The origin of ultra-high-energy cosmic rays (UHECRs; $\gtrsim10$ 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, $n_s > 10^{-3.5} \, \mathrm{Mpc}^{-3}$, with 95% C.L.