The linear Breit-Wheeler process (γ+γ→e−+e+) is one of the most basic predictions of quantum electrodynamics, formulated back in 1934. It plays a key role in astrophysics: fast radio bursts, pulsar emission, relativistic shock waves, gamma-ray bursts, and the vicinity of black holes. However, in the laboratory this process has never been observed with real photons because it requires simultaneously dense and energetic photon beams. The development of petawatt lasers with intensities around $10^{22}$ W/cm² has made it possible to create such photons in plasma. Previously proposed schemes suffered from a low signal-to-noise ratio and produced signals on the order of $10^3$–$10^4$ MeV^-1 sr^-1, which is below the detection threshold in typical laser-plasma experiments. This work belongs to a series of studies on quantum decoherence and uncertainty, but it stands out by proposing a concrete signal amplification mechanism rather than just an estimate.
The authors used three-dimensional particle-in-cell (PIC) simulations with the EPOCH code. A laser pulse with a wavelength of 800 nm, a peak intensity of 3×$10^{22}$ W/cm², and a duration of 25 fs was directed at a carbon-hydrogen foam target. They considered uniform targets with densities from 2 to 100 critical densities (nc) and a two-layer target: the first layer at 1 nc, the second at 20 nc. To account for quantum effects, Monte Carlo modules for synchrotron emission and pair production were used. An analytical model described the dynamics of positrons in the azimuthal magnetic field of the plasma, which arises from the longitudinal field gradient in a funnel-shaped channel.
In a uniform target with a density of 2 nc, most positrons unexpectedly moved backward (against the laser propagation), whereas at 100 nc they moved forward. The reason is a strong azimuthal magnetic field generated by the electron current in the transparent plasma. This field not only turns the positrons around but also compresses their angular distribution toward the axis due to the longitudinal field gradient. The analytical model showed that each time a positron crosses the axis, its polar angle decreases, leading to a 'pinching' (compression) of the beam. Numerical diagnostics confirmed that the azimuthal component of the magnetic field, Bφ, dominates the change in angle. To enhance the pair yield, a two-layer target is proposed: the laser first creates the magnetic structure in the low-density layer, then generates many pairs in the dense layer. In this scheme, the total pair yield was 5.5×$10^7$, and the backward signal reached about $10^6$ MeV^-1 sr^-1 within a cone with an opening angle of about 15°. This is 2-3 orders of magnitude higher than in previous single-pulse schemes and falls into the range where positrons have already been detected in laser-plasma acceleration experiments.
The work offers a concrete and relatively simple way to boost the signal of the linear Breit-Wheeler process to a detectable level. Unlike neighboring studies, where the signal was estimated in the direction of laser propagation, here the positrons are turned backward, where the background is much quieter. This reduces the requirements for noise filtering and increases the chances of the first laboratory observation of the process. The mechanism is self-consistent: the plasma itself creates a magnetic lens that focuses the born particles. This approach could also be applied to other pair production schemes.
The proposed scheme can be implemented on existing petawatt laser facilities. The two-layer foam target could be replaced by a pre-plasma created by a prepulse, simplifying the experiment. Further simulations could refine the laser and target parameters to maximize the signal. It is also interesting to investigate how this mechanism works at even higher intensities, where quantum effects become stronger. Possibly, similar self-consistent fields could help in studying other quantum electrodynamics processes in strong fields.
The results will impact experimental physics of strong laser fields, high-energy astrophysics, and tests of quantum electrodynamics under extreme conditions.
Conduct an experiment on a petawatt laser facility with a two-layer target and a positron detector in the backward direction. Optimize the target and laser parameters for maximum pair yield and minimal background.
This work is directly related to the unresolved problem of experimentally observing the linear Breit-Wheeler process in the laboratory. It also touches on the broader question of pair production in strong fields, which is important for understanding astrophysical sources such as pulsar magnetospheres and black holes. The proposed self-consistent positron focusing mechanism could be the key to overcoming the detection threshold.
This section is addressed to the author of the paper and their colleagues. Neighbouring works were found by meaning-based search across our archive, not by word overlap. We do not review: we show the field around the work.
What the work does
The authors propose a scheme for observing the linear Breit-Wheeler process: an ultra-intense laser pulse interacts with a double-layer target made of plastic foam, generating an azimuthal magnetic field in the plasma that turns the produced positrons backward and compresses them into a narrow cone. Using three-dimensional PIC simulations with the EPOCH code and accounting for quantum effects via Monte Carlo modules, they show that the backward signal reaches $$10^6$$ MeV$$^{-1}$$ sr$$^{-1}$$, which is 2-3 orders of magnitude higher than previous schemes and falls within the detectable range.
Where its strength is
The strong point is the specific signal enhancement mechanism via a self-generated magnetic field, which not only increases the positron yield but also collimates them into a narrow cone, crucial for detection.
Where it could go next
Worth trying
Why this may matter
This step matters because it offers a concrete path toward the first laboratory observation of a fundamental process that has so far eluded experimentalists, and it could open a new research area in strong-field QED.
Nearby works in our archive
The archive is sparse around this work: the closest review is only 0.53, against a usual 0.64. It looks like few have drilled nearby.
Positrons born in the plasma are turned 180 degrees and fly toward the laser beam, like swimmers carried back to shore by a current.
The idea of creating matter from light resembles science fiction plots, for example, in Arthur C. Clarke's novel 'The Light of Other Days', where light is used to create matter. In reality, the Breit-Wheeler process is the literal conversion of photon energy into particle mass.