Suppression of $ψ$(2S) production in p-Pb collisions at $\sqrt{s_{\rm NN}}$ = 5.02 TeV

TL;DR

The paper tackles why the weakly bound charmonium state $ψ(2S)$ experiences stronger suppression than $J/ψ$ in proton-nucleus collisions at LHC energies and whether cold nuclear matter (CNM) effects suffice to explain it. It presents ALICE measurements of inclusive $ψ(2S)$ production in $p$-Pb at $\sqrt{s_{NN}}=5.02$ TeV, using the dimuon channel to determine $\sigma^{\psi(2S)}$ (integrated and vs $p_T$) in forward and backward rapidities, and compares to $J/ψ$ through cross-section ratios, the double ratio relative to pp, and $R_{pPb}^{\psi(2S)}$, all anchored to pp references. The results show a pronounced $ψ(2S)$ suppression not reproduced by models with only shadowing or coherent energy loss, pointing to final-state interactions as a key mechanism. The authors discuss formation and crossing times to argue against simple breakup in CNM at forward rapidity and suggest the final-state hadronic environment as a likely contributor, providing differential $p_T$ dependence analyses to constrain theories with significant implications for interpreting quarkonium production in heavy-ion and CNM studies.

Abstract

The ALICE Collaboration has studied the inclusive production of the charmonium state $ψ(2S)$ in proton-lead (p-Pb) collisions at the nucleon-nucleon centre of mass energy $\sqrt{s_{NN}}$ = 5.02 TeV at the CERN LHC. The measurement was performed at forward ($2.03<y_{cms}<3.53$) and backward ($-4.46<y_{cms}<-2.96$) centre of mass rapidities, studying the decays into muon pairs. In this paper, we present the inclusive production cross sections $σ_{ψ(2S)}$, both integrated and as a function of the transverse momentum $p_{T}$, for the two $y_{cms}$ domains. The results are compared to those obtained for the 1S vector state (J/$ψ$), by showing the ratios between the production cross sections, as well as the double ratios $[σ_{ψ(2S)}/σ_{J/ψ}]_{pPb}/[σ_{ψ(2S)}/σ_{J/ψ}]_{pp}$ between p-Pb and proton-proton collisions. Finally, the nuclear modification factor for inclusive $ψ(2S)$ is evaluated and compared to the measurement of the same quantity for J/$ψ$ and to theoretical models including parton shadowing and coherent energy loss mechanisms. The results show a significantly larger suppression of the $ψ(2S)$ compared to that measured for J/$ψ$ and to models. These observations represent a clear indication for sizeable final state effects on $ψ(2S)$ production.

Figures (8)

• Figure 1: Opposite-sign dimuon invariant mass spectra for the p-Pb (left) and Pb-p (right) data samples, together with the result of a fit. For the fits shown here, Crystal Ball functions (shown as dashed lines) and a variable-width Gaussian have been used for the resonances and the background, respectively. The $\chi^{2}$/ndf refers to the goodness of the signal and background combined fit in the displayed mass range.
• Figure 2: The cross section ratios ${\rm B.R.}_{\psi(\rm 2S)\rightarrow\mu^+\mu^-}\sigma^{\psi(\rm 2S)}/{\rm B.R.}_{{\rm J}/\psi\rightarrow\mu^+\mu^-}\sigma^{{\rm J}/\psi}$ for p-Pb and Pb-p collisions, compared with the corresponding pp results at $\sqrt{s}=7$ TeV Lop14. The horizontal bars correspond to the width of the rapidity regions under study. The vertical error bars represent statistical uncertainties, the boxes correspond to systematic uncertainties.
• Figure 3: Double ratios $[\sigma_{\psi(\rm 2S)}/\sigma_{\rm J/\psi}]_{\rm pPb}/[\sigma_{\psi(\rm 2S)}/\sigma_{\rm J/\psi}]_{\rm pp}$ for p-Pb and Pb-p collisions, compared to the corresponding PHENIX result at $\sqrt{s_{\rm NN}}$ = 200 GeV Ada13. The horizontal bars correspond to the width of the rapidity regions under study. For ALICE, the vertical error bars correspond to statistical uncertainties, the boxes to uncorrelated systematic uncertainties, and the shaded areas to correlated uncertainties. For PHENIX, the various sources of systematic uncertainties were combined in quadrature.
• Figure 4: The nuclear modification factor for $\psi(\rm 2S)$, compared to the corresponding quantity for J/$\psi$Abe14. The horizontal bars correspond to the width of the rapidity regions under study. The vertical error bars correspond to statistical uncertainties, the boxes to uncorrelated systematic uncertainties, and the shaded areas to partially correlated uncertainties. The filled box on the right, centered on $R_{\rm pPb}=1$, shows uncertainties that are fully correlated between J/$\psi$ and $\psi(\rm 2S)$. Model calculations tuned on J/$\psi$, and including nuclear shadowing Alb13 and coherent energy loss Arl13 are also shown. The corresponding calculations for $\psi(\rm 2S)$ produce identical values for the coherent energy loss mechanisms and a 2-3% larger result for nuclear shadowing and therefore are not shown.
• Figure 5: Opposite-sign dimuon invariant mass spectra, in bins of transverse momentum, for the p-Pb and Pb-p data samples. For the fits shown here, Crystal Ball functions (shown as dashed lines) and a variable-width Gaussian have been used for the resonances and the background, respectively. The $\chi^{2}$/ndf refers to the goodness of the signal and background combined fit in the displayed mass range.