Measurement of an excess in the yield of J/$ψ$ at very low $p_{\rm T}$ in Pb-Pb collisions at $\sqrt{s_{\rm NN}}$ = 2.76 TeV

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2.76 TeV
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We report on the first measurement of J/$\psi$ yield excess at low transverse momentum ($p_{T} <$300 MeV/$c$) in heavy-ion collisions, performed by ALICE experiment at the LHC in Pb-Pb collisions at $\sqrt{s_{NN}}$ = 2.76 TeV. Remarkably, the measured nuclear modification factor ($R_{AA}$) of the J/$\psi$ in the rapidity range $2.5 < y < 4.0$ reaches more than 6.92 (1.93) units for Pb-Pb collisions in the $p_{T}$ range 0-300 MeV/$c$ and in the centrality bin 70$\%$-90$\%$ (50$\%$-70$\%$). The measured J/$\psi$ yield is compared to what is expected from proton-proton collisions scaled by the number of binary collisions and assuming a smooth evolution of the J/$\psi$ $R_{AA}$ at $p_{T}$ below 1 GeV/$c$. The J/$\psi$ production cross-section associated to the observed low $p_{T}$ yield excess is obtained under the hypothesis that coherent photo-production of J/$\psi$ is the underlying physics mechanism. The observation of J/$\psi$ coherent photo-production in Pb-Pb collisions at impact parameters smaller than twice the nuclear radius would open new theoretical and experimental challenges. Furthermore such a new J/$\psi$ production mechanism could become a novel signature of the QGP at LHC energies.

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