Coherent $ψ$(2S) photo-production in ultra-peripheral Pb-Pb collisions at $\sqrt{s}_{\rm NN}$ = 2.76 TeV

We have has performed the first measurement of the coherent $\psi$(2S) photo-production cross section in ultra-peripheral Pb-Pb collisions at the LHC. This charmonium excited state is reconstructed via the $\psi$(2S) $\rightarrow l^{+}l^{-}$ and $\psi$(2S) $\rightarrow$ J/$\psi \pi^{+}\pi^{-}$ decays, where the J/$\psi$ decays into two leptons. The analysis is based on an event sample corresponding to an integrated luminosity of about 22 $\mu\rm{b}^{-1}$. The cross section for coherent $\psi$(2S) production in the rapidity interval $-0.9<~y<~0.9$ is $\mathrm{d}\sigma_{\psi{\rm(2S)}}^{\rm coh}/\mathrm{d}y =0.83\pm 0.19\big(\mathrm{\rm{stat}+{\rm syst}}\big)$ mb. The $\psi$(2S) to J/$\psi$ coherent cross section ratio is $0.34^{+0.08}_{-0.07}(\rm{stat}+{\rm syst})$. The obtained results are compared to predictions from theoretical models.

Figures

Figure 1

Invariant mass (left) and $p_{T}$ distributions (right) for ultra-peripheral Pb-Pb collisions at $\sqrt{s_{\mathrm{NN}} } = 2.76$ TeV and $-0.9 < y < 0.9$ forevents satisfying the event selections in Sec.3 The channels $\psi(\rm{2S})$ $\rightarrow l^{+}l^{-}$ are shown on the top panel ($l^+l^-=e^{+}e^{-}$ and $\mu^+\mu^-$), the $\psi(\rm{2S})$ $\rightarrow \pi^{+}\pi^{-}\mu^{+}\mu^{-}$ channel is shown in the central and the channel $\psi(\rm{2S})$ $\rightarrow \pi^{+}\pi^{-}e^{+}e^{-}$ in thebottom one. The number of event is obtained by the fit (top panel) or by event counting in a selected invariantmass region (central and bottom panel), see text.
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Figure 2

Measured differential cross section of $\psi(\rm{2S})$ photo-production in Pb--Pb ultra-peripheral collisions at $\snn$=2.76 TeV at $-0.9 < y< 0.9$ in three different channels. The square represents the systematic uncertainties while the bar represents the statistic uncertainty The combined cross section uncertainty (shaded area) was obtained using the prescription from reference [33].
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Figure 3

Measured differential cross section of $\psi(\rm{2S})$ photo-productionin ultra-peripheral Pb-Pb collisions at $\snn$=2.76 TeV at $-0.9< y< 0.9$ The uncertainty was obtained using the prescription from reference [33]. The theoretical calculations described in the text are also shown.
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Figure 4

Ratio of the $\psi(\rm{2S})$ to $\jpsi$ cross section for pp and $\gamma$p interactions compared to theoretical predictions. The ALICE ratio measured in Pb-Pb collisions is shown as well. The uncertainty was obtained using the prescription from reference [33]
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Figure 5

Ratio of the $\psi(\rm{2S})$ to $\jpsi$ cross section measured by ALICE in Pb-Pb collisions. The uncertainty was obtained using the prescription from reference [33]. The predictions from different theoretical models are also shown.
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