Performance of the TALE inﬁll experiment as a TA-TALE extension down to the PeV region

. The TALE inﬁll experiment is a further extension of TA-TALE detectors to observe low-energy cosmic rays down to the PeV region. TALE inﬁll utilizes the existing TALE-FD detectors, and newly developed "inﬁll" surface detectors with 100m and 200m spacing. The new detectors will be deployed at the TALE site in October-November 2022. We present the design and performance of the TALE inﬁll array in the hybrid mode, in terms of the resolutions and biases of arrival direction, energy, and X max .


TALE infill : Science goals
This is the first experiment in the world to observe lowenergy cosmic rays using both a Surface Detector (SD) and Fluorescence Detector (FD). By using TALE infill hybrid, we want to accurately measure mass composition in the low energy range, where results vary from experiment to experiment (Fig.1), and unravel the knee structure which is thought to be the energy range where cosmic rays transition from lighter nuclei to heavier nuclei. Therefore, it is necessary to distinguish at least protons and iron in the PeV region.

TALE infill SD array
The TALE-SDs are located northwest of the TA-SDs. To observe cosmic rays with energies lower than the TALE experiment, the TALE infill SDs are deployed closer to the TALE-FD and denser than the TALE-SDs. TALE infill SDs have 100 m spacing area and 200 m spacing area. By creating a high density area, it's possible to observe low energy CRs. By creating a low density area, it's possible to eliminate energy gap between the TALE hybrid experiment. * e-mail: m21sa004@st.osaka-cu.ac.jp * * e-mail: sogio@icrr.u-tokyo.ac.jp * * * e-mail: o22412a@omu.ac.jp * * * * e-mail: tsunesada@omu.ac.jp Figure 2: The layout of the TA detector, the TALE detector, and the TALE infill detector. TALE infill SDs, represented by purple square, are located between TALE-FD and TALE-SDs.

Simulation
We simulated air showers by the CORSIKA for proton and iron primaries. The energy is 10 15.2 eV, 10 15.4 eV, 10 15.6 eV, 10 15.8 eV, 10 16.0 eV, 10 16.2 eV and 10 16.5 eV (fixed for each energy). Zenith angle and Azimuthal angle ranges are 0 • to 60 • and 0 • to 360 • with uniformly random distributions, respectively. The core position is uniformly random distributed within a semi-circle of 1.4 km radius shown in Fig.4. All of the calibration factors with time dependence are applied to SD and FD detector simulations.
We select events that satisfy the quality cuts conditions in Table.1 from those that pass reconstruction, in order to remove poorly reconstructed events and ensure good detector resolution. We define fluorescence events as fractional contribution to the total signal of Fluorescence Light (FL) > 0.75, and Cherenkov events as fractional contribution to the total signal of FL 0.75.

Parameter resolution
The resolution of X max and Energy are shown in Fig.6.  6 Mean X max bias Fig.7 shows thrown X true max without any reconstructions, triggered X true max , reconstructed X true max , and reconstructed X reconstructed max with quality cuts using TALE infill hybrid. The X max bias is estimated from difference between thrown X true max and reconstructed X reconstructed max with quality cuts.The maximum value of the bias os 50 g/cm 2 . The mass composition measurement of CRs at the knee region has been validated from this simulation studies. Figure 7: The X max bias for proton (orange) and iron(blue). These points represent thrown X true max (circle), triggered X true max (square), reconstructed X true max (plus), and reconstructed X reconstructed max with quality cuts (inverse triangle).

Future work
Considering the bias of the parameter resolution and mean X max , the X max resolution is better than 40 g/cm 2 by optimizing the quality cuts.

acknowledgements
The TALE SD production and the TALE hybrid operations are supported by the Japan Society for the Promotion of Science(JSPS) through Grants-in-Aid for Scientific Research (S) 15H05741 and 19H05607; by the joint research program of the Institute for Cosmic Ray Research (ICRR), The University of Tokyo. The experimental site became available through the cooperation of the Utah School and Institutional Trust Lands Administration (SITLA), U.S. Bureau of Land Management (BLM), and the U.S. Air Force. We appreciate the assistance of the State of Utah and Fillmore offices of the BLM in crafting the Plan of Development for the site. The people and the officials of Millard County, Utah have been a source of steadfast and warm support for our work which we greatly appreciate. We gratefully acknowledge the contribution from the technical staffs of our home institutions.