Ultra-peripheral Pb-Pb collisions highlighted in the EPC J front cover

Panos Charitos

For a third time in 2013, ALICE results were selected for the cover of EPJC. November's issue of EPJC featured a figure showing ALICE measurements of cross sections of the γγ -> e+ e- from ultra-peripheral Pb-Pb collisions for events in the invariant mass interval 2.2. inv 2 at mid rapidity [1].

In ultra-peripheral Pb-Pb collisions hadronic interactions are strongly suppressed as the two ions pass by each other with large impact parameters. The electromagnetic field surrounding these protons and ions can be treated as a source of quasi real photos. This allows the study of photon-photon and photon-hadron interactions at a novel kinematic regime [2].

ALICE measured this cross section in the invariant mass interval 3.7 inv 2. Results from STARLIGHT simulations are represented for comparison. ALICE seems to indicate that models predicting a strong contribution of higher-order terms (not included in STARLIGHT) to the cross section are not favoured.

In this same publication ALICE reported new results on exclusive photoproduction of J/ψ at mid-rapidity in ultra-peripheral Pb-Pb collisions, for both coherent and incoherent J/ψ photoproduction. These results add to the existing measurement [3] which are believed to be a good probe of the nuclear gluon distribution. The ALICE UPC results have also triggered lots of interest by the theory community.

In the next issue of ALICE Matter, Joakim Nystrand and Daniel Tapia Takaki, who are the ALICE UPC physics analysis coordinators, will discuss the status and future directions on UPC analyses in pp, p-Pb and Pb-Pb collisions at LHC.

Further reading

1. A. abbas et al. (ALICE Collaboration) Eur. J. Phys. C73 2617 2. Using the LHC as a photon collider, CERN Courier, 6 November 2012 http://cerncourier.com/cws/article/cern/51147 3. B. Abelev et al. (ALICE Collaboration) Phys.Lett. B718 (2013) 1273-1283

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