Study On The Characteristic Of PeV Cosmic-ray Large-scale Anisotropy By LHAASO-KM2A

Cosmic rays are high-energy particles from the cosmic space,the main components of which are various atomic nuclei,as well as small amounts of electrons,gamma rays and neutrinos.After being accelerated in the source region,cosmic rays propagate in a complex interstellar magnetic environment arrival to the earth.The cosmic rays are observed with a weak anisotropy on their arrival direction.The anisotropy may be related to the distribution of the cosmic ray source and the influence of the magnetic field.Therefore,the cosmic-ray anisotropy is one of the most important probes to understanding the origin and propagation of cosmic rays.From TeV to PeV,the structure and magnitude of large-scale anisotropy are obviously evolved with the energy increase.Above 100 TeV,a new "excess" large-scale structure,named “inverse anisotropy”,appears toward the direction of the galactic center.Due to the low intensity of the high-energy cosmic rays,it is difficult to measure the high-energy cosmic-ray large-scale anisotropy.At same time,due to the limitations of detector array size,the previous observations for anisotropy are generally below PeV.In addition,because of the different detector type,coverage of the sky area and energy range,the observed results of anisotropy by different experiments are not consistent well at high energies.More reliable results are expected to constraint the existing anisotropy-causation models.The Large High Altitude Air Shower Observatory(LHAASO),located on Mount Haizi in Dacheng,Sichuan Province(4410 m above sea level),consists of the Kilo-Meter Square Array(KM2A),the Water Cherenkov Detector Array(WCDA)and the Wide Field Cherenkov Telescope Array(WFCTA).LHAASO starts on operation during its installation period,and the deployment of the whole array is completed in July 2021.KM2 A has a coverage area of one square kilometer,detecting the cosmic rays from dozens TeV up to 100 PeV.The altitude corresponds to the location of the shower maximum of PeV cosmic rays,which has an advantage compared with the previous experiments in measuring the anisotropy of PeV cosmic rays.In this paper,we analyze the large-scale anisotropy of cosmic rays from dozens TeV to several PeV using KM2A’s data,and the accuracy at this energy is much better than the others.This result provides a reliable observation for the anisotropy studies.