Electron–positron pairs in physics and astrophysics: From heavy nuclei to black holes

Abstract

Abstract Due to the interaction of physics and astrophysics we are witnessing in these years a splendid synthesis of theoretical, experimental and observational results originating from three fundamental physical processes. They were originally proposed by Dirac, by Breit and Wheeler and by Sauter, Heisenberg, Euler and Schwinger. For almost seventy years they have all three been followed by a continued effort of experimental verification on Earth-based experiments. The Dirac process, e+e−→2γe+e−→2γ, has been by far the most successful. It has obtained extremely accurate experimental verification and has led as well to an enormous number of new physics in possibly one of the most fruitful experimental avenues by introduction of storage rings in Frascati and followed by the largest accelerators worldwide: DESY, SLAC etc. The Breit–Wheeler process, 2γ→e+e−2γ→e+e−, although conceptually simple, being the inverse process of the Dirac one, has been by far one of the most difficult to be verified experimentally. Only recently, through the technology based on free electron X-ray laser and its numerous applications in Earth-based experiments, some first indications of its possible verification have been reached. The vacuum polarization process in strong electromagnetic field, pioneered by Sauter, Heisenberg, Euler and Schwinger, introduced the concept of critical electric field Ec=me2c3/(eħ). It has been searched without success for more than forty years by heavy-ion collisions in many of the leading particle accelerators worldwide. The novel situation today is that these same processes can be studied on a much more grandiose scale during the gravitational collapse leading to the formation of a black hole being observed in Gamma Ray Bursts (GRBs). This report is dedicated to the scientific race. The theoretical and experimental work developed in Earth-based laboratories is confronted with the theoretical interpretation of space-based observations of phenomena originating on cosmological scales. What has become clear in the last ten years is that all the three above mentioned processes, duly extended in the general relativistic framework, are necessary for the understanding of the physics of the gravitational collapse to a black hole. Vice versa, the natural arena where these processes can be observed in mutual interaction and on an unprecedented scale, is indeed the realm of relativistic astrophysics. We systematically analyze the conceptual developments which have followed the basic work of Dirac and Breit–Wheeler. We also recall how the seminal work of Born and Infeld inspired the work by Sauter, Heisenberg and Euler on effective Lagrangian leading to the estimate of the rate for the process of electron–positron production in a constant electric field. In addition to reviewing the intuitive semi-classical treatment of quantum mechanical tunneling for describing the process of electron–positron production, we recall the calculations in Quantum Electro-Dynamics of the Schwinger rate and effective Lagrangian for constant electromagnetic fields. We also review the electron–positron production in both time-alternating electromagnetic fields, studied by Brezin, Itzykson, Popov, Nikishov and Narozhny, and the corresponding processes relevant for pair production at the focus of coherent laser beams as well as electron-beam–laser collision. We finally report some current developments based on the general JWKB approach which allows us to compute the Schwinger rate in spatially varying and time varying electromagnetic fields. We also recall the pioneering work of Landau and Lifshitz, and Racah on the collision of charged particles as well as the experimental success of AdA and ADONE in the production of electron–positron pairs. We then turn to the possible experimental verification of these phenomena. We review: (A) the experimental verification of the e+e−→2γe+e−→2γ process studied by Dirac. We also briefly recall the very successful experiments of e+e−e+e− annihilation to hadronic channels, in addition to the Dirac electromagnetic channel; (B) ongoing Earth-based experiments to detect electron–positron production in strong fields by focusing coherent laser beams and by electron-beam–laser collisions; and (C) the multiyear attempts to detect electron–positron production in Coulomb fields for a large atomic number Z>137Z>137 in heavy-ion collisions. These attempts follow the classical theoretical work of Popov and Zeldovich, and Greiner and their schools. We then turn to astrophysics. We first review the basic work on the energetics and electrodynamical properties of an electromagnetic black hole and the application of the Schwinger formula around Kerr–Newman black holes as pioneered by Damour and Ruffini. We only focus on black hole masses larger than the critical mass of neutron stars, for convenience assumed to coincide with the Rhoades and Ruffini upper limit of 3.2 M⊙M⊙. In this case the electron Compton wavelength is much smaller than the space–time curvature and all previous results invariantly expressed can be applied following well established rules of the equivalence principle. We derive the corresponding rate of electron–positron pair production and introduce the concept of dyadosphere. We review the recent progress in describing the evolution of optically thick electron–positron plasma in the presence of supercritical electric field, which is relevant both in astrophysics as well as in ongoing laser beam experiments. In particular we review the recent progress based on the Vlasov–Boltzmann–Maxwell equations to study the feedback of the created electron–positron pairs on the original constant electric field. We evidence the existence of plasma oscillations and its interaction with photons leading to energy and number equipartition of photons, electrons and positrons. We finally review the recent progress obtained by using the Boltzmann equations to study the evolution of an electron–positron–photon plasma towards thermal equilibrium and determination of its characteristic timescales. The crucial difference introduced by the correct evaluation of the role of two- and three-body collisions, direct and inverse, is especially evidenced. We then present some general conclusions. The results reviewed in this report are going to be submitted to decisive tests in the forthcoming years both in physics and astrophysics. To mention only a few of the fundamental steps in testing in physics we recall, the setting up of experimental facilities at the National Ignition Facility at the Lawrence Livermore National Laboratory as well as the corresponding French Laser Mega Joule project. In astrophysics these results will be tested in galactic and extragalactic black holes observed in binary X-ray sources, active galactic nuclei, microquasars and in the process of gravitational collapse to a neutron star and also of two neutron stars to a black hole giving rise to GRBs. The astrophysical description of the stellar precursors and the initial physical conditions leading to a gravitational collapse process will be the subject of a forthcoming report. As of today no theoretical description has yet been found to explain either the emission of the remnant for supernova or the formation of a charged black hole for GRBs. Important current progress toward the understanding of such phenomena as well as of the electrodynamical structure of neutron stars, the supernova explosion and the theories of GRBs will be discussed in the above mentioned forthcoming report. What is important to recall at this stage is only that both the supernovae and GRBs processes are among the most energetic and transient phenomena ever observed in the Universe: a supernova can attain an energy of ∼1054 ergs on a timescale of a few months and GRBs can have emission of up to ∼1054 ergs in a timescale as short as a few seconds. The central role of neutron stars in the description of supernovae, as well as of black holes and the electron–positron plasma, in the description of GRBs, pioneered by one of us (RR) in 1975, are widely recognized. Only the theoretical basis to address these topics are discussed in the present report.