Polarized Positrons at a Future Linear Collider and the Final Focus Test Beam

Abstract

Having both the positron and electron beams polarized in a future linear e + e − collider is a decisive improvement for many physics studies at such a machine. The motivation for polarized positrons, and a demonstration experiment for the undulator-based production of polarized positrons are reviewed. This experiment ('E-166') uses the 50 GeV Final Focus Test electron beam at SLAC with a 1 m-long helical undulator to make ≈ 10M eV polarized photons. These photons are then converted in a thin (≈ 0.5 radiation length) target into positrons (and electrons) with about 50% polarization. FOR THE E-166 COLLABORATION * Having both the positron and electron beams polarized in a future linear e + e − collider is a decisive improvement for many physics studies at such a machine. The motivation for polarized positrons, and a demonstration experiment for the undulator-based production of polarized positrons are reviewed. This experiment ('E-166') uses the 50 GeV Final Focus Test electron beam at SLAC with a 1 m-long helical undulator to make ≈ 10MeV polarized photons. These photons are then converted in a thin (≈ 0.5 radiation length) target into positrons (and electrons) with about 50% polarization.

Generation of circularly polarized photons for a linear collider polarized positron source

“…Below we consider a round collimator with the opening angle ϑ col and axially symmetric distributions f (θ e ) ≡ f (θ 2 e ). Remember that the quantities A, B depend on the angles n ⊥ and θ e only in the combination (see (8)) n ef = n ⊥ − θ e . The azimuth of n ef enters only the linear polarization via sin 2φ and cos 2φ, while the value of n 2 ef is fixed by δ-function ( (γn ef ) 2 = y 2 = n/ν − (1 + ξ 2 )).…”

“…where Θ(X n ) is the step function: Θ(x) = 1 for x > 0 and Θ(x) = 0 for x < 0 , the variable ν ∝ ω is defined in (8). The function F (X n , ϑ col ), which appears in integration over dn ⊥ in Eq.…”

“…This scheme is proposed in [7] for TESLA LC. It is also considered for the NLC project and a dedicated experiment [8] is to be done at SLAC as a "proof of principle". One of the challenges of this method, which requires a very long (∼ 200 m) undulator, is an accurate alignment.…”