Head-on collision of ultrarelativistic particles in ghost-free theories of gravity

TL;DR

This paper investigates ghost-free modifications of gravity, deriving regularized potentials, analyzing ultrarelativistic particle fields, and demonstrating a mass gap for mini-black-hole formation during head-on collisions.

Contribution

It provides explicit solutions for gravitational fields in ghost-free theories and establishes conditions for apparent horizon formation and black hole production.

Findings

Newtonian potential is finite and regular in any dimension

Existence of a mass gap for mini-black-hole production

Inner and outer apparent horizons behave differently with increasing energy

Abstract

We study linearized equations of a ghost-free gravity in four- and higher-dimensional spacetimes. We consider versions of such a theory where the nonlocal modification of the $\Box$ operator has the form $\Box \exp[(-\Box/\mu^2)^N]$, where $N=1$ or $N=2n$. We first obtain the Newtonian gravitational potential for a point mass for such models and demonstrate that it is finite and regular in any number of spatial dimensions $d\ge 3$. The second result of the paper is calculation of the gravitational field of an ultrarelativistic particle in such theories. And finally, we study a head-on collision of two ultrarelativistic particles. We formulated conditions of the apparent horizon formation and showed that there exists a mass gap for mini-black-hole production in the ghost-free theory of gravity. In the case when the center-of-mass energy is sufficient for the formation of the apparent horizon, the latter has two branches, the outer and the inner ones. When the energy increases the outer horizon tends to the Schwarzschild-Tangherlini limit, while the inner horizon becomes closer to $r=0$.