Online monitoring of the center-of-mass energy from real data at BESIII

Background Data Quality Monitoring system (DQM) was developed to monitor data quality at BESIII experiment in real time. The stable center-of-mass energy ( E_\mathrmcms ) is essential for the data taking. Online monitoring the E_\mathrmcms can help find the beam energy shift in time.
Purpose The purpose is to monitor the E_\mathrmcms in DQM system in real time.
Methods The E_\mathrmcms is measured using Bhabha scattering process in DQM system, due to its large cross section and low background level. The E_\mathrmcms is calculated from the invariant mass of e^+e^- , and the correction value from radiation effect and momentum calibration.
Result The E_\mathrmcms 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 from signal extraction and 2.54 MeV/c ^2 from calibration and radiation correction.
Conclusions The E_\mathrmcms 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.