Acceleration of particles by Janis-Newman-Winicour singularities

We examine here the acceleration of particles and high-energy collisions in the Janis-Newman-Winicour (JNW) spacetime, which is an extension of the Schwarzschild geometry when a massless scalar field is included. We show that while the center of mass energy of collisions of particles near the event horizon of a black hole is not significantly larger than the rest mass of the interacting particles, in an analogous situation, it could be arbitrarily large in the JNW spacetime near the naked singularity. The high-energy collisions are seen to be generic in the presence of a photon sphere in the JNW spacetime, whereas an extreme fine-tuning of the angular momentum of the colliding particles is required when the photon sphere is absent. The center of mass energy of collision near the singularity grows slowly for small and extremely large deviations from the Schwarzschild black hole, but for intermediate strengths of the scalar field it rises moderately fast. As a possible and potentially interesting application, we point out that the presence of such high-energy collisions may help the black hole configurations to be distinguished from a naked singularity.