All-Optical Generation of Dense, Multi-GeV, Longitudinally-Polarized Positron Beams

TL;DR

The work tackles the challenge of producing dense, longitudinally polarized positron beams for high-energy physics and demonstrates an all-optical approach that combines generation, acceleration, and spin control in a single laser–electron collision. A head-on interaction between an ultraintense circularly polarized laser and an unpolarized GeV electron beam seeds a QED cascade via nonlinear Breit–Wheeler pair production, yielding positrons born inside the strong laser field. These newborn positrons are captured by radiation reaction, directly accelerated to multi-GeV energies through direct laser acceleration, and experience spin precession that converts initial transverse polarization into longitudinal polarization. Simulations report a yield of about 25.8 e^+/e^-, average longitudinal polarization around 46.8% (peaking beyond 70%), and energies reaching up to ~9 GeV, indicating feasibility at upcoming ultraintense-laser facilities and offering a compact path toward polarized positron sources for future colliders and fundamental experiments.

Abstract

The production of high-yield, longitudinally polarized positron beams represents an outstanding challenge in advanced accelerator science. Laser-driven schemes offer a compact alternative but typically yield only transverse polarization, or require pre-polarized electron beams, and struggle to efficiently accelerate positrons to high energies. Here, we introduce an all-optical scheme that overcomes these limitations by integrating positron generation, acceleration, and spin manipulation in a unified framework. Through a head-on collision between an ultraintense, circularly polarized laser pulse and a counterpropagating unpolarized electron beam, we drive a robust QED cascade. The nonlinear Breit-Wheeler process within the cascade produces positrons that are born directly within the strong laser field. Crucially, these positrons are instantaneously captured and accelerated to multi-GeV energies (up to $\sim$9 GeV) via a direct laser acceleration mechanism, while their spins are simultaneously rotated to longitudinal alignment by the field dynamics. Our Monte-Carlo simulations confirm the simultaneous achievement of a high positron yield ($\sim$20 $e^+/e^-$), a high average longitudinal polarization ($\sim$50\%), and GeV-scale energies. This all-optical source, feasible at upcoming ultraintense laser facilities, presents a compact and efficient solution for applications in collider physics and fundamental high-energy experiments.

Figures (5)

• Figure 1: Generation of longitudinally polarized positrons via collision of a counterpropagating ultraintense laser pulse and unpolarized electron beam. NBW pair production generates $e^+e^-$ pairs from $\gamma$-photons created by NCS. Newborn positrons, initially propagating along $-z$, are reflected and backward-accelerated by the laser field. During their spiral trajectory, spin precession induced by the phase-dismatched field efficiently converts transverse polarization into longitudinal polarization.
• Figure 2: (a)Angular distribution of number density $\log_{10}(\mathcal{N}_{+})$, with $\mathcal{N}_{+}=dN_{e^+}/({\rm sin}{\theta}d\theta d\phi)$ , (b) average longitudinal polarization $\bar{S}_{\parallel}$ and (c) average energy $\bar{\varepsilon}_+$ of positrons vs $\theta$$\in [0,30^\circ]$ and $\phi$$\in [0,360^\circ]$. Here, $\theta$ and $\phi$ are the polar and azimuthal angles with respect to $+z$ axis. Purple curves are normalized density, $\bar{S}_{\parallel}$ and $\bar{\varepsilon}_{+}$ with respect to $\theta$ along $\phi=0, 180^\circ$. (d)Distribution of $\bar{S}_{\parallel}$ vs $\varepsilon_{+}$ and $\theta$. (e) Positron yield (black solid), average polarization (red dashed), and average energy (blue dash-dotted) as functions of the lower energy cutoff $\tilde{\varepsilon}_+$; (f) the same quantities as functions of the angular acceptance $\tilde{\theta}$. The energy selection corresponds to collecting positrons with energies from $\tilde{\varepsilon}_+$ to 10 GeV, while the angular selection corresponds to collecting positrons within a polar angle from $0^\circ$ to $\tilde{\theta}$.
• Figure 3: (a) Initial longitudinal ${S}_{\parallel}^i$ and transverse ${|{S}_{\perp}^i|}$ polarization of a positron at birth versus $\theta$ at $\phi=0^\circ$. (b) Corresponding initial momentum components $\bar{{p}_{z}^i}$, $\bar{{p}_{x}^i}$ (in units of $mc$). (c) Temporal evolution for a representative positron (radiation reaction neglected) in a plane-wave laser field: (c1) laser field components $E_{x^{'}}$, $E_{y^{'}}$, and the longitudinal Lorentz force component of $({\bm v}\times {\bf B})_{z^{'}}$; (c2) momentum components $p_{x^{'}}$, $p_{y^{'}}$, $p_{z^{'}}$; (c3) polarization components $S_\perp$, ${S}_{\parallel}$ (for physical and $g=2$$g$-factor), and $S_{y^{'}}$. (d) Spatial and dynamic evolution in the realistic laser field: (d1) spatial trace $r=\sqrt{(x^2+y^2)}$ with the laser focus profile (black dash-dotted lines); (d2) dynamics of ${|{S}_{\perp}|}$, ${S}_{\parallel}$, $p_z$ and $({\bm v}\times {\bf B})_{z}$.
• Figure 4: (a) Final longitudinal polarization $\bar{S}_{\parallel}$ (red dash-dotted) and creation phase $\bar{\eta^i}$ (blue solid) vs energy $\varepsilon_{+}$. (b) Birth phase distribution for positrons in Fig. \ref{['fig:2']}(b).
• Figure 5: Profile curves of $\bar{S}_{\parallel}$ vs $\theta$ in $\phi=0^\circ$ and $\bar{S}_{\parallel}$ vs energy $\varepsilon_{+}$ in $\theta\in[0,12^\circ]$ with different laser intensity $a_0$ (a), laser pulse duration $\tau$ (b), laser focal radius $w_0$ (c) and initial seed electron energy $\varepsilon_0$ (d). Other parameters are the same as those in Fig.\ref{['fig:2']}.