Christoph Baumann (Johann-Wolfgang-Goethe Univ. (DE)) Markus Kohler (GSI - Helmholtzzentrum fur Schwerionenforschung GmbH (DE))
Direct photons are an important probe of the hot and dense medium created in heavy-ion collisions because they do not interact with the medium and therefore carry information of all stages of the collision. At low momenta, thermal emission, which would provide the ideal measurement of temperature of the quark-gluon plasma, is expected to dominate over other sources of direct photons. However, since the fraction of thermal photon is small, the measurement of direct photons is notoriously difficult due to the large background from hadronic decay photons. These difficulties can be overcome by measuring virtual photons detected as internal conversion into electron-positron pairs. ALICE uses its excellent tracking and particle identification capabilities at very low momenta to perform this measurement in the Pb-Pb collisions collected in 2011 at sqrt(s_NN) = 2.76 TeV. Details of the measurement and status of the analysis will be presented.
Collaboration ALICE (CERN Geneva, Switzerland)