Charmonium and $e^+e^-$ pair photoproduction at mid-rapidity in ultra-peripheral Pb-Pb collisions at $\sqrt{s_{\rm NN}}$=2.76 TeV

ALICE Collaboration

The ALICE Collaboration at the LHC has measured the J/$\psi$ and $\psi^{'}$ photoproduction at mid-rapidity in ultra-peripheral Pb-Pb collisions at $\sqrt{s_{\rm NN}}=2.76$ TeV. The charmonium is identified via its leptonic decay for events where the hadronic activity is required to be minimal. The analysis is based on an event sample corresponding to an integrated luminosity of about 23 $\mu\rm{b}^{-1}$. The cross section for coherent and incoherent J/$\psi$ production in the rapidity interval $-0.9

Figures

Figure 1

d$E$/d$x$ of the positive lepton versus the negative one, as measured by the TPC for J/$\psi$ candidates in the ultra-peripheral Pb-Pb collisions at $\sqrt{s_{\mathrm{NN}} } = 2.76$ TeV in the invariant mass range $2.8< M_{\rm inv}< 3.2$ GeV/$c^{2}$ and $-0.9< \eta< 0.9$. Muon pairs and electron pairs are clearly separated, with the latter showing higher d$E$/d$x$ values.
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Figure 2

Invariant mass distributions for ultra-peripheral Pb-Pb collisions at $\sqrt{s_{\mathrm{NN}} } = 2.76$ TeV and $-0.9< y< 0.9$ for events satisfying the event selection in Table 1, in the invariant mass range $2.2< M_{\rm inv}< 6$ GeV/$c^{2}$. Coherent enriched sample and incoherent enriched sample for muons and electrons are shown. Blue (red) circles (triangles) are opposite-sign (like-sign) pairs. For like-sign pair the penultimate cut in Table 1 is replaced by the request of a same-sign pair. No LS events were found for coherent di-electron events.
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Figure 3

Invariant mass distribution for ultra-peripheral Pb-Pb collisions at $\sqrt{s_{\mathrm{NN}} } = 2.76$ TeV at $-0.9< y< 0.9$ for events satisfying the event selection in Table 1, in the invariant mass interval $2.2< M_{\rm ll}< 6$ GeV/$c^{2}$. Coherent di-electron and di-muon candidates are summed together. For like-sign pair the penultimate cut in Table 1 is replaced by the request of a same-sign pair.
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Figure 4

Di-muon and di-electron $\pt$ distribution for ultra-peripheral Pb-Pb collisions at $\sqrt{s_{\mathrm{NN}} } = 2.76$ TeV and $-0.9< y< 0.9$ for events satisfying the event selection in the invariant mass interval $3.0< M_{\rm inv}< 3.2$ GeV/$c^{2}$ and $2.2< M_{\rm inv}< 3.2$ GeV/$c^{2}$ respectively, with the $\pt$-range extended to $p_{\rm T}< 1$ GeV/$c$ and to $p_{\rm T}< 5$ GeV/$c$. The data points are fitted summing six different Monte Carlo templates: coherent J/$\psi$ production (black), incoherent J/$\psi$ production (red), J/$\psi$s from coherent $\psi^{'}$ decay (light blue), J/$\psi$s from incoherent $\psi^{'}$ decay (violet), $\gamma \gamma$ (green), and J/$\psi$ produced in peripheral hadronic collisions (grey). The solid histogram (blue) is the sum.
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Figure 5

Measured differential cross section of J/$\psi$ photoproduction in ultra-peripheral Pb-Pb collisions at $\sqrt{s_{\mathrm{NN}} } = 2.76$ TeV at $-0.9< y< 0.9$ for coherent a) and incoherent b) events. The error is the quadratic sum of the statistical and systematic errors. The theoretical calculations described in the text are also shown.
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Figure 6

$\gamma\gamma \rightarrow e^+ e^-$ cross section (blue circles) for ultra-peripheral Pb-Pb collisions at $\sqrt{s_{\mathrm{NN}} } = 2.76$ TeV at $-0.9< \eta< 0.9$ for events in the invariant mass interval $2.2< M_{\rm inv}< 2.6$ GeV/$c^{2}$ (top) and $3.7< M_{\rm inv}< 10$ GeV/$c^{2}$ interval (bottom) compared to STARLIGHT simulation (black line). The blue(green) bars show the statistical (systematic) errors, respectively.
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