Background Data Quality Monitoring system (DQM) was developed to monitor data quality at BESIII experiment in real time. The stable center-of-mass energy ( Ecms E cms ) is essential for the data taking. Online monitoring the Ecms E cms can help find the beam energy shift in time. Purpose The purpose is to monitor the Ecms E cms in DQM system in real time. Methods The Ecms E cms is measured using Bhabha scattering process in DQM system, due to its large cross section and low background level. The Ecms E cms is calculated from the invariant mass of e⁺e⁻ e + e - , and the correction value from radiation effect and momentum calibration. Result The Ecms E cms calculated with DQM system shows a good consistency with that from offline reconstruction within error. The results are validated with data taken in 2013. The overall systematic uncertainty includes 0.39 MeV/c ² 2 from signal extraction and 2.54 MeV/c ² 2 from calibration and radiation correction. Conclusions The Ecms E cms calculated from Bhabha scattering process is now available on DQM system in real time, which can be used as references for researchers to operate BESIII experiment.
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