A method to estimate efficiency of event start time determination at BESIII is developed. This method estimates the efficiency at the event level by combining the efficiencies of various tracks ($e$, $mu$, $pi$, K, $p$, $gamma$) in a Bayesian way. Ef
ficiencies results and difference between data and MC at the track level are presented in this paper. For a given physics channel, event start time efficiency and systematic error can be estimated following this method.
We developed a least squares fitter used for extracting expected physics parameters from the correlated experimental data in high energy physics. This fitter considers the correlations among the observables and handles the nonlinearity using lineariz
ation during the $chi^2$ minimization. This method can naturally be extended to the analysis with external inputs. By incorporating with Lagrange multipliers, the fitter includes constraints among the measured observables and the parameters of interest. We applied this fitter to the study of the $D^{0}-bar{D}^{0}$ mixing parameters as the test-bed based on MC simulation. The test results show that the fitter gives unbiased estimators with correct uncertainties and the approach is credible.
In 2009, the BESIII experiment has collected about 225M $jpsi$ and 106M $psip$ samples, both of which are the world largest on-peak charmonium production. Based on these dataset, BESIII has made great effort on the study of the charmonium decays, som
e important of which have been reviewed in this proceeding. In addition, a searching for new physics through the $CP/P$ violation process is reported.
