Vacuum Pair Research Articles

Nonadiabatic quantum Vlasov equation in spinor QED

The nonadiabatic quantum Vlasov equation in spinor QED is derived, and its relation to the well-known adiabatic one is established by three different methods. One is by deriving an explicit analytical expression. The second is by utilizing the Dirac equation in the length gauge, and the last is by introducing a weak and slowly varying counterfield. The relations obtained using the first two methods are instantaneous. Moreover, a comparative analysis of the nonadiabatic quantum Vlasov equations in both spinor and scalar QED is performed by investigating the time evolution of the distribution functions for a given momentum and the asymptotic momentum distributions of particles created by an electric field with and without an additional counterfield. It is found that after the external field is turned off, the oscillation periods of the distribution functions in both spinor and scalar QED equal pi divided by the positive eigenvalue of the Hamiltonian. The asymptotic momentum distributions in spinor and scalar QED show a novel out-of-phase behavior that cannot be explained by the conventional Stokes phenomenon. These results will further deepen our understanding of quantum Vlasov equations and their applications in vacuum pair production. Published by the American Physical Society 2024

Vacuum pair production in zeptosecond pulses: Peculiar momentum spectra and striking particle acceleration by bipolar pulses

We examine the phenomenon of electron-positron pair production from vacuum in a combination of two counterpropagating electromagnetic pulses having a duration of the order of the Compton time. We show that in this extreme short-time domain, the momentum distributions of the particles produced possess a peculiar structure which strongly depends on whether the electromagnetic pulses have a unipolar or bipolar profile. It is shown that bipolar pulses can predominantly generate particles with ultrarelativistic velocities along the propagation direction of the pulses, while unipolar ones are generally more favorable in terms of the total particle yield in the same regime. The highly nontrivial properties of the e+e− spectra revealed in our study provide strong experimental signatures paving the way to probe a complex vacuum response within the short-time domain of quantum electrodynamics. Published by the American Physical Society 2024

Kinetic theory of vacuum pair production in uniform electric fields revisited

We investigate the phenomenon of electron-positron pair production from vacuum in the presence of a uniform time-dependent electric field of arbitrary polarization. Taking into account the interaction with the external classical background in a nonperturbative manner, we quantize the electron-positron field and derive a system of ten quantum kinetic equations (QKEs) showing that the previously-used QKEs are incorrect once the external field rotates in space. We employ then the Wigner-function formalism of the field quantization and establish a direct connection between the Dirac-Heisenberg-Wigner (DHW) approach to investigating the vacuum pair-production process and the QKEs. We provide a self-contained description of the two theoretical frameworks rigorously proving their equivalence and present an exact one-to-one correspondence between the kinetic functions involved within the two techniques. Special focus is placed on the analysis of the spin effects in the final particle distributions. Published by the American Physical Society 2024

Dunkl algebra and vacuum pair creation: Exact analytical results via Bogoliubov method

This paper examines the generation of scalar particle pairs resulting from the application of an external electric field's Coulomb potential in both three- and two-dimensional scenarios, employing Dunkl's formalism. In doing this, we present two distinct sets of exact solutions to the Dunkl-Klein-Gordon equation. Subsequently, we employ the canonical approach based on the Bogoliubov transformation to link the “in” and “out” states in order to compute both the probability of pair creation and the density of particle creation. It is shown that the conditions necessary for the creation of pairs of spinless particles depend on several factors: nucleus charge Z, quantum numbers, and Dunkl parameter. It is also shown that the Dunkl formalism enhances particle pair production.

Spin effect induced momentum spiral and asymmetry degree in pair production

Spin effect on the pair production under circularly polarized fields is investigated. Significantly different from what momentum spirals caused by two counterrotating fields with a time delay, we find for the first time that the spirals can also be induced due to the particle spin effect even if in a single field. We further examine the bichromatic combinational fields. The inhomogeneous spiral structures can be observed in the momentum spectrum; in particular, the spiral not only does exist in two cases of spin, but also is about 2 orders of magnitude amplified compared with that in the single field. Meanwhile, the spin asymmetry degree on the momentum distributions is investigated, and it was found that the effect of spin flip exists with an increased time delay between two fields. The spin asymmetry degree on the number density can reach to 98% in certain conditions. These results indicate that the signatures of created particles, especially the spiral structures, are strongly associated with information pertaining to the laser field as well as the created particle spin, which can deepen the understanding of vacuum pair production. Published by the American Physical Society 2024

Pair production in rotating electric fields via quantum kinetic equations: Resolving helicity states

