All-optical nonlinear Breit-Wheeler pair production with $\gamma$-flash photons

High-power laser facilities give experimental access to fundamental strong-field quantum electrodynamics processes. A key effect to be explored is the nonlinear Breit-Wheeler process: the conversion of high-energy photons into electron-positron pairs through the interaction with a strong electromagnetic field. A major challenge to observing nonlinear Breit-Wheeler pair production experimentally is first having a suitable source of high-energy photons. In this paper we outline a simple all-optical setup which efficiently generates photons through the so-called $\gamma$-flash mechanism by irradiating a solid target with a high-power laser. We consider the collision of these photons with a secondary laser and systematically discuss the prospects for exploring the nonlinear Breit-Wheeler process at current and next-generation high-power laser facilities.

Figure 1

Proposed experimental configuration. The total available laser energy $E_{\text{total}}$ is split into two beams: $E_{\text{total}}=E_{\text{flash}}+E_{\text{pairs}}$. The beam with energy $E_{\text{flash}}$ is used to irradiate a solid lithium target, producing high-energy photons via the so-called $\gamma$-flash mechanism [51, 52]. Charged secondary particles from the target are deflected to minimize background. The $\gamma$-flash photons propagate from the rear surface of the lithium target and collide with a counterpropagating secondary pulse, energy $E_{\text{pairs}}$, to produce electron-positron pairs via the nonlinear Breit-Wheeler mechanism [20, 21, 22, 23]. Positrons are emitted at an angle $\vartheta$ relative to the axis of the colliding counterpropagating laser.

Figure 2

Angular distribution of radiant intensity of $\gamma$-flash photons along the laser polarization axis, averaged over the two-lobe angular structure. The $\gamma$-flash is produced by the interaction with the Li solid target and focused laser pulses with peak powers of 5 PW (blue circles), 15 PW (orange squares), and 50 PW (green stars). The shaded region denotes $\beta ={10}^{\circ }$ full-angle divergence, which maximizes the radiant intensity.

Figure 3

The $\gamma$-flash energy spectrum ${\omega }_{\gamma }d{\mathcal{N}}_{\gamma }/d{\omega }_{\gamma }$ generated by lasers with peak powers of 5 PW (blue circles), 15 PW (orange squares), and 50 PW (green stars). Dark solid lines show the energy spectrum over all emission angles. Light dashed lines show the energy spectrum over the peak with full-angle divergence $\beta ={10}^{\circ }$, corresponding to the respective shaded regions in Fig. 2.

Figure 4

Laser-to-photon energy conversion efficiency ${\kappa }_{\gamma }$ versus laser peak power for the full spectrum (solid line) and for the photons emitted within the full-angle divergence $\beta ={10}^{\circ }$ (dashed line).

Figure 5

Total number of produced pairs ${\mathcal{N}}_{e^{-}{e}^{+}}$ as a function of photon-laser interaction point distance $d$ calculated with Eq. (15) for $E_{\text{total}}=170$ J (blue circles), $E_{\text{total}}=510$ J (orange squares), and $E_{\text{total}}=1700$ J (green stars). The legend gives the values of the ratio $\mathrm{\Delta }$ [cf. Eq. (16)].

Figure 6

Comparison of initial photon spectrum $d{\mathcal{N}}_{\gamma }/d{\omega }_{\gamma }$ (solid line) with the differential number of pairs $d{\mathcal{N}}_{e^{-}{e}^{+}}/d{\omega }_{\gamma }$ (dashed line) for (a) $E_{\text{total}}=170$ J and $\mathrm{\Delta }=1$, (b) $E_{\text{total}}=510$ J and $\mathrm{\Delta }=1$, and (c) $E_{\text{total}}=1700$ J and $\mathrm{\Delta }=1$. Black dashed lines denote the value of ${\omega }_{\gamma }$ for which $\text{max}\left[{\chi }_{\gamma }\right]={\eta }_{\gamma }{\xi }_{\text{pairs}}=1$.

Figure 7

Double differential spectra $d^2{\mathcal{N}}_{e^{-}{e}^{+}}/d\vartheta d{E}_q$ for (a) $E_{\text{total}}=170$ J, (b) $E_{\text{total}}=510$ J, and (c) $E_{\text{total}}=1700$ J.

Figure 8

Single differential spectra: (a)–(c) energy spectra $d{\mathcal{N}}_{e^{-}{e}^{+}}/d{E}_q$ and (d)–(f) angular spectra $d{\mathcal{N}}_{e^{-}{e}^{+}}/d\vartheta$ for (a) and (d) $E_{\text{total}}=170$ J and $\mathrm{\Delta }=1$, (b) and (e) $E_{\text{total}}=510$ J and $\mathrm{\Delta }=1$, and (c) and (f) $E_{\text{total}}=1700$ J and $\mathrm{\Delta }=1$.