Performance of LHAASO-WCDA and Observation of Crab Nebula as a Standard Candle

The first Water Cherenkov detector of the LHAASO experiment (WCDA-1) has been operating since April, 2019. The first 10 months of data have been analyzed to test its performance by observing the Crab Nebula as a standard candle. The WCDA-1 achieves the sensitivity of 65 mCU per year with a statistical threshold of 5 $\sigma$. In order to do so, 97.7\% cosmic ray background rejection rate around 1 TeV and 99.8\% around 6 TeV with an approximately photon acceptance about 50\% by using the $compactness$ of the shower footprints to be greater than 10 as the discriminator between gamma induced showers and the cosmic ray backgrounds. The angular resolution is measured using the Crab Nebula as a point source about 0.45$^\circ$ at 1 TeV and better than 0.2$^\circ$ above 6 TeV with the pointing accuracy better than 0.05$^\circ$. They are all matching the design specifications. The energy resolution is found 33\% for gamma rays around 6 TeV. The spectral energy distribution of the Crab Nebula in the range from 500 GeV and 15.8 TeV is measured and in agreement with results of other TeV gamma ray observatories.