We investigate the phenomenon of electron-positron pair production from vacuum in the presence of a strong electric field of circular polarization. By means of a nonperturbative approach based on the quantum kinetic equations (QKEs), we numerically calculate helicity-resolved momentum distributions of the particles produced and analyze the corresponding helicity asymmetry. It is demonstrated that the external rotating field tends to generate left-handed and right-handed particles traveling in opposite directions. Generic symmetry properties of the momentum spectra are examined analytically by means of the QKEs and also confirmed and illustrated by direct numerical computations. The helicity signatures revealed in our study are expected to provide a firmer basis for possible experimental investigations of the fundamental phenomenon of vacuum pair production in strong fields. Published by the American Physical Society 2024

Enhanced vacuum pair production by combination of two spatially separated electric fields

The effective enhancement of the pair production rate using a subcritical electromagnetic field is an active area of interest in quantum electrodynamics. Enhanced pair production was investigated using the Dirac–Heisenberg–Wigner (DHW) formalism with spatially separated electric fields. Symmetric and asymmetric combinations of two separate electric fields with the same temporal shape were considered. The interplay and dynamical assistance between the fields were examined from a numerical perspective to elucidate the effects of the spatial distance and asymmetry of the field on the pair production efficiency. For the symmetric combination, the total particle number was reduced with an increase in the distance between the two fields up to one Compton length and then remained constant with distance. For the asymmetric combination, the total number of created pairs increased significantly up to a distance of one Compton length. It was also determined that not only was the separation distance crucial for enhancing pair production, but the spatial field asymmetries were also important, both of which had nontrivial effects on the particle momentum distribution. Finally, we have studied the validity of the DHW and analytical approximation in the spatially separated two electric fields. We found that the analytical approximation could almost estimate the DHW results.

Efficient vacuum pair production in frequency chirped electric fields with spatiotemporal inhomogeneity

Vacuum pair production under intense electromagnetic fields is an intriguing prediction in quantum electrodynamics (QED), and has become a topic of considerable interest with the rapid advancements in laser technologies. The pair production efficiency can be enhanced using chirped electric fields. In this study, the effects of electric field chirping on electron–positron pair production in different oscillating electric fields were investigated using the Dirac–Heisenberg–Wigner (DHW) formalism. We primarily considered quadratic and sinusoidal chirped electric fields. The results revealed that chirping significantly influences the momentum spectra of the generated particles in an electric field dominated by low-frequency tunneling effects, resulting in pronounced oscillations of the momentum spectrum, particularly in a sinusoidal chirped field. However, multiphoton processes gradually emerge as the chirp parameter increases. Consequently, in a high-frequency electric field dominated by multiphoton processes, the number density of the generated particles increases with the chirping parameter in a quadratic chirped field, whereas the momentum distribution remains relatively symmetric. However, in a sinusoidal chirped electric field, introducing a chirp parameter immediately alters the momentum distribution and enhances pair production rate. Moreover, there exists an optimal chirp parameter and the total particle number is enhanced approximately more than an order of magnitude with optimal chirping in a sinusoidal chirped electric field. These findings provide valuable insights into the intricate dynamics of pair production under the influence of chirping in different oscillating electric fields, and offer potential directions for future research in this domain.

Exploring the higher-order QED effects on the differential distributions of vacuum pair production in relativistic heavy-ion collisions

Extensive studies have been conducted in the past few decades to investigate potential signatures of higher-order QED effects in high-energy electromagnetic scattering processes. In our previous work, we have identified evidence of higher-order corrections in the total cross-section for the vacuum pair production in relativistic heavy-ion collisions. However, the presence of higher-order QED corrections cannot be unambiguously proven solely based on total cross-section measurements due to substantial experimental and theoretical uncertainties. The objective of this paper is to explore the sensitivity of specific differential observables in the vacuum pair production to higher-order QED effects in high-energy heavy-ion collisions. These investigations will provide guidance in determining the presence or absence of higher-order QED processes by conducting precise measurements in future experiments.

Webs, Nijenhuis operators, and heavenly PDEs

In 1989 Mason and Newman (Commun. Math. Phys. 121 659–68) proved that there is a 1-1-correspondence between self-dual metrics satisfying Einstein vacuum equation (in complex case or in neutral signature) and pairs of commuting parameter depending vector fields which are divergence free with respect to some volume form. Earlier Plebański (1975 J. Math. Phys. 16 2395–402) showed instances of such vector fields depending of one function of four variables satisfying the so-called I or II Plebański heavenly PDEs. Other PDEs leading to Mason–Newman vector fields are also known in the literature: Park (1992 Int. J. Mod. Phys. A 7 1415), Husain (1994 Phys. Rev. Lett. 72 800), Grant (1993 Phys. Rev. D 48 2606–12), Schief (Phys. Lett. A 223 55–62). In this paper we discuss these matters in the context of the web theory, i.e. theory of collections of foliations on a manifold, understood from the point of view of Nijenhuis operators. In particular we show how to apply this theory for constructing new ‘heavenly’ PDEs based on different normal forms of Nijenhuis operators in 4D, which are integrable similarly to their predecessors. Relation with the Hirota dispersionless systems of PDEs and the corresponding Veronese webs, which was recently observed by Konopelchenko–Schief–Szereszewski, is established in all the cases. We also discuss some higher dimensional generalizations of the ‘heavenly’ PDEs and the existence of related vacuum Einstein metrics in 4D-case.

