Analysis of Inclined Air Shower Events Observed by the TA SD and the TAx4 SD

The Telescope Array (TA) experiment has been observing extensive air showers (EAS) induced by ultrahigh-energy cosmic rays (UHECR) since 2008. The TA$\times$4 upgrade aims to expand the detection area of TA at the highest energies to four times its original size with 500 additional surface detectors (SD) and with the nearest neighbor spacing extended from 1.2 km to 2.08 km. Half of the new detectors were installed in 2019 and have been operating since then. The objective of the upgrade is to significantly increase the number of events for analysis. In addition to the physical expansion, TA experiment is also investigating improvements of its reconstruction procedure. In particular, TA has previously accepted events only up to a maximum zenith angle of 55 degrees. In this paper, we describe the results of extending the acceptance to 65 degrees in zenith, which required the energy estimation table generated from Monte Carlo simulation (MC) to be expanded to include events up to 70 degrees. To validate the accuracy of MC, which is crucial for both the energy estimation and for the calculation of detector acceptance, we compared distributions of key parameters between observed and simulated TA$\times$4 data. We confirmed that there was no significant discrepancy between the two, in particular for the extended range of zenith angles. We also show the measured TA$\times$4 SD energy spectrum, including events with large zenith angles, based on three years of observations. Finally, we applied the same extension to the TA SD analysis, generating a new energy estimation table for zenith angles up to 70 degrees.