A pr 2 00 3 Is it possible to obtain polarized positrons during multiple Compton backscattering process ?

1. In existing projects of electron-positron colliders, the option of polarized electron and positron beams is considered [1,2]. While one can consider the problem of producing the polarized electron beams with required characteristics as having been solved [3], the existing approaches to polarized positrons generation [4-7] do not provide required parameters. In quoted papers the schemes were offered, in which by means of various methods a beam of circularly-polarized (CP) photons with energy of ∼ 10 1 MeV is generated to be subsequently used for producing the longitudinally polarized positrons during the process of pair creation in the amorphous converter. In this paper an alternate approach is discussed-at the first stage the unpolarized positrons are generated by the conventional scheme (interaction of an electron beam with energy of ∼10 1 GeV with an amorphous or crystalline converter), which are accelerated up to energy ∼ 5 ÷ 10 GeV and then interact with intense CP laser radiation. In the scheme of " laser cooling " of an electron beam suggested in the paper [8], electrons with energy of 5 GeV in head-on collisions with laser photons lose their energy practically without scattering. Thus, as a result of a multiple Compton scattering (MCS), the electron beam " is decelerated " resulting in some energy distribution, which variance is determined by the electron energy and laser flash parameters. It is clear that the laser cooling process will accompany also the interaction of positrons with laser photons. If we consider unpolarized positron beam as a sum of two fractions of the identical intensity with opposite signes of 100% longitudinal polarization, its interaction with CP laser radiation results in different Compton effect cross-sections for positrons with opposite helicity. In other words, positrons polarized in opposite directions lose a various part of the initial energy, therefore, by means of momentum selection of the resulting beam, it is possible to get a polarized positron beam with some intensity loss. 2. Let us write the Compton effect cross-section of CP photons on relativistic positrons after summing over scattered photon polarization [9] (the system of units being used hereinafter is ¯ h = m = c = 1): dσ dy = πr 2 0 x 1 1 − y + 1 − y − s 2 − ξ 0z P c cy 2 − y 1 − y − − ξ z P c