Reconstruction of Higgs -- gamma gamma in CMS

The mass resolution of Higgs decaying into two photons in the centralrapidity region of the CMS electromagnetic calorimeter for m_H=100 GeV has been simulated for the low luminosity phase of LHC. Emphasis is put on events with a photon converting in the material in front of the electromagnetic calorimeter. The events where a photon converts after the measuring layers of the tracker do not degrade the energy measurement noticeably. Including these events, the mass resolution is shown to be 0.66 GeV. In the technical proposal tracker design, 18.4% of events have a conversion in the tracker, 53% of which can be recovered with a simple algorithm. If a more sophisticated algorithm is applied for the remaining events 87% of conversion events can be recovered with only a small increase in the mass resolution.