The origin of ultra-high-energy cosmic rays (UHECRs; ≳ 10 EeV) is unknown. Gamma-rays and neutrinos produced in CR-induced hadronic interactions can serve as the smoking gun pointing back to sources. Motivated by the fact that IceCube-measured diffuse TeV neutrino flux is comparable to Waxman-Bahcall bound derived from the detected UHECR flux, we assume a common origin of UHECRs and TeV neutrinos, and expect TeV hadronic gamma-rays associated with UHECRs as well, the detection probability of which depends on UHECR source density. Here we use LHAASO-WCDA to search for TeV gamma-rays associated with UHECRs. A detailed data analysis based on LHAASO-WCDA sky map and UHECR events detected by Telescope Array results in non-detection of gamma-ray signals. A lower limit is put on the source number density, 𝑛 𝑠 > 10 −3.5 Mpc −3 , with 95% C.L.

IceCube measures a diffuse neutrino flux comparable to the Waxman-Bahcall bound, which suggests the possibility that the ultra-high energy cosmic rays (UHECRs) have a common origin with diffuse high energy neutrinos. We propose high energy gamma-ray and/or neutrino observations toward the arrival directions of UHECRs to search for the sources and test this possibility. We calculate the detection probability of gamma-ray/neutrino sources, and find that the average probability per UHECR of >10 EeV is ∼10% if the sensitivity of the gamma-ray or neutrino telescope is ∼10−12 erg cm−2 s−1 and the source number density is ∼10−5 Mpc−3. Future gamma-ray and neutrino observations toward UHECRs, e.g., by LHAASO-WCDA, CTA, IceCube/Gen2, are encouraged to constrain the density of UHECR sources or even identify the sources of UHECRs.

LHAASO-KM2A is a sub-array of the Large High Altitude Air Shower Observatory (LHAASO) with an area of kilometer squared scale, consisting of 5195 electromagnetic detectors and 1171 muon detectors. Horizontal Air Showers (HAS) are a fundamental tool to detect penetrating particles like neutrinos and to study hadronic interactions. HAS detected at the ground are mainly constituted by secondary muons. In this contribution first observations of HAS with half array of LHAASO-KM2A are reported. We show that the zenith angle distribution of extensive air showers (EAS) and the transition from electromagnetic-dominated showers to muon-dominated ones above a zenith angle of 60 • .

The origin of cosmic ray (CR) spectral knee around ∼4 PeV is unknown. Composition studies around the knee can provide information on the CR sources and propagation. LHAASO-KM2A is capable of simultaneously measuring the EM and muonic components in extensive air showers. The triggered events of LHAASO-KM2A are dominated by CRs of higher energies at larger zenith angles because of the increasing atmospheric attenuation. In this work, we use the KM2A observed spectrum of muon-to-electron number ratio to constrain CR composition around the knee. The muon-to-electron number ratio spectra are compared between Monte Carlo (MC) simulation and experiment data over a wide zenith angle range. We report a MC excess at large zenith angles, which sets an upper limit on CR Fe flux at PeVs. Systematic uncertainties of interaction models and atmospheric models are briefly discussed.