Searches for correlation between UHECR events and high-energy gamma-ray Fermi-LAT data

The astrophysical sources responsible for ultra high-energy cosmic rays (uhecrs) continue to be one of the most intriguing mysteries in astrophysics. we present a comprehensive search for correlations between high-energy (greater than or similar to 1 Gev) gamma-ray events from the fermi large area telescope (LAT) and uhecrs (greater than or similar to 60 Eev) detected by the telescope array and the pierre auger observatory. we perform two separate searches. first, we conduct a standard cross-correlation analysis between the arrival directions of 148 uhecrs and 360 gamma-ray sources in the second catalog of hard fermi-LAT sources (2fhl). second, we search for a possible correlation between uhecr directions and unresolved fermi-LAT gamma-ray emission. for the later, We use three different methods: a stacking technique with both a model-dependent and model-independent background estimate, and a cross-correlation function analysis. we also test for statistically significant excesses in gamma rays from signal regions centered on cen A and the telescope array hotspot. No significant correlation is found in any of the analyses performed, except a weak (less than or similar to 2 sigma ) hint of signal with the correlation function method on scales similar to 1 degrees . upper limits on the flux of possible power-law gamma-ray sources of uhecrs are derived.