Ultra High Energy Cosmic Rays may include strangelets, a form of Strange Quark Matter, among their components. We briefly review their properties and discuss how they can be accelerated via Penrose process taking place in singular rotating Kerr black holes or in their smooth, horizonless counterparts in string theory, according to the fuzzball proposal. We focus on non-BPS solutions of the JMaRT kind and compute the efficiency of Penrose process that turns out not to be bounded unlike for Kerr BHs.
Introduction and motivationsCosmic rays (CR) and in particular Ultra High Energy CR (UHECR) tend to play an important role in any progress of high-energy physics, from the identification of new elementary particles in the past to the recent confirmation of rare phenomena such as neutrino oscillations. Although the flux is tremendously suppressed at very high energies [1,2,3] and practically ends at the ZeV scale set by the GZK cutoff [4,5], the question remains as for how UHECR can be accelerated up to such high energies.Among the components of UHECR one can include the so-called "strangelets", a form of Strange Quark Matter (SQM) [22,28,30] that can be present in the dense core of a Neutron Star (NS) or in Quark Stars (QS) [23], where the temperature and density may be significantly higher than on the crust. When the mass of a NS exceeds the Chandrasekhar-Oppenheimer-Volkoff bound (around a few solar masses), it becomes unstable wrt gravitational collapse and produces a Black Hole (BH). In turn, BHs may provide both tidal tearing of captured astrophysical objects, including NS and QS, and a powerful acceleration mechanism of UHECR, including strangelets, aka Penrose process [6]. This can take place in rotating (Kerr) BHs surrounded by an 'ergo-region' where a time-like Killing vector becomes space-like. Thanks to Penrose mechanism Kerr/rotating BHs can be used as cosmic slings to accelerate UHECR and reach the GZK cutoff scale. BHs are the epitome of quantum gravity (QG), which is still poorly understood with quantum fieldtheory means. Luckily there is a leading contender: string theory. Based on the idea that point-like particles be replaced by one-dimensional objects, string theory can accommodate gravity (mediated by closed strings) together with gauge interactions (mediated by open strings) in an entirely consistent framework. General Relativity or higher dimensional extensions thereof, coupled to gauge fields and fermions, governs the dynamics at very low energies, compared to the string mass scale. The realization that string theory in addition to fundamental strings admits stable, extended solitons with p spatial dimensions, called p-branes, allows to represent BHs as bound states of strings and branes and to quantitatively address and partly solve some long-standing issues in the physics of BHs [7], including BH production in high-energy collisions [8]. In particular there are classes of charged BPS 1 BHs for which one can precisely count the micro-states responsible for the macroscopic entropy, which accordin...
