Evidence for nuclear gluon shadowing from the ALICE measurements of PbPb ultraperipheral exclusive J/ψ production

We show that the recent ALICE measurements of exclusive J/psi production in ultraperipheral PbPb collisions at 2.76 TeV provide the first direct experimental evidence for the strong nuclear gluon shadowing in lead at x similar to 10(-3). The evidence is based on the comparison of the nuclear suppression factor S(x approximate to 0.001) = 0.61(-0.04)(+0.05) found in the analysis of the coherent J/psi photoproduction cross sections measured by ALICE with the nuclear gluon shadowing predicted by the global fits of nuclear parton distributions and by the leading twist theory of nuclear shadowing. (C) 2013 Elsevier B.V. All rights reserved.