Measurement Of Cosmic Ray Mean Mass Around Knee Region By Muon Content In Air Shower

A“knee”structure at a few PeV in the all-particle energy spectrum of cosmic rays observed by many experiments.The physical properties of the all-particle energy spectra of primary cosmic rays in the knee region are essential for testing theoretical hy-potheses of the cosmic ray origin,acceleration and propagation.The mass composition of very high-energy cosmic rays can provide crucial clues about the origin and acceleration mechanisms of these most energetic particles in the universe.The Large High Altitude Air Shower Observatory(LHAASO)project aims to explore the origin of high energy cos-mic rays and observe a large fraction of the northern sky to hunt for gamma-ray sources.The measurement of cosmic ray composition in the knee region is an important research content of the LHAASO project.The LHAASO experiment is installed at high altitude 4410 m a.s.l.with an atmo-spheric depth of about 600 g/cm2 in the Daochen,Sichuan province,P.R.China.The altitude is close to the maximum development of the cosmic rays air showers in the knee region.The high altitude of the observatory enables LHAASO to measure showers close to their maximum depth,which can minimize the shower fluctuations.The LHAASO is a hybrid extensive air shower array,in which electromagnetic detector(ED)and muon de-tector(MD)covering an area of 1 km2 forms the KM2A,which can measure the spectrum of cosmic rays within the energy range 1013?1017 eV.KM2A can measure the size and muon content of an air shower simultaneously with high precision for cosmic rays and the muon detector array is the largest at high altitude in the world.In an extensive air shower,the ? meson and K meson produced by the strong in-teraction decay quickly into muon.However,the muon undergoes fewer atmospheric interactions and carries almost all the information generated to the ground,so the muon content of an extensive air shower is sensitive to the strong interaction model.The muon contents produced by various cosmic ray components in the atmospheric shower is dif-ferent,which is a predominant physical quantity to identify cosmic ray components.The measurement of the muon contents in atmospheric showers to identify cosmic ray compo-nents and test strong interactions model has scientific significance for studying the origin of cosmic rays.The KM2A reconstructs the direction,energy and core position of the primary cos-mic ray through the arrival time of the secondary particle,the signal of ED or MD,and the position of the detector.The Monte Carlo simulation(CORSIKA)is used to realize the cascade shower of high-energy primary cosmic rays entering the atmosphere.We have selected two strong interaction models,EPOS-LHC and QGSJETII-04,which contain 5 cosmic ray components-Proton,He,CNO,MgAlSi and Fe.The primary cosmic ray en-ergy range of the simulation data is from 1013 to 1017 eV that generated according to the energy spectrum index of E-2.The detector simulation part uses the G4KM2A simulation developed based on Geant4.The experimental data is selected from July to August 2020.It is necessary to calibrate the charge and time of each detector by taking into account the different response of the detectors and checking the operating status of each detector.The purpose is to provide better data for physical analysis.To measure the mean mass of cosmic rays in the knee region,the ln(A)derived from the N? which the signal recorded by the muon detector.The N? represents the sum of the number of ? recorded by the unit detector in the 40-200 m ring from the shower core.The purpose of excluding detectors within 40 m from the shower core is to reduce the punch-through effect caused by high-energy electromagnetic particles penetrating the soil.The number of secondary particles Nsize in the cosmic ray shower is calculated by NKG formula.We compare the electromagnetic particles Nsiz and the muon component N? obtained from the experimental and simulated data.The distribution shows that the detector performance is consistent with the model expectation,and the simulation results can also describe the detector better.We construct a new variable N?e based on the Nsize and N? measured at the same time by KM2A to reconstruct the energy,which the energy is weakly dependent on the components of cosmic ray.In this paper,the mean number of muons in air showers is measured by analyzing muon detectors signal in the KM2A quarter-array for cosmic rays with energies from hundreds of TeV to tens of PeV.There is no obvious excess in muon abundance at least up to 30 PeV from comparison with the simulation results of protons and irons.The mean logarithmic mass of cosmic rays as derived from the mean number of muons and the simulation of five cosmic ray components ln(A)and ln(N?)is presented.We find the mean mass of cosmic rays becomes heavy above several PeV.We also compared the differences between different models,which will bring about an 8%difference in the mean mass measurement.The different energy spectrum models will bring about 4%difference in the mean mass measurement.