Production of Highly Polarized Positron Beams via Helicity Transfer from Polarized Electrons in a Strong Laser Field

The production of a highly polarized positron beam via nonlinear Breit-Wheeler processes during the interaction of an ultraintense circularly polarized laser pulse with a longitudinally spin-polarized ultrarelativistic electron beam is investigated theoretically. A new Monte Carlo method employing fully spin-resolved quantum probabilities is developed under the local constant field approximation to include three-dimensional polarization effects in strong laser fields. The produced positrons are longitudinally polarized through polarization transferred from the polarized electrons by the medium of high-energy photons. The polarization transfer efficiency can approach 100% for the energetic positrons moving at smaller deflection angles. This method simplifies the postselection procedure to generate high-quality positron beams in further applications. In a feasible scenario, a highly polarized (40%–65%), intense (${10}^5–{10}^6$/bunch), collimated (5–70 mrad) positron beam can be obtained in a femtosecond timescale. The longitudinally polarized positron sources are desirable for applications in high-energy physics and material science.

Figure 1

Scenarios of generation of a LSP ultrarelativistic positron beam via an ultraintense laser pulse head-on colliding with a counterpropagating LSP electron beam. First, longitudinal polarization (helicity) is transferred from electron to photon during NCS, then, from high-energy photon to positron through NBW process, as shown in the inset.

Figure 2

Angular distribution of number density ${\log }_{10}(d^2{N}_{e+}/d{\theta }_{x}d{\theta }_y)({\mathrm{mrad}}^{-2})$ (a), Longitudinal polarization $P_{\parallel }=-{\overline{S}}_z$ (b), and transverse polarization degree $|P_{\perp }|=\sqrt{{{\overline{S}}_x}^2+{{\overline{S}}_y}^2}$ for $P_{\perp }=({\overline{S}}_x,{\overline{S}}_y)$ (c), vs positron deflection angles of ${\theta }_x=p_x/p_z$ and ${\theta }_y=p_y/p_z$. (d): $P_{\parallel }$ (black-solid), $P_{\perp }$ (blue dash-dotted), and number density ${\log }_{10}(d{\tilde{N}}_{e+}/d{\theta }_y)({\mathrm{mrad}}^{-1})$ (cyan-dotted) vs ${\theta }_y$, for positrons with ${\theta }_x$ into [$-20$, 20]. Here, $\mathrm{d}{\tilde{N}}_{e+}/d{\theta }_y={\int }_{-20}^{20}{d}^2{N}_{e+}/(d{\theta }_{x}d{\theta }_y)d{\theta }_x$; ${\overline{S}}_x=0$ after averaged over [$-20$, 20], $P_{\perp }={\overline{S}}_y$. (e): $P_{\parallel }$ (black-solid line) and ${\log }_{10}(d{N}_{e+}/d{ϵ}_{+})({\mathrm{GeV}}^{-1})$ (cyan dotted) vs positron-energy ${ϵ}_{+}$. The red stars indicate positrons with deflection angle $\theta =\sqrt{{\theta }_x^2+{\theta }_y^2}$ within 5, 10, and 20 mrad, from right to left, respectively.

Figure 3

(a) Circular polarization of photons $P_{\parallel }^{\gamma }=-{\overline{\xi }}_2$ vs the energy ratio parameter ${\delta }_{\gamma }={\omega }_{\gamma }/{ϵ}_i$, from NCS. (b) Longitudinal polarization of positrons $P_{\parallel }$ vs the energy ratio parameter ${\delta }_{+}={ϵ}_{+}/{\omega }_{\gamma }$, from NBW of an initial photon beam with ${\omega }_{\gamma }=10\mathrm{GeV}$: calculated numerically including (black-dash-dotted) or excluding radiation reaction (RR) effect (blue-solid, using the instantaneous ${\chi }_{e,\gamma }$ parameter), and analytically (red-dashed, employing the constant average value of ${\overline{\chi }}_e=0.97$ or ${\overline{\chi }}_{\gamma }=4.45$). The other parameters are the same with those in Fig. 2. (c) Normalized field components of $E_{x^{\prime }}$ (grey solid), $E_{y^{\prime }}$ (cyan dotted) and vector potential $A_{y^{\prime }}$ (red dash dotted), vs laser phase $\eta -{\eta }_{+}$, with a positron created at ${\eta }_{+}$ (marked with blue point). The blue arrow represents the spin is antiparallel to ${\widehat{\mathbf{e}}}_{y^{\prime }}$. (d) Average energy ${\overline{ϵ}}_{+}$ vs ${\theta }_y$, for positrons into $|{\theta }_x|\le 20\mathrm{mrad}$. Panels (c) and (d) refer to the simulation case in Fig. 2.