Mini Black Holes in the Atmosphere?
April 2021
Before the biggest proton accelerator on Earth - the Large Hadron Collider (LHC) at CERN - was activated, scientists speculated that the particle collisions may lead to the creation of tiny black holes [1]. In Einstein's theory of general relativity, the minimal energy required to create such a black hole should be close to the so-called Planck energy. This is at least a trillion times larger than the energies reached at the LHC or in collisions of ultra-high-energy cosmic rays with the atmosphere. However, there are theories in which gravitational interactions are modified at subatomic distances so that the energy necessary to create a mini black hole can be achieved at the LHC or in collisions of cosmic rays with nuclei in the atmosphere.
What happens with such tiny black holes? Could these be stopped inside the Earth, accrete the surrounding material and eventually destroy the Earth?
In 1975 Stephen Hawking speculated [2] that black holes radiate due to quantum fluctuations. The vacuum is not totally empty according to quantum field theory, but is instead a very dynamical medium with pairs of particles and anti-particles appearing and disappearing all the time. The evaporation of a black hole can be visualized as a process in which a pair of a particle and its anti-particle is created near the black hole, such that one of them falls into the black hole while the other one escapes. This process is so quick that for a black hole with a mass of about 1 TeV (one thousand times heavier than the proton), the evaporation time is shorter than the time necessary for light to cross an atom [3]. When a cosmic ray interacts within the Earth's atmosphere, it produces a shower of particles, but within this framework it can also produce a black hole. If such a black hole was created by a collision of a cosmic ray within the atmosphere, it would immediately evaporate by emitting ordinary particles such as protons, pions, electrons, gamma rays, neutrinos, etc. These are the same particles created in the ordinary air-showers resulting from such collisions. As a result, the mini black hole creation event would be lost inside an ordinary air-shower induced by a cosmic ray.
Although Hawking radiation is a generally accepted theoretical concept with several independent cross-checks [3], it is still possible that some aspect of the black hole physics near the horizon has been overlooked and the mini black holes can survive for longer times. If such a mini black hole travels several kilometers in the atmosphere before it completely evaporates, it would create a second shower at the end of its lifetime below the original impact point of the cosmic ray in the atmosphere. This would then produce a "di-shower event" and could be detected by telescopes such as H.E.S.S.. Interestingly, evaporating mini black holes are not the only potential source for this exotic signal. If the collision of a high-energy cosmic ray with the atmosphere produces a new meta-stable particle, this particle could travel several kilometers, and eventually decay into ordinary particles producing a sub-shower. A search for di-shower events hence simultaneously probes several exotic scenarios for new physics.
Searches for di-shower events mainly have to contend with the muon background. These events can also be created by a high-energy muon produced in a collision of a cosmic ray with a nucleus, if the muon undergoes a deep inelastic scattering off a proton or a neutron several kilometers below the start of the original shower. A muonic di-shower event typically has a much brighter upper shower than the lower one - we will call this a normal di-shower event. The inverted di-shower events, for which the lower shower is brighter than the upper one, are very unlikely for the muonic events. These inverted di-shower events would then potentially point to mini black holes or new particles, if the production of the new particle or the black holes is on-resonance, so that most of energy in the collision goes into the mass of the new object.
The H.E.S.S. collaboration has performed a search for the inverted di-shower events in the archival data since the beginning of the mission. One such candidate event from April 1, 2020 was identified (Figure 1). The probability that this event comes from a muonic deep inelastic scattering is about 0.004% (or 4.1 sigma) as derived from Monte Carlo simulations. A further study of inverted and partially inverted di-shower events is on-going. This study should answer the question, whether the observed inverted di-shower event is a signature of a new particle or is instead due to a high energy muon. In view of a possible new particle discovery, we suggest the name for the new hypothetical particle (in case it is not a black hole): aprilion.
References:
[1] Peskin, M.E., 2008, Physics Online Journal, 1, 14
[2] Hawking, S.W., 1975, Commun. Math. Phys. 43, 199
[3] Giddings, S.B. and Mangano, M.L., 2008, Phys. Rev. D 78, 035009