Photoproduction In Pb-Pb Collisions Research Articles

Gluon shadowing at small x from coherent J/ψ photoproduction data at energies available at the CERN Large Hadron Collider

The cross section for coherent $\mathrm{J/}\psi$ photoproduction in Pb-Pb collisions is the sum of two contributions, one from low-, the other from high-energy photon-nucleus interactions. A novel method to disentangle both contributions allowing one to extract the coherent photo-nuclear cross section for coherent $\mathrm{J/}\psi$ production, $\sigma_{\gamma \rm Pb}$, is presented. The utility of the method is demonstrated using measurements from peripheral and ultra-peripheral Pb-Pb collisions at the LHC. Applying the proposed method to the available data it is possible to obtain $\sigma_{\gamma \rm Pb}$ up to a photon-lead centre-of-mass energy of 470 GeV, which corresponds to $x$ of 4.4x10$^{-5}$. To illustrate a possible use of the extracted photo-nuclear cross sections, the corresponding nuclear suppression factors are computed and they are compared to predictions of gluon shadowing calculated in the leading-twist approximation approach.

Measurement of an Excess in the Yield of J/ψ at Very Low p_{T} in Pb-Pb Collisions at sqrt[s]_{NN}=2.76 TeV.

We report on the first measurement of an excess in the yield of J/ψ at very low transverse momentum (p_{T}<0.3 GeV/c) in peripheral hadronic Pb-Pb collisions at sqrt[s_{NN}]=2.76 TeV, performed by ALICE at the CERN LHC. Remarkably, the measured nuclear modification factor of J/ψ in the rapidity range 2.5<y<4 reaches about 7 (2)in the p_{T} range 0-0.3 GeV/c in the 70%-90% (50%-70%) centrality class. The J/ψ production cross section associated with the observed excess is obtained under the hypothesis that coherent photoproduction of J/ψ is the underlying physics mechanism. If confirmed, the observation of J/ψ coherent photoproduction in Pb-Pb collisions at impact parameters smaller than twice the nuclear radius opens new theoretical and experimental challenges and opportunities. In particular, coherent photoproduction accompanying hadronic collisions may provide insight into the dynamics of photoproduction and nuclear reactions, as well as become a novel probe of the quark-gluon plasma.

J/ψ and υ Photoproduction in Pb-Pb Collisions at Large Hadron Collider

Hard photoproduction of J/ψ and T at large hadron collider Pb-Pb collisions with center of mass energy 6300 AGeV is discussed in the process Pb + Pb → Pb + J/ψ(Γ) + X with three types of equivalent photon spectrum function. It turns out that in hard photoproduction the J/ψ cross section is dominated by the gluon and heavy quark fragmentation, especially the gluon fragmentation is very important at large pt, while the T cross section is dominated by the leading order process. It may be conclude from the result that the hard photoproduction processes can be used to test several important problems such as the gluon distribution in the nucleus, gluon and heavy quark fragmentation of J/ψ, and so on. In addition the process also provide a new way of testing the EMC effects.