Tribochemistry of TiN films sliding against ceramic counterparts in vacuum condition and N2 atmosphere

With the rapid development of aerospace technology, the tribological performance of moving parts under extreme operating conditions has attracted a great deal of attention and interest. The application of solid lubricant coatings has become a major means of improved performance to ensure stable operation. Although molybdenum disulfide (MoS2) and diamond-like carbon (DLC) films have excellent low coefficients of friction, they are prone to failure in vacuum because they cannot overcome the challenges of assembly in atmospheric environments. Surprisingly, unexpected results were obtained in this study using conventional nitride films. Specifically, the friction coefficient of TiN/SiC friction pair in vacuum is 0.21 and the wear rate is 8.8 × 10−7 mm3/mN. The relatively stable friction coefficient is mainly attributed to the formation of carbonaceous lubricating layer at the interface, which is the decisive factor in reducing wear. The friction coefficient of TiN/WC friction pair under N2 atmosphere is 0.31 and the wear rate is 4.5 × 10−7 mm3/mN. It can be summarized as follows: first, the mechanochemical induced chemical reaction of the interface, and secondly, the thermally excited nitrogen atoms saturate the dangling bonds of the transfer film. The results further reveal the friction mechanism of TiN films with advanced ceramic materials under harsh conditions and suggest a guide for engineering applications.

Dissociation of the Watson-Crick base pairs in vacuum and in aqueous solution: a first-principles molecular dynamics study

The damage of the DNA structure can affect the correct functioning of the cellular processes. This work investigates the required forces to dissociate the Watson-Crick (WC) base pairs AT into A and T, and GC into G and C. The WC base pairs are immersed in water under realistic conditions of temperature, volume, and density that reproduce the main characteristics of a biological system. The simulations are based on first-principles molecular dynamics combined with steering atomic forces. In addition to the force intensities, the charge transfers between the nucleic acid bases, energy variations, and temperature fluctuations in the cleavage moments are reported. With the purpose of evaluating the effects of the aqueous medium, simulations of the WC base pairs in vacuum are included. The results considering the solvated medium are consistent with the experimental measurements, and show the importance of the aqueous solution to regulate the structural modifications of the nucleic acid bases. The investigation contributes with a novel molecular model in molecular simulations, and to better understand the biological processes where the DNA compounds play an active role in life forms. Communicated by Ramaswamy H. Sarma

Collisional strong-field QED kinetic equations from first principles

Starting from nonequilibrium quantum field theory on a closed time path, we derive kinetic equations for the strong-field regime of quantum electrodynamics (QED) using a systematic expansion in the gauge coupling $e$. The strong field regime is characterized by a large photon field of order $\mathcal{O}(1/e)$, which is relevant for the description of, e.g., intense laser fields, the initial stages of off-central heavy ion collisions, and condensed matter systems with net fermion number. The strong field enters the dynamical equations via both quantum Vlasov and collision terms, which we derive to order $\mathcal{O}(e^2)$. The kinetic equations feature generalized scattering amplitudes that have their own equation of motion in terms of the fermion spectral function. The description includes single photon emission, electron-positron pair photoproduction, vacuum (Schwinger) pair production, their inverse processes, medium effects and contributions from the field, which are not restricted to the so-called locally-constant crossed field approximation. This extends known kinetic equations commonly used in strong-field QED of intense laser fields. In particular, we derive an expression for the asymptotic fermion pair number that includes leading-order collisions and remains valid for strongly inhomogeneous fields. For the purpose of analytically highlighting limiting cases, we also consider plane-wave fields for which it is shown how to recover Furry-picture scattering amplitudes by further assuming negligible occupations. Known on-shell descriptions are recovered in the case of simply peaked ultrarelativistic fermion occupations. Collisional strong-field equations are necessary to describe the dynamics to thermal equilibrium starting from strong-field initial conditions.

Discovery of higher-order quantum electrodynamics effect for the vacuum pair production

The higher-order quantum electrodynamics (QED) effect for vacuum pair production has been searched without success since 1954. In this paper, we show that the combined world-wide data of lepton pair vacuum production is about 20% smaller than the latest lowest order QED calculation with a 5.2 sigma-level of significance and is consistent with the corresponding higher-order QED result. We claim the discovery of higher-order effect for the QED pair production, which settles the dust of previous debates for several decades. The verification of higher-order QED effect is a fundamental scientific problem, which is an important milestone towards the nonperturbative and nonlinear regime of QED vacuum.

