as a function of $p_{{\mathrm T}}$ at backward (top), mid (center) and forward (bottom) rapidities. Statistical uncertainties are represented by vertical error bars, while open boxes correspond to uncorrelated uncertainties and the shaded areas to uncertainties partially correlated in $p_{{\mathrm T}}$. The boxes around $R_{\rm pPb}=1$ show the size of the correlated uncertainties. The horizontal bars correspond to the widths of the $p_{{\mathrm T}}$ bins. Results from various models are also shown, including a pure shadowing calculation Albacete:2013ei based on the EPS09 parameterization, a CGC-inspired model Fujii:2013gxa, and the results of the coherent energy loss calculation Arleo:2013zua, with or without the inclusion of an EPS09 shadowing contribution.