| Measuring the energy spectral structure and composition of cosmic rays is one of the important methods to study the origin of cosmic rays.Large High Altitude Cosmic Ray Observatory(LHAASO)is currently the world’s highest altitude,largest scale,and most sensitive cosmic ray detection device.One of its scientific goals is to measure the energy spectrum of cosmic rays within the energy of 1014-1018 eV,build a bridge between space experiments and ground experiments and achieve consistent measurement of the energy spectrum of cosmic rays.The measurement of the structure and composition of the energy spectrum of cosmic rays near the knee region is an important part of achieving this goal.And this work focuses on the measurement of the light component energy spectrum(proton plus helium spectrum)of cosmic rays near the knee region.In this work,we used the combined observation of the wide field-of-view Cherenkov telescope array(WFCTA)and the kilometer square array(KM2A)to measure the light component energy spectrum of cosmic rays near the knee region.Combined data contains the information from two arrays at the same time.The criteria of combined event was studied in order to improve the data quality.Based on the Heitler model,we proposed a new energy reconstruction parameter Ncμ,by combining the Cherenkov light of WFCTA with the number of muons of KM2A.This parameter was proportionate to the primary energy and almost independent of comic ray components.The energy reconstruction resolution of the light component was about 12%with bias 2%around 1 PeV.The difference in the energy bias between proton and iron was less than 8%above 200 TeV.In this work,two component identification parameters were proposed based on the superposition model:No(the ratio of the number of muons and the number of electromagnetic particles measured by KM2A)and Dist0(The angular distance between the gravity of the Cherenkov image and the shower axis measured by WFCTA).Both of them had strong identification power and the correlation between them was weak.Based on these two identification parameters,this paper compared traditional component identification methods and the toolkit for multivariate analysis(TMVA).Under the same compoiient selection efficiency.the TMVA can achieve higher purity of the light components.So the TMVA method was used to select light components of cosmic rays.By studying the impact of different cosmic ray component models and component selection efficiencies on the light component spectrum,the component selection efficiency 60%was used for measurement of the light component spectrum,with corresponding purity of the light component reaching 96%around 1 PeV.With the combined data collected by LHAASO from November 2020 to March 2021,the light component is identified with these parameters and the light component energy spectrum was reconstruted based on the energy reconstruction with the combined parameter Ncμ.By testing the hypothesis of the single power function and the piecewise power function of the energy spectrum,the significance of the knee position of the energy spectrum was about 10σ.The systematic errors eaused by component selection filters,component models,and hadronic interaction models were discussed at the end of the article.Component models aecounted for the most significant proportion of systematic errors.In this stndy piecewise power functions were used to fit the measured results of the QGSJET-Ⅱ-04 hadronic interaction model and the EPOS-LHC hadronic interaction model,respectively.For the QGSJET-Ⅱ-04 hadronic interaction model,the knee position of the light component energy spectrum was E0=2.7±0.2(stat)PeV.and the energy spectral indices before and after the knee were γ1=-2.615±0.007(stat)and γ2=-3.3±0.1(stat),respectively.For the EPOS-LHC hadronic interaction model.the knee position of the light component energy spectrum is E0=2.4±0.2(stat)PeV,and the energy spectrum indices before and after the knee are γ1=-2.580±0.007(stat),γ2=-3.1 ±0.1(stat)respectively. |