Chevrel phase-based solid lubrication coating for wear resistant electrocontact friction pairs in vacuum

The work comparatively studies the wear kinetics of copper friction pairs with Chevrel phase-based coating (Cu-Mo-S) and without it in vacuum of 1.5x10−3 Pa. The coating is deposited on test pads by the ion-plasma method. The tribological investigations scheme were used the “disk – coated pad”. The detected falling of the wear rate of the friction pair with the coating is explained by the change of the adhesive wear mechanism to fatigue one.

Exact WKB analysis of the vacuum pair production by time-dependent electric fields

We study the vacuum pair production by a time-dependent strong electric field based on the exact WKB analysis. We identify the generic structure of a Stokes graph for systems with the vacuum pair production and show that the number of produced pairs is given by a product of connection matrices for Stokes segments connecting pairs of turning points. We derive an explicit formula for the number of produced pairs, assuming the semi-classical limit. The obtained formula can be understood as a generalization of the divergent asymptotic series method by Berry, and is consistent with other semi-classical methods such as the worldline instanton method and the steepest descent evaluation of the Bogoliubov coefficients done by Brezin and Izykson. We also use the formula to discuss effects of time-dependence of the applied strong electric field including the interplay between the perturbative multi-photon pair production and non-peturbative Schwinger mechanism, and the dynamically assisted Schwinger mechanism.

Multi-photon enhancement of the Schwinger pair production mechanism under strong FEL radiation

In this work we study the production of electron-positron pairs from vacuum in presence of a field resulting from the collision of two counter-propagating FEL beams that undergo Gaussian amplitude fluctuations. Our work aims to the extension of previous works based on the standing wave hypothesis, by including the inherent stochastic amplitude fluctuations of the individual FEL beams. As shown, depending on the order of the process, a large non-linear enhancement in the number of created pairs can be expected over a big intensity window in the multi-photon regime. Vacuum pair creation in view of future plans on the production of ultra-strong and high energy radiation in FEL facilities is also discussed.

Dark energy and neutrino mass from a transient vacuum particle pair model

We propose that the transient vacuum fluctuations of quantum fields can be represented by transient vacuum quanta where both the lifetime and spatial extent are given by the Heisenberg Uncertainty Principle, HUP. Accordingly, a quantum field fluctuation at any spatial site results in the creation of a transient HUP-limited vacuum pair, corresponding to the particle and the antiparticle associated with that quantum field. All HUP-limited vacuum pairs, both massive and massless, are found to be on-shell particles, and are indistinguishable from each other except for differences related to their exchange symmetry. We find that the net vacuum energy density of all HUP-limited vacuum pairs is identically zero at any energy where both bosonic and fermionic transient vacuum pairs are present. The net vacuum energy density, from zero energy to the Planck limit, is found to be proportional to that of the rest mass of the lightest fermion, the lowest mass neutrino eigenstate. This net energy density is equal to the observed dark energy density if the mass of the lightest neutrino eigenstate is 3.2[Formula: see text]meV. The model predicts the absolute values of the masses of the three neutrino mass eigenstates and of the three neutrino flavor eigenstates for both normal and inverted ordering. The sum of the neutrino masses is found to be below the Planck satellite upper limit of 120[Formula: see text]meV, and the neutrino mass sum for normal ordering is below the most stringent upper bound of 78[Formula: see text]meV, indicating a preference for normal ordering.

Chirp effects on pair production in oscillating electric fields with spatial inhomogeneity

Chirp effects on the vacuum pair creation are studied with space and time dependent electric field models. For rapidly oscillating electric fields, the total particle number enhancement only occurs for larger chirp values and the very large chirp may render the effect from the finite spatial size trivial. For slowly oscillating electric fields, the chirp and carrier phase parameters have crucial effects on the particle momentum distribution. The quasihomogeneous results are analyzed using local density approximation where the consequence of the field strength dependence of the homogeneous results is noticed.

Antifriction lubricant for vacuum

The work presents the results of experimental studies of a solid lubricant based on indium and gallium, for operation in vacuum up to 10–6 mm Hg, having a low melting point and good wettability. This lubricant provides long-term operation of the friction unit with a stable low coefficient of friction at specific loads of up to 2.5 MPa. The composition of the lubricant introduced fine powders of copper and nickel. The design of a vacuum-stand for studying the work of friction pairs in vacuum and gases at pressures up to 10–6 mm Hg, loads up to 2.5 MPa and sliding speed of the samples 0.6 ÷ 2 m/s is described. As a result of experimental studies, it was found that the introduction into the composition of a solid lubricant containing molybdenum disulfide, indium, and gallium, additionally finely dispersed powders of copper and nickel, allows to increase the durability of the friction unit by 3–10 times and reduce the friction coefficient by 1.2–2 times.