Polarized Positron Beams via Intense Two-Color Laser Pulses

The generation of ultrarelativistic polarized positrons during the interaction of an ultrarelativistic electron beam with a counterpropagating two-color petawatt laser pulse is investigated theoretically. Our Monte Carlo simulation, based on a semiclassical model, incorporates photon emissions and pair productions, using spin-resolved quantum probabilities in the local constant field approximation, and describes the polarization of electrons and positrons for the pair production and photon emission processes, as well as the classical spin precession in between. The main reason for the polarization is shown to be the spin asymmetry of the pair production process in strong external fields, combined with the asymmetry of the two-color laser field. Employing a feasible scenario, we show that highly polarized positron beams, with a polarization degree of $\zeta \approx 60\%$, can be produced in a femtosecond timescale, with a small angular divergence, $\sim 74\mathrm{mrad}$, and high density, $\sim {10}^{14}{\mathrm{cm}}^{-3}$. The laser-driven polarized positron source raises hope for providing an alternative for high-energy physics studies.

Figure 1

Scheme of laser-based polarized positron beam production. An intense linearly polarized two-color laser pulse collides head-on with an unpolarized relativistic electron beam, resulting in emission of $\gamma$ photons in a forward direction which decay into polarized $e^{+}$ and $e^{-}$, with spin parallel and antiparallel to the laser’s magnetic field direction, respectively, and with a small divergence angle in the propagation direction.

Figure 2

(Top row) Angular distribution $d^{2}N/(d\theta d\varphi )$, with the polar angle $\theta$ (rad), and the azimuthal angle $\varphi$ (rad) (a) for seed electrons $e_0^{-}$, (b) for produced electrons $e^{-}$, and (c) for positrons $e^{+}$. (Middle row) The averaged spin distribution along the magnetic field direction ${\overline{\zeta }}_y$ vs $\theta$ and $\varphi$ (d) for $e_0^{-}$, (e) for $e^{-}$, and (f) for $e^{+}$. (Bottom row) The particle number distribution $dN/d\theta$ (solid blue lines), and the average spin component $\overline{{\zeta }_y}$ (dashed-dotted red lines): (g) for $e_0^{-}$, (h) for $e^{-}$, and (i) for $e^{+}$. In (i), $dN/d\theta$ (green line) and $\overline{{\zeta }_y}$ (magenta line) are for $e^{+}$ at the production points [the beam average of $\overline{{\zeta }_y}$ are indicated on panels (g)–(i); the beam average of $\overline{{\zeta }_y}$ at the production points is ${\overline{\zeta }}_{y0}=0.6$]; ${\xi }_0=83$, ${\epsilon }_0=2\mathrm{GeV}$.

Figure 3

(a) Averaged spin ${\overline{\zeta }}_y$ vs laser cycle $\psi$: in a one-color laser pulse, for seed electrons $e_0^{-}$ (solid green line) and positrons $e^{+}$ (dashed-dotted blue line); in a two-color laser pulse, $e_0^{-}$ (dotted pink line) and $e^{+}$ (dashed red line). (b) Positron number $dN/d\psi$ vs $\psi$ for one- (solid blue line) and two-color (dashed red line) laser pulses. (c) Photon number $d{N}_{\gamma }/d\psi$ emitted by $e_0^{-}$ (solid blue line) at emission points and positron number $d{N}_{+}/d\psi$ (dashed red line) at production points. (d) Ratio of electron radiation probability with spin antiparallel to the magnetic field direction (solid blue line), and ratio of pair production probability with the positron spin parallel to magnetic field direction (dashed-dotted red line); ${\xi }_0=83$, ${\epsilon }_0=2\mathrm{GeV}$. The faint dotted line is the two-color laser field.

Figure 4

(a),(d) The averaged polarization ${\overline{\zeta }}_y$. (b),(e) FWHM of ${\theta }_{+}$ and (c),(f) the positron number $N_{+}$. (Top row) $R=1$ (solid blue lines), $R=2$ (dashed red lines), and $R=3$ (dotted-dashed green lines) vs laser peak intensity ${\xi }_0$ for ${\epsilon }_0=2\mathrm{GeV}$. (Bottom row) ${\xi }_0=83$ (solid blue lines), ${\xi }_0=117$ (dashed red lines), and ${\xi }_0=150$ (dotted-dashed green lines) for $R=2$. The number of initial electrons is $N_{-}=2\times 10^5$.