Spiral Momentum Structures in Electron and Positron Emission from One- or Two-color Laser Field Positronium Breakup

Positronium (Ps), a hydrogen-like bound state of an electron and a positron, provides an interesting opportunity to study strong-field dynamics in quantum systems. Najjari[1] recently investigated positronium breakup via Compton scattering in a photon-energy regime where both the electron and positron remain nonrelativistic. Under intense laser fields, positronium can dissociate into free electron–positron pairs, providing direct access to correlated momentum dynamics with no analogue in ordinary atomic ionization.

 

Our preliminary results show that, for one-photon absorption driven by time-delayed oppositely circularly polarized laser pulses, the electron and positron momentum distributions display spiral-arm structures that are exact mirror images of each other. This behavior originates from the opposite charges of the electron and positron and reflects the underlying charge-conjugation symmetry of positronium in strong circularly polarized fields. In contrast, a two-color laser field introduces quantum-path interference that breaks this symmetry, resulting in distinct, phase-dependent differences between the electron and positron momentum distributions.

 In contrast to the single-color linear regime, a two-color   field introduces quantum-path interference that breaks charge-conjugation symmetry, leading to distinct, phase-dependent differences between the electron and positron momentum distributions. A finite center-of-mass momentum acts as an additional symmetry-breaking control knob that, when combined with two-color circularly polarized fields, leads to strongly inequivalent and directionally separated electron and positron momentum distributions following positronium breakup.

 

In summary, we show that tailored laser fields can controllably break charge-conjugation symmetry in positronium breakup, enabling tunable and strongly distinct electron and positron momentum distributions with no counterpart in conventional atomic ionization.

Reference:

[1] B. Najjari et al., Phys. Rev. A 111, 023109 (2025).