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Precise in situ calibration of the CMS electromagnetic calorimeter (ECAL) will be of crucial importance to fully exploit the physics reach of the CMS detector. In particular a precise calibration is needed to fully benefit from the excellent energy resolution of the calorimeter, approaching 0.5% for high energy unconverted photons. The resolution for photons is particularly important for the discovery of the Higgs boson in the two-photon decay channel. An inter-calibration technique based on low-mass resonance decays, p^(0)(?) ? ?? has been developed with simulated events, and was successfully tested with p^(0)(?) ? ?? decays collected at a CERN test beam. To accommodate the high rate of such decays at the LHC, a dedicated High Level Trigger (HLT) has been developed to select and store the events that can be used by this calibration algorithm. In this paper we describe this online procedure, including the regional unpacking of ECAL data and the selection procedure to be performed on the CMS HLT online computing farm, as well as the final calibration to be performed on the CERN Analysis Facility. In a low-luminosity scenario of the LHC, L = 2·10^(30) cm^(-2) s^(-1), we estimate that the majority of the barrel electromagnetic calorimeter can be inter-calibrated to 0.5% (1%) precision with p^(0)(?) ? ?? decays after a few weeks of data-taking.

Computer architecture --- Data transmission systems

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