Speaker
Description
Simone Ragoni$^1$, on behalf of the ALICE Collaboration
$^1$ University of Birmingham, Birmingham, B15 2TT, UK
simone.ragoni@cern.ch
Ultra-peripheral collisions (UPC) are events characterised by large impact parameters between the two projectiles, larger than the sum of their radii. As a consequence, the protons and ions accelerated by the LHC are beyond the reach of the strong interaction and they can be considered as photon sources.
Vector mesons produced in UPC i.e. \rhozero, \jpsi, and \psip, are of particular interest: vector meson photoproduction in UPC is sensitive to the low-$x$ gluon parton density.
As the photons involved in the interactions are \textit{quasireal}, the vector mesons should retain the polarisation of the photon, as postulated by the s-channel helicity conservation hypothesis.
ALICE has provided measurements of the production cross section at forward rapidity for \jpsi and at mid-rapidity for coherent \jpsi, \psip and \rhozero. The collaboration has also measured the $t$-dependence of coherent \jpsi production and compared it with models incorporating nuclear shadowing effects, thus providing a new tool to investigate the gluon structure at low Bjorken-$x$. The measurement of photoproduction accompanied by neutron emission allows us to use a new technique to resolve the ambiguity in Bjorken-$x$ which arises in symmetric A--A UPC collisions.
