Laser-induced selection of longitudinal $Z$ bosons in resonant $e^+e^-$ annihilation

arXiv:2606.09394v3 Announce Type: replace-cross Abstract: Longitudinal $Z$-boson production is strongly suppressed in collinear $e^+e^-$ annihilation in vacuum due to current conservation and helicity selection. We show that this suppression is lifted when the incoming leptons are nonperturbatively dressed by an intense circularly polarized plane wave. The dressing produces two distinct effects. The Volkov quasi-momenta shift the collision invariant and allow an initially subresonant collision to reach the $Z$-boson pole, whereas the Volkov spinor prefactors generate additional Dirac structures in the neutral current. In the collinear geometry, the field-quadratic contribution to the $n=0$ harmonic obeys a helicity-selection rule: it vanishes identically for the transverse states ($\lambda=\pm1$) of $Z$-boson and survives only for the longitudinal state ($\lambda=0$). At sufficiently large $\xi$ along the physically allowed $n=0$ resonance trajectory, this helicity-selective contribution becomes dominant, simultaneously driving the growth of the total rate and the enrichment of the longitudinal helicity fraction. We further show that incident-energy asymmetry does not appreciably shift this crossover, because its apparent enhancement of the light-front denominator cancels in the dominant spin-summed matrix element. Instead, the asymmetry controls the laboratory-frame boost of the resonantly produced $Z$ boson. These results identify a nonlinear helicity-selection mechanism that separates laser control of resonant kinematics from laser control of electroweak-boson polarization.

Abstract:Longitudinal $Z$-boson production is strongly suppressed in collinear $e^+e^-$ annihilation in vacuum due to current conservation and helicity selection. We show that this suppression is lifted when the incoming leptons are nonperturbatively dressed by an intense circularly polarized plane wave. The dressing produces two distinct effects. The Volkov quasi-momenta shift the collision invariant and allow an initially subresonant collision to reach the $Z$-boson pole, whereas the Volkov spinor prefactors generate additional Dirac structures in the neutral current. In the collinear geometry, the field-quadratic contribution to the $n=0$ harmonic obeys a helicity-selection rule: it vanishes identically for the transverse states ($\lambda=\pm1$) of $Z$-boson and survives only for the longitudinal state ($\lambda=0$). At sufficiently large $\xi$ along the physically allowed $n=0$ resonance trajectory, this helicity-selective contribution becomes dominant, simultaneously driving the growth of the total rate and the enrichment of the longitudinal helicity fraction. We further show that incident-energy asymmetry does not appreciably shift this crossover, because its apparent enhancement of the light-front denominator cancels in the dominant spin-summed matrix element. Instead, the asymmetry controls the laboratory-frame boost of the resonantly produced $Z$ boson. These results identify a nonlinear helicity-selection mechanism that separates laser control of resonant kinematics from laser control of electroweak-boson polarization.

arXiv:2606.09394 [hep-ph]

(or arXiv:2606.09394v3 [hep-ph] for this version)

https://doi.org/10.48550/arXiv.2606.09394 Focus to learn more arXiv-issued DOI via DataCite

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Laser-induced selection of longitudinal $Z$ bosons in resonant $e^+e^-$ annihilation

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