Indications of Intermediate-Scale Anisotropy of Cosmic Rays with Energy Greater Than 57 EeV in the Northern Sky Measured with the Surface Detector of the Telescope Array Experiment

We have searched for intermediate-scale anisotropy in the arrival directions of ultrahigh-energy cosmic rays with energies above 57~EeV in the northern sky using data collected over a 5 year period by the surface detector of the Telescope Array experiment. We report on a cluster of events that we call the hotspot, found by oversampling using 20${}^{\circ }$-radius circles. The hotspot has a Li-Ma statistical significance of 5.1$\sigma$, and is centered at R.A.=146.7${}^{\circ }$, Dec.=43.2${}^{\circ }$. The position of the hotspot is about 19${}^{\circ }$ off of the supergalactic plane. The probability of a cluster of events of 5.1$\sigma$ significance, appearing by chance in an isotropic cosmic-ray sky, is estimated to be 3.7$\times$10${}^{-4}$ (3.4$\sigma$).