Abstract
We report on the first measurement of an excess in the yield of at very low transverse momentum () in peripheral hadronic Pb-Pb collisions at , performed by ALICE at the CERN LHC. Remarkably, the measured nuclear modification factor of in the rapidity range reaches about 7 (2) in the range in the 70%–90% (50%–70%) centrality class. The production cross section associated with the observed excess is obtained under the hypothesis that coherent photoproduction of is the underlying physics mechanism. If confirmed, the observation of 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.
- Received 12 October 2015
DOI:https://doi.org/10.1103/PhysRevLett.116.222301
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© 2016 CERN, for the ALICE Collaboration