Probing the parton densities of polarized photons at a linear e^+e^- collider

TL;DR

This paper reviews the current understanding of spin-dependent parton densities in polarized photons and proposes that measurements at future linear e^+e^- colliders, especially using backscattered laser photons, can effectively determine these densities.

Contribution

It demonstrates that measurements of spin asymmetries and di-jet distributions at a linear collider can uniquely determine polarized photon parton densities, highlighting the importance of backscattered laser photons.

Findings

Backscattered laser photons are highly effective for polarized photon measurements.

Deep-inelastic spin asymmetry measurements can determine quark and gluon densities.

Theoretical review of spin-dependent photon parton densities.

Abstract

The present theoretical status of spin-dependent parton densities Delta f^{\gamma}(x,Q^2) of circularly polarized photons is briefly reviewed. It is then demonstrated that measurements of the deep-inelastic spin asymmetry A_1^{\gamma}\simeq g_1^{\gamma}/F_1^{\gamma} and of di-jet rapidity distributions at a future linear e^+e^- collider appear to be particularly suited for a determination the spin-dependent photonic quark and gluon densities, respectively. Special emphasis is devoted to a comparison of the different sources of polarized photons at a linear collider: the equivalent photon approximation and backscattered laser (Compton) photons. It is shown that backscattered laser photons are highly favorable, even indispensable, for decent measurements of the Delta f^{\gamma}(x,Q^2).